GIFT  OF 
Dean  Frank  H.  Probert 


Mining  Dept 


A  TEXT-BOOK 


OF 


QUANTITATIVE 
CHEMICAL  ANALYSIS. 


BY  FRANK  JULIAN. 


FIRST  EDITION. 


THE  RAMSEY  PUBLISHING  CO. 
ST.  PAUL,  MINN. 

COPYRIGHT  I9O2,  BY    FRANK   JULIAN. 


""•  rrPT. 


"//  is  ufual  with  Chymifts  to  affert  their  Principles,  Salt,  Sul- 
phur, Mercury,  and  fame  add  Caput  Mortuum  and  Flegm;  butwhenwe 
exactly  confider,  there  is  nothing  f olid  to  be  built  upon;  for  fome  call 
that  which  is  Oyly,  fulphur;  that  which  vapoureth,  Mercury;  that 
which  concreteth,  fait;  but  this  doth  notfatisfy  us  of  the  wifer  fort. 
For  we  know  there  is  fomething  that  is  not  Oyly  which  is  fulphurous, 
as  Aqua  Vitae,  &c.  and  fometimes  vapours,  which  differs  front 
Mercury,  as  the  Flegma  in  Stillings,  Alfo  there  are  many  things 
that  want  the  oyly  part,  do  they  therefore  want  Sulphur?  And  the 
examples  they  bring  to  prove  it,  are  more  difficult,  taken  from  com- 
buftible  wood  and  anatomy  of  Vegetables.  In  the  firft  example  they 
will  have  Sulphur  reprefented  by  Butter,  Mercury  by  Whey,  Salt 
'  by  Cheefe.  In  the  fecond  they  call  that  fulfur  that  fumes,  Mercury 
that  which  fmoaks,  fait  that  which  remaineth  in  the  afhes.  In  the 
anatomy  of  Vegetables  they  fay  there  is  oyl  that  is  Sulphur,  Water 
that  is  Mercury,  and  afhes  full  of  fait.  But  who  knows  not,  but 
that  in  Whey  there  is  more  fait  than  in  the  Cheefe  ?  That  foot 
{which  is  congealed  fume)  contains  in  it  self,  oyl,  fait,  and  fpiritual 
water?  And  fome  vegetables  have  not  a  drop  of  Oyl." 


CONTENTS. 

PART  1. 

PAGE 

Chapter  1.  Introduction 7 

2.  Sampling.    Preparation  of  the  sample  for  analysis      ....  18 

3.  The  Balance  and  Weights 29 

4.  The  Operations  of  Analysis 45 

Weighing  the  Sample 45 

Solution 46 

Evaporation 67 

Distillation 62 

Precipitation 68 

Separation 74 

Filtration 86 

Washing  precipitates 96 

Ignition 100 

6.  Volumetric  Analysis 110 

6.  Gasometry 139 

7.  Attributive  Methods 155 

8.  Calculation  of  Analyses 174 

9.  Errors  and  Precautions 190 

PART  2. 

Reagents 205 

Exercises    1.  Alcohol 213 

2.  Lead  carbonate  — Ferfous  sulf ate 215 

3.  Sodium  chloride 216 

4.  Coffee  —  Ginger 217 

6.  Cast  Iron 219 

6.  Ether 220 

7.  Standard  acid  and  alkali 221 

8.  Vinegar  —  Lemon  juice 223 

9.  Chloral  hydrate 223 

10.  Acetic  acid  .     .     . 225 

11.  Hydrastis 227 

12.  Guarana 227 

13.  Standard  permanganate 229 

14.  Potassium  chlorate  —  Forge  scale 229 

15.  Chrome  yellow 231 

16.  Metol 233 

17.  Sodium  thiosulfate  — Steel 234 

18.  Galena 237 

19.  Barium  chloride        238 

(3) 


4  QUANTITATIVE    CHEMICAL    ANALYSIS. 

PAGE 

Exercises  20.  Lard       240 

21.  Potassium  permanganate 242 

22.  Air  —  Ammonium  sulf ate 244 

23.  Nickel-Copper  alloy 247 

24.  Wollastonite 251 

Additional  analyses .    .  266 

PART  8. 

/ 

SPECIAL   METHODS   AND   TECHNICAL  ANALYSIS. 

Colorimetry 259 

The  Fire  Assay 268 

Electrolysis 278 

The  Metals  and  Common  Acids 289 

Ultimate  Organic  Analysis 295 

Proximate  Organic  Analysis 311 

Chlorimetry 322 

Iron  and  Steel  —  Iron  Ores 828 

Coal 369 

Natural  Water 366 

Fertilizers 382 

The  Alcohols  —  Glycerol 393 

The  Alkaloids 1  . 409 

The  Tannins 421 

The  Carbohydrates * 427 

The  Oils  and  Fats 462 

Soaps 469 

Milk  and  Butter • 476 

Urinalysis 493 

The  Organic  Dyestuffs 606 

PART  4. 

Notes  on  the  Methods  of  Analysis 621 

APPENDIX. 

Technical  and  Industrial  Analysis      . 667 

Tables 692 

Index  599 


PEEFACB. 

This  volume  is  intended  for  the  aid  of  students  who,  having  a  fair  acquaint- 
apce  with  the  elements  of  general  chemistry,  can  devote  a  limited  time  to 
quantitative  analysis  concurrent  with  or  following  the  usual  qualitative  course; 
and  as  an  introduction  to  the  monographs  on  special  departments  of  technical 
analysis  for  those  purposing  to  engage  in  some  particular  branch  as  a  future 
occupation. 

In  Part  1,  after  outlining  the  general  principles  of  the  art,  there  are  described 
the  operations  of  solution,  precipitation,  etc.,  and  the  appliances  commonly 
employed  for  the  purposes. 

Following  is  a  graded  series  of  exercises  chosen  with  a  view  to  illustrate  the 
leading  principles  in  analysis  and  afford  practice  in  the  usual  manipulations. 
They  are,  for  the  most  part,  simple  and  easy  of  execution,  and  call  for  only 
such  apparatus  as  is  commonly  found  in  the  laboratories  of  educational  insti- 
tutions. Directions  are  given  in  full  detail  and  have  been  closely  followed  in 
the  analyses  whose  results  are  appended. 

In  Part  3  is  considered  the  analytical  behavior  of  a  number  of  articles  of 
commercial  importance.  It  has  been  attempted  to  outline  the  most  approved 
methods  for  their  analysis  and  to  annotate  some  others  that  are  of  interest 
from  their  promise  of  future  development  or  as  suggesting  the  application  of 
less  familiar  principles.  Working  details  and  criticisms  have  been  largely 
omitted  as  they  would  be  useless  unless  accompanied  by  particulars  and  pre- 
cautions too  voluminous  for  insertion  here;  for  these  there  may  be  consulted 
the  standard  treatises  on  the  various  subjects  and  the  references  given,  which 
are,  wherever  possible,  to  original  articles  or  abstracts  in  English. 

In  Part  4  are  presented  some  notes  and  observations  relating  to  the 
principles  and  practice  of  the  art  in  general  that  may  be  of  interest  to  the 
student. 

To  treat  with  any  degree  of  completeness  within  the  compas^  of  a  text-book,  a 
subject  so  extensive  and  so  essentially  one  of  detail  as  quantitative  analysis  is 
of  course  out  of  the  question,  and  in  presenting,  as  is  here  attempted,  a  general 
view  of  the  art  as  practiced  at  the  present  time,  there  arises  the  difficulty  of 
deciding  as  to  what  matters  should  be  given  prominence  and  what  but  touched 
upon.  In  any  event,  much  of  importance  can  only  be  referred  to  and  much  of 
interest  must  be  omitted  altogether,  leaving  to  the  instructor  to  amplify  and 
particularize  to  the  extent  he  may  consider  most  profitable  to  the  student. 

More  prominence  than  is  usual  m  treatises  on  quantitative  analysis  has  been 
accorded  to  the  principles  underlying  the  methods  of  analysis.  This  course 
may  not  appeal  to  those  who  regard  analytical  chemistry  only  as  a  means  to  a 
financial  end,  but  a  broader  view  must  perceive  that  the  art  as  a  whole  will 
only  advance  in  proportion  as  the  basic  principles  are  better  understood,  and  if 
it  is  ever  to  attain  the  dignity  of  a  science  those  who  contribute  to  this  end 
will  need  a  more  comprehensive  knowledge  of  the  art  than  is  afforded  by  the 
study  of  a  string  of  detailed  recipes,  however  practically  useful  they  may  be. 

In  an  appendix  I  have  ventured  to  discuss  at  some  length  certain  phases  of 

(5) 


6  QUANTITATIVE  CHEMICAL   ANALYSIS. 

the  important  subject  of  the  practice  of  technical  and  industrial  chemical  analy- 
sis. Though  much  of  any  comments  on  a  theme  of  this  kind  must  necessarily 
be  but  personal  opinions  and  accepted  as  such,  a  somewhat  extended  experi- 
ence and  observation  confirms  my  belief  in  the  correctness  of  the  views  there 
set  forth. 

The  author  will  be  grateful  if  his  attention  is  called  to  any  errors  that  may 
be  noted. 

F.  J. 


CHAPTEK   1. 

INTRODUCTION. 

Quantitative  chemical  analysis  is  the  art  of  ascertaining  the  relative  propor- 
tions of  the  constituents  of  any  form  of  complex  matter  through  the  applica- 
tion of  physical  forces  aided  by  chemical  reactions. 

It  is  based  mainly  on  these  laws :  that  the  extent  of  a  measurable  physical 
or  chemical  attribute  of  a  body  varies  with  the  mass  of  the  body ;  especially 
that  at  any  given  locality,  weight  bears  a  constant  ratio  to  mass ;  that  among 
the  atoms  of  every  chemical  compound  there  exists  a  definite  invariable  pro- 
portion ;  and  that  every  synthesis  or  decomposition  of  a  molecule  or  rearrange- 
ment of  molecules  is  governed  by  fixed  laws.  A  successful  practice  of 
the  art  calls  for  a  knowledge  of  the  principles  of  chemistry,  the  physical 
properties  of  bodies  and  their  chemical  behavior,  and  manual  skill  to  perform 
the  mechanical  operations  of  analysis  without  loss  of  the  substance  analyzed. 

Under  a  strict  construction  of  the  term  quantitative  chemical  analysis  there 
are  comprehended  only  those  processes  involving  and  depending  on  chemi- 
cal reactions,  but  custom  has  widened  the  scope  to  include  also  methods 
based  on  the  measurement  of  some  physical  attribute  of  the  substance  analyzed 
and  even  purely  mechanical  separations. 

Any  form  of  complex  matter,  animal,  vegetable  or  mineral,  may  be  subjected  to 
a  quantitative  examination  with  an  outcome  more  or  less  successful.  For  as 
every  art  is  hedged  by  current  limitations,  so  an  analysis  may  be  easy  or  difficult 
or  impossible.  The  common  metals,  the  inorganic  acids,  a  few  organic  bodies 
and  their  compounds  have  such  pronounced  and  clean-cut  relations  toward 
solvents  and  precipitants  that  their  separation  from  solutions  and  from  each 
other  can  be  done  with  ease  and  precision,  and  with  a  fair  degree  of  skill, 
exact  determinations  can  be  made,  or  at  least  sufficiently  exact  for  all  practi- 
cal purposes. 

But  it  is  quite  the  contrary  with  many  mixtures,  such  as  those  of  the  rare 
earths,  the  alkaloids,  oils,  vegetable  extracts,  etc.,  which  oppose  peculiar  diffi- 
culties against  their  separation  and  determination,  from  their  indifference  to 
most  reagents  and  similarity  of  behavior  towards  others ;  and  so  one  is  often 
restricted  to  reactions  ill-defined  and  modified  or  obscured  by  known  or  un- 
known influences,  and  in  general  must  be  content  with  but  approximate  result* 
even  under  the  most  favorable  circumstances. 

The  object  for  which  an  analysis  is  undertaken  may  be  either  (A)  to  ascer- 
tain or  prove  the  composition  of  a  chemical  compound  or  derive  its  empirical 
or  rational  formula;  or  (B)  to  separate  the  components  of  a  mechanical 
mixture,  or  find  the  percentage  of  one  or  more  of  the  valuable  constituents 
or  detrimental  impurities  in  a  natural  product  or  an  article  of  commerce.  In 
general,  the  analysis  of  a  substance  belonging  to  either  class  follows  the  same 
lines,  the  only  difference  being  in  the  preliminary  treatment  of  the  material, 
as  for  the  former  it  is  essential  that  the  aggregate  of  the  impurities  contained 
shall  at  most  not  exceed  the  minimum  of  error  inherent  to  the  method  adopted 
for  the  analysis,  and  so  it  is  admissible  and  often  imperative  that  some  mode 

(7) 


t   *, :  -k        \  .QUANTITATIVE    CHEMICAL   ANALYSIS . 


of  purification  be  resorted  to ;  while  with  the  latter  class  the  only  alterations 
allowable  are  those  of  comminution  and  the  removal  of  hygroscopic  water  or 
foreign  bodies  known  to  have  been  accidentally  introduced. 

The  manipulations  in  solution,  filtration,  evaporation,  etc.,  are  practically 
the  same  as  those  familiarized  by  qualitative  analysis,  and  only  call  for  greater 
care  to  avoid  losses  and  gains  during  these  operations.  In  addition,  there 
must  be  learned  the  arts  of  weighing,  accurate  measuring  of  liquids  and  gases, 
and  the  comparison  of  shades  of  colors,  besides  a  certain  manual  dexterity  and 
lightness  in  working  with  fragile  glassware  and  instruments  for  precise 
measurements.  As  in  other  arts,  some  will  quickly  become  proficient,  others 
by  dint  of  practice,  while  a  few  seem  devoid  of  any  sort  of  mechanical  knack. 

TERMINOLOGY. 

1.  In  general,  a  gravimetric  analysis  is  performed  by  separating  successively 
from  a  weighed  amount  of  the  substance,  each  of  the  constituents  in  the  solid 
form,  either  isolated  or  in  combination  with  other  elements,  and    from  its 
weight  calculating   its  proportion  in  the  original   substance.    Of  volumetric 
analysis  the   basis  is  a  chemical  reaction  between  a  constituent    and   some 
reagent;    the  weight  of  the  former  being  calculated  from  the  weight  of  the 
latter  required  to  produce  an  exactly  complete  reaction  between  them,  neither 
remaining  in  excess.    In  gasometry,  from  a  measured  volume  of  a  mixture  of 
gases,  each  is  absorbed  in  turn  by  a  suitable  solid  or  liquid  reagent,  the  dimin- 
ution in  volume  showing  its  proportion  therein.    Attributive  methods  are  based 
on  the  rule  that  the  extent  to  which  a  measurable  attribute  is  exhibited  by  a 
body  is  proportional  to  the  mass  of  the  body. 

2.  In  a  proximate  analysis  the  component  species  or  groups  of  a  chemical 
compound  or  a  mixture  are  determined,  while  an  ultimate  analysis  shows  the 
proportions  of  the   elements  present.    For  example,  the  composition  of  a 
mixture  of  gases  may  be  stated  in  either  of  the  following  ways :  — 

Proximate  Analysis.  Ultimate  Analysis. 

Hydrocarbons 71.80  Carbon 68.98 

Carbon  monoxide 25.60  Hydrogen 14.63 

Carbon  dioxide 1.50  Oxygen 16.39 

Water 50 

The  purpose  for  which  the  result  is  to  be  employed  decides  which  of  the  two 
will  be  of  the  greater  service;  also  whether  a  partial  analysis,  showing  but  a 
few  of  the  constituents,  will  suffice,  or  a  complete  one  giving  all  (or  as  many  as 
can  be  determined)  will  be  necessary. 

3.  Finding  the  proportion  of  any  constituent  of  a  compound  or  mixture  is  called 
a  determination,  estimation,  trial,  or  test;  if  present  in  the  quantity  of  the  sub- 
stance ordinarily  weighed  for  analysis  in  so  small  a  proportion  as  to  be  un- 
weighable,  it  is  reported  as  a  trace  or  color.    If  only  the  most  valuable  or 
important  constituent  is  determined,  the  process  is  termed  assaying  and  the 
result  an  assay;*  thus,  quinometry,  the  assay  of  cinchona  bark  for  quinine; 
morphiometry,  the  assay  of  the  poppy  for  morphine.    In  metallurgical  analysis 
a  wet  assay  is  one  made  by  ordinary  methods,  the  reactions  taking  place  in 
aqueous  solutions,  while  in  a  dry  assay  or  fire  assay  the  substance  for  examina- 
tion is  melted  with  the  proper  fluxes  in  an  earthen  crucible  heated  in  a  furnace. 

4.  A  method  or  scheme  comprises  the  directions  for  performing  an  analysis, 
a  scheme  more  usually  applying  to  the  complete  analysis  of  a  complex  ma- 


*  Chem.  News,  1888-1—140, 160, 170. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  9 

terial.  If  carried  out  exactly  as  formulated  with  the  exception  that  the  sub- 
stance to  be  analyzed  is  omitted,  the  determination  is  called  a  blank  or 
"dummy."  If  there  is  analyzed  along  with  the  sample  to  be  tested  and  under 
the  same  conditions,  another  sample  whose  composition  is  known  from  a 
previous  analysis  by  another  method,  the  latter  is  termed  a  parallel  deter- 
mination; while  a  proof  or  synthetic  proof  is  a  mixture  made  up  with  chemi- 
cally pure  constituents  to  the  same  composition,  as  near  as  may  be,  of  the 
substance  to  be  analyzed,  and  both  proceeded  with  under  uniform  conditions. 

'5.  In  volumetric  or  titrimetric  analysis  the  addition  of  a  measured  volume 
of  one  solution  A  (the  titrand)  to  another  B  (the  titrate  —  a  solution  of  the 
substance  to  be  tested)  producing  an  exactly  complete  reaction  between  the 
two  is  called  a  titration.  The  amount  of  reagent  dissolved  in  a  unit  volume 
of  A  or  the  weight  of  substance  in  B  that  this  volume  of  A  exactly  com- 
bines with  is  termed  the  titre,  strength,  or  standard  of  A,  and  A  a  standard 
or  set  solution. 

Acidimetry  is  the  process  of  finding  the  strength  of  a  solution  of  an  acid, 
usually  by  the  aid  of  a  standard  solution  of  an  alkali;  alkalimetry  is  there- 
verse  of  this.  Chlorimetry  and  iodimetry  are  respectively  the  measurement  ot 
free  chlorine  and  iodine  by  a  standard  solution  of  seme  reducing  agent. 

6.  The  decomposition  of  a  solution  of  a  metallic  salt  by  the  agency  of  an 
electric  current  is  called  electrolysis,  and  the  salt  an  electrolyte.  The  con- 
ductors through  which  electricity  passes  from  the  solution  are  termed  elec- 
trodes and  are  usually  platinum  plates,  the  one  by  which  the  positive 
electricity  leaves  being  the  cathode,  and  the  other  the  anode,  the  metal  from 
the  decomposition  of  the  electrolyte  being  deposited  on  the  former. 


The  methods  available  in  practice  are  as  diversified  in  their  nature  as  the 
genera  of  the  material  analyzed,  every  known  qualitative  reaction  having  been 
scrutinized  with  the  object  of  pressing  it  into  service,  though  often  without 
success ;  and  frequently  when  called  on  to  analyze  a  substance,  the  chemist 
will  find  a  purely  physical  method  to  give  more  accurate  results  and  with 
greater  expedition  —  in  fact  he  at  times  has  no  alternative  from  the  lack  of  a 
suitable  chemical  method. 

Omitting  details,  the  principal  methods  of  analysis  may  be  outlined  as 
follows : — 

A.    GRAVIMETRIC   METHODS. 

1.  By  direct  weight.  In  which  each  element  is  separated  and  weighed,  either 
alone  or  more  commonly  in  combination  with  another  or  others.  Thus  gold  is 
always  separated  and  weighed  as  an  element,  while  zinc,  difficult  to  obtain  in 
the  metallic  form,  is  usually  weighed  in  combination  with  oxygen  as  zinc 
protoxide,  a  simple  calculation  giving  the  weight  of  the  metal  in  the  oxide. 

The  routine  is  about  as  follows:  the  material  to  be  analyzed  is  dried  if 
necessary,  and  a  small  portion  weighed  and  dissolved  in  water  or  an  acid.  Many 
substances  require  a  special  preliminary  treatment  before  solution  can  be 
effected.  A  reagent  is  then  introduced  which  will  precipitate  one  of  the  com- 
ponents in  an  insoluble  form,  leaving  the  remainder  in  solution;  this  precipi- 
tate is  caught  on  a  paper  filter,  the  clear  liquid  passing  through,  and  after 
removing  what  solution  adheres  to  it  by  washing  with  water,  and  the  water  by 
heating,  it  is  weighed.  Should  the  precipitate  be  of  such  a  nature  that  it  can- 
not be  brought  to  a  definite  formula  for  weighing,  it  is  redissolved  and  again 


10  QUANTITATIVE    CHEMICAL   ANALYSIS. 

precipitated  in  a  more  suitable  combination.     The  other    constituents  are  in 
turn  thrown  out  of  the  solution  and  weighed,  as  before  described. 

For  example,  of  an  alloy  of  tin,  lead,  and  copper,  a  weighed  portion  is  treated 
with  nitric  acid;  the  copper  and  lead  dissolve  as  nitrates,  while  the  tin  is 
oxidized,  mainly  to  insoluble  hydrated  metastannic  acid  (HaSngOu.iEkO).  After 
nitration,  the  metastannic  acid  is  freed  from  any  adhering  solution  of  the 
copper  and  lead  nitrates  by  washing  it  with  water,  then  heated  until  anhydrous, 
becoming  stannic  oxide  (SnC>2) ,  and  weighed.  As  pure  stannic  oxide  invariably 
contains  a  certain  fractional  part  of  its  weight  of  tin,  the  product  of  the  weight 
of  the  former  by  this  fraction  is  the  weight  of  the  tin  contained. 

The  nitrate  and  washings  from  the  metastannic  acid  are  united  and  mixed 
with  an  excess  of  sulf uric  acid.  The  lead  and  copper  nitrates  become  sulfates 
liberating  nitric  acid.  The  lead  sulfate  being  insoluble  precipitates;  and  filter- 
ing, etc.,  as  before  gives  its  weight  and  that  of  the  lead  by  a  similar  calculation. 
From  the  filtrate  from  the  lead  sulfate  the  copper  is  precipitated  as  cupric  sul- 
fide  by  means  of  hydrogen  sulfide;  the  precipitate  loses  on  ignition  one-half  of 
its  sulfur  becoming  cuprous  sulflde  (Cu2S)  and  is  weighed  as  such. 

Other  elements  such  as  iron  and  zinc  are  generally  present,  but  in  minute 
proportions  only.  If  it  is  desired  to  determine  them,  separate  larger  weights 
of  the  alloy  are  dissolved  and  treated  according  to  special  methods. 

It  must  not  be  supposed  that  the  actual  analysis  is  as  simple  as  would 
appear  from  the  above.  The  metastannic  acid  always  retains  small  amounts 
of  copper  and  lead  which  must  be  reclaimed  before  the  stannic  oxide  is 
weighed;  the  lead  sulfate  is  somewhat  soluble  in  nitric  acid  and  water,  so  the 
former  must  be  removed  by  evaporation,  and  dilute  alcohol  substituted  for  the 
latter;  and  precautions  must  be  taken  during  the  ignition  of  the  copper  sul- 
flde to  preserve  it  from  oxidization  by  the  air,  and  of  the  lead  sulfate  from 
reduction  to  the  sulfide  or  metal  by  the  carbon  of  the  filter  paper. 

When  a  non- volatile  stable  compound  is  held  in  a  solution  it  may  be  deter- 
mined by  evaporating  the  latter  to  dryness  and  weighing  the  residue;  this 
presupposes  that  other  solid  compounds  are  either  absent  or  volatilized  during 
the  evaporation  or  on  heating  the  residue  to  a  temperature  insufficient  to  affect 
the  first;  but  as  these  conditions  can  seldom  be  easily  obtained  the  method  is 
of  limited  application. 

The  Fire-assay.  Applied  exclusively  to  metaliferous  ores,  mattes  and  slags, 
and  a  few  alloys,  and  principally  to  ores  of  gold  and  silver  which  contain  but 
minute  amounts  of  these  metals  disseminated  through  a  silicious  or  earthy 
gangue.  According  to  circumstances  either  the  crucible  or  scoriflcation  pro- 
cess is  employed. 

Crucible  Fusion.  A  fire-clay  crucible  contains  a  mixture  of  the  ore  with 
suitable  fluxes  like  lead  protoxide,  sodium  carbonate,  etc.,  together  with  a  half- 
gram  or  so  of  carbon.  It  is  covered  and  heated  to  redness  in  a  furnace,  when 
the  gangue  of  the  ore  unites  with  the  fluxes  forming  a  fluid  slag.  Simulta- 
neously the  carbon  reduces  its  equivalent  of  the  lead  oxide  to  metallic  lead 
(2PbO +C-f  heat=2Pb -|- CO2);  the  minute  particles  sinking  through  the 
mobile  slag  collect  the  gold  and  silver  on  the  way,  the  alloy  eventually  gather- 
ing in  a  globule  at  the  bottom  of  the  crucible. 

Scoriflcation.  When  certain  interfering  elements,  as  arsenic,  antimony  or 
copper,  are  present  in  an  ore  the  scorification  process  is  usually  preferred  to 
the  crucible  fusion.  The  ore  is  mixed  with  metallic  lead  and  a  trifle  of  borax 
and  heated  in  an  open  shallow  clay  dish  (a  scorifler)  standing  in  a  muffle  (a 
thin  semi-cylinder  of  fire  clay)  heated  to  whiteness  in  a  furnace.  The  front 
of  the  muffle  is  open  and  allows  free  access  of  air  to  the  scorifler,  the  oxygen 


QUANTITATIVE    CHEMICAL    ANALYSIS.  11 

Slowly  converting  part  of  the  lead  to  oxide.  The  lead  oxide  gives  up  its 
oxygen  to  the  arsenic,  etc.  The  silica  and  bases  of  the  ore,  the  oxide  of  lead, 
and  the  oxides  of  copper,  etc.  form  a  fluid  slag  floating  on  the  surface  of  the 
lead -silver-gold  alloy. 

Cupellation.  Subsequently  the  alloy  is  placed  in  a  porous  dish  of  bone-ash 
(a  cupel)  and  heated  in  the  muffle.  The  lead  is  converted  to  protoxide  and 
partly  escapes  as  fume  and  is  partly  absorbed  by  the  cupel.  The  gold  and 
silver  are  left  as  a  metallic  button  since  they  do  not  oxidize  in  this  process. 
They  may  be  separated  by  '  parting  '  with  diluted  nitric  acid  which  dissolves 
the  silver  only. 

Electrolysis.  Precipitation  of  metals  through  the  agency  of  dynamic  elec- 
tricity has  come  into  extended  use  during  late  years.  Many  metalfic  salts  in 
aqueous  solution  are  decomposed  by  a  current  of  moderate  strength  and  the 
metal  deposited  on  an  electrode  connected  with  the  zinc  pole  of  a  galvanic 
battery,  while  oxygen  is  liberated  at  the  surface  of  another  electrode  connected 
with  the  copper  pole. 

In  practice,  the  solution  of  the  metal  to  be  determined  is  held  in  a  large 
platinum  dish  connected  by  a  copper  wire  with  the  battery.  In  the  solution  is 
suspended  a  platinum  plate,  not  touching  the  dish,  also  connected  with  the 
battery.  After  the  current  has  passed  for  several  hours  all  the  metal  will  be 
found  deposited  on  the  dish  as  a  coherent  film;  the  weight  is  found  from  the 
increase  in  weight  of  the  dish.  A  few  metals  like  lead,  manganese  and  thal- 
lium are  deposited  as  peroxides  on  the  positive  electrode. 

An  old  method,  now  supplanted  by  that  described  above,  is  the  decompo- 
sition of  a  metallic  salt  in  aqueous  solution  by  a  metal  more  electropositive 
than  the  one  in  solution,  the  latter  separating  as  a  powder  or  in  a  spongy 
mass,  while  an  equivalent  of  the  other  metal  dissolves.  Thus,  when  a  rod  of 
zinc  is  introduced  into  a  solution  of  copper  sulfate,  copper  separates  and 
zinc  dissolves;  similarly,  lead  is  thrown  out  of  a  solution  of  lead  chloride 
by  metallic  aluminum.  The  method  is  more  appropriate  as  a  means  for  the  sep- 
aration of  a  metal  from  others  than  for  its  determination. 

Elementary  Analysis.  In  an  elementary  or  ultimate  analysis  of  an  organic 
or  semi-organic  body,  the  elements  carbon  and  hydrogen  are  always  to  be 
determined,  frequently  nitrogen  and  oxygen,  and  occasionally  sulfur,  the  halo- 
gens, etc.  The  principle  on  which  hinges  most  of  the  methods  is  the  con- 
version of  the  elementary  constituents  into  gaseous  compounds  by  burning 
in  oxygen  or  otherwise;  the  mixture  of  resultant  gases  is  passed  through  a 
series  of  tubes  filled  with  different  solid  or  liquid  reagents.  Each  tube  ab- 
sorbs and  retains  one  of  the  gases  of  the  mixture,  and  being  weighed  before 
and  after  the  operation,  the  difference  in  weight  is  that  of  the  gas  absorbed; 
or  the  contents  of  the  tube  may  be  treated  to  determine  the  gas  by  a  gravi- 
metric or  volumetric  process.  From  the  weight  of  the  gas  —  a  definite 
chemical  compound  —  maybe  calculated  that  of  the  element  originally  in  the 
body  analyzed. 

For  carbon  and  hydrogen  a  weighed  portion  of  the  substance  is  mixed  with 
cupric  oxide  and  placed  in  the  middle  of  a  long  glass  tube  T,  Fig.  1,  whose  rear 

end  is  closed. 

To  the  * ront 
end  is  attach- 
ed two  ab- 
sorption 
tubes,  A  con- 
taining a 
hygroscopic  solid  (calcium  chloride),  to  retain  water,  and  B,  a  strong  solution 


12 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


of  o  caustic  alkali  to  combine  with  the  carbon  dioxide.  On  heating  the  tube- 
the  substance  burns  with  the  oxygen  of  the  copper  oxide,  the  water  and  carbon 
dioxide  passing  out  through  A  and  B.  As  the  tubes  have  been  weighed  before 
the  combustion,  the  increase  in  weight  gives  the  amount  of  water  and  carbon 
dioxide  they  have  respectively  absorbed.  To  sweep  out  the  gases  remaining  in 
the  tube  T,  the  posterior  end  is  broken  off  and  a  current  of  pure  air  forced 
through  the  train. 

Nitrogen  is  determined  either  by  conversion  to  ammonia  or  by  separation 
in  the  elementary  form.  For  the  former,  the  substance  is  mixed  with  sodium 
hydrate  and  lime  and  heated  in  a  tube  similar  to  the  above;  by  the  decomposi- 
tion of  water  the  nitrogen  assimilates  three  atoms  of  hydrogen  and  becomes 
ammonia,  which  passing  into  a  vessel  containing  a  dilute  mineral  acid,  com- 
bines with  it.  The  ammonium  salt  is  then  determined  gravimetrically  or 
volumetrically.  Or  the  substance  may  be  decomposed  by  boiling  with  strong 
sulfuric  acid,  when  the  nitrogen  becomes  ammonia  (3C  -f-  N2  +  SE^O  =  2NHs  -J- 
3CO2),  this  uniting  immediately  with  sulfuric  acid  to  form  ammonium  sulfate. 
The  solution  is  diluted  and  made  alkaline  by  sodium  hydrate  which  combines 
with  the  sulfuric  radical  and  liberates  ammonia  ((NH4)2SO4-f  2NaOH  = 
Na2S04-f  2NH4OH).  The  ammonia  is  distilled  into  hydrochloric  acid  forming 
ammonium  chloride,  and  this  compound  treated  by  the  usual  process  for  the 
gravimetric  determination  of  ammonium. 

If,  instead  of  soda-lime,  the  substance  be  heated  with  copper  oxide,  nitrogen 
is  liberated  along  with  the  carbon  dioxide  and  water  generated;  the  mixture  is 
passed  into  a  gas-measuring  tube  standing  over  mercury.  The  water  vapor 
condenses  and  the  carbon  dioxide  is  absorbed  by  an  alkali  solution,  and  from 
the  volume  of  the  remaining  nitrogen  is  calculated  its  weight. 

Oxygen  is  nearly  always  determined  by  difference,  and  sulfur,  phosphorus, 
the  metals,  etc.,  by  the  usual  gravimetric  methods  for  inorganic  bodies,  previ- 
ously destroying  the  organic  matter  by  oxidation  with  nitric  acid  or  a  similar 
reagent. 

2.    BY  INDIRECT   WEIGHT. 

By  loss  in  weight.  In  a  compound  or  mixture,  one  of  the  constituents  may  be 
volatile  at  a  temperature  insufficient  to  affect  the  others,  so  that  its  weight  may 
be  found  from  the  decrement  on  heating.  The  method  is  appropriate  for  com- 
bined water  and  carbon  dioxide  in  minerals;  water  of  crystallization  and  combi- 
nation of  stable  salts ;  etc.  Small  portions  of  organic  in  inorganic  matter  may 
be  burned  away  by  ignition  in  a  current  of  air  or  oxygen. 

A  mixture  of  several  bodies  may  be  treated  by  a  reagent 
which  will  dissolve  one  body  leaving  the  remainder  prac- 
tically unacted  on,  the  difference  between  the  original 
weight  and  that  of  the  residue  being  the  weight  of  the  body 
dissolved.  Thus,  copper  is  dissolved  from  associated  car- 
bon, alumina,  etc.,  by  a  solution  of  ferric  chloride;  silica, 
from  metals  by  hydrofluoric  acid;  gold  from  quartz  and 
silicates  by  bromine  water. 

The  radical  of  a  mineral  acid  may  displace  that  of  a 
weaker  volatile  acid,  and  the  weight  of  the  latter  found  by 
the  loss  in  weight  of  the  mixture.  For  example,  sulfuric 
acid  acts  on  a  carbonate  to  liberate  carbon  dioxide;  as 
MgCO3  (magnesium  carbonate)  -f  H2SO4  =  MgS04  +  COa  -h 
H2O. 
The  determination  is  made  by  means  of  an  apparatus 


Fig.  2. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  13 

termed  an  alkalimeter,  one  of  the  many  forms  being  shown  in  Fig.  2.  The 
magnesium  carbonate  is  weighed  and  introduced  into  the  light  glass  flask  A  to- 
gether with  a  little  water.  The  tube  B  is  filled  with  dilute  sulfuric  acid  which 
may  be  run  into  A  by  opening  the  stop -cock  C;  the  exit  tube  D  contains  con- 
centrated sulfuric  acid  to  retain  water  from  the  gas  passing  out  through  it. 
The  flask  is  weighed  and  an  excess  of  acid  run  into  A  from  B;  as  the  carbonate 
dissolves,  the  carbonic  acid  bubbles  through  D  leaving  it  as  anhydrous  carbon 
dioxide.  When  action  has  ceased  the  solution  of  magnesium  sulfate  is  boiled, 
and  a  current  of  air  drawn  through  the  apparatus  to  sweep  out  the  last  traces 
of  the  gas.  Finally  the  flask  is  weighed,  the  loss  from  the  former  weight  being 
carbon  dioxide. 

From  the  weight  of  another  element  or  radical  with  which  it  combines.  If 
two  atoms  or  radicals  a  and  x  unite  to  form  the  molecule  a  x,  it  follows  that  if 
the  weight  of  a  is  determined,  the  corresponding  weight  of  x  may  be  calculated 
from  their  combining  weights.  For  example,  one  way  of  determining  am- 
monium (NH4)  is  to  combine  it  with  chlorine  and  platinum  to  form  the  com- 
pound ammonium  platinic  chloride  (NH^aPtCle.  On  igniting  this  compound 
all  the  elements  except  platinum  volatilize,  and  from  the  weight  of  the  residual 
platinum  may  be  calculated  that  of  the  ammonium,  the  ratio  being  as  194.9 
(atomic  weight  of  platinum)  to  36.144:  (twice  the  molecular  weight  of  am- 
monium). 

A  product  of  the  reaction  between  the  body  to  be  determined  and  a  reagent 
may  be  weighed  or  measured,  and  the  weight  of  the  body  calculated  therefrom. 

(1)  The  product  may  be  determined  in  the  same  solution  in  which  the  reac- 
tion takes  place;  thus,  a  neutral  solution  of  cadmium  sulfate  treated  with 
hydrogen  sulflde  yields  sulfuric  acid  and  a  precipitate  of  cadmium  sulflde;  the 
free  sulfuric  acid  is  then  determined  by  a  volumetric  process  and  the  weight  of 
cadmium  calculated. 

This  principle  Is  extended  in  the  following  example.*  Let  it  be  required  to  determine 
the  percentage  of  phenol  in  a  commercial  sample ;  a  weighed  quantity  of  the  sample  is 
treated  as  follows :  — 

A.  On  evaporation  with  an  excess  of  concentrated  sulfuric  acid  phenol  is  converted  into 
soluble  (para)  phenolsulfonic  acid. 

2(C6H5OH )  +  2H2SO4= 2(C6H5HSO4)  +  2H2O (1) 

Phenol  Phenolsulfonic  acid 

B.  The  solution  is  diluted  with  water  and  (insoluble)  barium  carbonate  stirred  in;  the 
phenolsulfonic  acid  combines  with  barium  to  form  soluble  barium  phenolsnlfonate. 

2(C6HsHS04)  +BaCO3=Ba(C6H5S04)2  +  H2C03 (2) 

Phenolsulfonic  acid       Barium  phenolsulfonate 

At  the  same  time  the  excess  of  sulfuric  acid  necessarily  employed  in  A  reacts  with 
barium  carbonate  to  form  Insoluble  barium  sulfate. 

C.  The  precipitate  of  barium  sulfate  mixed  with  the  excess  of  barium  carbonate  is  fil- 
tered off,  and  to  the  clear  filtrate,  containing  only  barium  phenolsulfonate,  is  added  an 
excess  of  sodium  carbonate,  the  metathesis  giving  soluble  sodium  phenolsnlfonate  and  in- 
soluble barium  carbonate. 

Ba(C6H5HSO4)2  +  Na2COs  =  Na2(C6Hs8O4)2  +  BaCOs (3) 

Barium  phenolsulfouate  Sodium  phenolsulfonate 

The  liquid  is  filtered  and  the  barium  carbonate  weighed. 

D.  From  the  weight  of  the  barium  carbonate  is  calculated  that  of  the  barium  it  con- 
tains; from  the  weight  of  barium,  the  weight  of  the  rest  (CeH5SO4)2  combined  with  it  by 
equation  (3) ;  from  the  (C6HsSO4)2,  the  weight  of  the  phenolsulfonic  acid  by  equation  (2) ; 
and  from  the  phenolsulfonic  acid,  by  equation  (1),  the  weight  of  phenol  generating  it  and 
the  percentage  of  phenol  in  the  sample  (page  176). 

(2)  If  a  product  of  the  reaction  is  an  insoluble  gas  it  may  be  carried  from 
the  solution  to  a  suitable  receiver  and  there  weighed  or  measured.  Thus, 


Druggists  Circular,  1896-158. 


14  QUANTITATIVE    CHEMICAL    ANALYSIS. 

when  aspartic  acid  is  boiled  with  a  solution  of  nitrous  acid  there  is  evolved 
nitrogen  (H2C4H5NO4  +  HNO2  =  N2  +  H2C4H4O5  + HaO)  ;  the  nitrogen  is  passed 
into  a  graduated  gas -tube  and  the  volume  measured,  one-half  coming  from  the 
aspartic  acid  and  one-half  from  the  nitrous  acid. 

Or  instead  of  directly  weighing  or  measuring  a  gas,  it  may  be  passed  through 
a  solution  of  a  reagent  and  an  insoluble  product  of  this  second  reaction 
weighed.  Thus,  ferrous  sulflde  with  hydrochloric  acid  yields  gaseous  hydro- 
gen sulflde;  the  latter  is  passed  through  a  solution  of  silver  nitrate  and  the 
precipitated  silver  sulflde  filtered  out  and  weighed. 

Volumetric  Analysis.  This  designation  is  commonly  applied  to  the  following 
described  process  'though  It  would  appear  to  belong  more  properly  to  the 
methods  of  Division  3,  below. 

In  every  metathesis  the  rearrangement  of  the  molecules  takes  place  in  a  fixed 
ratio,  and  from  the  general  equation  AB-\-  XY  =  AX-{-  BY,  if  the  weight  of 
the  element  or  radical  A  or  B  is  known,  the  weight  of  Xor  Y  can  easily  be 
calculated. 

In  volumetric  determinations  small  weighed  quantities  of  AB  are  added  in 
succession  to  a  solution  of  XY until  the  reaction  is  just  complete,  this  point 
made  manifest  by  some  visible  change  in  color  of  the  solution  or  otherwise. 
It  is  more  convenient,  and,  with  a  few  exceptions,  the  rule,  to  employ  a  solution 
of  AB  of  known  strength,  reckoning  the  weight  of  AB  used  from  the  volume 
of  the  solution  required  —  hence  the  name  of  " volumetric"  analysis. 

For  example  (taking  the  simplest  of  the  various  modifications),  the  per- 
centage of  silver  in  a  coin  is  to  be  determined.  Exactly  equal  weights  of  the 
coin  and  of  pure  silver  are  separately  dissolved  in  nitric  acid;  into  the  first 
solution  is  cautiously  poured  from  a  graduated  tube  a  dilute  solution  of  sodium 
chloride,  forming  a  white  precipitate  of  silver  chloride.  On  vigorously  stirring 
the  solution,  the  precipitate  collects  into  one  mass  leaving  the  liquid  clear. 
Successive  additions  of  the  salt  solution  are  made  until  a  cloud  ceases  to  form, 
showing  that  all  the  silver  has  combined  with  chlorine.  The  volume  of  the 
salt  solution  used  is  noted,  and  the  above  process  repeated  with  the  second 
solution.  The  proportion  is  then  solved  — 

Volume  of  salt  solution  required  to  precipitate  the  pure  silver:  volume 
required  for  the  coin: :  weight  of  pure  silver:  weight  of  silver  in  the  coin. 

If  we  ascertain  that    G  volumes  of  the  salt  solution  precipitates  D  grams  of 

silver,  then  one    volume  precipitates  -~  grams,   and  we  have  a  standard  solu- 

o 

tion  of  sodium  chloride;  the  number  of  volumes  used  for  any  alloy  multiplied  by 
this  fraction  giving  the  weight  of  silver  contained. 

By  difference.  When  the  sum  of  the  weights  of  all  the  constituents  except 
one  is  subtracted  from  the  weight  of  the  substance  taken  for  analysis,  or  the 
sum  of  their  percentages  from  100  per  cent,  the  determination  of  the  one  con- 
stituent is  said  to  be  made  ««  by  difference." 

3.    BY   VOLUME. 

Gasometry.  To  analyze  a  mixture  of  gases,  a  convenient  volume  is  brought 
into  a  long  graduated  glass  tube  supported  in  a  vertical  position,  the  upper 
end  being  sealed  while  the  lower  end  is  open  and  immersed  in  a  vessel  of 
mercury.  The  volume  of  the  gas  being  noted,  a  solid  or  liquid  reagent  capable 
of  absorbing  one  gas  only  is  introduced  and  after  a  time  withdrawn,  when  the 
diminution  in  volume  shows  the  proportion  of  this  gas  in  the  original  mixture. 
The  other  gases  are  successively  eliminated  in  the  same  manner,  except  nitro- 
gen, hydrogen,  and  some  hydrocarbons  for  which  no  absorbent  is  available. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  15 

Hydrogen  is  determined  by  introducing  a  measured  volume  of  pure  oxygen 
and  kindling  the  mixture  by  an  electric  spark;  the  water  formed  by  the  combi- 
nation of  these  elements  condenses,  and  the  diminution  of  the  total  volume 
is  the  volume  of  the  hydrogen.  The  same  plan  is  pursued  with  gaseous  hydro- 
carbons, their  combustion  producing  carbon  dioxide  and  water;  the  former 
is  then  removed  by  a  suitable  absorbent.  Nitrogen  is  determined  by  difference. 

A  gaseous  constituent  of  a  solid  or  liquid  may  be  evolved  by  displacement 
by  a  stronger  radical;  as  carbon  dioxide  from  carbonates  on  solution  in  a  min- 
eral acid;  the  nitrogen  of  urea  set  free  by  sodium  hypobromite ;  etc.  The 
product  of  the  decomposition  is  passed  into  a  gas-measuring  tube  and  its 
weight  calculated  from  its  volume. 

The  weight  of  a  liquid  suspended  or  in  contact  with  another  may  often  be 
found  more  conveniently  by  measuring  its  volume  than  by  actual  separation 
and  weighing.  In  computing  the  weight  the  specific  gravity  of  the  liquid  is  a 
factor.  A  simple  illustration  is  the  well-known  method  of  testing  milk  by  the 
"  creamometer  " ;  a  tall,  graduated  jar  is  filled  with  milk,  and  when  the  fat- 
globules  have  risen  to  the  surface  the  volume  of  cream  is  read  on  the  gradua- 
tions of  the  jar.  The  fusel  oil  in  alcoholic  liquors  is  estimated  by  mixing  the 
liquor  with  a  measured  volume  of  chloroform ;  the  chloroform  extracts  the  oil 
with  a  proportional  increase  in  its  volume.  Chloral  hydrate  mixed  with  a  solu- 
tion of  sodium  hydrate  decomposes  into  soluble  sodium  formate  and  insoluble 
chloroform;  the  volume  of  the  latter  is  a  function  of  the  weight  of  chloral 
reacting. 

The  volume  of  a  precipitate  formed  in  a  solution  under  fixed  conditions  of 
temperature,  dilution,  agitation,  and  time  of  repose,  bears  a  fairly  constant 
ratio  to  its  weight.  In  methods  based  on  this  principle,  not  the  actual  volume 
of  the  precipitate  is  measured  but  the  space  it  occupies  after  settling  inter- 
spersed with  the  surrounding  liquid.  But  from  the  difficulty  of  adhering 
strictly  to  the  conditions  prescribed  and  through  unavoidable  errors  in  measur- 
ing, the  method  is  limited  to  the  determination  of  such  elements  as  form  but  a 
small  proportion  by  weight  of  the  precipitate  or  of  the  substance  analyzed, 
where  these  sources  of  inaccuracy  have  the  least  effect. 

One  application  is  for  the  approximate  determination  of  phosphorus  in  steel.*  One  gram 
of  the  metal  is  dissolved  in  dilute  nitric  acid  in  a  pear-shaped  glass  bulb,  Fig.  3,  whose 
lower  extremity  is  narrowed  into  a  nearly-capillary  graduated  tube  A. 
The  phosphorus  being  oxidized  completely  to  phosphoric  acid,  a  solution 
of  molybdic  acid  in  ammonium  nitrate  and  nitric  acid  is  added,  produc- 
ing a  dense,  finely  granular  precipitate  of  ammonium  phosphomolybdate. 
The  precipitate  slowly  subsides  into  A  or  is  quickly  forced  in  by  whirling 
the  bulb  in  a  centrifugal  machine.  In  the  latter  case  each  graduation 
of  A  represents  .01  per  cent  of  phosphorus  in  the  steel. 

4.    BY  THE   EXTENT    OF    SOME   SPECIFIC    PROPERTY. 

Colorimetry .  When  a  substance  or  one  of  its  constituents 
gives  a  colored  solution,  the  intensity  of  the  color  is  assumed 
to  be  in  a  direct  ratio  to  the  amount  of  substance  dissolved; 
and  in  two  solutions  of  equal  depth  of  tint,  the  weights  of  sub- 
stance  dissolved  are  in  direct  proportion  to  the  volumes  of  the 
solutions,  the  known  weight  of  one  serving  to  establish  that  Fig.  3. 
of  the  other. 

The  methods  are  only  comparative  and  for  several  reasons  are  best  suited 
for  technical  work  where  strict  accuracy  is  not  essential. 


*  Zeits.  angew.  1889—638. 


16  QUANTITATIVE    CHEMICAL    ANALYSIS. 

The  most  common  of  the  methods  is  to  prepare  a  solution,  in  water  or  other 
medium,  of  a  certain  weight  of  the  sample  to  be  tested,  and  pour  it  into  a  long 
graduated  test-tube.  Into  another  tube  of  the  same  diameter  is  placed  a  solu- 
tion containing  a  known  weight  of  the  pure  chromogenous  constituent  of  the 
sample,  or  of  a  body  containing  a  known  proportion  of  it.  The  darker  solution 
is  diluted  with  water  until  the  tints  of  the  two  have  the  same  intensity,  and  a 
calculation  from  the  weights  employed  and  the  respective  volumes  gives  the 
percentage  of  the  constituent  in  the  sample. 

By  divergent  values  of  a  constant.  When  two  allied  bodies  possess  a  com- 
mon  physical  characteristic  to  an  unequal  extent  or  chemically  react  with  a 
third  body  in  dissimilar  ratios,  the  constant  of  a  homogeneous  mixture  of  the 
two  lies  between  those  of  the  constituents,  its  distance  from  either  being  a 
function  of  the  proportion  of  the  constituents  in  the  mixture.  In  general,  this 
proportion  may  be  computed  from  the  simple  equations,  JT-f-  F=100,  and 

d—b  d—a 

aX-{-  bY=10Qd;  whence X=  100  ^rb    and  Y=  100  ^^  in  which  Xis  the 

percentage  of  one  constituent ;   T,  that  of  the  other :  a,  a  given  constant  of  X; 
b,  of  T;  and  d,  of  the  mixture. 

For  example,  when  linseed  oil  is  digested  under  certain  conditions  with  an  excess  of 
iodine  it  combines  with  about  175  per  cent  of  its  weight  of  the  halogen,  while  cottonseed 
oil  combines  with  only  108  per  cent.  If  it  be  proved  by  qualitative  tests  that  a  sample  of 
the  former  oil  is  adulterated  with  the  latter,  the  proportion  of  each  oil  may  be  calculated 
after  finding  the  percentage  of  iodine  taken  up  by  the  sample  — should  it  be,  for  example, 
161.6  per  cent,  the  proportion  of  cottonseed  oil  in  the  mixture  is  20  per  cent. 

It  is  clear  that  this  method  is  not  applicable  where  the  constant  of  the  mix- 
ture is  affected  by  reactions  consequent  on  the  bringing  together  of  the  con- 
stituents, and  that  the  degree  of  accuracy  that  can  be  attained  in  any 
determination  is  dependent  on  the  extent  of  the  disparity  of  the  constants  and 
the  exactness  with  which  they  can  be  —  or  have  been  —  determined  Confidence 
in  the  deduction  is  enhanced  if  the  results  obtained  by  two  or  more  constants 
agree  within  reasonable  limits,  when  the  average  of  these  results  will  probably 
closely  approximate  the  true  percentage. 

Of  a  variety  of  constants  that  may  be  employed,  such  as  melting  and  solidify- 
ing temperatures,  refraction  of  light,  rotation  of  polarized  light,  solubility,  etc., 
one  in  particular  is  in  extended  use,  namely  specific  gravity.  It  may  be  applied 
to  a  mixture  of  golids,  as  alloys  and  amalgams;  to  gases,  as  for  the  determina- 
tion of  carbon  dioxide  in  a  chimney-gas;  and  especially  to  liquids.  It  must  be 
observed,  however,  that  the  volume  of  a  mixture  is  often  not  so  great  as  the 
sum  of  the  volumes  of  the  constituents,  and  in  this  case,  of  course,  the  above 
formulae  will  not  apply.  Where  such  is  the  case,  tables  may  be  drawn  up  from 
data  obtained  by  mixing  the  pure  constituents  in  progressive  ratios,  carefully 
noting  the  densities  of  the  mixtures,  and  supplying  intermediate  values  by 
interpolation;  for  liquids,  tables  may  be  fitted  to  special  hydrometers  (page  159), 
showing  at  a  glance  the  proportion  of  one  constituent,  and  by  their  aid  results 
quite  accurate  enough  for  most  technical  work  can  be  had  immediately  and 
with  a  minimum  of  labor. 

Aqueous  solutions  of  certain  bodies  deviate  the  plane  of  a  ray  of  polarized 
light  according  to  a  specific  coefficient  of  rotation  and  to  an  extent  commen- 
surate with  the  concentration  of  the  solution.  Of  the  optically  active  bodies, 
some  turn  the  plane  to  the  right  (dextro -gyrate),  others  to  the  left  (laevo- 
gyrate).  The  extent  of  the  rotation  is  observed  by  an  instrument  known  as 
the  polariscope.  Of  the  many  forms  all  agree  in  having  four  principal  parts  — 
a  calcite  prism  to  polarize  a  beam  of  light;  a  long  metal  tube  with  glass  ends 


QUANTITATIVE    CHEMICAL    ANALYSIS.  17 

filled  with  the  solution  to  be  tested  and  through  which  the  polarized  beam 
passes;  an  arrangement  of  prisms  and  lenses  to  exhibit  to  the  eye  the  extent  of 
the  rotation;  and  a  scale  for  measuring  the  rotation.  The  angle  of  deviation 
is  shown  in  one  form  of  apparatus  by  the  identity  in  color  and  tint  of  two 
luminous  semi-circles,  in  others  by  the  position  of  black  bands  on  a  white 
field  or  their  disappearance,  etc.  The  polariscope  is  used  for  the  determina- 
tion of  alkaloids  and  essential  oils,  in  pathological  examinations,  and  exten- 
sively for  cane  sugar  and  glucose. 

Aqueous  solutions  of  many  salts  refract  a  ray  of  light,  the  angular  displace- 
ment increasing  with  the  proportion  of  the  solid  in  solution.  Alcohol,  albumen, 
the  oils,  etc.,  also  possess  this  property  to  a  greater  of  less  extent.  The  devi- 
ation is  measured  in  an  instrument  knpwn  as  the  refractometer. 

Attempts  have  been  made  to  employ  the  spectroscope,  so  useful  in  qualita- 
tive analysis,  for  quantitative  examinations  of  alloys,  dye-stuffs,  and  other  mate- 
rials. Various  other  principles  have  been  proposed  for  use  in  special  cases,  as 
the  difference  in  height  to  which  liquids  ascend  in  capillary  tubes,  the  vapor 
tension  of  volatile  liquids,  electric  conductivity,  viscosity,  melting,  freezing  and 
boiling  points,  etc. 

Separation  by  mechanical  means  comprises  sifting  through  various  sized 
meshes,  applied  for  example  in  the  separation  of  fibrous  from  granular  par- 
ticles; elutriation,  in  floating  a  light  mineral  in  fine  powder  from  one  much 
heavier;  extraction  of  particles  of  iron  by  the  magnet  from  non-magnetic  mat- 
ter, e.  g.,  boneblack;  vanning,  etc. 


18  QUANTITATIVE    CHEMICAL    ANALYSIS, 


CHAPTER  2. 

SAMPLING— PREPARATION  OF  THE  SAMPLE  FOR  ANALYSIS. 

The  market  value  of  a  raw  material  or  commercial  product  is  now  commonly 
decided  on  the  basis  of  its  analysis,  and  some  judgment  and  experience  are 
called  for  to  withdraw  a  representative  portion  of  a  suitable  size  in  such  a 
manner  as  to  preclude  any  suspicion  of  discrimination  or  selection,  either 
unconsciously  or  fraudulently,  of  a  quality  superior  or  inferior  to  the  aver- 
age of  the  original.  Where  the  material  is  a  solid  in  the  form  of  small  par- 
ticles or  powder,  fairly  homogeneous  and  well  mixed,  and  with  liquids  and 
gases  the  operation  is  mechanical  only,  and  the  proportion  by  weight  or 
volume  that  the  sample  bears  to  the  original  may  be  comparatively  small. 
But  for  a  heterogeneous  material  where  it  is  impracticable  to  pulverize  the 
whole,  only  a  large  and  judiciously  chosen  sample  is  of  any  value;  examples 
are  found  in  some  ores,  made  up  of  large  and  small  lumps  and  coarse  and 
fine  powder  all  differing  in  composition  —  perhaps  the  interior  of  a  lump  dif- 
fering from  the  exterior;  vegetable  products  where  the  granular  parts  and 
fiber  contain  unequal  amounts  of  some  active  principle;  photographic  wastes; 
paint-skins;  scrap  metal;  waste  manufactured  rubber;  etc.,  etc. 

In  withdrawing  a  representative  from  a  commercial  article  prone  to  undergo 
spontaneous  change  or  be  altered  by  variations  in  temperature,  contact  with 
air  or  moisture,  or  partial  volatilization  of  a  constituent,  it  is  important  that 
the  sample  should  be  divided  between  the  buyer  and  seller  and  analyzed 
without  delay.  Changes  in  composition  to  the  detriment  of  the  quality  and 
value  are  likely  to  take  place  more  rapidly  in  the  original  packages  exposed 
to  air  and  moisture  than  in  the  sample  kept  in  a  closely  stoppered  bottle; 
and  if  the  analyses  accord  to  the  satisfaction  of  both  parties  to  the  trans- 
action, there  will  be  less  difficulty  in  reaching  an  agreement  as  to  the  extent 
of  any  depreciation  of  the  original  should  the  sale  or  transfer  be  delayed. 

The  following  examples  illustrate  the  general  procedure  to  be  followed  in 
sampling  commercial  articles. 

Car  lots  of  ores  or  like  materials  are  often  sampled  in  this  way :  The  surface 
is  divided  into  squares  by  equidistant  perpendicular  lines  one  or  two  feet  apart, 

and  at  each  intersection  a  spadeful  is  taken 
out,  or  if  the  intersection  falls  at  a  lump,  a 
fragment  is  broken  away.  Uniting  these,  the 
whole  is  weighed,  dried  at  100°  and  again 
weighed  to  show  the  proportion  of  moisture 
contained,  then  crushed  to  pass  a  screen  of 
say  one-quarter  inch  mesh.  The  pile  is 
Fig.  4.  spread  out  in  a  large  circle  and  two  opposite 

quadrants  rejected,  the  remainder  crushed 

somewhat  finer  and  again  halved,  and  so  on,  the  size  of  the  fragments  and  the 
weight  of  the  sample  being  correspondingly  reduced  until  only  a  few  ounces  of 
fine  powder  remain.* 


School  of  Mines  Quart.  1892—364. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  19 

Instead  of  dividing  the  pilesas  described,  some  preferto  transfer  ahalf  with  the 
sampling-  or  split-shovel,  Fig.  4,  a  series  of  alternate  rectangular  cups  and  spaces. 
A  device  for  dividing  the  final  powder  into  four  parts 
is  shown  in  Fig.  6,  the  funnel  A  being  moved  rapidly 
to  and  fro  over  the  distributer  B.  It  is  the  custom 
for  the  purchaser  to  retain  one  bottle  for  analysis,  the 
second  is  sent  to  the  shipper,  the  third  to  a  referee, 
and  the  fourth  sealed  and  preserved  for  emergencies.* 

In  sampling  ores,  coal,  limestone,  etc.,  from  stock- 
piles or  cars  it  is  often  assumed  that  the  surface  rep- 
resents the  interior,  but  if  at  all  doubtful,  it  is  the  safer  fig.  5. 
plan  to  dig  in  so  far  as  to  cut  through  all  the  layers 

formed  as  the  ore  was  dropped  on  the  pile  or  loaded  into  the  car.    This  applies 
also  to  ensilage,  fodder,  haystacks,  etc. 

During  the  unloading  of  a  cargo  of  ore  or  like  material,  a  small  portion  of 
every  fifth  or  tenth  barrow  furnishes  an  unexceptionable  sample.  In  a  mine 
or  quarry  the  face  of  ore  or  rock  is  marked  off  into  squares  of  suitable  dimen- 
sions and  a  piece  picked  at  each  intersection  —  always  exercising  caution 
against  the  possibility  of  the  mine  being  ts  salted." 

In  sampling  commercial  metals,  from  bullion  and  bars  little  cylinders  are  cut 
out  with  a  punch.  Ingots,  pig  iron,  and  metal  shapes  are  drilled  through  in 
several  places  and  the  drillings  well  mixed  to  annul  any  segregation  or  non- 
uniformity  of  structure,  unless  it  is  desired  to  ascertain  the  existence  of  this 
condition. 

The  fairest  sample  is  obtained  by  remelting  and  dipping  out  a  small  ladle  - 
ful,  which,  after  solidification,  is  drilled  or  pulverized.  If  this  is  impractica- 
ble, a  fair  representative  may  perhaps  be  cut  from  just  below  the  surface  where 
immediate  solidification  has  checked  segregation.  From  a  pig  or  bar,  a  thin 
section  perpendicular  to  the  axis;  or  fine  drillings  from  several  holes  as  deep 
as  possible  and  well  mixed,  best  by  drilling  out  a  large  quantity  and  subdi- 
viding as  before  described.  Easily  fusible  alloys  are  previously  melted  and 
cast  into  ingots,  stirring  rapidly  while  cooling  to  promote  homogeneity. 

The  uniform  distribution  of  the  impurities  in  the  metals  of  commerce  has 
often  been  the  cause  of  discrepancies  between  analyses,  resulting  in  friction 
and  controversy.  Admitting  an  originally  perfect  mixture  of  the  molten  metal, 
by  segregation  an  undue  proportion  of  the  impurities  pass  toward  the  median 
line  of  an  ingot  and  rise  near  to  the  upper  end  where  a  u  spot "  may  form 
charged  with  double  or  triple  the  impurities  held  by  the  metal  nearer  the 
surface.f  Obviously  duplicate  determinations  of  any  impurity  will  not  agree 
should  one  determination  be  made  on  borings  cut  from  near  the  surface,  the 
other  from  further  in,  or  both  from  an  imperfect  mixture  of  the  two. 

When  a  molten  metal  is  to  be  sampled,  as  iron  from  a  cupola,  it  is  best  to 
allow  a  thin  stream  to  fall  into  water,  where  solidification  takes  place  so 
rapidly  that  but  little  segregation  can  occur,  and  there  is  consequently  less 
danger  of  a  non-uniform  sample.  The  chilled  spheroids  cannot  be  drilled  or 
filed,  but  are  so  brittle  that  they  may  easily  be  crushed  to  powder.  For  melted 
slags  a  cold  iron  rod  is  thrust  in  and  quickly  withdrawn;  as  the  chilled  coating 
of  adhering  slag  is  amorphous  in  structure  its  powder  is  more  readily  attacked 
by  acids  (an  advantage  in  the  analysis)  than  if  the  slag  were  allowed  to  cool 
slowly  and  become  crystalline. 

Recent  vegetables,  roots  and  seeds  are  rasped,  divided  in  a  tobacco-cutter* 


*  Trans.  Amer.  Inst.  Min.  Engrs.  1891—155. 
t  Journ.  Anal.  Chem.  2—70. 


20  QUANTITATIVE    CHEMICAL    ANALYSIS. 

or  contused  in  a  lignum- vitae  or  boxwood  mortar.  If  the  sap  or  juices  are  to 
be  examined,  the  vegetable  is  passed  through  a  pair  of  rolls  at  a  suitable  pres- 
sure or  expressed  in  a  fruit-press  or  small  cider-mill.  Sugar  beets  are  pierced 
with  a  rapidly  rotating  hollow  drill  roughened  at  the  point,  withdrawing  the 
interior  in  the  form  of  pulp.  Dyewoods  are  sawed  through  transversely  to  the 
fiber  and  the  sawdust  mixed  and  "  quartered  "  as  described  for  ores.  Of  the 
softer  drugs  like  opium  received  in  case  lots,  from  every  tenth  lump  is  excised 
a  cone  whose  apex  is  the  center  of  the  lump,  and  from  each  cone  a  narrow 
sector;  the  sectors  are  worked  into  one  homogeneous  mass  by  the  fingers,  and 
slices  taken  from  it  for  the  analysis. 

For  the  valuation  of  oil-cake  and  solids  marketed  in  similar  shape,  a  narrow 
strip  is  cut  from  one  corner  to  the  corner  diagonally  opposite,  or  a  whole  cake 
is  broken  in  half  and  a  strip  cut  from  each  half.  Packages  of  butter,  cheese, 
bins  of  grain,  bags  of  ground  fertilizers,  etc.,  have  a  long  tube  of  thin 
metal  passed  entirely  through  the  package  cutting  a  cylinder  which  is  assumed 
to  be  representative.  With  shipments  of  tallow  or  other  solid  fat,  a  core  is 
cut  from  each  cask,  and  from  the  cores  weights  proportional  to  the  weights  of 
the  casks  from  which  they  were  taken ;  these  are  melted  together  at  a  low  heat 
and  constantly  stirred  during  cooling,  and  from  the  granular  mass  is  with- 
drawn the  portions  for  analysis.  Arranged  to  retain  a  liquid,  the  tube  be- 
comes a  "  thief  "  used  for  abstracting  oils,  liquors,  etc., 'from  barrels  and  tanks. 

Viscous  liquids  are  drawn  by  a  glass  syringe  with  a  large  orifice;  whe« 
samples  from  several  barrels  are  to  be  united  to  form  a  composite  one,  the 
syringe  can  be  graduated  so  that  a  volume  proportional  to  the  weight  of  the 
contents  of  each  barrel  may  be  included  in  the  mixture.  Packages  of  liquids 
containing  insoluble  suspended  matter,  such  as  paints  and  semi- solids  gener- 
ally, are  transferred  entire  to  a  large  dish,  well  mixed,  and  suitable  portions 
withdrawn  for  analysis  before  any  deposition  can  take  place.  The  importance 
of  thoroughly  mixing  a  sufficiently  large  quantity  of  a  heterogeneous  solid  be- 
fore dividing  down  to  a  sample  applies  equally  to  liquids  that  stratify  on  stand- 
ing or  deposit  a  sediment.  Slimes  are  passed  through  a  filter-press  and  a  sec- 
tion cut  from  each  cake;  the  sections  are  together  rubbed  up  with  water  and 
the  liquid  filtered,  and  from  the  resulting  cake  is  cut  a  section  for  assay. 

For  spring  or  other  natural  waters,  an  empty  stoppered  bottle  is  submerged 
to  the  proper  depth,  the  stopper  withdrawn  and  reinserted  when  the  air  has 
been  displaced  by  water.  The  excess  of  carbonic  acid  in  aqueous  solutions 
supersaturated  under  pressure,  as  table  waters,  beers,  or  sparkling  wines,  is 
drawn  slowly  and  without  loss  through  a  champagne  tap  or  its  equivalent, 
passing  through  a  rubber  tube  into  the  solution  for  its  absorption. 

Gases  diffuse  one  into  another  so  readily  that  after  contact  for  a  reasonable 
time  any  portion  of  a  mixture  represents  the  whole.  With  heating  and  illumi- 
nating gases,  the  conducting  pipe  may  be  tapped  at  any  point  and  the  gas 
drawn  through  a  dry  meter  into  a  gasometer  or  directly  through  the  train  of 
apparatus  for  its  analysis.  A  simpler  plan  is  to  connect  to  a  branch  of  the  gas- 
pipe  a  large  syringe  provided  with  a  three-way  stop-cock  at  the  orifice,  and  fill 
it  by  fetracting  the  piston,  this  appliance  avoiding  any  contact  of  the  gas  with 
water  and  absorption  of  a  soluble  constituent  like  carbon  dioxide  or  hydrogen 
sulflde. 

For  the  determination  of  minute  proportions  of  a  gas  in  a  mixture,  as 
carbon  dioxide  in  air,  a  large  volume  is  passed  through  an  absorbing  fluid  con- 
tained in  tubes  of  a  special  design,  then  through  a  gasmeter  to  learn  the  vol- 
ume transmitted.  When  the  average  quality  of  a  gas  generated  in  a  furnace 
or  producer  in  a  given  length  of  time  — say  24  hours  — is  to  be  determined,  a 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


21 


large  aspirator  is  filled  with  mercury  or  water  and  the  tap  at  the  bottom  opened 
only  so  far  that  the  mercury  will  be  entirely  withdrawn  in  the  specified  time, 
the  gas  entering  through  a  capillary  tube  as  the  mercury  flows  out. 

Schloessing's  apparatus  for  the  determination  of  carbon  dioxide  in  a  soil  is  a 
steel  tube  of  a  bore  of  two  millimeters,  pointed  at  the  lower  end.  The 
bore  is  closed  by  a  wire  of  slightly  less  diameter,  and  the  tube  driven  into  the 
soil.  The  wire  is  withdrawn  and  the  gases  aspirated  into  a  bulb-tube  by  means 
of  a  mercury  aspirator. 

During  the  manufacture  of  certain  commercial  products  samples  may  have  to 
be  taken  out  periodically  to  show  the  progress  of  the  chemical  reaction  or  to 
indicate  its  completion.  Small  ladles  of  iron  are  dipped  from  a  furnace  making 
open -hearth  steel  to  find  the  rate  at  which  the  carbon  or  phosphorus  is  being 
removed  from  the  bath;  hourly  samples  of  the  mixture  of  soap-stock  and 
alkali  from  the  autoclave  to  ascertain  how  rapidly  the  saponification  is  pro- 
gressing; gas  from  a  gas  producer  for  the  variation  in  the  ratio  of  carbon 
monoxide  to  carbon  dioxide;  etc. 

In  technical  work  there  is  sometimes  required  the  analysis  of  an  article  from 
which  only  a  small  part  can  be  removed  for  a  sample,  such  as  a  roll  of  paper,  a 
tanned  hide,  or  a  sheet  of  metal  bearing  a  u  coupon."  Here  deductions  from 
the  results  of  analysis  must  be  drawn  mindful  of  a  possibility  of  the  non- 
homogeneity  of  the  original. 

With  large  lots  of  manufactured  articles,  or  where  it  is  impracticable  to 
open  all  original  packages,  from  one  to  ten  per  cent  of  the  lot  is  picked  at  ran- 
dom and  assumed  to  be  representative.  Obviously  this  plan  is  unsatisfactory 
at  best,  though  often  the  only  one  possible,  since  an  unscrupulous  dealer  will 
often  take  advantage  of  the  opportunity  to  include  a  small  proportion  of  an 
inferior  quality,  relying  on  the  improbability  of  any  one  of 
these  entering  the  number  picked  for  the  test. 

Mechanical  sampling.  Several  machines  have  been  in- 
vented for  sampling  large  consignments  of  ore  or  matte  in 
powder  or  for  liquids.  They  are  so  constructed  that  the 
entire  lot  falls  in  a  continuous 
stream  through  an  orifice  in  an 
elevated  bin  to  another  bin  some 
distance  below.  A  suitable 
Fig.  C.  1/3  mechanism  cuts  out  for  the 

sample  a  section  of  the  stream  continuously,  or 
t'ae  whole  stream  is  momentarily  diverted  at  reg- 
ular intervals  of  time. 


Pulverizing.  Easily  soluble  compounds  and  crys- 
talline salts  in  general  need  only  be  coarsely 
powdered  for  the  removal  of  any  foreign  matter, 
moisture  in  the  lammalae,  or  inclosed  mother- 
liquor,  and  for  convenience  in  weighing.  Minerals, 
less  easy  to  dissolve,  must  be  ground  to  a  fine 
powder.  They  are  broken  into  small  fragments,  Fig  7.  l/4 

and  clean  pieces  freed  from  gangue  by  picking  out  under  a  magnifying  glass, 
or  by  separation  with  a  solution  of  intermediate  density.  The  fragments  are 
crushed  in  a  diamond  steel  mortar,  Fig.  6.  In  a  cavity  in  the  hardened  steel 
base  A  stands  an  iron  collar  B  containing  the  fragments  to  be  pulverized;  the 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


pestle  C  slides  loosely  in  the  collar  and  is  struck  by  a  hammer  until  they  are 

reduced  to  granules.    Fig.  7  shows  in  section  a  mor- 
tar made  of  hardened  steel  used  for  powdering  white 

iron  and  similar  substances. 
Small  parcels  of  ores  may  be  broken  down  in  an  iron 

mortar,  both  mortar  and  pestle  of  a  hard,  tough  grade 

of  cast  iron,  Fig.  8.    The  mortar  should  have  a  solid 

support,  best  the  upper  end  of  a  wooden  post  sunk 

several  feet  in  the  earth,  and  the  manipulation  of  the 

pestle  is  less  laborious  when  it  is  suspended  at  the       

eud  of  a  spiral  spring  hung  from  the  ceiling.  Fig.  8. 

Ores  and  other  hard  materials  in  lumps  are  quickly  reduced  to  a  moderately 

fine  powder  in  a  hand-crusher  of  which  several  styles  are  on  the  market.    One 

is  shown  in  section  in  Fig.  9.  The 
ore  is  fed  between  the  serrated  jaws 
A  and  B.  The  former  is  fixed  to  the 
frame  of  the  machine,  and  the  latter 
held  by  the  rod  E  bearing  on  the 
rubber  spring  F,  against  two  pivotal 
bearings  C  and  D.  C  is  supported  by 
the  frame,  and  D  presses  against  the 
short  arm  of  the  bent  lever  H.  The 
lever  oscillates  on  J,  and  the  long  arm 


Fig.  9. 


is  actuated  by  the  cam  G  turned  by  a  hand-wheel.  As  the  cam  rotates,  the  jaw 
B  swings  toward  A  and  crushes  the  ore  between  them,  and  as  it  recedes  the 
powder  falls  into  a  pan  below.  For  large  lots  of  ores  a  rotatory  crusher  is 
more  suitable,  and  a  ball- mill  has  some  advantages  for  certain  materials. 

Shavings  of  iron  may  be  cut  from  a  steel  or  iron  mortar  by  the  harder  min- 
erals, to  be  removed  by  stirring  the  powder  with  a  magnet;  but  if  a  magnetic 
mineral  is  to  be  pulverized  some  other  means  of  extraction  must  be  used,  such 
as  dissolving  out  the  iron  with  a  solution  of  some  chemical  that  will  not  affect 
the  mineral.  For  the  assay  of  ores  other  than  iron,«the  presence  of  such  a 
relatively  small  amount  of  iron  is  of  less  consequence  and  may  safely  be  dis- 
regarded; 

Very  hard  anhydrous  minerals  may  be  broken  up  by  heating  to  redness  and 
quenching  in  water,  provided  that  the  composition  is  not  changed  by  this 
operation. 

A  metal  or  alloy  dissolving  readily  and  completely  in  an  acid  may  be  weighed 
in  the  form  of  one  or  more  pieces  as  it  is  often  difficult  to  subdivide  without 
contamination  by  foreign  matter  from  the  file  or  other  instrument.  If  brittle 
it  may  be  powdered  in  the  same  way  as  a  mineral,  but  if  malleable,  as  is  more 
often  the  case,  is  rolled  or  hammered  into  a  sheet  and  cut  by  shears  into  strips 
of  a  convenient  weight,  or,  if  very  soft,  may  be  whittled  by  a  knife  or  chisel. 

A  metal  may  be  finely  divided  by  alloying  it  with  a  large  proportion  of 
another  and  dissolving  out  the  latter  with  an  acid  which  will  not  attack  the 
former;  as  platinum  with  zinc,  the  zinc  extracted  by  nitric  acid  leaves  the 
platinum  as  a  powder  far  more  readily  dissolved  in  aqua  regia  than  if  in  the 
massive  form. 

Soft  and  tenacious  solids  and  semi -solids  are  difilcult  to  pulverize  and  mix 
uniformly,  and  this  operation  may  be  facilitated  by  the  incorporation  of  a 
harder  substance,  like  sand  or  asbestos,  so  chosen  that  it  shall  not  interfere  In 
the  analysis.  This  is  called  <l  pulverizing  by  intervention." 

Mortars.    The  powder  is  finally  passed  through  a  sieve  having  not  less  than 


QUANTITATIVE    CHEMICAL    ANALYSIS.  23 

forty  meshes  to  the  linear  inch;  if  an  ore  is  being  pulverized,  or  in  fact  any 
mixture,  every  particle  must  go  through,  for  it  is  evident  that  what  renmins  on 
the  sieve  contains  an  undue  proportion  of  the  harder  and  tougher  particles,  or 
those  of  a  malleable  or  fibrous  character.  The  resulting  powder  is  to  be 
thoroughly  mixed  before  dividing  down. 

A  few  grams  are  then  ground  to  the  proper  degree  of  fineness  in  an  agate, 
wedgewood  or  porcelain  mortar.    The  first  named,  Fig.  10,  is  to  be  preferred, 
as  by  reason  of  its  flinty  nature  it  suffers  less  from 
abrasion  by  a  hard  mineral  than  either  of  the  others 
and  consequently  the  powder  is  impurifled  to  a  less 
extent.    In    selecting  one,    the    interior    should  be 
examined  for  perceptible  cracks  and  fissures  apt   to 
harbor  a  little  of  one  powder  to  contaminate  the  next 
one  ground.    It  should  be  slightly  roughened  before         Fi     ~       i/""Ii/ 
using   by   grinding  a  little  emery  flour,  and  can  be 

cleaned  in  the  same  way.  For  greater  stability  it  may  be  imbedded  in  a  hard- 
wood block. 

The  pestle  should  be  long  enough  to  be  comfortably  grasped  or  be  set  in  a 
wooden  handle,  and  its  grinding  face  have  a  convexity  of  somewhat  less  radius 
than  the  interior  of  the  mortar.  It  is  manipulated  with  a  rubbing  motion  only, 
as  pounding  is  less  effective  and  liable  to  chip  its  edge  or  crack  the  mortar,  for 
it  is  well  to  remember  that  banded  agate  is  quite  brittle  and  easily  fractured  by 
a  slight  blow.  The  powder  must  be  occasionally  brushed  down  to  the  center 
of  the  mortar  during  the  grinding,  that  every  part  receive  its  share  of  the 
trituration. 

Wedgewood  and  porcelain  mortars  will  answer  for  the  softer  minerals  such 
as  talc  or  graphite,  crystalline  salts  and  organic  substances,  while  those  of 
glass  are  only  suitable  for  mixing  pow- 
ders and  like  operations.  The  '  bucking- 
board,'  Fig.  11,  is  used  in  technical  work 
for  rapidly  breaking  down  ores.  It  is  a 
hard  steel  plate,  A,  with  amuller,  B,  hav-  FiS-  n-  (Vie) 

ing  a  curved  base  and  provided  with  a  long  handle.  The  grinding  is  done  by 
running  the  muller  back  and  forth  with  a  slight  oscillatory  motion,  at  the 
same  time  pressing  it  down  with  the  left  hand. 

Grinding  by  power.*  Pulverizing  large  amounts  of  ore  by  hand  is  rather  slow  and  tire, 
some,  and  machines  are  in  use  for  this  purpose  aetuated  by  pulleys  from  a  shaft  or  by  a 
water-motor.  In  one  machine  the  pestle  is  hinged  at  the  center  of  the  arc  of  the  con- 
cavity of  the  mortar  and  oscillates  across  it,  both  the  mortar  and  pestle  revolving  slowly 
at  the  same  time.  The  mortar  is  supported  on  one  end  of  a  lever,  and  is  pressed  up 
against  the  pestle  by  a  weight  at  the  other  end. 

In  a  simpler  contrivance  the  mortar  is  stationary  and  the  grinding  is  done  by  hand  in 
the  usual  way  with  the  difference  that  the  hand  grasps  a  sleeve  within  which  the  pestle 
is  rapidly  rotated  by  means  of  a  flexible  shaft  joined  to  the  upper  end. 

Sifting—  Levigation.  The  fineness  to  which  the  pulverization  is  to  be  carried 
is  dictated  mainly  by  the  solubility  or  ease  of  decomposition  of  the  substance. 
Chalk,  for  example,  dissolves  in  acids  so  readily  that  it  is  ground  merely  to 
obtain  a  homogeneous  mixture  of  the  mineral  and  its  impurities,  while  on  the 
other  hand,  minerals  like  chromite,  the  titanic  acid  and  alumina  families,  cas- 
siterite,  and  zircon  are  so  dense  and  refractory  that  their  trituration  can 
scarcely  be  prolonged  to  an  unprofitable  extent.  But,  as  a  rule,  a  powder  may 
be  considered  sufficiently  fine  if  no  grittiness  is  observed  when  rubbed 
between  the  fingers. 


*  '"hem.  \ew?,  1888-2;  Blair,  Chem.  Anal,  of  Iron,  13;  Journ.  Anal,  (,'hein.  2—81. 


24  QUANTITATIVE    CHEMICAL    ANALYSIS. 

A  uniformly  fine  powder  results  from  sifting  through  some  thin  closely  woven 
fabric,  such  as  bolting-silk.  Another  plan  is  that  of  levigation,  stirring  the 
powder  into  an  inactive  fluid  and  after  a  time  decanting  the  liquid  holding  the 
finer  particles  in  suspension,  these  subsiding  after  the  liquid  has  been  allowed 
to  stand  undisturbed  for  some  time.  Unfortunately  but  few  powders  are 
entirely  unaffected  by  water  or  other  liquids,  which  frequently  acts  as  a  solvent 
to  a  degree  that  would  not  be  expected,  and  the  levigated  powder  differ  con- 
siderably from  the  original  by  reason  of  the  constituents  dissolving  to  an 
unequal  extent.  But,  on  the  whole,  it  is  seldom  that  a  thorough  grinding  in  an 
agate  mortar  will  not  suffice  for  analytical  purposes  without  need  of  recourse  to 
either  of  the  above  expedients. 

PREPARATION  OF  THE  SAMPLE  FOR  ANALYSIS. 

When  the  composition  of  a  definite  chemical  compound  is  to  be  ascertained 
for  the  deduction  of  its  formula,  entire  freedom  from  other  bodies  must  be 
assured  by  qualitative  tests,  and  impurities,  if  detected,  removed  by  a  suitable 
treatment  chosen  according  to  the  nature  of  the  substance  and  the  impurity. 

Crystalline  salts  are  purified  by  dissolving  in  hot  water  or  other  solvent  to 
the  specific  gravity  suitable  for  crystallization,  filtering  as  hot  as  possible 
through  paper  or  asbestos,  and  allowing  to  cool  during  continuous  stirring  that 
only  small  crystals  may  be  deposited.  Precautions  must  be  taken  with  a 
double  salt  that  the  product  shall  not  partially  dissociate  into  the  simple  ones 
composing  it.  The  crystals  are  dried  by  pressing  between  filter  paper,  or  if 
decomposed  by  organic  matter,  on  a  porous  tile  or  in  a  funnel  connected  with  a 
vacuum  pump;  more  quickly  by  inclosing  them  in  a  perforated  basket  of 
porcelain  or  wire  gauze  and  revolving  it  in  a  centrifugal  machine. 

When  a  salt  is  easily  oxidized  by  exposure  to  the  air  or  otherwise  affected 
on  evaporation,  a  better  plan  is  to  precipitate  it  in  the  crystalline  form  by 
reducing  the  solvent  capacity  of  the  menstruum,  as  by  the  addition  of  alcohol 
or  an  acid  to  an  aqueous  solution,  or  ether  to  an  alcoholic  one.  Thus  a  solu- 
tion of  commercial  ferrous  sulfate  always  contains  more  or  less  ferric  sulfate; 
the  addition  of  alcohol  precipitates  only  the  former,  as  the  latter  is  compara- 
tively quite  soluble  in  dilute  alcohol. 

An  impurity  in  a  salt  may  also  be  eliminated  from  its  solution  by  precipita- 
tion with  an  appropriate  reagent,  provided  that  the  reagent  has  no  action  on 
the  salt  and  that  its  excess  can  easily  be  removed  by  evaporation  or  otherwise. 
An  expedient  often  resorted  to  is  that  of  replacing  the  base  of  an  impurity  by 
the  base  of  the  salt  to  be  purified,  the  former  being  precipitated.  Thus  if  into 
a  solution  of  zinc  sulfate  containing  ferric  sulfate,  is  stirred  a  little  zinc 
oxide,  all  the  iron  will  be  precipitated  as  ferric  hydrate  — 

Fe2(SO4)s  +  3ZnO  -{-  3H2O  =  Fe2(OEn6  +  3ZnSO4. 

and  it  is  obvious  that  zinc  oxide  will  yield  an  iron-free  solution  if  treated  with 
dilute  sulfuric  acid  in  quantity  insufficient  for  entire  solution. 

Dialysis  may  be  resorted  to  with  advantage  to  free  an  inorganic  salt  from 
organic  impurities  of  such  a  nature  that  only  repeated  recrystallization  would 
eliminate  them,  or  to  separate  a  colloidal  compound  from  a  crystalloid. 

A  solid  volatile  on  moderate  ignition,  as  indigotin,  camphor,  or  benzoic  acid, 
is  purified  by  the  process  of  sublimation.  After  drying  the  substance  in  the 
desiccator  it  is  gently  heated  in  a  porcelain  dish  covered  by  an  inverted  funnel 
in  which  the  fume  condenses,  or  by  inclosing  it  between  two  watch-glasses 
and  heating  the  lower.  Metals  of  comparatively  low  boiling  points,  like 
cadmium  and  zinc,  can  be  distilled  in  vacuo  leaving  a  residue  of  whatever  ia 
not  volatile  at  the  temperature  employed. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  25 

A  liquid  is  freed  from  suspended  matter  by  subsidence  and  decantation  or 
filtration  through  paper  or  felt,  and  from  dissolved  impurities  by  lowering  the 
temperature  to  such  an  extent  that  they  separate  in  the  solid  state,  by  extrac- 
tion with  another  immiscible  liquid,  by  treatment  with  a  strong  acid  to  carbon- 
ize organic  matter,  etc.  Optically  active  liquids  or  solutions  are  clarified  for 
examination  in  the  polariscope  or  refractometer  by  basic  lead  acetate,  or  di- 
gestion with  animal  charcoal,  fuller's  earth,  talc,  or  a  mixture  of  wood-meal 
and  finely  powdered  infusorial  earth. 

Volatile  liquids  are  distilled  from  non-volatile  matter  and  the  vapor  condensed 
(page  62),  after  digestion  over  some  hygroscopic  solid  if  water  is  to  be  elimi- 
nated. Repeated  fractional  distillation  serves  to  isolate  one  of  a  mixture  of 
volatile  liquids  or  a  fraction  of  a  homologous  series  coming  over  between 
certain  specific  temperatures. 

A  gas  is  transmitted  into  the  apparatus  for  analysis  through  a  series  of  wash- 
bottles  or  tubes  containing  granular  solids  to  absorb  water.  From  gases 
produced  or  used  in  technical  processes,  tar,  soot  and  dust  are  retained  when 
the  gas  is  conducted  through  a  column  of  some  closely  packed  fibrous  mate- 
rial ;  otherwise  the  gases  are  analyzed  as  obtained  or  delivered  to  the  consumer. 

Various  other  modes  of  purification  may  be  adopted  as  circumstances  admit; 
for  the  metals  and  other  elements,  elaborate  directions  will  be  found  in  the 
literature  on  the  determination  of  their  atomic  weights. 


Drying  the  sample.  The  amount  of  moisture  condensed  in  a  porous  body 
exposed  to  the  air  varies  with  the  atmospheric  temperature  and  humidity,  so  it 
is  the  rule  to  dry  all  powders  previous  to  their  analysis  and  preserve  them  in 
tight  receptacles  until  actual  work  is  begun.  The  point  at  which  a  substance 
is  altered  or  decomposed  limits  the  degree  of  heat  to  be  applied;  moisture, 
however,  should  not  be  confounded  with  water  of  crystallization  or  constitu- 
tion, neither  of  which  must  be  disturbed  in  the  drying,  their  removal  being 
usually  considered  as  constituting  a  part  of  the  analysis  proper.  With  some 
few  organic  bodies,  exposure  to  a  heat  above  ordinary  (or  freezing),  causes  a 
marked  reduction  in  solubility,  and  such  are  analyzed  as  received,  the  water 
determined  in  a  separate  portion  and  the  percentage  calculated.  An  instance 
of  the  mal-effect  of  prolonged  drying  is  found  in  linseed  cake,  the  amount  of 
linseed  oil  soluble  in  ether  being  largely  diminished. 

Minerals,  ores,  and  commercial  raw  materials  and  products  in  general, 
after  having  been  sampled  and  pulverized  as  described  in  the  preceding  pages, 
need  only  a  removal  of  the  hygroscopic  water  to  be  ready  for  analysis.  The 
drying  is  often  omitted,  as  when  the  object  of  the  analysis  is  to  decide  the 
value  of  a  material  in  the  condition  as  found  on  the  market,  but  it  is  to  be 
remembered  that  any  extensive  preparation  for  analysis  in  the  way  of  grinding 
or  mixing  gives  an  opportunity  for  either  evaporation  or  absorption  from  the 
air  of  a  considerable  amount  of  moisture  with  a  correspondingly  inaccurate 
result.  It  is  better  therefore  to  determine  at  once  the  percentage  of  moisture 
in  the  material,  thoroughly  dry  the  portion  taken  for  analysis,  and  calculate 
the  results  obtained  to  the  undried  material  (page  177). 

Certain  amorphous  bodies,  cellulose  for  example,  appear  to  have  a  normal 
water-content,  and  frequently  the  analysis  is  expressed  on  this  basis.  Aerhy- 
drous  bodies,  when  only  coarsely  ground  may  not  part  with  all  the  water  they 
contain  at  a  temperature  of  100°,  but  if  in  a  fine  powder  lose  it  at  ordinary 
temperatures  when  dried  in  a  desiccator  over  sulfuric  acid.  Soils  are  analyzed 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


in  an  air-dried  condition,  that  may  only  be  arrived  at  by  several  days'  exposure 
in  thin  layers. 

A.  Crystalline  salts  are  coarsely  powdered  and  pressed  between  bibulous 
papers  until   the  crystals  show  no  tendency  whatever  to  adhere  together, 
deliquescent  and  efflorescent  salts  to  be  handled  as  expeditiously  as  possible. 
Volatile  liquids  are  dried  by  long  contact  with  some  hygroscopic  solid  like 
calcined  potassium  carbonate,  anhydrous  copper  sulfate,  quicklime,  baryta,  or 
other  dehydrating  agent  that  has  no  chemical  action  on  the  liquid ;  gases,  by 
passing  through  a  column  of  anhydrous  calcium  chloride  or  phosphoric  acid. 

B.  Substances  altered  at  temperatures  above  the  ordinary  are  dried  in  a 
"  desiccator  "  surrounded  by  an  atmosphere  which  acts  as  a  conveyor  of  the 
moisture  of  the  substance  to  a  hygroscopic  material. 

The  desiccator  or  exsiccator,  Fig.  12,  is  a  heavy  glass 
cnp  with  a  cover  fitted  by  a  ground  joint.  A  sheet  metal 
shelf  perforated  with  various  sized  holes  rests  on  a  flange 
near  the  middle  of  the  cup,  or  a  wire  triangle  is  supported 
on  legs  encased  in  glass  tubes  to  protect  the  wire  from 
corrosion  by  the  exsiccant.  The  shelf  or  triangle  carries 
different  sized  crucibles  or  dishes  holding  the  substance 
to  be  dried.  In  the  bottom  of  the  desiccator  is  a  layer  of 
iused  calcium  chloride  or  sand  saturated  with  concen- 
trated sulfuric  acid,  the  latter  drying  the  air  to  a  greater 
degree  than  the  former.*  The  time  required  for  drying  a  Fig.  12.  ll\- 
substance  Cat  least  several  hours)  may  be  considerably  shortened  by  ratifying 
the  inclosed  air  by  exhaustion  through  a  tube  projecting  from  the  desiccator 
or  its  cover. 

A  modification  of  the  above  Is  by  Hempel. 
The  edge  of  the  cover  is  curled  inwardly  lorm 
ing  an  annular  cup  also  filled  with  the  exsic- 
cant, thus  providing  a  drjing  medium  both 
above  and  below  the  substance.  Another  form, 
not  so  portable,  is  a  shallow  porcelain  dish 
with  several  radial  ribs  to  support  dishes  con- 
taining the  material  to  be  dried  or  preserved 
from  moisture.  The  dish  Is  partly  filled  with 
concentrated  sulfuric  acid  and  stands  on  a 
ground-glass  plate  and  covered  by  a  large  bell- 
jar. 


Fig.  13. 


For  substances  decomposed  by  sunlight,  desiccators  are  made 
of  yellow  glass  to  exclude  actinic  rays. 

C.  A  number  of  organic  bodies,  though  partially  de- 
composed at  100°  may  safely  be  dried  at  a  temperature 
somewhat  below  this  and  much  more  rapidly  than  in  a 
desiccator. 

The  air-bath,  Fig.  13,  is  a  box  of  sheet  copper  with 
riveted  joints,  supported  over  a  Bunsen  burner  by  four 
detachable  legs,  and  the  front  a  door  hinged  at  the  side. 
A  thermometer  passes  through  a  cork  inserted  in  a  ring 
at  the  top  of  the  box,  the  bulb  situated  near  the  center 
of  the  bath.  The  containing  vessels  are  supported  on 
a  tin-plate  shelf  about  half-way  up,  since  the  bottom  of 
the  bath  is  at  a  higher  temperature  than  is  shown  by  the 
thermometer. 
The  flame  of  the  burner  may  be  adjusted  to  give  any 


*  '  h.-in.  News,  1)O!— 1-151. 


Qr.\  NTITATIVE    CHKMICAL    ANALYSIS.  27 

degree  of  bent  up  to  300s  —  higher  than  this  is  destructive  to  the  bottom 
plate.  To  maintain  a  uniform  temperature,  many  form  of  regulators  or  ther- 
mostats have  been  devised,  to  control  the  flow  of  gas  to  the  burner  independent 
of  any  irregularity  in  the  gas  pressure.  One  is  shown  in  Fig.  14;  the  glass 
bulb  A  filled  with  mercury  bangs  in  the  bath,  and  whenever  the  desired  tem- 
perature is  slightly  exceeded  the  mercury  expands  and  rises  so  far  as  to  partly 
close  au  aperture  B  in  the  tube  C  througa  which  the  gas  flows  to  the  burner, 
diminishing  the  size  of  the  flame.  A  screw  E  presses  against  the  mercury  In 
the  branch  tube  F  and  provides  for  adjusting  the  height  of  the  mercury  col- 
umn to  maintain  any  given  temperature.  Cooling  of  the  bath  below  the  proper 
temperature  is  immediately  followed  by  an  increased  flame.  Since  a  sudden 
increase  in  the  volume  or  pressure  of  the  gas  admitted  to  the  burner  would 
result  in  an  abrupt  closing  of  the  orifice  and  extinguish  the  light,  a  small  hole 
at  D  allows  just  enough  gas  to  pass  to  keep  it  alight  or  feed  a  small  pilot-light.* 
Through  a  large  glass  tube  passing  horizontally  through  the  oven  and 
projecting  several  inches  at  each  end  may  be  passed  a  current  of  some  neu- 
tral gas,  the  tube  holding  boats  containing  bodies  affected  by  the  oxygen  of 
the  air.f 

Venable  J  describes  a  drying  apparatus  constructed  of  a  glass  bell- jar  set  on  a  sand-bath 
or  a  ring  of  asbestos  board  resting  on  an  iron  plate.  The  bell-jar  has  three  mouths,  one 
holding  a  thermometer,  the  other  two  for  entrance  of  air  and  egress  of  aqueous  vapor. 
An  electrically  heated  oven  is  described  by  Richards.§ 

D.  Most  oxides,  minerals,  anhydrous  salts  and  the  like  are  heated  to  the 
temperature  of  boiling  water;  usually  an  exposure  for  an  hour  is  sufllcient  to 
remove  all  moisture  from  a  fine  powder. 

The  water  oven  has  the  advantage  over  the  air  bath  of  maintaining  nearly  the 
temperature  of  100°  without  attention  or  special  appliances.  It  is  of  tbe  same 
construction  with  the  exception  that  the  top,  bottom  and  three  sides  are  double. 
The  space  between  the  walls  is  partly  filled  with  water  kept  boiling  by  a  Bunsen 
burner.  The  temperature  of  the  interior  approximates 
100°  as  nearly  as  is  necessary  for  most  purposes. 

As  the  water  evaporates  through  an  opening  at  the  top  it 
must  be  replenished  from  time  to  time  unless  the  oven  is 
provided  with  an  appliance  to  refill  it  automatically,  such 
as  the  one  shown  in  Fig.  15.  A  continuous  stream  of 
water  enters  the  cup  A  from  B  and  flows  to  the  oven 
through  C ;  the  height  of  the  upper  orifice  of  an  overflow 
pipe  D  determining  the  water-level  in  A  and  consequently 
in  the  oven.  Or  if  the  opening  into  the  water  chamber  is 
closed  with  a  cork  carrying  a  vertical  open  glass  tube  many 
feet  long,  the  arising  steam  will  be  largely  condensed  by 
the  air  and  returned. 

A  hygroscopic  solid  or  semi-solid  is  best  dried  in  a  broad,  light  glass  bottle 
with  ground  stopper.  After  cooling,  the  stopper,  inserted  while  the  bottle  is 
still  hot,  is  apt  to  stick  fast  from  rarefaction  of  the  air,  and  Mason  proposes  to 
provide  it  with  a  stopcock  that  may  be  opened  to  admit  air  to  the  bottle. 

Viscous  bodies  are  more  quickly  and  thoroughly  dried  if  previously  admixed 
with  an  equal  weight  of  sand  or  other  anhydrous  powder.  In  some  cases 
there  is  an  additional  advantage  in  that  the  residue  is  left  in  a  pellicular  form 
easily  permeated  by  solvents. 


*  Chem.  News,  1890— 1—4 and  24. 
t  Journ.  Socy.  Chem.  Ind.,  1896—417. 
J  Journ.  Amer.  Chem.  Socy.  1898—272. 
§  Amer.  Chem.  Journ.  1899—45. 


28  QUANTITATIVE    CHEMICAL    ANALYSIS. 

When  a  constituent  other  than  water  is  lost  on  drying  it  may  be  retained  by  the  addi- 
tion of  some  reagent  that  will  combine  with  it,  as  anhydrous  oxalic  acid  for  ammonia,  or 
an  alkali  carbonate  for  a  volatile  acid.  Or  the  vapors  may  be  passed  through  an  absorbent 
to  retain  the  volatile  constituent. 

For  a  direct  determination  of  moisture  the  drying  is  done  in  a  glass  tube  one 
end  connected  with  a  reservoir  of  dry  air,  the  other  with  weighed  bulbs  filled 
with  dried  calcium  chloride  or  other  desiccator.  If  the  amount  of  water  is 
considerable,  the  substance  is  placed  in  a  small  flask  connected  with  a  gradu- 
ated tube ;  the  combination  is  exhausted  by  a  vacuum  pump,  the  tube  cooled 
in  a  freezing  mixture,  and  the  flask  gently  heated.  The  moisture  distills  into 
the  tube  where  its  volume  can  be  measured  or  the  water  weighed. 

E.  A  comparatively  few  bodies  must  be  dried  at  temperatures  above  100°  for 
specific  reasons. 

When  it  is  desired  to  maintain  an  even  temperature  somewhat  above  100°,  a 
small  oven  may  contain  between  the  walls  an  organic  liquid,  of  a  suitable  boil- 
ing point.  Thus,  toluol  boils  at  107°,  xylol  at  136°,  anisol  at  150°,  pinene  at 
160°,  anilin  at  180°,  naphthalin  at  200°,  and  diphenylamine  at  310°.  An  aqueous 
solution  of  calcium  chloride  boils  at  from  101°  to  178°  according  to  concentra- 
tion; to  prevent  the  solution  from  becoming  more  concentrated  by  evaporation 
of  the  water,  an  inverted  condenser  is  held  in  a  cork  closing  the  hole  at  the 
top,  to  condense  and  return  the  steam.  Usually,  however,  the  drying  is  done 
in  the  air-bath. 

For  the  very  hygroscopic  manganese  ores  (superoxides),  Fresenius  advises  a  thick  iron 
plate  with  a  number  of  cup-shaped  depressions  arranged  in  a  circle  near  its  periphery, 
into  each  of  which  fits  a  metal  dish.  The  Bunsen  burner  flame  is  under  the  center  of  the 
plate,  and  one  of  the  dishes  is  filled  with  iron  filings  surrounding  the  bulb  of  a  thermom- 
eter showing  its  temperature  and  consequently  that  of  the  ores  in  the  other  dishes. 

Liquids  unaltered  at  100°  may  be  dried  at  this  temperature,  but  it  is  safer  to 
employ  one  considerably  higher,  as  for  physical  reasons  all  the  water  may  not 
be  driven  off  at  its  boiling  point;  this  phenomenon  may  be  noticed  with  the 
oils  and  fats,  and  many  precipitates  are  said  to  be  subject  to  it.  Yet  it  must 
not  be  forgotten  that  many  apparently  stable  bodies  are  somewhat  volatile  or 
are  decomposed  even  at  100°  and  one  must  be  cautious  in  this  respect.  A 
good  plan  is  to  finish  by  moistening  the  substance  with  absolute  alcohol  and 
evaporating  at  a  moderate  temperature. 

For  soaps,  wood  fiber  and  the  like  it  is  recommended  to  heat  a  suitable 
quantity  of  paraffin  oil  to  250°,  cool  and  weigh.  The  weighed  sample  is  intro- 
duced, the  oil  heated  to  240°,  and  reweighed.  All  the  moisture  is  said  to  be 
expelled  without  decomposition  of  the  substance. 

In '  drying  above  300°,  a  considerable  variation  in  temperature  is  seldom  of 
consequence,  and  the  heat  may  be  applied  by  a  sand  or  metal-bath  (a  shallow 
iron  dish  filled  with  sand  or  an  easily -fusible  alloy^,  or  by  a  direct  flame  as  is 
most  convenient. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


CHAPTER  3. 

THE  BALANCE  AND  WEIGHTS. 

Practically  our  only  means  of  measuring  mass  is  by  weight,  and  since  the 
correctness  of  every  analysis,  with  scarcely  an  exception,  depends  primarily  on 
this  operation,  it  is  evident  that  the  balance  used  should  be  of  the  highest 
attainable  precision.  The  obvious  importance  of  this  subject  calls  for  a  some- 
what extended  discussion  of  the  principles  of  its  construction  and  rules  for  its 
employment. 

As  at  present  built  by  a  number  of  European  and  American  manufacturers, 
the  analytical  balance  is  always  of  one  form  differing  only  in  details.  The 
emulation  between  them  has  resulted  in  establishing  a  high  standard,  and  one 
may  now  rely  on  receiving  a  first-class  instrument  when  purchasing  of  a 
reputable  firm,  and  at  a  price  quite  reasonable  considering  the  care  and 
skill  bestowed  on  its  manufacture,  and  the  patience  in  its  adjustment. 
The  few  variations  of  construction  and  fittings  are  of  minor  importance  and 

need  be  given  little  consideration  —  a 
belief  in  the  intrinsic  superiority  of 
the  productions  of  any  one  maker  is 
often  only  the  reflection  of  a  personal 
preference  born  of  familiarity  with 
their  manipulation  and  behavior. 

Brief.  The  analytical  balance  is 
essentially  a  light  metal  beam  oscil- 
lating on  frictionless  bearing  located 
exactly  in  the  middle,  and  having  a 
pan  suspended  from  a  similar  bearing 
near  each  extremity,  with  an  arrange- 
Fig.  16.  Vio  ment  to  support  the  beam  and  pans 

when  the  balance  is  not  in  actual  service;  all  being  inclosed  in  a  glass  case. 
The  parts  of  a  modern  balance,  Fig.  16,  of  the  style  originated  and  perfected  by 
American  builders  may  be  described  as  follows :  — 

The  beam,  Fig.  17.  This  was  formerly  made  of  some  light  wood,  but  at 
present  of  brass,  bronze  or  hardened  aluminum,  lacquered,  to  prevent  corro- 
sion by  acid  fumes.  The  form  is  a  truss  or  hollow  triangle  braced  at  intervals 
by  cross-bars,  thus  uniting  the  important  qualities  of  rigidity  and  lightness. 
Across  the  center  is  fixed  a  triangular  knife-edge,  A,  either  of  hard  steel,  agate 
or  carnelian  set  in  metal,  or  iridosmine.  The  ends  of  the  beam  are  each  pro- 
vided with  a  similar  knife-edge  B,  B,  standing  in  the  reverse  position,  the  edge 
being  upward ;  the  three  are  firmly  clasped  in  the  beam  and  correctly  located 
once  for  all  by  the  maker.  A  vertical  needle  C,  about  as  long  as  the  beam, 
depends  from  its  center  to  exhibit  the  angle  of  its  deviation  from  the  horizon- 
tal, and  from  one  or  both  ends  extends  a  threaded  wire  D,  carrying  a  nut  E, 
that  may  be  screwed  in  or  out  to  restore  the  equilibrium  if  disturbed  from  any 
cause.  In  the  older  balances  equilibrium  was  obtained  by  altering  the  angular 
position  of  a  light  vane,  a  short  metal  strip  centered  at  the  bottom  of  the  post 
F,  sometimes  terminated  by  a  threaded  wire  carrying  a  nut. 


n 


o 


30 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


The  center  of  gravity  of  the  balance  is  brought  up  to  the  proper  height  by  a 
post  F,  rising  from  the  center  of  the  beam ;  in  some  balances  the  weight  of  the 


beam  is  so  distributed  in  re- 
post  is  dispensed  with.  The 
by  the  position  of  a  light  collar 
secured  by  a  set-screw.  When 
rests  on  two  studs  H  H,  one 
cups  fixed  in  the  beam  sup- 
the  beam,  and  the  studs  to  the 


Fig.  18. 


lation  to  the  knife-edges  that  the 
center  of  gravity  is  closely  adjusted 
G,  which  slides  on  the  needle,  and  is 
the  balance  is  not  in  use,  the  beam 
convex,  the  other  conical,  fitting  in 
ports ;  or  the  cups  may  be  fixed  to 
supports. 

The  Pillar,  Fig.  18.  A  hollow  col- 
umn mounted  on  a  brass  plate  held 
by    two   thumb-screws    to    another 
plate  bolted  to  the  floor  of  the  bal- 
ance-case; in  a  bracket  A  projecting 
anteriorly  from  the  top  is  imbedded 
a  flat  agate  plate  B  supporting  the 
center  knife-edge  of  the  beam.    Near 
the  bottom  is  an  ivory  scale  C  tra- 
Fig.  17.     versed  by  the  needle,  an  arc  divided 
right  and  left  from  the  middle  zero; 
the  balance  is  usually  adjusted  so  that  a  space  of  five 
divisions  corresponds  to  a  weight  of  one  milligram. 
A  reading  gla«s  of  medium  refraction  in  front  of  the 
scale  is  an  agreeable  addition  especially  in  an  illy- 
lighted  balance  room. 

Fixed  to  the  pillar  at  the  rear  is  a  circular  spirit- 
level  D,  or  two  tubular  levels  at  right  angles,  showing 
any  deviation  of  the  pillar  from  the  vertical,  and  con- 
sequently of  the  beam  from  the  horizontal  when  the 
needle  points  to  zero. 

The  Stirrups,  Fig.  19,  are  two  hollow  rectangles  of 
thin  sheet  German  silver,  the  top  bar  A  inclosing  the 
flat  agate  plate  B  which  rests  on  the 
end  knife-edge.  The  projecting  ends 


of  the  bar  have  edges  C  C  beneath  caught  by  the  V's  of  the 
beam -supports.  Below  is  a  double  hook  D,  the  upper  recess 
for  the  suspension  wires  of  the  scale-pan,  the  lower  for  hanging 
U-tubes,  etc.,  to  be  weighed. 

The  Scale -pans  are  circles  of  light  sheet  metal  stiffened  by 
a  down-turned  rim  and  hung  from  the  stirrup  hooks  by 
bent  wires.  The  length  of  the  wires  is  immaterial  (as- 
suming that  there  is  no  friction  between  the  end  knife- 
edges  and  their  bearings)  but  it  is  essential  that  the  pans  be 
suspended  in  such  a  manner  that  they  may  swing  freely  at 
right  angles  to  the  beam,  so  that  if  an  object  be  placed  on  the 
pan  away  from  its  center,  the  pan  will  shift  in  the  opposite 


Fig.  19. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


direction  so  far  that  the  center  of  gravity  will  lie  vertically 
below  the  middle  of  the  knife-edge,  which  will  consequently 
sustain  the  load  uniformly  throughout  its  length,  Fig  20.  This 
is  provided  for  by  the  flexible  joints  in  the  stirrups. 

Beam  Supports.  It  being  indispensable  for  preserving  intact 
the  keenness  of  the  knife-edges  that  they  press  upon  the 
agate  planes  only  when  the  balance  is  in  actual  use,  a  system 
of  supports,  Fig.  21,  carries  the  beam  and  stirrups.  It  con- 
sists of  a  reeded  wheel,  A,  fitting  on  the  anterior  end  of  a 
horizontal  shaft  B  extending  back  to  the  center  of  the  balance 
case;  immediately  under  the  pillar  is  keyed  a  cam,  cardiod,  or 
eccentric  C,  on  the  periphery  of  which  rests  the  lower 
end  of  a  rod  D  sliding  up  and  down  within  the  pillar.  On 

the  upper  end  of  the 
rod  is  a  cross-head  E. 
Through  this  pass  two 
screws,  F  F,  held  in 
position  by  jam -nuts; 
on  their  points  rest 
the  two  arms,  G  G, 
working  on  a  central 
pivot  H.  Near  the  Fig.  20. 
end  of  each  arm  is  a  small  stud,  II* 
receiving  the  cup  beneath  the  beam, 
and  also  a  bifurcated  extension  J  with 
a  pair  of  V-shaped  grooves  catching 
the  upper  bar  of  the  stirrup,  Fig.  21  A. 

As  the  cam  is  rotated  by  turning  the 
wheel,  the  rod  and  crosshead  drops, 
lowering  the  arms  GG  to  the  position  shown  by  the  dotted  lines.  It  will  be 
noticed  that  when  the  wheel  is  turned  at  a  uniform  speed  the  rapidity  of  the 
motion  of  the  rod  and  arms  is  not  correspondingly  uniform,  the  variation  due 
to  the  eccentricity  of  the  cam;  the  upward  motion  is  continuously  retarded,  so 

that  the  impact  of  the  supports  with  the 
beam  and  stirrups  is  free  from  shock  pro- 
vided the  wheel  is  turned  with  reasonable 
slowness. 

The    whole    arrangement    demands    the 
Fig.  21  A.  most  careful  construction  to  insure  that  the 

beam  and  stirrups  shall  invariably  be  lowered  to  the  same  position  and  the  beam 
raised  from  an  inclination  without  any  sliding  of  the  knife-edges  along  or 
across  the  planes. 

For  rigidity,  the  inner  ends  of  the  arms  GG  are  broadened  and  dove-tailed, 
the  firm  hinge- joint,  Fig.  22,  preventing  any  lateral  motion  of  their  extremities; 
the  axis  of  the  pivot  is  located  exactly  in  the 
line  of  junction  of  the  center  knife-edge  that 
the  arcs  described  by  the  cups  II  and  the  studs 
HH  (Fig.  17)  shall  coincide;  and  (unless  the 
stirrups  do  not  engage  with  their  supports  until 
after  the  beam  is  brought  to  a  horizontal  posi-  Fig.  22. 

tion)  the  edges  CC  (Fig  19)  of  the  stirrups  are  situated  in  the  plane  of  the  in- 
closed agate  plates  BB.  Adjusting  screws  are  provided  to  compensate  for  any 
wear  or  derangement. 


Fig.  21. 


32  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Pan  -supports.  The  pans  rest  on  the  plush  covered  tips  of  a  pair  of  light  fin- 
gers A,  Fig.  23,  which  are  pivoted  at  B.,  and  the  posterior  ends  fixed  to  the 
extremities  of  a  shaft  C  running 
across  the  rear  of  the  balance  case. 
Near  the  middle  of  C  depends  an 
arm  D,  the  lower  end  hooked  to  a 
rod  E  moving  longitudinally  and 
terminating  in  a  button  F  to  the 
left  of  the  wheel.  A  light  spiral 
spring  encircles  E  forcing  it  and  the 
arm  D  forward,  thereby  depress- 
ing C  and  raising  A.  When  the 

button  is  pressed  the  reverse  takes  place,  and  the  pans  are  set  free.  The  ten- 
sion of  the  spring  is  regulated  by  turning  the  button,  and  equality  in  height  of  A 
and  A%  by  the  screw  H.  On  the  rod  E  near  the  button  is  a  transverse  groove, 
into  which  drops  a  catch  when  the  button  is  pressed  in,  keeping  the  fingers  AA' 
down  until  the  catch  is  released  by  pressing  a  wire  near  the  button.  A  better 
arrangement  provides  for  holding  down  the  fingers  when  the  button  is  rotated 
through  a  small  arc. 

The  tips  of  the  pan-supports  should  press  against  the  exact  centers  of  the 
pans  to  prevent  deflection  of  the  stirrups.  In  a  late  improvement  the  tips  are 
conical  in  form  and  rise  into  corresponding  cavities  beneath  the  pans  thereby 
insuring  an  exact  centering  of  the  latter  when  their  swinging  is  arrested. 

The  Rider-arm.  This  is  a  brass  rod  hung  horizontally  above  and  behind  the 
beam.    It  extends  from  the  middle  to  the  exterior  of  the  balance  case,  termi- 
-  ^  ,    nating  in  a  knurl  by  which  it  can  be  rotated  or  moved 

longitudinally.  Across  the  inner  end  is  an  adjust- 
able hooked  wire  A,  Fig.  24,  which  catches  the  loop 
of  a  bent  wire  weight  B  called  a  '*  rider."  By  manip- 
ulating the  rider-arm  the  rider  may  be  hung  at  any 
point  on  the  right-hand  half  of  the  beam,  or  sus- 
pended above  it. 

The  Balance  Case.    A  frame  of  well-seasoned  wood 
24  or  metal,  glazed  on  the  sides  and  top,  is  mounted  on 

leveling  screws  and  incloses  the  entire  mechanism 
for  protection  from  dust,  drafts  and  acid  fumes.  The  front  and  back  panes 
slide  upward,  the  former  counterpoised  by  sash-weights ;  both  may  be  locked 
by  the  wheel.  The  floor  may  be  of  wood,  plate-glass  or  black  marble,  prefer- 
ably the  last  named,  as  wood  is  apt  to  warp  and  glass  to  crack. 

Special  Balances.  To  save  time  in  transferring  weights  to  and  from  the  pans,  balances 
have  been  designed  where  any  weights  of  a  set  may  be  placed  on  the  pan  or  removed 
while  the  case  is  closed,  by  manipulating  the  corresponding  key  of  a  key-board  situated  in 
front  of  the  case. 

Bunge,  Hase,  and  others  have  devised  attachments  to  show  at  once  the  weight  of  an 
object  to  within  a  centigram.  The  object  to  be  weighed  being  in  the  left-hand  pan,  a 
spring  or  bent  lever  is  brought  in  contact  with  the  beam,  and  on  releasing  the  supports,  a 
pointer  actuated  by  the  spring  or  lever  shows  the  approximate  weight  on  a  scale  gradu- 
ated in  grams  and  fractions.  The  apparatus  is  then  thrown  out  of  contact  with  the  beam, 
and  the  exact  weight  found  in  the  usual  way.  In  the  plan  of  Serrler,*  one  end  of  a  long 
chain  is  attached  to  the  balance  beam,  the  other  end  to  a  lever  supported  by  the  balance 
case.  As  the  lever  is  raised  or  lowered  a  correspondingly  shorter  or  longer  part  of  the 
chain  hangs  from  the  beam.  The  height  of  the  lever  is  read  on  a  scale. 

In  Taylor's  apparatus  a  long  thread  made  of  glass  or  celluloid  is  firmly  held  horizon- 
tally at  one  end,  and  to  the  other  is  suspended  a  pan  carrying  a  pointer  traversing  an 


Analyst,  1897-196. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  33 

arbitrary  scale;  the  sample  is  filled  into  the  pan  until  the  pointer  sinks  to  the  division 
previously  indicated  when  a  standard  weight  was  in  the  pan.  It  is  specially  adapted  for 
weighing  a  number  of  equal  quantities  of  metal  drillings,  and  for  this  purpose  the  drill- 
ings as  they  are  cut  from  the  metal  bar  may  fall  directly  into  the  pan. 

Phillipsf  proposes  for  similar  purposes  an  instrument  in  the  shape  of  a  hydrometer,  the 
bulb  being  a  gilded  brass  ball  and  the  stem  an  aluminum  tube.  Across  the  top  of  the  stem 
is  fixed  a  horizontal  bar,  the  ends  perforated  to  slide  up  and  down  on  two  rods  attached  to 
the  hydrometer  jar.  On  the  cross-piece  is  fastened  a  small  scale-pan,  and  below  It  a 
pointer.  Another  pointer  extends  from  a  collar  that  slides  on  one  of  the  uprights  and  may 
be  fixed  at  any  point  by  a  set-screw.  The  jar  being  filled  with  water,  an  analytical  weight 
of  the  proper  denomination  is  placed  in  the  scale-pan  and  when  the  instrument  has 
come  to  rest,  the  pointers  are  made  to  coincide.  The  weight  is  then  removed  and  the 
sample  dropped  in  the  pan  until  the  pointers  again  coincide.  The  range  of  weight  is 
necessarily  small  and 'the  delicacy  depends  on  the  diameter  of  the  stem,  e.  g.,  one  of  a 
diameter  of  1-16  inch  sunk  a  depth  of  3  7-8  inches,  with  a  weight  of  .2  gram. 

The  elasticity  of  a  delicate  coiled  spring  is  applied  in  the  well  known  Jolly's  balance, 
designed  particularly  for  taking  the  specific  gravity  of  minerals. 

Attempts  have  been  made  to  provide  a  substitute  for  the  easily  dulled  knife-edges, 
such  as  hemispheres  rolling  on  the  plates,  etc.,  though  none  of  them  have  come  into  ex- 
tended use  for  analytical  work.  In  the  '  torsion  balance '  the  beam  grips  the  middle  of  a 
taut  horizontal  wire  held  firmly  at  the  ends  by  two  supports;  the  ends  of  the  beam  hold 
simi  ar  wires  terminating  in  the  upright  bars  of  the  stirrups.  As  the  beam  Is  inclined  the 
wires  are  twisted,  the  moment  of  their  torsion  being  for  small  angles  proportional  to  the 
load.  A  "  dynamic  balance  "  has  been  proposed  on  the  principle  that  a  load  applied  above 
the  fulcrum  of  a  compound  pendulum  raises  the  center  of  gravity  and  correspondingly 
alters  the  rate  of  vibration. 

A  comparison  with  another  style  of  balance  of  German  manufacture  shows 
several  points  of  difference  from  the  one  just  described.  The  case  has  three 
doors,  and  is  without  a  drawer  for  weights,  etc.  The  beam  is  much  wider, 
and  inverted,  the  obtuse  angle  being  above,  and  its  center  is  subtended  by  a 
vertical  threaded  wire  carrying  a  nut  which  may  be  screwed  up  or  down  to 
adjust  the  center  of  gravity.  To  the  front  of  the  lower  bar  of  the  beam  and 
parallel  with  it  is  fixed  a  strip  of  ivory  whose  upper  edge  carries  the  rider 
and  is  in  a  horizontal  plane  with  the  line  joining  the  knife-edges.  The  beam 
supports  are  not  pivoted  but  firmly  attached  to  the  cross-head  and  move 
vertically;  and  the  pan-supports  have  a  positive  motion,  generated  by  a  second 
cam  on  the  shaft  so  adjusted  as  to  raise  them  slightly  in  advance  of  the  beam- 
supports  in  order  that  the  latter  engage  with  the  beam  only  when  it  is  in  a 
horizontal  position.  The  spirit-level  is  replaced  by  a  plumb  bob  hanging  at 
the  rear  of  the  pillar,  and  the  joints  in  the  stirrups  by  a  second  knife-edge 
below  and  at  right  angles  to  the  first. 

Short-beam  Balances.  There  has  been  a  tendency  of  late  years  to  consider- 
ably shorten  the  beam,  reducing  it  from  10  or  12  to  6  or  7  inches,  thereby  giving 
a  more  rapid  movement  to  the  needle.  The 
sensibility  of  the  balance  is  theoretically  de- 
creased, as  the  beam  may  be  considered  as  a 
lever  of  the  first  class,  but  it  is  claimed  that  this 
is  offset  by  the  greater  stiffness  and  lightness  of 
the  shorter  beam.  While  there  is  a  diversity  of 
opinion  as  to  whether  the  operation  of  weigh- 
ing is  really  expedited  to  any  great  extent, 
certainly  they  have  met  with  favor  and  are  sup  - 
planting  the  older  form. 

The  Assay  Balance,  Fig.  25,  for  weighing  sil- 
ver buttons  and  gold  from  the  fire-assay  is  far  ™. 
lighter  and  more  delicate  in  construction,  as  it 


f  Chem.News,  1895  —  216. 


34 


QUANTITATIVE    CHEMICAL,    ANALYSIS. 


is  designed  for  a  load  not  exceeding  a  gram;  one  one-hundredth  of  a  milli- 
gram is  shown  on  one  of  the  finer  grades.  A  noticeable  difference  is  the  de- 
sign of  the  beam  supports  as  well  as  the  end  knife-edges  and  the  agate  plates. 
.--.  As  shown  in  the  figure  no  supports  are  pro- 

|V' "7\     vided  for  the  stirrups.    The  end  agate  bearings 

A  VpTIL/  "      are  kept  in  position  on  the  knife-edges  by  a 
semicircular  furrow  cut  in   the  former  termi- 


Fig.  26. 


nated  by  thin  metal  plates;  friction  between 


these  and  the  ends  of  the  knife-edges  is  prevented  by  cutting  the  latter  at 
an  angle,  Fig.  26.  In  the  more  precise  instruments  a  system  of  supports 
similar  to  that  of  the  analytical  balance  is  provided. 

An  equation  expressing  the  sensibility  of  the  balance  may  be  deduced  as  follows:  — 

In  Fig.  27  let  A  and  B  be 
the  end  knife-edges,  and  A 
B  a  line  joining  them  ;  O  G 
a  line  perpendicular  to  A  B 
with  the  point  of  intersec- 
tion at  C;  O,  the  center  of 
oscillation,  and  G,  the  cen- 
ter of  gravity  of  the  beam. 
Let  M  be  the  weight  of  the 
beam ;  P,  a  weight  In  the 
pan  supported  by  A;  and 
F',  an  equal  weight  in  the 
pan  under  B.  The  line  A 
B  being  horizontal  we  have 
Fig.  27.  by  the  law  of  levers, 

PX  AC  =  P'X  BO (1), 

If  now  a  small  weight  p  be  dropped  into  the  right-hand  pan  the  beam  will  incline, 
finally  coming  to  rest  In  the  position  a  O  b  while  O  G  moves  to  O  g.  Obviously  the 
greater  the  angle  g  O  G  produced  by  p,  the  more  sensitive  the  balance. 

Draw  the  perpendicular  a  h,  b  k,  and  g  i,  and  call  the  angle  AOa  =  B  O  b  =  G  O  g=  0, 
and  AOC  =  BOC  =  a.  Since  the  balance  is  now  in  equilibrium  at  a  O  b,  we  have  p 
balanced  by  MX  g  i,  and  therefore 

Pxah  +  MXgl=(P'  +  p)X  bk (2). 

the  "  equation  of  equilibrium." 

Nowgi  =  0gsin  0  =  OGsln0  (3). 

Also  a  h  =  a  O  sin  (A  O  a  +  A  O  C). 

Or  a  h  =  A  O  sin  (a  -j-  0)  since  a  O  =  A  O. 

And  (according  to  the  well-known  theorem,  sin  (x  -f  y)  =  sin  x  cos  y  +  sin  y  cos  x), 

a  h  =  A  O  sin  a  cos  &  +  A  O  sin  6  cos  a, 
and  since  A  O  sin  a  =  A  C,  and  A  O  cos  a  =  O  0,  then 

a  h  =  AC  cos  6  +  OCsin0 (4). 

In  the  same  manner  wa  find 

b  k  =  A  Ocos  6  —  OCsin0 (5). 

Substituting  the  values  of  (3),  (4),  and  (5)  In  equation  (2), 

P  X  A  C  cos  6  +  P  X  O  0  sin  0  +  M  x  O  G  sin  d  =  (P'  +  p)  X  A  0  cos  6  —  (P'  +  p)  X 
O  C  sin  d. 
Reducing, 

sin  6  _  (P'  +  p)  X  A  0  —  P  X  A  O 

C080 


QUANTITATIVE    CHEMICAL    ANALYSIS.  35 

or 

tan  g  O  G  =  (p  +  p,  +  p)  XOC+MXOG  .....................  (6) 

From  equation  (6)  it  will  be  seen  that  tan  g  O  G  —  a  measure  of  the  deflection  of  the 
needle  or  the  inclination  of  the  beam  —  is  the  greater, 

1.  The  greater  the  difference  between  the  weights  in  the  pans  (Increase  in  p). 

2.  The  longer  the  beam  (increase  in  A  0). 

3.  The  less  the  weight  of  the  beam  (decrease  in  M). 

4.  The  nearer  the  axis  of  oscillation  to  the  line  joining  the  end  knife-edges  (decrease  In 
OC). 

5.  The  less  the  load  on  the  pans  (decrease  of  P  +  P'  +  p)  ;  but  if  O  and  C  are  made  to 
coincide  (as  they  do  in  all  balances),  O  C  becomes  zero,  P  +  P'  +  P  vanishes,  and  the  bal- 
ance is  equally  sensitive  in  this  respect  with  every  load. 

6.  The  less  the  distance  between  the  line  joining  the  end  knife-edges  and  the  center  of 
gravity  of  the  beam  (decrease  of  O  G).    If,  however,  O  and  G  as  well  as  O  and  C  are  made 
to  coincide,  the  equation  becomes 


and  for  every  difference  in  weight  in  the  pans,  however  small,  the  needle  will  tend  to  as- 
sume a  horizontal  position.  And  if  O  C  and  C  G  were  negative,  the  needle  would  describe 
more  than  a  quadrant. 


Details  of  construction.  Several  essential  features  must  be  given  consider- 
ation by  the  builder. 

1.  The  aggregate  weight  of  the  beam,  pans  and  hangers  should  be  as  small  as 
is  consistent  with  their  rigidity,  that  the  inertia  of  the  system  may  be  more 
easily  overcome  by  a  small  accession  of  weight  in  the  pan  or  on  the  beam. 

2.  Considering  the  balance  as  a  compound  pendulum,  it  is  clear  that  the 
center  of  gravity  must  lie  below  the  bearing  line  of  the  center  knife-edge  and 
as  near  to  it  as  is  consistent  with  a  reasonable  rapidity  of  vibration  (say  10  to 
15  seconds)  for  if  above  the  line,  the  beam  will  be  in  unstable  equilibrium  — 
top-heavy;  if  exactly  in  the  line,  in  neutral  equilibrium  and  refuse  to  vibrate; 
and  in  proportion  to  its  distance  below  will  a  greater  weight  be  needed  to  in- 
cline the  beam,  for  the  lower  the  center  of  gravity  is  situated  from  the  ful- 
crum—  that  is,  away  from  the  position  of  maximum  sensitiveness  —  the  more 
stable  will  be  the  equilibrium.     And  clearly,  any  increase  of  weight  applied  to 
the  end  knife-edges  (i.  e.  on  the  pans)  will  tend  to  raise  the  center  of  gravity 
nearer  the  axis  of  the  center  knife-edge;  the  gain  in  sensibility,  however,  is 
neutralized  in  a  great  measure  by  the  inertia  and  friction  engendered. 

3.  The  edges  of  the  three  knives  must  lie  in  the  same  plane,  as  otherwise  the 
center  of  gravity  will  shift  to  an  unallowable  degree  under  different  loads;  for 
if  the  center  knife-edge  is  below  a  line  joining  the  end  ones,  any  increase  in 
weight    on    the    latter  will  tend  to  raise  the  center  of 

gravity,  and  conversely.  For  this  reason  only  the  slight- 
est flexure  of  the  beam  under  the  maximum  load  is  per- 
missible, and  since  perfect  rigidity  cannot  be  attained 
without  a  sacrifice  of  lightness,  the  maker  endeavors  to 
secure  an  alignment  when  the  pans  are  weighted  with 
about  one-half  of  the  greatest  load  the  balance  is  designed 
to  carry. 

Each  knife-edge  should  be  horizontal  in  order  that  a 
load  may  be  sustained  equally  throughout  its  length,  and 
the  three  edges  be  at  exact  right  angles  to  the  axis  of  the 
beam  —  the  center  edge,  in  order  that  the  beam  at  every 
inclination  shall  align  with  its  supports,  and  the  end 
edges,  that  they  remain  horizontal  when  the  beam  is 


36  QUANTITATIVE    CHEMICAL    ANALYSIS. 

inclined.  The  knives  must  also  be  as  keen  as  possible,  not  only  to  lessen 
friction,  but  also  that  the  distances  from  one  another  of  their  lines  of  con- 
tact with  the  agate  plates  shall  be  identical  in  every  position  the  beam  may 
assume ;  the  effect  of  a  blunt  edge  is  shown  exaggerated  in  Fig.  28. 

The  agate  plates  must  be  exact  planes  and  highly  polished  that  their  contact 
with  the  knife-edges  shall  approach  geometric  lines.  Under  a  load  we  may 
conceive  that  the  junction  of  the  edge  of  the  knife  and  the  plate  beneath 
becomes  an  arc  whose  radius  increases  with  the  load. 

4.  Theoretically,  the  end  knife-edges  should  be   exactly  equidistant  from 
the    center,  and  a  remarkably   close  approximation  is  attained  by   modern 
builders.    The  inevitable    slight  inequality  may  be  counteracted  by   certain 
modes  of  weighing. 

5.  Good  workmanship  throughout  is  to  be  insisted  on  when  selecting    a 
balance,  particularly  that  the  wood  of  the  case  shows  no  signs  of  warping  or 
cracking;  that  the  rider -arm  is  so  located  that  the  hook  will  fairly  catch  and 
lift  the  rider  without  striking  the  beam ;  that  the  needle  swings  close  to  the 
scale  without  touching  it  at  any  point;  and  that  the  mechanism  of  the  beam 
supports  and  pan  arrests,  the  movement  of  the  rider -arm,  and  the  sliding  of 
the  front  pane,  all  work    smoothly  without  undue    friction.    Finally,  it  is 
well  to   consider  that,    other  things  being  equal,  the  less  complicated  the 
mechanism  and  the  fewer  the  movable  parts  the  better.    The  profusion  of 
adjusting  screws  and  other  accessories,  a  distinguishing  feature  of  the  instru- 
ments of  some  manufacturers,  are  not  only  practically  useless  but  corrodible 
and  dust -catching,  a  positive  detriment. 

Before  sending  a  balance  into  the  market  the  builder  carefully  examines  it  to 
be  convinced  that  these  requirements  have  been  complied  with.  The  equi- 
distance  of  the  knife-edges  is  tested  by  transferring  equal  loads  to  opposite 
pans;  for  their  being  at  right  angles  to  the  beam  by  hanging  each  pan  by  a< 
wire  hook  to  the  rear  extremities  of  the  end  knives  and  weighting  one  pan  until 
the  balance  is  in  equilibrium,  then  transferring  one  of  the  hooks  to  the 
anterior  end,  when  the  equilibrium  should  be  unaltered;  for  their  collimation 
by  raising  the  center  of  gravity  until  with  empty  pans,  the  vibrations  are  very 
slow,  and  observing  the  rate  with  the  maximum  load;  and  finally  lowers  the 
gravity-bob  on  the  needle  until  the  beam  swings  with  the  proper  velocity.  It 
will  be  found  that  the  sensibility  of  a  balance  can  be  increased  by  raising  the 
bob,  but  this  is  not  as  a  rule  necessary  or  advisable,  as  the  errors  in  weighing 

with  a  reasonably  precise  adjust- 
ment are  usually  the  least  impor- 
tant of  those  affecting  every 
analysis. 

Care  of  a  balance.  The  foregoing 
shows  the  importance  of  proper 
treatment  of  an  instrument  of  such 
refinement.  Carelessness  and  neg- 
lect will  quickly  impair  its  sensitive- 
ness,andonceitisin  properworking 
order  the  less  experimenting  with 
the  various  provisions  for  adjust- 
ment, the  better.  Such  minor  cor- 
rections as  an  occasional  turn  of 
the  nut  at  the  end  of  the  beam  may 

Fig.  29  be  required,  but  any  further  alter- 

ation is  to  be  made  with  caution, 


QUANTITATIVE    CHEMICAL   ANALYSIS.  37 

and  only  competent  hands  ought  attempt  the  rectification  of  a  serious  fault.  The 
balance  should  be  reserved  for  analytical  purposes  exclusively,  a  less  delicate 
scale  (us  in  Fig.  29;  serving  to  weigh  out  reagents,  etc. 

It  may  stand  on  a  heavy  table  in  a  place  free  from  vibration  of  the  floor,  on  a 
shelf  bracketed  to  a  wall,  or  best  on  a  pillar  rising  from  the  ground,  and  away 
from  draughts  and  direct  sunlight.  Where  the  jar  of  passing  vehicles  or  trains 
causes  a  tremor  of  the  needle,  a  thick  rubber  sheet  laid  under  the  leveling  screws, 
or  inserting  them  in  rubber  stoppers,  will  absorb  the  vibration  partially  if  not  en- 
tirely. The  room  should  be  well  lighted  and  have  a  fairly  uniform  temperature, 
as  sudden  changes  may  cause  irregularities  by  expanding  or  contracting  the 
beam  ununiformly. 

The  air  in  the  balance  case  may  be  kept  fairly  dry  and  the  steel  parts  pre- 
served from  corrosion  by  a  beaker  holding  anhydrous  calcium  chloride  or  quick 
lime,  or  partly  filled  with  concentrated  sulfuric  acid.  The  case  should  always 
be  kept  closed  when  the  balance  is  not  in  actual  use,  and  a  baize  or  enameled 
cloth  bag  drawn  over  it  during  the  night  will  aid  in  keeping  out  dust  and  fumes. 
Agate  knives  advance  the  original  cost  of  a  balance  somewhat,  but  are  advisable 
where  the  balance  room  communicates  with  a  laboratory  where  acid  fumes  are 
abundant. 

Setting  up  a  balance.  As  received  from  the  dealer  the  beam  is  found  fas- 
tened to  the  bottom  of  the  drawer  of  the  balance  case.  The  needle  is  to  be 
tightly  screwed  into  the  beam,  taking  great  care  that  it  is  not  bent 
in  the  operation.  The  supports  are  raised  and  the  beam  slowly  and 
carefully  lowered  on  them,  observing  that  the  needle  goes  in  front 
of  the  scale.  The  stirrups  are  then  slipped  over  the  ends  of  the  beam 
into  the  V's  of  the  supports,  with  the  openings  of  the  hooks  to  the  front;  the 
upturned  ends  of  the  pan- wires  are  inserted  in  the  holes  beneath  the  pans, 
and  hung  on  the  upper  hooks.  All  the  detachable  parts  are  marked  with 
points  or  letters  to  show  their  contiguity  at  the  time  the  balance  was  adjusted 
by  the  maker,  and  this  arrangement  must  b'e  exactly  reproduced.  The  bubble 
is  then  brought  directly  under  the  ring  at  the  center  of  the  spirit-level  by 
adjusting  the  leveling  screws  of  the  balance  case.,  the  rider  is  hung  on  the  hook 
of  the  rider-arm,  and  the  balance  is  in  readiness  for  examination  by  the  direc- 
tions on  page  41;  but  no  attempt  at  equilibration  is  to  be  made  before  some 
hours  have  elapsed  for  the  beam  to  assume  or  return  to  the  temperature  of 
the  balance  room. 

Weighing.  Before  proceeding  to  make  a  weighing,  the  pans  are  to  be 
dusted  with  a  camels -hair  pencil,  and  the  equilibrium  of  the  balance  tested, 
seeing  that  the  rider  is  off  the  beam,  and  that  the  balance  stands  level.  The 
wheel  is  turned  very  slowly  until  the  knives  rest  on  their  bearings;  especially 
is  this  important  when  the  supports  separate  them  too  far  —  they  should  barely 
part.  The  wheel  is  then  turned  as  far  as  it  will  go  and  the  button  pushed  in. 
Should  the  needle  remain  stationary  the  rider  is  momentarily  dropped  on  the 
beam,  but  the  extent  of  the  first  vibration  should  not  be  so  great  as  to  cause 
the  beam  to  touch  its  supports,  for  a  slight  shifting  may  take  place  if  their 
axis  does  not  coincide  with  the  bearing  line  of  the  center  knife  edge. 

Nothing  is  to  be  placed  on  the  pans  or  removed  while  the  beam  is  unsupported, 
and  the  pan-rests  should  bring  the  needle  to  zero  before  the  location  of  the 
rider  is  changed  or  the  beam -supports  raised.  The  case  should  always  be 
closed  while  taking  an  exact  weight.  Any  twist  or  shift  of  the  stirrup  while 
the  beam  is  free  is  to  be  avoided,  not  that  the  beam  length  is  altered  thereby, 
but  because  the  edges  of  the  knives  may  be  injured,  and  if  the  stirrup-case 
comes  in  contact  with  the  knife-edge  the  beam  will  refuse  to  vibrate. 


38 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


Fig.  30. 


Only  glassware,  crucibles,  dishes  and  the  like  are  set  directly  on  the  scale 
pans,  other  materials  being  held  in  suitable  containers.  A  pair  of  watch  -glasses 
of  exactly  equal  weight  usually  accompanies  the  balance  for  weighing  powders, 
as  well  as  a  light  German  silver  stand,  Fig.  30,  to  support  a  test-tube  contain- 
ing a  liquid.  Most  powders,  crystalline  salts,  and  dried 
or  ignited  precipitates  acquire  moisture  from  the  air  so 
slowly  as  to  need  no  further  protection  than  covering 
the  dish  or  crucible  containing  them,  while  hygroscopic, 
efflorescent,  or  volatile  substances  are  held  in  a  test 
tube  closed  by  another  of  equal  length  and  slightly  greater 
diameter,  or  a  weighing-bottle.  Paper  filters  are  inclosed 
between  two  watch-glasses,  bound  together  by  a  spring 
brass  clip,  Fig.  81. 

A.  gas  is  weighed  in- 
closed in  a  light  glass 
bulb  approximately 
counterpoised  by  an- 
other of  the  same  size, 
and  as  these  are  too  FiS-  31. 

bulky  to  enter  the  balance  case,  they  are  hung  to  the  stirrups  by  thin  wires 
passing  through  the  bottom  of  the  case  and  the  shelf  on  which  it  rests  into  a 
brick-walled  cell  wherein  the  temperature  is  not  subject  to  sudden  changes. 
Temperature  in  weighing.  After  heating  a  crucible  or  dish  it  must  be  allowed 
to  cool  to  the  temperature  of  the  balance-room  before  placing  on  the  scale-pan 
as  the  air  currents  arising  from  a  body  warmer  than  the  surrounding  air  tend 
to  raise  one  pan  and  depress  the  other.  In  especially  delicate  work  the  vibra- 
tions of  the  needle  are  observed  through  a  telescope  from  a  distance  of  sev- 
eral feet  that  the  radiated  heat  of  the  body  may  not  affect  the  length  of  the 
beam  or  induce  air-currents. 

It  is  plain  that  the  more  extensive  the  surface  of  a  body  exposed  to  the  air, 
the  greater  its  capacity  for  condensing  moisture  and  consequent  variation  in 
weight  at  different  times  through  a  change  in  the  hygroscopic  condition  of  the 
air,  and  on  rainy  days  it  is  almost  impossible  to  obtain  accordance  in  several 
weighings  of  a  bulky  object.  Hence  vessels  containing  substances  for  weigh- 
ing should  be  as  small  and  compact  in  form  as  circumstances  will  permit,  and 
remain  in  the  balance  for  fifteen  minutes  or  more  before  taking  their  exact 
weight.  It  is  said  that  in  weighing  an  open  wide-mouth  vessel  containing  a 
volatile  liquid,  air  currents  are  set  up  by  evaporation,  the  more  extensive  the 
drier  the  air  of  the  balance  case. 

Weighing  by  reversal  and  substitution.  Besides  the  ordinary  direct  mode 
there  are  two  others  which  eliminate  or  correct  for  an  unequal  length  of  the 
beam  arms.  In  the  method  of  reversal,  the  object  is  balanced  first  on  one  pan, 
then  on  the  other;  the  arithmetical  (strictly  the  geometrical)  mean  of  the  two 
being  the  true  weight.  In  that  of  substitution,  a  counterweight  somewhat 
heavier  than  the  object  is  weighed ;  the  object  is  then  placed  in  the  pan  with  the 
weights,  and  by  the  sum  of  such  of  these  as  must  be  removed  to  restore  the 
equipoise,  the  true  weight  of  the  object  is  given.  And  in  weighing  in  the 
usual  manner  it  should  be  the  rule  to  reserve  the  right-hand  pan  for  the  weights 
only. 

Weighing  by  vibrations.  Theoretically,  equality  in  weight  is  shown  by  a 
journey  of  the  needle  the  same  number  of  divisions  to  each  side  of  the  zero, 
also  when  the  needle  remains  at  zero  after  the  supports  are  disengaged.  The 
former  indication  is  to  be  preferred  since  the  accession  of  a  weight  inadequate 


QUANTITATIVE    CHEMICAL    ANALYSIS.  39 

to  move  the  needle  from  a  state  of  rest  will  perceptibly  shift  the  arc  of  its 
vibration.  But  as  the  excursions  continually  lessen  in  amplitude,  uniformly  in 
theory,  but  more  or  less  irregularly  in  practice,  several  consecutive  readings  on 
each  side  are  averaged  in  preference  to  accepting  a  single  swing. 

With  the  load  in  one  pan  slightly  the  heavier  and  a  representing  the  number 
of  divisions  swung  to  one  side  and  a'  to  the  other,  then  £  (a  —  a')  locates  the 
secondary  point  of  rest,  and  if  the  displacement  corresponding  to  an  overweight 
of  one  milligram  is  determined  for  different  loads,  in  practice  fractions  of  this 
weight  may  be  quickly  and  quite  accurately  estimated  by  the  eye. 

Weighing  in  vacuo.  A  correction  is  made  in  the  most  refined  analyses  for  the 
buoyancy  of  the  air  which  diminishes  the  true  weight  to  the  extent  of  the  weight 
of  a  volume  of  air  equal  to  the  volume  of  the  object  weighed.*  The  latter  may 
be  taken  as  the  product  of  its  apparent  weight  by  the  reciprocal  of  its  specific 
gravity,  and  the  average  weight  of  one  cubic  centimeter  of  air  as  .0012  gram. 

When  an  object  has  the  same  specific  gravity  and  consequently  an  equal 
volume  with  the  sum  of  the  weights  balancing  it,  both  are  equally  lightened, 
and  the  apparent  is  also  the  true  weight.  But  when  the  object  has  a  lower 
specific  gravity  and  greater  bulk,  its  apparent  weight  must  be  increased  by  the 
weight  of  a  volume  of  air  equal  to  the  difference  between  the  volume  of  the  ob- 
ject and  that  of  the  weights,  and  conversely.  Assuming  the  weights  to  have 
been  standardized  in  vacuo,  if  G  represents  the  specific  gravity  of  the  object ;  G', 
that  of  the  weights;  and  W,  the  weight  of  the  object;  then 

Weight  in  vacuo  =  W  f  1  -f  .0012  (  -  _  JL  \  ~j    grams. 

Thus,  a  platinum  wire  sp.  gr.  21,  balancing  a  one-gram  weight  sp.  gr.  8.6 
weighs  in  a  vacuum,  .999916  gram. 

In  analysis  the  error  for  solids  and  liquids  is  unimportant  in  ordinary  work  as 
it  affects  both  the  weight  of  the  substance  taken  for  analysis  and  those  of  the 
separated  constituents,  one  largely  compensating  the  other.  For  gases  it  can- 
not be  neglected,  so  for  simplicity  the  glass  globe  inclosing  one  is  nearly  coun- 
ter-balanced by  another  of  the  same  size  and  slightly  less  in  weight. 

A  new  balance  of  the  best  quality  suitable  for  general  analytical  work:  — 

Should  carry  up  to  100  grams  in  each  pan. 

A  weight  of  one  milligram  should  alter  the  point  of  rest  of  the  needle  at  least 
4  mm.,  and  the  sensibility  be  not  greatly  decreased  with  the  maximum  load. 

On  reversing  a  load  of  one  gram  in  each  pan,  a  difference  of  not  more  than 
.0003  gram  should  be  found. 

Several  consecutive  weighings  of  the  same  object  should  not  differ  by  more 
than  .0002  gram. 

Not  more  than  15  seconds  should  be  required  for  one  vibration  of  the  needle, 
and  they  should  slowly  diminish  in  amplitude. 

For  general  work  the  balance  need  only  show  plainly  a  change  of  one -half  a 
milligram  in  the  load. 

THE   WEIGHTS. 

The  metric  system  of  weights  and  measures  is  now  almost  universally 
employed  in  analysis.  Although  the  standard  grams  of  different  makers  may 
vary  a  trifle  from  the  normal  gram  and  from  each  other,  yet  this  is  not  a  matter 
of  much  moment,  since  the  analyst  concerns  himself  almost  exclusively  with 
relative  rather  than  actual  mass.  But  it  is  of  the  highest  importance  that  in 
any  one  set  each  weight  shall  be  as  nearly  as  possible  an  exact  multiple  or 
factor  of  every  other  weight,  and  it  is  indispensable  that  the  extent  of  the 


*  Them.  News,  1895—1—4. 


40 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


variation  be  known  to  the  operator  if  it  be  measurable  by  the  means  at  his 
disposal,  for  the  delicacy  of  a  balance  counts  for  nothing  if  the  weights  are  not 
correspondingly  accurate. 

Denominations  —  The  Rider.  A  set  (Fig.  32),  containing  two  twenty-gram 
pieces  running  down  to  one  milligram  is  ample  for  most  work.  The  gram  and 
its  multiples  are  usually  of  brass,  lacquered,  gilded  or  platinized.  They  are 


Fig.  32. 

cylindrical  in  form,  capped  by  a  stem  for  handling.  By  some  manufacturers 
each  weight  is  made  a  trifle  light  originally,  the  stem  screwing  into  the 
cylinder  seals  a  cavity  containing  shot  and  strips  of  foil  to  bring  up  the 
weight  to  standard. 

A  number  of  materials  are  in  use  or  .have  been  proposed  for  analytical 
weights,  but  none  are  free  from  some  objectionable  features.*  Lacquered  brass 
can  be  fashioned  and- adjusted  at  a  moderate  cost,  but  the  weights  are  quickly 
attacked  by  acid  fumes  of  the  laboratory,  the  lacquer  is  hygroscopic,  and  it  is 
said  that  the  internal  blowholes  formed  in  casting  the  brass  are  liable  to  be- 
come oxidized  and  increase  the  weight  of  the  piece.  Gilded  and  platinized 
brass  weights  resist  acid  fumes,  yet  the  platings  are  so  soft  and  heavy  that 
slight  wear  perceptibly  reduces  the  weight  of  the  piece.  Quartz  and  hard 
glass  are  incorrodible,  though  expensive  to  make,  brittle,  and  liable  to  become 
electrically  charged.  Iridio-platinum  is  undoubtedly  the  best  material,  but  for 
ordinary  use  the  cost  of  a  set  of  weights  is  almost  prohibitive. 

The  denominations  below  one  gram  are  squares  of  platinum  or  aluminum 

foil  with  an  edge  or  corner  bent  up  to  be  grasped  by  the  forceps.    Fractions 

of  the  milligram  are  made  of  aluminum    wire,   but  they  are  so 

A  minute  and  difficult  to  handle  that  a  substitute  is  provided  in  this 
way:  the  front  of  the  top  bar  of  the  balance  beam  is  divided 
from  the  center  to  the  right-end  knife-edge  into  60,  100,  or  120 
equal  parts,  and  a  bent  aluminum  wire,  Fig.  33,  called  a  " rider" 
weighing  respectively  6,  10,  or  12  milligrams,  may  be  hung  at 
,  will  over  any  division  by  means  of  the  rider-arm  of  the  balance. 

l^'  '  'l  Each  division  therefore  corresponds  to  a  weight  of  one-tenth 
of  a  milligram,  and  the  set  of  weights  need  have  no  lower  denomination  than 
ten  milligrams,  or  five  when  the  beam  has  only  60  divisions.  In  some  bal- 
ances this  arrangement  is  duplicated  on  the  left-hand  side. 


*  Chen.  News,  1888-2-197. 


QUANTITATIVE    CHEMICAL.  ANALYSIS.  41 

Assay  weights.  For  gold  and  silver  ores  it  is  the  custom  with  assayers  to  report  their 
results  as  so  many  ouuces  (or  the  money  value  thereof),  in  one  ton  of  ore,  and  a  special 
system  of  weights  has  been  devised  by  Chandler  to  dispense  with  the  tedious  calculation 
from  the  metric  system  employed  in  the  assay.  The  basis  is  an  "assay  ton"  of  29.167 
grams;  this  weight  of  ore  being  taken  for  an  assay,  each  milligram  of  gold  or  sliver 
recovered  represents  one  Troy  ounce  per  ton  of  2000  Avoirdupois  Ibs.  In  some  mints 
for  assaying  alloys,  the  weight  of  .500  gram  is  taken  as  1000,  and  the  set  contains  decimals 
of  this  down  to  .0001. 

Ivory-tipped  forceps  are  best  for  handling  weights,  never  the  fingers  or 
sharp  pointed  forceps.  The  box  should  ^ be  kept  closed  when  the  weights  are 
not  in  use  to  exclude  dust  and  corrosive  fumes.  Warping  of  the  box  may 
distort  the  holes  and  cause  the  brass  weights  to  bind,  when  the  holes  should 
be  enlarged  by  scraping  with  a  knife- blade,  not  with  sand  or  emery-paper  as 
abrasive  particles  may  be  left  imbedded  in  the  wood. 

Experience  will  show  a  gain  in  time  by  trying  the  weights  in  a  methodical 
way  rather  than  at  random ;  beginning  with  the  one  judged  to  be  nearest  the 
weight  of  the  object,  finding  two  between  which  the  true  weight  lies,  and 
gradually  narrowing  the  limits.  Thus  for  a  crucible  of  15.752  grams  the 
sequence  will  be  about  as  follows:  20,  10,  15,  17,  16,  15.500,  15.700,  15.800, 
15.750,  15.770,  15.760,  and  by  the  rider,  15.752.  Some  of  the  latter  trials  may 
be  omitted  when  the  retarded  vibrations  of  the  needle  indicate  an  approxima- 
tion to  equilibrium. 

Precision  of  weights.  A  set  is  tested  by  trying  each  against  the  correspond- 
ing one  of  a  set  of  known  integrity  if  at  hand,  or  by  choosing  one  piece  as  a 
standard  and  finding  the  relative  weights  of  the  others,  comparing  each  with 
the  sum  of  such  lesser  ones  as  nominally  equals  it,  and  those  of  the  same 
denomination  with  each  other.  A  new  set,  professedly  of  the  best  quality, 
should  be  rejected  if  a  discrepancy  exceeding  .0002  gram  be  found  in  the 
denominations  below  the  five-gram  piece.  Above  this  they  need  not  agree  so 
closely,  being  generally  used  merely  as  counterpoises  for  crucibles,  etc.,  w.here 
their  actual  weight  is  a  matter  of  no  consequence.  Where  a  greater  variance 
is  the  result  of  corrosion  or  wear  of  an  old  set,  or  with  an  inferior  quality,  a 
table  of  corrections  must  be  drawn  up  and  applied  to  every  weighing;  this 
plan  will  be  found  more  feasible  than  an  attempt  to  adjust  the  weights 
themselves.  The  testing  should  be  repeated  from  time  to  time,  especially  if 
they  are  in  continuous  use. 

TESTING   THE   BALANCE. 

Raise  the  front  and  back  panes  of  the  case,  after  unscrewing  the  locking-bolt  of  the 
latter  with  the  wheel,  and  dust  the  entire  mechanism  with  a  camel's  hair  brush. 

Notice  that  the  stirrups  and  pan-wires  are  arranged  in  conformity  with  the  marks 
thereon,  and  that  the  pan-rests  touch  the  bottom  of  the  pans  when  the  beam-supports 
are  raided.  Turn  the  screws  under  the  case  until  the  bubble  rests  at  the  center  of  the 
spirit-level. 

Hang  the  rider  on  the  hook  of  the  rider-arm,  close  the  balance  case  and  slowly  turn  the 
wheel  as  far  as  It  will  go.  If  the  needle  does  not  now  point  to  zero,  raise  the  beam- 
supports,  and  adjust  the  pan -rests  by  the  screw  H,  Fig.  23. 

Lower  the  beam- supports,  push  in  the  button  and  start  the  beam  to  swinging  by 
momentarily  dropping  the  rider  on  it;  but  at  the  first  vibration  the  beam  should  not 
come  in  contact  with  its  supports. 

If  from  a  derangement  of  the  position  of  the  center  of  gravity  the  beam  refuses  to 
swing  or  oversets,  or  the  time  of  one  vibration  exceeds  15  seconds,  the  collar  on  the 
needle  should  be  lowered;  but  raised  if  the  vibration  occurs  in  less  than  8  seconds. 
Should  the  arcs  traversed  to  each  side  of  the  zero  be  not  equal  or  nearly  so,  raise  the 
beam  supports  and  turn  the  nut  on  the  wire  at  the  end  of  the  beam;  allow  a  few  minutes 
for  cooling,  and  test  as  before:  repeat  this  until  the  difference  is  not  over  one  scale- 
division. 


Left-hand 
No  load 
6.9 

Right-hand 
No  load 
65 

6.3 

6.5 

6.3 

6.0 

5.8     

6.0 

58 

5.5 

53 

.5.5 

53 

»       5.1 

4.8  .. 

.  .  .5.1 

42       .  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Again  start  the  beam  to  oscillating  and  when  the  excursions  have  shortened  to  about  six 
divisions  on  each  side,  begin  to  record  them,  marking  the  swings  to  the  right-hand  B,  and 
to  the  left-hand,  L.  If  the  bearings  were  frictionless  and  the  balance  in  vacuo  the  point 
equidistant  from  the  extremes  of  the  first  journey,  say  L  6.9  to  R  6.5  divisions  would  be  the 

position  (00  at  which  the  needle  would  eventually  come  to  rest  —  that  is  at  1(6.9-6  5)  =.20 

division  to  the  left  of  the  zero  of  the  scale.  To  reduce  errors  of  observation  and  to  com- 
pensate for  the  somewhat  irregular  shortening  of  consecutive  swings,  the  average  of  the 
means  of  an  even  number  of  vibrations  is  taken  as  0' :  thus, 

0'  division  from  0. 
Left  of  0,     Right  of  0. 
.20 

.10 
.15 

.10 
.15 

.10 
.10 

.15 

Mean  position  of  0',  .08  division  to  the  left  of  0. 

Now  place  a  one  or  one- half  milligram  weight  in  one  pan  and  note  the  oscillations  as 
before. 

Left  Right 

No  load.  .5  mrag.  Point  of  rest,  0". 

6.0 1.4  2.30  divisions  to  left  of  0. 

5.7 1.4  2.15         "  right    " 

5.7 1.1  2.30        "  left      «« 

5.4 1.1  2.15         "  right    " 

5.4 .8  2.30         "  l,eft      " 

5.0 8  2.10         "  right    " 

5.0 6  2.20         «  left      " 

4.7 6  2.05         "  right    " 

Mean  position  of  0",  2.19  divisions  to  the  left  of  0. 

Subtracting  0'  (.08)  from  0"  (2.19)  gives  the  net  variation  for  .5  mmg.  of  2.11  divisions 
from  the  zero  of  the  scale;  therefore  one  mmg.  moves  the  zero  to  2  X 2.11  =  4.22  divisions 
to  the  left,  and  one  division  on  the  scale  with  empty  pans  shows  .001  +-  4.22  =  .00023  gram ; 
assuming  that,  for  small  angles,  the  weight  is  proportional  to  the  angular  displacement  of 
the  needle. 

The  above  examination  is  repeated  with  loads  of  say  15,  25  and  50  grams,  when  some- 
what different  results  will  be  obtained;  for  example,  a  balance  of  American  manufac- 
ture:— 

Length  of  beam  between  the  end  knife-edges,  256  mm. 
Length  of  needle  from  center  knife-edge,  250  mm. 
Length  of  center  knife-edge,  20  mm. 

Length  of  end  knife-edges,  10  mm. 

Weight  of  beam,  stirrups  and  pans,  110  grams. 

A  space  of  10  divisions  on  the  scale  equals         15  mm. 

Deviation  for  one  One  scale -division  Time  of 

Weight  on  each  pan.  mmg.  equals.  vibration. 

No  load.  4.22  divisions.  .00023  gram.  15  seconds. 

15  grams.  4.08       "  .00024      "  17       " 

25      •«     .  4.00        "  .00025      "  18       " 

60     «  3.48        "  .00029      "  20        " 

Length  of  arms.  The  relative  distances  of  the  end  knife-edges  from  the  center  are 
found  from  the  law  of  levers  —  the  equality  of  the  products  of  the  length  of  each  arm  times 
its  load. 

Let  L  be  the  length  of  the  left  arm,  and  R,  that  of  the  right;  A,  the  load  in  the  left-hand 
pan,  and  S,  that  in  the  right;  and  a,  a  small  weight  added  to  A  (or  shown  by  needle  vibra- 
tions) needed  to  produce  equilibrium.  Then, 

LX(A  +  a)  =  RX3  (1) 

If  the  arms  are  exactly  equal  In  length,  on  reversing  the  loads, 

LxB  =  RX(A  +  a) (2) 

But  if  say  the  right  arm  be  longer,  then  a  lesser  weight  b  replaces  a,  and 

LXB=RX  (A  +6) (3) 


QUANTITATIVE    CHEMICAL    ANALYSIS.  43 

Multiplying  (1)  and  (3)  together  and  reducing, 


Since  a  and  6  are  very  small  as  compared  with  A  and  B,  this  may  be  expressed, 

a  —  6 
R:L::l+-^:l. 

Example.  A  =  50  grams  ;  B  =  20  +  10  +  10  +  5+2  +  2  +  1  grams.  Left  pan  A  +.0008  = 
£,  right  pan.  Reversing,  B  =  A  +  .0003. 

Hence  a  =  .0008  gram,  and  6  =  .0003  gram  ;  and 

jt  :  L  :  :  i  +    '°°^~^^    :  1  ;  that  is,  the  length  of  the  right-hand  arm  is  1.000005  times 

that  of  the  left 

Variation  on  repeated  weighings.  A  metal  object  weighing  50  to  100  grams  is  coun- 
terbalanced by  weights  and  the  rider  so  that  the  needle  swings  an  equal  number  of  di- 
visions on  each  side  of  the  zero.  The  beam-supports  are  then  raised  and  lowered  ten 
times  in  succession;  a  variation  of  more  than  one-tenth  of  a  milligram  indicates  faulty 
workmanship  or  maladjustment  of  the  beam  supports.* 

TESTING   THE   WEIGHTS. 

Before  testing  the  relative  correctness  of  the  weights  each  should  be  carefully  cleaned 
by  brushing  or  wiping  with  chamois.  Those  below  the  denomination  of  the  gram  are 
made  of  platinum  foil  generally  down  to  .050  gram,  below  that  of  aluminum.  The  former 
may  be  cleaned  by  heating  to  redness  for  a  moment  in  the  flame  of  a  Bungen  burner,  but 
aluminum  will  not  admit  of  this  operation. 

Assuming  the  set  to  contain  two  twenty-gram  pieces  down  to  one  milligram,  the  most 
common  arrangement  is  as  follows  :  — 

Grams,  20A  —  20B  —  10  —  5  —  2A  —  2B  —  1. 

Milligrams,  500  -  200  —  100A  —  100B  -  50  —  20  —  10A  —  10B  —  5A  —  2A  —  2B  —  1. 

Rider,  10  or  12  milligrams. 

Some  makers  depart  from  the  above  order  in  having  two  one-gram  pieces,  and  one 
two  -gram  piece,  etc.  No  particular  advantage  appears  to  lie  in  either  system. 

One  of  the  pieces  is  selected  as  a  standard  and  the  deviation  of  the  others  therefrom 
ascertained  f  It  is  customary  to  take  the  heaviest  for  this  comparison,  but  for  weights 
to  be  used  in  ordinary  analyses  the  one-gram  is  preferable  (page  41). 

Placing  it  in  one  pan,  and  in  the  other  those  of  lesser  denominations  whose  sum  nom- 
inally equals  it,  the  difference  between  them,  if  any,  is  noted  by  needle  vibrations  and  If 
necessary  by  small  weights,  with  a  correction  if  the  arms  are  of  unequal  length.  We 
obtain  for  example  .0001  gram  which  gives  the  following  equation,  the  figures  inclosed  in 
parentheses  being  those  stamped  on  the  weights  —  their  nominal  value. 

A.  1.000=(500)  +  (200)  +  (100A)  +  (100B)  +  (50)+(20)+(10A)+(10B)  +  (5)  +  (2A)+(2B)  +  (l)—  .0001. 
In  the  same  way  are  found, 

B.  (500)  =  (200)+(100A)  +  (100B)+(50)+(20)+(10A)  +  (10B)+(5)+(2A)+(2B)+(l)+.0007. 

C.  (200)  =  (iOOA)+(50)  +  (20)  +  (10A)+(10B)  +  (5)+(2A)  +  (2B)  +  (l)+.0005. 

D.  (2UO)  =  (100A)  +  (100B)  +  .0001. 

E.  (100A)  =  (50)+(20)  +  (10A)  +  (10B)  +  (5)  +  (2A)  +  (2B)  +  a)+.0009. 
P.  (100A)  =  (100B)+.0005. 

G.  (50)  =  (20)  +  (10A)  +  (10B)  +  (5)  +  (2A)  +  (2B)+(l)—  .0004. 
H.  (20)  =  (10A)  +  (5;  +  (2A)+(2B)+(l)+.0006. 
I.  (20)  =(10A)  +  (10B)+.0002. 
J.  (10A)  =  (5)+(2A)+(2B)  +  (l)+.0002. 
K.  (10A)  =  (10B)—  .0002. 
L.   (5)  =  (2A)+(2B)  +  (1)+.0001. 
M.  Rider  (10)  =  (10A)+.0003. 

The  equations  are  now  to  be  combined  to  deduce  the  actual  weight  of  each  piece  as 
compared  with  the  gram  taken  as  normal.    Substracting  B  from  A  gives 
N.  1.000—  (500)  =  (500)—  .0008  ;  or  (500)  =  .5004  gram. 

Substituting  .5004  in  equation  B  ami  transposing, 
O.  (200)  +  (100A)  +  (100B)  +  (50)+(20J+(10A)+(10B)+(5)+(2A)  +  (2B)  +  (l)  =  .4997. 

Transposing  equation  C, 
P.  (200)—  (100A)—  (50)—  (20)—  (10A)—  (10B)—  (5)—  (2A)—  (2B)—  (1)  ==•  .0005. 

Adding  O  and  P, 
Q.  2  (200)  +  (100B)  =  .5002. 


*  Journ.  Chem.  Socy.  47—116. 

T  Crookes'  Select  Methods,  685;  Journ.  Amer.  Chem.  Socy.  1900—144. 


44  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Doubling  equation  D  and  transposing, 
B.  2  (200)— 2  (100A)—  2  (100B)  —  .0002. 

Subtracting  R  from  Q, 
S.  2  (100A)+3  (100B)  =  .5000. 
Multiplying  F  by  3  gives, 
T.  3  (100A)— 3  (100B)  =  .0015. 

Adding  S  and  T, 

U.  5  (100A)  =  .5015,  or  (100A)  =  .1003. 
From  F,  (100B)  =  .0998. 
And  from  D,  (200)  =  .2002. 

Substituting  the  value  of  (100A)  in  E  and  transposing, 
V.  -(50)  =  (20)+(10A)  +  (10B)  +  (5)  +  (2A)+(2B)-KD-  .0994. 

Subtracting  V  from  G, 
W.  2  (50)  =  .0990,  or  (50)  =  .0495. 

The  values  of  the  (20),  (10A),  (10B),  and  (5)  are  calculated  in  the  same  way  as  the  above. 
The  weight  of  the  rider  is  found  from  M,  and  weights  below  .005  gram  compared  with  It. 
The  two-gram  weights  are  tested  against  the  one-gram  and  its  fractions;  the  five-gram, 
by  the  twos  and  one,  and  so  on  to  the  highest  of  the  set. 

A  table  like  the  following  is  drawn  up  and  kept  in  the  balance-case. 

Grams.  Milligrams.  Milligrams. 

(20A)  =  19.9997  (500)      =  .5004  (10B)  =  .0101 

(20B)  =  20.0013  (200)      »  .2002  (5)       =  .0049 

(10)      =  10.0002  (100A)  =  .1003  (2A)    =  .0019 

(5)        =  4.9986  (100B)  =  .0998  (2B)    =  .0020 

(2A)     =2.0002  (50)      =.0495  (1)       =.0009 

(2B)     =2.0008  (20)     '-.0202  Rider  =  .0102 

;.l)      «.  Standard.  (10A)   =.0099 


QUANTITATIVE    CHEMICAL    ANALYSIS.  45 


CHAPTER  4. 

THE  OPERATIONS  IN  ANALYSIS. 

Having  the  sample  pulverized  and  dried  or  otherwise  prepared,  a  portion  is 
to  be  weighed  for  the  analysis. 

The  amount  of  material  to  be  used  for  any  given  determination  depends  on 
several  conditions,  such  as  the  accuracy  of  the  method  employed,  the  degree  of 
exactness  required  for  the  results,  the  skill  of  the  analyst,  and  the  time  allow- 
able. Many  of  the  errors  incurred  in  an  analysis  affect  the  result  inversely 
with  the  weight  of  substance  taken,  while  on  the  other  hand  the  smaller  the 
weight  the  less  time  is  required  for  evaporations  and  nitrations,  and  the  more 
quickly  will  the  analysis  be  completed. 

Of  minerals,  alloys,  crystalline  salts  and  inorganic  bodies  generally,  for  con- 
stituents present  in  reasonably  large  proportions,  say  five  per  cent  or  more, 
and  where  the  methods  are  unexceptionable,  one  gram  of  a  solid  or  so  much  of 
a  solution  as  contains  this  weight  will  be  found  satisfactory  as  regards  both 
accuracy  and  speed.  A  constituent  of  less  than  say  one  per  cent  calls  for  a  pro- 
portionally greater  weight  of  sample,  and  when  It  is  desired  to  express  centesi- 
mally  the  proportion  of  one  so  small  as  to  be  ordinarily  reported  as  '  a  trace,1 
50, 100,  or  even  200  grams  may  be  required.  In  ultimate  organic  analysis  from 
.1  to  .5  gram  is  sufficient,  while  in  proximate  organic  analysis  with  questionable 
methods,  from  5  to  50  grams  is  not  excessive.  Of  gases,  50  and  100  Cc.  are  stand- 
ard volumes  and  gas  burettes  are  generally  of  one  of  these  capacities.  For 
traces  of  an  important  gas  in  a  mixture  many  litres  are  drawn  through  the  ab- 
sorbing medium  to  obtain  a  weighable  amount  of  the  constituent. 

It  is  seldom  necessary  to  weigh  exactly  the  amount  that  is  prescribed  in  the 
method  followed;  although  where  many  analyses  of  the  same  kind  are  to  be 
made  the  calculation  of  the  results  is  somewhat  facilitated  by  taking  a  fixed  or 
a  '  factor  '  weight;  yet  the  samples  may  be  received  by  the  chemist  in  such  a 
form  (e.  g.,  coarse  drillings  of  a  metal)  as  to  make  this  a  more  tedious  affair 
than  the  calculation.  However,  such  matters  as  these  are  best  left  to  be 
decided  by  the  individual. 

When  an  analysis  is  to  be  made  in  duplicate  or  triplicate,  instead  of  weigh- 
ing two  or  three  portions  of  the  sample  it  is  often  feasible  to  dissolve  one 
macro-weight  and  make  up  the  solution  to  a  definite  volume  with  water;  then 
draw  out  with  a  pipette  such  aliquot  parts  as  will  contain  proper  amounts  of 
the  substance  for  analysis.  This  is  especially  advantageous  when  the  sub- 
stance is  a  mixture  of  a  character  that  does  not  admit  of  fine  subdivision  or 
thorough  blending,  as  the  greater  the  weight  the  more  nearly  should  it  rep- 
resent the  original. 

The  allowable  error  in  weighing  the  portion  for  analysis  may  be  taken  as  one 
part  in  one  thousand  (one  milligram  per  gram)  except  in  the  determination  of 
a  constituent  forming  nearly  the  whole  of  the  substance  or  where  the  method 
is  a  highly  accurate  one.  That  this  limit  is  sufficiently  close  is  manifest  when 
It  is  considered  that  a  difference  of  one  one-thousandth  on  the  result  of  the  de- 
termination of  a  constituent  forming  say  50  per  cent  of  the.  whole  is  only  .05 
per  cent,  decreasing  as  the  proportion  of  the  constituent  decreases.  In  general 


46  QUANTITATIVE    CHEMICAL    ANALYSIS. 

In  an  assay  the  smaller  the  proportion  of  the  constituent  determined,  the  less 
accurate  need  be  the  weighing  of  the  substance. 

Powders  that  tend  to  ab'sorb  but  little  moisture  from  the  air  can  be  weighed 
in  an  open  watch-glass,  using  a  steel  or  horn  spatula  for  transferring,  or  what 
is  handier,  a  small  square  of  heavy  platinum  foil  sealed  in  the  end  of  a  glass 
rod.  The  watch-glass  is  then  held  over  a  beaker  or  dish  and  the  powder  allowed 
to  fall,  tapping  the  edge  to  loosen  any  adhering  clumps.  What  little  powder 
remains  is  brushed  down  with  a  clean  small  camels-hair  brush.  If  the  powder 
is  to  be  introduced  into  a  flask,  a  wide -stemmed  funnel  is  used, 
afterward  rinsed  down  with  the  solvent;  a  counterpoised  metal 
scoop  is  better  for  the  purpose  than  a  watch-glass. 

The  comparatively  few  powders  that  are  so  hygroscopic,  efflo- 
rescent or  volatile  as  to  increase  or  diminish  in  weight  during  the 
operation  sufficiently  to  affect  the  accuracy  of  the  analysis  are 
contained  in  tightly  closed  vessels,  like  the  light  glass-stoppered 
weighing  bottle  Fig.  34.  The  bottle  is  partly  filled  and  weighed, 
and  approximately  the  quantity  required  for  the  analysis  poured 
out  into  a  beaker;  on  reweighing,  the  loss  sustained  is  the  weight 
abstracted.  A  pair  of  test-tubes  without  flanges,  one  fitting 
closely  in  the  other,  can  be  substituted  for  the  weighing  bottle. 

In  dealing  with  a  liquid  it  is  usually  more  convenient  to  measure 

g.          '2  a  certain  volume  and  calculate  its  weight  from  the  specific  gravity, 

and  this  is  the  usual  procedure,  the  exceptions  being  where  greater  exactness 

is  desirable  than  can  be  attained  with  the  ordinary  appliances  for  measuring. 

A  non-volatile  liquid  may  be  weighed  in  a  counterpoised  beaker  or  flask  into 

which  it  is  conveniently  introduced  from  a  pipette  or  a  test-tube  fitted  up  like 

awash  bottle  (page  97).  For  quantities  less 
than  a  gram  a  short  narrow  glass  tube  with 
one  end  drawn  out  to  a  fine  orifice  is  weighed ; 
a  few  drops  of  the  liquid  are  drawn  in  by  suc- 
tion and  held  by  capillarity  during  the  re- 
weighing,  then  washed  out  by  the  solvent 
Fig.  35.  into  the  vessel  for  analysis.  A  pipette  for 

weighing  and  delivering  small  volumes  of  a  liquid  is  shown  in  Fig.  35. 

When  a  weighed  quantity  of  an  aqueous  solution  is  to  be  evaporated  in  & 
tared  dish,  a  flask  containing  the  liquid  is  weighed,  about  the  required  volume 
poured  out  into  the  dish,  and  the  flask  re  weighed.  This  plan  avoids  any  error 
that  would  arise  from  evaporation  were  the  liquid  weighed  in  an  open  vessel. 

A  very  hygroscopic  liquid,  such  as  Nordhansen  snlfnric  acid,  Is  drawn  into  a  thin  tared 
glass  bulb  with  capillary  prolongs  and  the  ends  of  the  capillary  tubes  sealed  by  ihe  blow- 
pipe. The  bulb  is  reweighed,  then  dropped  into  a  bottle  contai  ning  the  solvent,  and  the 
bottle  shaken  until  the  bulb  breaks. 

A  volatile  liquid  is  held  in  a  weighing-tube  which  is  afterward  dropped  into 
the  solvent  before  removing  the  stopper.  Or  a  thin  glass  bulb  with  a  capillary 
stem  is  warmed  and  the  stem  held  iu  the  liquid,  a  small  volume  entering  as  the 
bulb  cools;  this  is  boiled  until  the  air  is  expelled,  and  the  stem  is  again  sub- 
merged when  the  bulb  will  fill  completely  and  may  be  weighed.  It  is  dropped 
into  the  solvent  and  crushed  by  a  glass  rod. 

SOLUTION. 

*'  Solutions  may  be  defined  as  homogeneous  mixtures  which  cannot  be  sepa- 
rated into  their  constituent  parts  by  mechanical  means,  the  proportion  be  - 
tween  the  parts  being  continuously  variable  between  certain  limits,  with 


QUANTITATIVE    CHEMICAL    ANALYSIS.  47 

a  corresponding  continuous  variation  in  properties."  (Whetham).  In  a  broad 
sense  the  term  includes  the  union  of  either  a  solid,  liquid  or  gas  with  any  one 
of  the  three,  but  popularly  speaking,  refers  rather  to  the  disappearance  of  a 
solid  or  a  gas  in  a  liquid.  When  no  more  of  a  solid  can  be  taken  up  and  per- 
manently held  the  solution  is  said  to  be  *  saturated  ' ;  with  most  salts  an  un- 
stable stronger  one  may  be  obtained,  called  '  supersaturated  '.  In  analysis  only 
comparatively  dilute  solutions  are  the  rule. 

A  strict  construction  of  the  term  « insoluble  '  implies  an  attribute  of  matter 
of  which  we  can  furnish  no  unqualified  example,  but  as  commonly  used  it  sig- 
nifies that  the  substance  in  question  is  not  affected  by  a  given  solvent  to  such 
a  degree  as  to  materially  decrease  its  mass.  And  so  the  word  *  soluble  »  com- 
prehends not  only  the  bodies  dissolving  easily  and  abundantly,  but  those  re- 
quiring a  prolonged  digestion  with  a  large  proportion  of  the  solvent  as  well, 
one  class  merging  into  the  other.  Calcium  hydrate  has  been  proposed  as  the 
dividing  line,  it  dissolving  in  about  584  parts  of  water  at  15°  C. 

Solvents.  It  is  at  least  a  matter  of  convenience  to  consider  all  solutions  as 
involving  chemical  decomposition  or  combination,  whether  the  substance  can 
or  cannot  be  recovered  unaltered  on  evaporation  of  the  solvent.  With  the  ex- 
ception of  a  few  solids,  like  carbon,  and  some  gases,  little  difficulty  is  experi- 
enced in  finding  a  solvent  for  every  substance,  and,  in  general,  this  is  the  first 
step  in  every  analysis.  Water  dissolves  most  crystalline  salts  and  acids,  the 
sugars,  alkali  soaps,  tannins,  alkaloidal  salts,  gums,  albumenoids,  and  starches 
(when  hot) ;  the  inorganic  acids,  metals,  their  oxides  and  hydrates,  and  many 
native  and  artificial  compounds ;  alcohol,  most  deliquescent  salts,  the  alkaloids, 
tannins,  resins,  essential  oils,  chlorophyll,  and  many  organic  acids;  ether, 
chloroform,  and  ligroin,  the  fixed  oils  and  fats,  some  resins,  and  the  alkaloids; 
glycerol,  many  inorganic  salts,  the  gums,  starch,  albumin,  pepsin,  tannin,  and 
many  of  the  analine  dyes;  carbon  disulfide,  phosphorus  and  sulfur;  acetone,  the 
camphors  and  fixed  oils,  etc. 

Special  solvents  are  employed  (1),  when  those  of  general  application  are  in- 
effectual; as  concentrated  sulfuric  acid  for  ferrochrome,  alizarin,  and  indigotin ; 
(2),  where  perduction*  or  reduction  of  an  insoluble  solid  changes  it  to  a  soluble 
combination;  (3),  to  induce  polymerization,  hydrolysis, or  saponification;  (4), 
to  hasten  the  time  of  solution  by  some  chemical  reaction  or  by  catalysis;  or 
(5),  to  prevent  loss  of  a  constituent  or  reaction-product  through  volatiliza- 
tion. 

An  acquaintance  with  the  composition  and  solubility  in  various  menstrua  of 
the  substance  under  examination  should  be  gained  before  the  quantitative 
analysis  is  proceeded  with,  and  If  a  choice  of  solvents  is  allowed,  the  one  is 
selected  that  is  best  adapted  to  the  reactions  in  the  several  determinations  fol- 
lowing, or  will  the  least  interfere  with  then. 

Usually  the  solvent  is  a  clear  liquid,  but  occasionally  a  solid  in  mass  or 
powder  is  incorporated  therewith  to  produce  some  specific  effect  without  the 
introduction  of  an  acid  or  soluble  base.  Occasionally  one  meets  a  precipitate 
or  solid  body  that  best  dissolves  when  suspended  in  water  and  a  gas  passed 
through  the  mixture.  Oxygenation  is  sometimes  brought  about  by  passing  a 
current  of  air  through  the  water. 

lu  general  an  amount  of  solvent  iu  considerable  excess  of  that  actually 
required  for  solution  is  employed.  Exceptions  are  where  a  clear  solution  ob- 
tained with  a  limited  proportion  of  solvent  becomes  cloudy  on  dilution  with  a 
further  portion,  from  separation  of  part  of  the  matter  dissolved.  However,  as 


*  Journ.  Franklin  Inst.  1901—201. 


48  QUANTITATIVE    CHEMICAL    ANALYSIS. 

a  rule  it  is  well  to  restrict  the  amount  of  solvent  to  a  moderate  excess,  as  more 
than  this  is  often  objectionable  for  several  reasons. 

The  concentration  of  a  solution  or  mixture  used  as  a  solvent  is  a  matter  of 
importance.  The  solvent  capacity  of  many  is  at  a  maximum  at  a  certain  con- 
centration, less  effective  when  weaker  or  stronger;  for  example,  silver  chlo- 
ride in  ammonia,  gliadin  in  dilute  alcohol,  keratin  in  solution  of  potassium 
hydrate,  etc.,  Of  a  mixture  of  two  liquids  the  solvent  power  toward  a  given 
solid  depends  not  only  on  the  rate  of  solubility  in  each,  but  on  the  predomi- 
nance of  one  or  the  other  in  the  mixture  as  well. 

An  acid  of  medium  strength  will  generally  give  better  results  than  if  more 
concentrated,  as  many  metals  and  their  compounds  are  insoluble  in  a  strong, 
though  readily  so  in  a  dilute  acid.  Immersed  in  strong  nitric  acid,  iron  be- 
comes passive  and  insoluble,  said  to  be  protected  by  an  impervious  layer  of 
nitrate  or  oxide  formed  on  the  surface ;  when  strong  hydrochloric  acid  is  used 
to  decompose  a  silicate,  the  liberated  silica  (leucone)  gelatinizes  at  once  and 
encysts  particles  of  the  unattacked  mineral,  shielding  them  from  contact  with 
the  acid ;  etc.  So  as  a  rule  one  should  employ  as  dilute  a  solvent  as  will 
accomplish  the  purpose  unless  otherwise  directed  for  special  reasons,  always 
recognizing  that  during  the  operation  the  acid  becomes  continually  weaker  by 
neutralization  or  decomposition.  Practically  from  five  to  fifteen  cubic  centi- 
meters of  either  of  the  mineral  acids,  concentrated  or  diluted  with  water  as 
directed,  is  sufficient  to  dissolve  one  gram  of  a  metal  or  its  compounds. 

As  a  rule  solution  is  promoted  by  heating  the  solvent.  A  specific  formula 
for  the  solubility  at  different  temperatures  may  be  deduced  from  the  formula 
S  =  A  +  Bt  +  Ct2+Dt3,  where  Sis  the  proportion  of  solid  dissolved;  A,  the 
solubility  at  zero  Cent.;  t,  the  temperature;  and  B,  G,  and  Dt  empirical  co- 
efficients determined  by  experiment.*  The  solubility-curves  of  a  few  salts  are 
nearly  straight  lines,  while  a  few  compounds,  such  as  calcium  butyrate,  show  a 
decreasing  solubility  with  increase  of  temperature » 

Although  heating  a  solvent  is  usually  allowable  and  advantageous  in  point  of 
time  required  for  solution,  yet  a  boiling  heat  is  often  interdicted  on  account  of 
the  decomposition  of  the  solute  or  volatilization  of  some  constituent.  Heat 
should  not  be  applied  to  a  concentrated  volatile  acid  or  alkali  to  such  a  degree 
that  it  becomes  weakened  by  the  escape  of  its  gas  before  solution  is  accom- 
plished; a  temperature  of  40°  to  60°  will  usually  give  the  best  results. 

Solution  is  also  promoted  by  finely  dividing  the  solid  to  increase  the  surface 
exposed  to  the  solvent,  or  in  the  case  of  a  liquid  by  the  mixture  of  another 
liquid  to  form  a  semi-solution  or  emulsion.  Thus  a  slag,  but  slowly  and 
imperfectly  act.ed  on  by  an  acid  when  in  a  moderately  fine  powder,  is  readily 
dissolved  when  it  occurs  disseminated  through  an  unrefined  metal,  and  the 
metal  treated  with  an  acid. 

Powders  Inclined  to  agglutinate  are  best  mixed  with  some  granu- 
lar inert  solid  such  as  sand  or  quartz  powder,  that  tends  to  keep  the 
particles  separated. 

The  time  allowed  for  a  solution  is  of  no  consequence  ex- 
cept for  organic  substances  where  ferments  or  bacteria  may 
effect  changes.  Sterilization  by  heat  or  germicides  may  be 
advisable  where  long  contact  with  the  solvent  is  essential. 

On  dissolving  one  of  the  higher  metallic  oxides  in  an  acid 
a  partial  reduction  to  the  compound  corresponding  to  the 
most  stable  oxide  takes  place.  A  total  transformation  gen- 
erally follows  boiling  for  a  time,  or  may  be  accomplished  at 
Fig.  36.  ~  once  bv  tne  addition  of  some  reducing  agent.  Rarely,  a  sol- 


QUANTITATIVE    CHEMICAL    ANALYSIS.  49 

vent  may  be  found  that  will  dissolve  a  higher  oxide  without  reduction  (e.  g. 
glacial  acetic  acid  for  lead  peroxide).  Metals  and  oxides  lower  than  the  nor- 
mal may  be  dissolved  without  change  of  valence  in  a  non- oxidizing  acid  with 
exclusion  of  air. 

The  layer  of  fluid  in  contact  with  a  powder  lying  at  the  bottom  of  a  vessel 
soon  becomes  saturated,  and  so  a  frequent  and  sometimes  co  ntmual  stirring  or 
shaking  is  necessary.  A  mechanical  stirrer 
saves  time  and  labor  with  a  substance  slow  to 
dissolve;  it  is  simply  a  bent  glass  rod,  Fig.  36, 
rapidly  turned  by  a  small  electric  or  water 
motor,  a  toy  steam  engine,  or  a  light  sheet- 
metal  fan  driven  by  a  blast  of  air.  Another 
means  is  to  inclose  the  powder  in  a  porous  con- 
tainer such  as  a  linen  bag  or  a  cage  of  plat- 
inum gauze,  Fig.  37,'  suspended  just  below  the 
surface  of  the  solvent;  the  specifically  heavier 
solution  as  it  sinks  to  the  bottom  of  the  beaker 
displaces  the  virgin  solvent  and  establishes  a 
current  through  the  container;  or  if  oxidation 
is  not  to  be  feared,  a  stream  of  air  may  be  forced 
into  the  liquid  through  a  narrow  glass  tube  Fig.  37. 

reaching  to  the  bottom  of  the  vessel,  the  stream  of  bubbles  diffusing  the  solid 
through  the  solvent. 

Vessels  for  solution.  The  solution  may  take  place  in  a  beaker -glass,  dish  or 
flask.  Beakers  are  made  in  two  shapes,  the  tall  or  ordinary  form  about  two 
diameters  high,  and  the  Griffon,  somewhat  wider  in  proportion.  The  former 
is  better  for  dissolving  metals,  carbonates  and  the  like  where  gases  are  evolved, 
while  the  latter  is  more  convenient  for  evaporations  and  precipitations.  Sizes 
may  be  purchased  from  15  to  4,000  Co.  capacity,  those  holding  from  150  to  600 
Cc.  being  of  more  general  use.  They  are  covered  with  watch-glasses  or  round 
glass  plates  of  slightly  greater  diameter  than  the  top  of  the  beaker  for  the 

exclusion  of  dust. 

Beakers  containing  hot  liquids  are  con- 
veniently handled  by  the  use  of  a  tong,  Fig.  38, 
made  of  spring  brass  wire.  Langmuir  pro- 
poses the  use  of  a  leather  strap  an  inch  wide  by 
twenty  inches  long  passed  around  the  beaker 
Fig.  38.  Ve  aud  the  ends  held  between  the  thumb  and 

finger. 

Flat  bottomed,  triangular,  or  Erlenmyer  flasks,  Fig.  39,  are  preferred  by  some 
to  beakers  for  general  work,  the  sizes  grading  about  the  same.  Large  test 
tubes  about  one  inch  in  diameter  by  twelve  or  more  inches  in 
length  serve  well  for  dissolving  metals  in  acids  and  similar 
operations,  since  losses  from  bubbles  of  gas  and  contact  with 
air  are  prevented,  and  offensive  gases  may  be  led  into  the  open 
air  by  a  cork  and  tubing. 

All  glassware  must  be  of  a  composition  free  from  lead  and 
designed  to  resist  the  solvent  action  of  water  and  solutions,  and 
well  annealed  to  stand  heating  without  fracture.  As  glass  and 
porcelain  are  corroded  by  hydrofluoric  acid,  in  solution  or  gen- 
erated when  a  fluoride  is  dissolved  in  an  acid,  a  platinum  dish 
or  large  crucible  is  substituted  when  dealing  with  such  ma-  Fig.  39. 
terial.  On  the  other  hand,  a  platinum  dish  cannot  be  used  for 


50 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


hot  liquids  containing  or  evolving  chlorine  or  bromine,  such  as  nitro-hydro- 
chloric  acid,  a  nitrate  or  bromate  with  hydrochloric  acid,  etc. 

Since  solution  takes  place  more  readily  as  a 
rule  when  the  solvent  is  heated,  it  is  often  an 
advantage  to  raise  the  temperature  above  its 
boiling  point  which  can  be  done  by  preventing 
the  escape  of  its  vapor. 

A  bottle  of  heavy  glass  closed  with  a  well- 
ground  glass  stopper  secured  by  a  clamp,  Fig. 
40,  or  a  porcelain  cap  with  a  soft  rubber  ring, 
Fig.  40A,  may  be  used  for  this  purpose ;  it  is 
also  of  service  where  it  is  desired  to  heat  with- 
out loss  a  volatile  liquid  or  one  containing  or 
Fig.  40  A.  evolving  a  volatile  constituent,  or  to  hasten  a 

chemical  change  in  a  solution  that  occurs  but  slowly  at  a 
boiling  temperature.  For  greater  safety  the  mouth  of  the 
flask  may  be  expanded  to  form  a  gutter  around  the  stop- 
per; the  gutter  being  filled  with  a  suitable  liquid  entraps 
any  vapor  that  may  escape  between  the  neck  of  the  flask 
and  the  stopper. 

Allen  advises  that  a  heavy  flask  be  closed  by  a  rubber  stopper, 
wired  down,  through  which  passes  the  short  leg  of  a  glass  tube 
bent  twice  at  right  angles,  the  longer  leg  dipping  into  a  tall  cyl- 
inder filled  with  mercury.  The  distance  the  mercury  is  depressed 
In  the  tube  shows  the  pressure  in  the  flask,  thirty  inches  corre- 
sponding to  an  additional  atmosphere.  The  flask  is  heated  in  a 
saline  solution  of  a  suitable  boiling  point. 

An  autoclave  or  miniature  Papin's  digester,  a  kettle  of  heavy     ****" 
sheet  metal  with  a  steam-tight  cover  and  pressure-gauge,  can  be  Fig.  40.    V4 

used  for  the  extraction  of  animal  or  vegetable  matter  at  a  heat  above  the  boiling  point  of 
water  under  atmospheric  pressure. 

Many  refractory  minerals  are  dissolved  when  digested  with  an  acid  at  an 
elevated  temperature.  A  thick-glass  tube,  Fig.  41,  is  sealed  at  A,  the  powdered 
mineral  and  acid  s  — — 

introduced,    the  V       A i 

upper  end  drawn 

out  into  a  nar-  Fi£*      ' 


X 


row  prolong  BC,  and  the  end  sealed  by  a  blowpipe.  The  tube  is 
laid  horizontally  in  a  special  form  of  air-bath  and  heated  for  sev- 
eral hours  to  a  temperature  considerably  above  100°.  After  cool- 
ing,  the  tube  is  held  upright  and  C  softened  in  a  blowpipe  flame, 
when  any  gas  evolved  during  the  solution  forces  its  way  through 
the  plastic  glass;  the  prolong  is  then  cut  off  and  the  solution 
poured  out. 

When  it  is  desired  to  boil  a  solvent  without  its  volume  being 
diminished,  a  condenser  may  be  attached  as  in  A,  Fig.  42;  the 
lower  end  passes  through  the  cork  of  the  flask, 
and  the  vapor  rising  into  the  inner  tube  is  con- 
densed and  the  liquid  drops  back.  This  position 
of  the  condenser  is  termed  '  inverted,'  *  reversed,* 
or  *  reflux.'  A  simpler,  though  less  effective  con- 
trivance, is  a  small  U-tube  of  thin  glass  cooled 
by  transmitted  water,  hanging  in  the  neck  of  the 
flask  B. 
Fig.  42.  Goeckel*  describes  a  cover  for  a  lipless  beaker,  in  the 


*  Zelts.  angew  1899—494. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


51 


ft>*        f  a  glass  bulb,  conical  below,  of  somewhat  greater  diameter  than  the  beaker;  a 
streu.._;  of  cold  water  is  conducted  through  the  bulb  by  two  sealed-in  tubes. 

Where  a  stream  of  water  is  not  available,  a  fair  degree  of  condensation  may 
be  had  by  inserting  the  lower  end  of  an  open  glass  tube  through  the  cork  of 
the  flask  and  supporting  the  tube  in  a  nearly  vertical  position.  The  tube  should 
be  several  feet  long  (less  unwieldy  if  coiled  into  a  spiral),  and  the  glass  thin 
since  the  air  is  the  only  refrigerant. 

An  apparatus  allowing  the  treatment  of  a  substance  in  an  open  dish  with  a 
volatile  solvent  is  shown  in  section  in  Fig.  43.*  The  heavy  line  is  a  thin  cast 

iron  cup  whose  periphery  has  the  form  of  a  gutter, 
and  is  heated  by  a  burner  below.  The  evaporating 
dish  containing  the  solvent  and  substance  is  placed 
in  the  depression  of  the  cup,  a  thin  bell-glass  with 
open  top  set  in  the  gutter,  and  a  little  mercury 
poured  in  to  make  a  vapor- tight  joint.  The  mouth 
of  the  bell-jar  is  closed  by  a  cork  carrying  a 
reversed  condenser.  Since  the  bell-jar  becomes 
heated,  no  condensation  occurs  on  the  interior 
surface.  After  solution,  if  it  is  desired  to  distill 
most  or  all  of  the  solvent,  a  slant  condenser  is 
substituted  for  the  reversed  condenser. 

In  dissolving  a  metal  or  compound  it  is  often 
essential  that  contact  with  the  air,  or  entrance  of 
dust  and  laboratory  fumes,  be  prevented.  The 
solution  may  take  place  in  a  flask  closed  by  a  cork 
or  rubber  stopper  through  which  pass  two  glass 
tubes  transmitting  a  current  of  some  non -oxidiz- 
ing gas,  such  as  nitrogen  or  carbonic  acid. 


Fig.  43. 


Where  an  acid  is  the  solvent,  the  flask  may  be  connected  by  a  cork  and 
tubing  to  a  beaker  containing  a  solution  of  sodium  bicarbonate.  A  little 
sodium  bicarbonate  is  placed  in  the  flask  with  the  substance  to  be  dissolved, 
the  acid  poured  on,  and  the  flask  stoppered.  Heat  is  applied  until  solution  is 
complete,  and  on  cooling  bicarbonate  solution  is  drawn  back  into  the  flask  and 
generates  suflicient  carbon  dioxide  to  prevent  entrance  of  air.  Water  boiled 
free  from  air  is  used  to  dilute  the  solution. 

Offensive  or  noxious  gases  or  vapors  evolved  during  a  solution  should  be  led 
into  the  open  air  by  operating  under  a  "hood  "  or  draught-chamber,  or  by  dis- 
solving in  a  flask  provided  with  a  cork  (or  rubber  stopper  if  unobjectionable) 
in  which  is  fitted  a  glass  tube  connected  by  rubber  tubing  to  another  passing 
through  a  hole  in  a  window-frame. 


Percolation.  Certain  varieties  of  vegetable  matter  and  many  drugs  yield  their 
active  principles  to  solvents  with  such  reluctance  that  a  prolonged  digestion 
and  a  large  volume  of  solvent  is  needed.  The  advantage  of  the  process  of  '  per- 
colation '  over  simple  digestion  is  that  the  constituents  more  diflScult  of  ex  - 
traction  are  continually  brought  in  contact  with  the  pure  solvent.  The  percola- 
tor, Fig.  44,  is  a  glass  or  metal  cylinder  contracted  to  a  small  outlet  at  the 
bottom ;  in  this  is  fitted  a  cork  holding  a  short  glass  tube  over  the  lower  end  of 
which  is  drawn  a  narrow  rubber  tube  leading  to  the  receiving  flask.  Above  the 


Blyth,  Foods,  Their  Composition  and  Analysis,  70. 


52 


QUANTITATIVE    CHEMICAL   ANALYSIS, 


either 


cork  the  neck  of  the  percolator  is  plugged  with  cotton,  and  over  this  is  a 
layer  of  clean  sand.   The  powdered  bark,  leaves,  etc.,  are  introduced 

loosely  or  tightly  packed  according  to  their 

permeability  and  other  properties,  nearly  fill  - 

ing  the  percolator.    The  solvent,  cold  or  hot 

as  required,  is  then  poured  in,  the  rubber 

tube  compressed  by  a  pinchcock,  and  the 

mixture  allowed  to  macerate  or  digest  for 

several  hours.      The  pinchcock  is  released 

and  the  liquid  run  out.     More  of  the  solvent 

is  then  run  through,  becoming  the  more  im- 
44  pregnated  as  it  descends,  until  the  color  or  a 

qualitative  test  shows  that  the  extraction  is 
complete. 

A  simple  form  of  percolator  that  requires  no  atten- 
tion after  it  has  been  started  is  shown  in  Fig.  45.  The 
test-tube  A  has  a  small  orifice  at  the  bottom,  and  is 
packed  to  the  depth  of  an  inch  or  more  with  cotton ; 
on  this  rests  the  drug.  The  narrow -necked  flask  B  is 


Fig.  45. 


filled  with  the  solvent  and  inverted  into  A,  the  solvent  being  retained  in  B  by 
atmospheric  pressure  and  fed  into  A  only  as  fast  as  air  can  enter  and  displace  it. 

The  ratio  of  the  height  of  the  percolator  to  the  diameter  is  governed  by 
the  character  of  the  drug  and  the  degree  of  comminution,  but  obviously  the 
longer  and  narrower,  the  less  solvent  is  required  for  complete  exhaustion,  and 
this  fact  is  practically  applied  in  another  way  in  the  processes  of  '  repercola- 
tion '  and  c  sectional  percolation.' 

In  many  powders,  especially  those  rich  in  extractive,  channels  may  form 
through  which  the  solvent  passes  in  preference,  greatly  augmenting  the 
quantity  of  solvent  required  for  thorough  extraction.  An  occasional  stirring 
of  the  powder  will  break  up  any  channels  formed;  or  the  powder  may  be  mixed 
with  a  quantity  of  shredded  filter  paper  before  filling  the  percolator. 


Continuous   percolation.    In    an    e  extraction    apparatus '     a     still    more 
thorough  exhaustion  is  had  with  a  minimum  of  solvent.    Many  forms  have 

been  proposed,  all  on  the  same  principle 
however,  namely,  that  the  percolate  is  re- 
ceived in  a  heated  flask  from  which  the 
solvent  distills  leaving  the  non-volatile  ex- 
tractive; the  vapor  is  condensed  and  returns 
to  the  percolator. 

Eaikow's  apparatus  is  shown  in  Fig.  46. 
It  consists  of  three  parts,  a  distilling  flask 
A,  partly  filled  with  ether  or  other  volatile 
solvent;  a  condenser  B;  and  a  funnel  C 
containing  the  powder  to  be  extracted  rest- 
jiing  on  a  plug  of  cotton  in  the  stem.  The 
flask  A  is  heated  on  a  water-bath  and  the 
vapor  of  the  boiling  ether  rises  in  the  tube  D. 
The  inclined  portion  of  D  is  surrounded  by  a  larger  tube  B  through  which  cir- 
culates a  stream  of  cold  water;  here  the  ether  vapor  is  condensed  and  drops 
upon  the  substance  in  C,  and  after  percolating  it,  runs  back  into  A  by  way  of 


Fig.  46. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


53 


y^ 


the  tube  E.  Thus  the  circuit  of  the  ether  goes  on  continuously 
until  all  the  extractive  has  collected  in  A.  To  prevent  the  vapor 
of  the  ether  from  passing  into  C,  the  tube  E  has  a  trap  at  F. 

The  most  popular  form  of  extractor  for  general  work  is  that 
of  Soxhlet,*  Fig.  47.  The  vapor  from  a  flask  A  rises  through 
the  tubes  B  and  C  into  the  condenser,  the  form  shown  being  two 
concentric  spheres  of  glass  or  thin  metal,  the  inner  one  cooled 
by  water  flowing  through  it  entering  at  F.  The  vapor  is  con- 
densed on  the  surface  of  E  and  drops  upon  the  substance  in  the 
cup  G,  contained  in  a  plaited  filter,  a  thimble  of  filter  paper,  or  a 
tube  closed  with  a  plug  of  cotton  or  glass  wool.  When  G  is 
filled  to  the  dotted  line,  the  tube  H  acts  as  the  longer  leg  of  a 
syphon  and  transfers  all  the  liquid  to  the  flask  A.  After  the  ex- 
traction, the  substance  may  be  removed  from  G,  a  short  test-tube 
substituted  for  it,  and  the  ether  distilled  up  into  the  test-tube, 
leaving  the  extractive  in  the  flask  ready  for  weighing.  Disad- 
vantages of  the  Soxhlet  apparatus  are  fragility,  the  danger  of 
carrying  some  of  a  finely  powdered  substance  into  the  flask  dur- 
ing the  rapid  outflow  of  the  solvent,  and  relatively  high  cost. 
Various  modifications  have  been  proposed.  -p.  \- 

A  simple  form  is  shown  in  Fig.  48.  |    A  large  test-tube  A  contains  the  solvent  and 
is  closed  at  the  top  by  a  metal  cap  B.    The  condenser  is  a  tube  made  up  of  a  series  of 

double  cones  of  sheet  metal  cooled  by  water  entering  at  E  and  leaving  a 

D.    Below  the  tube  hangs  a  porcelain  crucible  with  a  perforated  bottom 

containing  the  substance  to  be  extracted. 

Lnmsden  \  describes  an  apparatus  suitable  as  well  for  the  finer  powders 

as  the  coarsely  granular,  the  solvent  being 

forced  through  the  mass  by  vapor  pressure. 

The  flask  a,  Fig.  49,  of  about  80  cubic  centi- 
meters capacity,  is  fitted  with  a  cork  through 

which  projects  the  contracted  end  of  the 

exhaustion  tube  c.    The  tube  contains  the 

sample  held  between  plugs  of  glass-wool. 

From  the  top  of  c  passes  a  tube  through  a 

condenser  d  nearly  to  the  bottom  of  a  flask 

6.    The  flask  is  immersed  in  a  jar  of  water 

kept  at  a  practically  uniform  temperature 

by  the   overflow  of  the  condenser.    The 

cork  of  6  is  notched  to  prevent  the  pressure 

in  the  apparatus  from  rising  above  atmos- 
pheric. 

The  flask  b  is  half  filled  with  ether  and  a 
L  ]      few  cubic  centimeters  poured  into  a.    The 

^-  -S       apparatus  is  connected   as  shown   and   a 

Immersed  in   a  beaker  of  hot  water;  the 

ether  vapor  generated  expels  the  air  from 
the  apparatus.  Then  a  is  placed  in  cold  water,  the  vacuum 
formed  by  the  condensation  of  the  inclosed  vapor  drawing 
the  ether  from  &  into  a  via  the  material  in  c.  Afterward  a 
in  transferred  from  cold  to  hot  water,  causing  the  ether  to 
return  to  &.  The  cycle  is  continued  until  the  exhaustion  is  complete. 

In  the  apparatus  of  Wollny,  Fig.  50,  the  powder  to  be  extracted  is  continually  per- 
meated by  the  hot  vapor  of  the  solvent  and  intermittently  drenched  with  small  volumes 


Fig.  48 


Fig.  49. 


*  Chem.  News,  1888—1—56,91,  and   235,   and  1891—1—86;   Journ.  Amer.   Chem.  Socy. 
1901-338. 

t  Journ.  Amer.  Chem.  Socy.  1893—121. 
\  Chem.  News  1888—130. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


of  the  liquid.  The  solvent  is  contained  in  the  flask  A  and  the  substance 
in  C;  as  the  solvent  boils  the  vapor  rises  in  B,  passes  down  through 
the  substance  into  F,  thence  rises  through  D  into  a  condenser  at- 
tached above  E.  The  condensed  liquid  drops  into  E  filling  it  to  the 
level  of  the  dotted  line,  when  the  inclosed  syphon  transfers  the 
entire  contents  Into  O.  After  percolating  the  substance  the  solvent 
runs  through  F  into  A.  If  the  extractive  Is  somewhat  volatile  or 
affected  by  the  temperature  of  the  boiling  solvent,  the  side  tube  G 
may  be  opened  and  vapor  generated  in  another  flask  inducted. 

Ordinary  corks -contain  resin,  etc.,  soluble  in  ether  and  like  fluids, 
and  rubber  is  also  attacked,  so  that  both  are  objectionable  for  con- 
nections exposed  to  these  fluids  or  their  vapors.  The  three  connec- 
tions shown  in  the  figure  are  made  in  a  different  way,  namely  by  a 
'mercury  seal'.  The  lower  tube  has  a  short  tube  of  larger  diameter 
fused  on  near  the  top  forming  a  gutter  in  which  stands  the  bottom  of 
the  upper-tube.  The  gutter  is  nearly  filled  with  mercury  making  a  gas- 
tight  trap,  the  weight  of  the  mercury  withstanding  the  slight  increase 
over  atmospheric  pressure  of  the  inclosed  vapor. 

An  apparatus  for  the  extraction  of  oil  from  seeds  is  due  to  Lehmann. 
It  is  a  small  conical  mill  In  which  the  seeds  are  ground  fine.  After 
removing  the  handle  01  the  interior  cone,  the  entire  mill  is  placed  In 
a  large  extraction  apparatus  and  the  oil  dissolved  out.* 

Instead  of  packing  the  sample  in  the  exhaustion  tube  of 
an  apparatus,  it  may  be  held  in  a  plaited  filter,  or  an  « extrac- 
tion thimble',  a  narrow,  seamless  cup  made  of  a  special  quality 
of  filter  paper.     Many  substances  are  more  quickly  and  easily 
Fig.  50.         extracted  if  mixed  with  an  insoluble  powder.     An  emulsion  of 
a  fatty  matter  in  water  or  an  aqueous  solution  may  be  im- 
bibed in  a  tight  roll  of  filter  paper,  which  after  drying  is  extracted  directly  by 
ether  or  gasoline. 

An  extraction  will  consume  from  an  hour  to  a  day  or  longer,  according  to 
the  nature  of  the  substance  and  the  percentage  of  extractive  it  contains.  The 
length  of  time  required  is  not  a  great  objection  as  the  apparatus  is  automatic 
and  needs  little  attention,  but  it  can  be  inferred  only  by  experience  as  there  is 
usually  no  provision  short  of  disconnecting  the  apparatus  to  ascertain  when  the 
extraction  is  complete.  It  has  been  proposed  to  provide  a  tap  below  the  per- 
colator (as  at  F,  Fig.  46)  that  a  small  amount  of  the  solvent  may  be  drawn 
off  occasionally  and  tested  by  evaporation. 

The  process  of  extraction  finds  use  principally  in  dissolving  fats,  gums, 
alkaloids,  etc.,  from  accompanying  insoluble  matters.  The  solvents  are  com- 
monly ether,  chloroform,  gasoline,  alcohol  and  benzene,  less  frequently  water 
and  volatile  acids  and  ammonia.  In  most  cases  the  solvents  have  a  boiling 
point  of  less  than  100°,  though  for  a  few  extractions  such  liquids  as  anilin  and 
napthalin  are  more  efficacious. 

Of  the  organic  solvents,  ether,  gasoline  and  alcohol  are  most  in  use,  Com- 
mercial ether  always  contains  some  water  and  alcohol  and  must  be  purified  by 
washing  out  the  alcohol  with  water,  and  removing  the  water  by  distillation 
from  a  hygroscopic  solid.  Gasoline,  also  known  as  petroleum  ether  and 
ligroin,  is  a  mixture  of  the  hydrocarbons  of  the  second  fraction  of  the  distilla- 
tion of  petroleum.  Commercial  gasoline  is  unsuitable  for  extractions ;  the  grade 
of  a  density  of  87°  Baume  is  distilled  fractionally,  rejecting  all  that  comes  over 
below  40°  and  above  60°  Cent.  For  some  purposes  other  fractions  of  the  dis- 
tillate are  more  suitable,  as  the  higher  the  boiling  point  the  higher  the  tem- 
perature of  the  solvent  in  contact  with  the  substance  to  be  extracted.  In  any 
case  the  narrower  the  limits  of  temperature  between  which  the  fraction  begins 


*  Chem.  News,  1890-1-15. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  55 

to  boil  and  completely  volatilizes,  the  more  regularly  will  the  process  of  ex- 
traction proceed.  When  using  a  mixture  of  liquids,  such  as  diluted  alcohol, 
or  a  solution  of  a -gas,  it  must  be  remembered  that  the  vapor  has  generally  a 
different  composition  from  the  original  liquid. 


Insoluble  inorganic  substances  can  usually  be  transformed  to  a  soluble  state 
or  combination  by  some  preliminary  igneous  treatment. 

Simple  ignition  may  alter  the  form  of  combination  of  an  insoluble  material 
so  as  to  allow  a  subsequent  partial  or  complete  solution  in  an  acid  or  alkali; 
thus  the  mineral  talc,  Mg3H2Si4Oi2  =  SMgSiOs  +  H2O  -f-  SiCte,  the  silica  becoming 
soluble  in  a  solution  of  sodium  carbonate.  Several  minerals  can  be  made  com- 
pletely soluble  in  this  way. 

Ignition  in  a  current  of  air  or  oxygen  will  oxidize  sulphur  and  burn  out 
bituminous  or  other  organic  matter  that  may  hinder  solution,  and  some  of  the 
Tare  metals,  indifferent  to  all  acids,  calcine  to  soluble  oxides.  Ignition  in  hy- 
drogen reduces  higher  to  lower  oxides,  and  some  oxides  to  the  metallic  state. 
At  a  moderate  heat  the  vapor  of  sulphur  or  carbon  disulflde  forms  volatile  or 
fixed  sulfldes  with  many  metals  and  alloys,  and  a  current  of  air  loaded  with  the 
vapor  of  bromine  has  been  proposed  for  the  decomposition  of  certain  native 
suifides. 

Fluxing.  The  solvent  or  decomposing  power  of  a  reagent  is  much  greater 
-when  applied  above  the  temperature  of  fusion  than  when  in  aqueous  solution, 
and  many  insoluble  compounds  are  changed  to  a  soluble  form  by  fluxing.  The 
choice  of  a  flux  depends  mainly  on  the  composition  of  the  substance  to  be 
treated,  though  differences  in  aggregation  or  crystalline  character  of  the  sample 
may  lead  to  the  substitution  of  a  flux  more  effective  though  perhaps  less  suit- 
able chemically. 

Native  silica  and  titanic  acid  and  many  silicates  react  with  the  oxide  of  po- 
tassium or  sodium  at  a  melting  heat,  the  product  being  soluble  in  water  or  in 
acid.  A  large  excess  of  sodium  carbonate  is  generally  the  flux,  the  silica, 
alumina,  etc.,  replacing  the  carbon  dioxide  radical,  while  the  bases  are  con- 
verted into  oxides  or  carbonates. 

Other  fluxes  in  less  common  use  are  sodium  carbonate  with  potassium 
nitrate;  sodium  hydrate  alone  or  with  sodium  nitrate,  sulphur  or  charcoal; 
sodium  peroxide;  sodium  thiosulf ate;  potassium  or  sodium  pyrosulf ate;  boric 
acid;  sodium  fluoride;  and  borax-glass;  also  soda-lime  and  various  metallic 
oxides  as  sinters. 

The  fusion  is  nearly  always  made  in  a  platinum  crucible,  though  with  a  flux 
of  a  caustic  alkali  or  baryta  a  silver  or  gold  crucible  is  better  because  less  at- 
tacked. The  mineral  to  be  decomposed  is  ground  to  a  fine  powder  and  mixed 
with  a  large  excess  of  the  flux  also  in  powder,  and  the  mixture  placed  in  a 
crucible  so  large  that  it  is  not  more  than  half  filled.  The  crucible  is  covered 
and  supported  on  a  platinum  triangle  resting  on  the  ring  of  a  retort-stand, 
and  heated  by  a  burner,  gently  at  first,  finally  to  complete  fusion.  A  large 
Bunsen  burner  or  a  blast-lamp  furnishes  sufficient  heat  for  the  fusion  which 
is  known  to  be  complete  when  the  mass  settles  down  to  a  quiet  liquid  evolving 
no  more  gas. 

When  the  crucible  and  contents  have  completely  cooled  the  solidified  lump 
is  sometimes  difficult  to  remove,  but  if  the  crucible  be  inclined  as  far  as  may 
be  while  the  contents  are  still  liquid  and  solidification  take  place  in  that 
position,  the  uvo  will  readily  part  in  mot-t  cases.  Or  a  heavy  platinum  wire 


56 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


may  be  plunged  into  the  fusion,  and  after  solidification,  the  crucible  hung  over 
a  platinum  triangle  and  heated  by  a  strong  flame  until  the  crucible  separates 
from  the  mass  within  and  falls  into  the  triangle.  The  projecting  end  of  the 
wire  is  then  bent  and  hooked  over  the  edge  of  a  beaker  so  as  to  suspend  the 
lump  just  below  the  surface  of  the  solvent  in  the  beaker. 


The  volume  of  a  gas  absorbed  by  a  liquid  when  no  apparent  chemical 
combination  takes  place,  depends  on  the  nature  of  the  gas  and  liquid,  and 
varies  directly  with  the  pressure  and  inversely  with  the  temperature  of  the 
liquid.  Most  gases  dissolve  in  water  according  to  the  equation  F=  a  -f  bt  +  cf2, 
where  V  is  the  volume  dissolved  in  a  unit  weight  of  water;  t,  the  temperature 
of  the  water;  and  «,  6,  and  c,  empirical  coefficients  determined  by  experiment. 
The  ready  absorption  in  a  liquid  of  a  gas  or  some  one  of  a  mixture  of  gases 
is  favored  by  a  low  temperature,  a  high  coefficient  of  solubility,  and  their 
protracted  and  intimate  contact.  Usually,  only  a  partial  absorption  is  had 
A  during  the  bubbling  of  a  gas  through  a  liquid,  com- 
pleted, however,  if  it  be  allowed  to  remain  standing 
over  the  surface  for  a  time ;  therefore  the  space  in  the 
vessel  unoccupied  by  the  absorbent  should  be  ample 
to  retain  the  gas  for  a  considerable  period,  the  current 
being  reasonably  slow.  And  in  rising  through  a  liquid 
the  smaller  the  bubbles  and  the  more  obstructed  their 
path,  the  more  ready  and  complete  the  absorption. 
With  this  object  in  view,  many  forms  of  apparatus 
have  been  devised. 

The  most  common  is  the  well-known  gas-washing 
bottle,  of  which  one  form  is  shown  in  Fig.   51,  the 
gas  entering  at  A,  bubbling  through  the  liquid,  and 
Fig.  61.  passing  out  through  B.    Usually  two  or  more  are 

joined  tandem. 

In  Pig.  52  is  shown  a  form  due  to  Thoerner.  A  large  glass  tube  is  slightly  inclined  from 
the  horizontal;  the  gas  enters  from  the  large  bulb  in  separate  bubbles  each  rising  along 

the  upper  part  of  the 
tube  to  the  exit. 
Wlnkler's  modifica- 
tion is  more  com- 
pact, the  tube  being 
coiled  into  a  spiral; 
while  Meyer  would 
replace  the  straight 
tube  by  a«  series  of 

Fig.  52.  small  bulbs. 

Emmerling's  tube,  Fig.  53,  is  on  the  principle  of  the  Glover  tower;  the  gas  entering  at  A 
wanders  through  a  column  B  of  broken  glass  or  glass  beads  drenched  with  the  absorbing 
fluid  dripping  from  the  funnel  O,  and  finally  emerges  from  D;  as  the  absorbent  collects  in 
B  it  Is  occasionally  tapped  out  into  a  flask  below. 

Usually  a  solid  granular  absorbent  is  held  in  a  large  glass  tube  bent  in  the 
form  of  the  letter  U,  the  openings  being  fitted  with  corks  through  which  pass 
narrow  glass  tubes;  or  the  stoppers  may  be  of  glass,  acting  as  stop-cocks 
when  turned  at  an  angle,  Fig.  64. 


There  are  a  few  instances  where  a  determination  of  a  compound  in  solution 
may  be  made  by  finding  the  weight  of  an  insoluble  solid  that  reacts  with  the 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


57 


compound  and  enters  into  solution.  Examples  are  the  determination  of  aceton 
in  urine,  done  by  boiling  a  measured  quantity  of  urine  with  mercuric  oxide  and 
afterward  determining  the  weight  of  mercury  that  has  passed  into  solution ;  when 
an  acetyl  derivative  dissolved  <—- ^ 
in  water  is  boiled  for  some  f\ 

hours  with  magnesia  there  is  c         i 
formed   magnesium    acetate          f~\ 
which  dissolves  and  can  be          \     ) 
determined  gravi metrically ; 
commercial  pepsin  is  valued 
by  the  weight  of  white  of  egg 
(coagulated  albumen)  that  is 
dissolved  by  an  aqueous  solu- 
tion of  the  pepsin,  etc.    The 
method  is  usually  applied  in 
cases  where  the  direct  de- 
termination of  a  compound 
is  difficult,    and  is  not   very 
solutions. 


Fig.  54. 
reliable    for   complex: 


EVAPORATION  -  DISTILLATION  — 
SUBLIMATION. 

Evaporation  of  a  liquid  is  resorted  to  (1),  for  re- 
ducing the  volume  when  too  dilute  for  certain  opera- 
tions or  convenient  manipulation;  (2)  to  expel  some 
volatile  compound  whose  presence  is  objectionable  for 
a  subsequent  operation;  (3),  to  obtain  a  dissolved 
body  in  the  solid  form;  or  (4),  to  change  the  aggre- 
Fig.  53.  gation  of  a  suspended  precipitate  or  residue  from  solu- 

tion to  one  more  compact  and  manageable  in  filtration. 

Practically  the  rate  at  which  evaporation  proceeds  is  determined  by  the  tem- 
perature of  the  liquid,  the  extent  of  surface  exposed,  and  the  rapidity  with, 
which  the  vapor  is  displaced  by  air,  and  is  also  influenced  by  the  density  of  the 
liquid,  the  amount  of  solid  matter  in  solution,  and  the  pressure  and  .humidity 
of  the  atmosphere. 

For  economy  in  point  of  time  an  evaporation  is  conducted  as  rapidly  as  may 
be  done  without  danger  of  loss  by  projection  from  the  bursting  bubbles  of 
steam. 

A  moderately  dilute  solution  is  most  quickly  concentrated  in  a  porcelain  or 
platinum  dish  over  a  low  flame,  heating  nearly  or  quite  to  gentle  ebullition  . 
The  dish  is  supported  at  the  proper  height  above  the  burner  _ 

by  the  ring  of  an  iron  retort  stand.  If  the  evaporation  is  made 
in  a  beaker  or  flask  the  support  is  a  heated  metal  or  soapstone 
plate  or  a  sheet  of  wire  gauze  covering  a  retort-stand  ring. 

One  of  the  many  forms  of  hot-plate  is  illustrated  in  Fig.  55; 
it  is  a  metal  plate,  preferably  of  cast  iron,  about  one-eighth 
inch  in  thickness,  supported  on  legs  of  a  length  to  bring  the 
plate  a  few  inches  above  the  top  of  the  burner.  A  flask  or 
porcelain  dish  may  be  supported  an  inch  or  more  above  the  plate  by  a  Ting 
of  tin-plate  of  slightly  less  diameter. 

A  shallow  iron  dish  filled  with  clean  sand  (sand-bath)  supported  on  the  ring 
of  a  retort-stand  transmits  a  more  uniform  heat  than  the  plate  and  lessens  the 
danger  following  the  breaking  of  a  vessel  standing  on  it,  but  has  the  drawback 
that  the  sand  tends  to  scratch  glassware,  making  it  more  liable  to  crack. 


4 


F. 


58 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


In  the  'electric  hot  plate  '  *  is  applied  the  moderate  and  uniform  heat  diffused 
when  a  current  of  electricity  is  passed  through  a  wire  inadequate  in  section  for 
its  conduction.  It  is  essentially  a  form  of  rheostat  in  the  shape  of  a  shallow 
«ast  iron  box  filled  with  magnesia  in  which  is  imbedded  a  coil  of  fine  nickel 
wire.  Where  a  laboratory  is  fitted  with  incandescent  lights,  wires  may  be  run 
from  a  socket,  and  the  amperage  of  the  current  regulated  by  a  resistance  coil 
so  as  to  give  any  temperature  up  to  one  capable  of  gently  boiling  water  in  a 
covered  vessel. 

As  a  source  of  heat  the  Bunsen  burner  shown  in  section  in  Fig.  56  provides 
&  clean  smokeless  flame  under  perfect  control.  Coal  gas  entering  at  A  emerges 

from  a  fine  hole  in  the 
jet  B,  and  rising  in  the 
tube  C  carries  with  it 
a  stream  of  air  drawn 
in  at  the  holes  D  D; 
the  mixture  burns  at 
F  with  a  conical  blue 
flame  surrounded  by 
an  almost  invisible 
mantle.  The  highest 
temperature  is  just 
above  the  point  of  the 
cone  which  should  be 
sharply  defined. 
Should  the  openings 
the 


Fig.  5G. 

D  D  admit  too  preat  a  proportion  of  air, 
mixture  will  refuse  to  burn,   while  too  small  a 
Fig.  56.  proportion    makes    the    flame  luminous    and 

smoky  (though  not  less  calorific  f)  ;  so  the  supply  of  air  is  regulated  by  turning 
a  loose  sleeve  E  encircling  the  burner  and  having  holes  corresponding  to  D  D. 
The  burner  is  connected  to  a  gas  tap  by  rubber  tubing. 

A  modification  of  the  above  has  a  stopcock  at  the  base  for  controlling  the  flow  of  gas  into 
the  burner,  simultaneously  admitting  the  proper  proportion  of  air  to  deluminate  the  flame. 

A  low  or  dwarf  form  is  shown  in  Fig  57.  It  is 
less  liable  to  be  injured  by  the  drippings  from 
solutions  that  have  a  tendency  to  boll  up  and 
overflow  the  evaporating  dish. 

When  the  gas  is  turned  low  and  the  burner  ex- 
posed to  draughts,  the  small  flame  is  liable  to 
drop  in  the  tube  C  and  burn  at  B.  This  may  be 
prevented  by  reducing  the  size  of  the  orifice  of  C 
by  the  insertion  of  a  short  piece  of  glass  tubing 
with  the  upper  edge  expanded  to  rest  on  F.  A 
burner  is  made  with  an  arrangement  to  tnrn  off 
the  gas  when  this  happens,  on  the  principle  that 
when  the  tube  C  becomes  heated  by  the  flame  at 
Fisj.  57.  */4  B  it  expands  and  releases  the  upper  end  of  a 

weighted  lever  connected  with  a  stopcock  at  the  base  of  the  burner  admitiing  the  gas; 
the  lever  falls,  closing  the  stopcock. 

Gasoline-gas  burners.  Special  forms  of  Bunsen  burners  have  been  designed  for  the 
highly  carburetted  gas  made  by  loading  air  with  the  vapors  of  the  lighter  petroleum  dis- 
tillates. The  carburetter  (a  closed  tank  on  the  principle  of  a  gas-washing  bottle,  Fig.  51) 
is  filled  with  a  high  degree  gasoline  and  connected  with  an  air  pump.  The  air  bubbling 
through  the  gasoline  takes  up  at  first  mainly  the  lightest  of  the  mixed  hydrocarbons  and 


*  Journ.  Amer.  Chem.  Socy.  1897—790. 
t  Journ.  of  Gas  Lighting,  38—878. 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


59 


Fig.  58. 


enters  the  burner  so  rich  in  their  vapors  that 
not  enough  air  can  be  drawn  in  through  D  D  to 
prevent  the  separation  of  free  carbon  in  the 
flame.  After  a  time  'the  lightest  fractions  of 
the  gasoline  having  been  carried  off,  those  of 
medium  density  are  taken  up  by  the  air  yield- 
ing an  entirely  satisfactory  flame  in  an  ordinary 
Bunsen.  Finally  only  the  least  volatile  frac- 
tions are  left  and  the  gas  refuses  to  burn  even 
when  D  D  are  entirely  closed,  and  the  carbu- 
retter must  be  emptied  and  recharged. 

The  special  burners  mentioned  above  en- 
deavor to  obviate  the  smoky  flame  of  the  first 
issue  of  a  newly  filled  carburetter  by  lessening 
the  volume  of  gas  passing  through  B,  Fig.  56,  while  maintaining  its  normal  pressure.  But 
an  ordinary  Bunsen  burns  satisfactorily  when  an  auxiliary  blast  of  air  is  led  from  the  air 
pump  of  the  gas  machine  and  introduced  into  the  rubber  tube  leading  to  the  burner  or 
into  the  gas-pipe  leading  from  the  carburetter,  regulating  the  volume  Inducted  by  means 
of  a  valve  or  stop -cock. 

The  ordinary  Bunsen  is  ill  adapted  for  heating  a  dish  directly  since  the  heat 
is  concentrated  around  one  point.  A  burner  with  many  small  flames,  such  as 
the  '  radial  burner '  shown  in  Fig.  58,  distributes  the  heat  over  the  bottom  of 
the  dish  and  diminishes  the  tendency  to  explosive  boiling.  An  inverted  Bunsen 
clamped  above  the  dish,  the  flame  nearly  reaching  the  liquid,  hastens  an  evap- 
oration to  a  remarkable  extent  and  diminishes  any  tendency  to  boil  over.* 

Substitutes  for  gas.  If  not  provided  in  a  laboratory,  or  where  there  is  required  a  flame 
giving  off  no  sulfurous  gases,  some  form  of  an  alcohol  or  gasoline  lamp  may  be  substi- 
tuted. The  Berzilins  alcohol  lamp  is  so  well  known  as  to  need  no  description. 

Of  the  several  forms  of  gasoline  lamps,  that  of  Kellogg  can  be  highly  recommended.  It 
Is  constructed  on  the  principle  of  the  common  gasoline  stove,  the  burner  having  three 
movable  tips  affording  different  sized  Bunsen  flames,  the  largest  being  almost  equal  in 
heating  effect  to  a  blast-lamp;  there  are  also  rose  and  wing-top  attachments.  With 
reasonable  care  the  lamp  is  perfectly  safe. 

Water-bath.  With  more  concentrated  solutions  it  is  safer,  though  less  ex- 
peditious, to  evaporate  by  steam  heat  in  a  "  water-bath."  The  most  common 

form  is  a  hemispherical  copper,  aluminum,  or 
porcelain  dish,  Fig.  59,  covered  by  a  series  of 
concentric  rings  that  can  be  removed  to  provide 
holes  of  various  sizes  to  support  evaporating 
dishes  and  beakers.  The  bath  is  half  filled  with 
water,  and  may  be  connected  to  a  constant 
water-level  (page  27)  with  advantage.!  A  glass 
Fig.  59.  V4-V8  crystallizing  dish  half  filled  with  water  and 

heated  on  the  hot  plate  is  a  substitute  more  sightly  than  corroded  copper;  the 
dishes  can  be  purchased  in  nests,  the  sizes 
grading  from  2.5  to  8.5  inches  in  diameter. 
For  the  technical  laboratory  may  be  pro- 
vided a  large  rectangular  copper  box,  the 
top  having  a  number  of  holes  fitted  with 
rings  allowing  several  evaporations  to  be 
carried  on  at  one  time;  it  is  usually  heated 
by  steam  entering  from  a  steam  pipe  at  the 

side,  the  condensed  water  led  off  through  Fig.  60. 

a  pipe  at  the  bottom. J 


*  Chem.  News,  1888-1— 250. 

t  Chem.  News,  1889-2—  250  and  269. 

t  Jour.  Anal.  Chem.  3—269. 


60  QUANTITATIVE   CHEMICAL    ANALYSIS. 

Vessels  for  evaporation.  Evaporating  dishes,  Fig.  60,  are  made  of  thin  glazed 
porcelain  of  the  shape  of  a  segment  of  a  sphere  or  with  a  flat  bottom,  the  vari- 
ous sizes  holding  from  30  to  1,000  cc.  or  more.  They  allow  a  more  rapid 
evaporation  than  a  beaker  or  flask  by  reason  of  the  greater  surface  of  liquid 
and  less  condensation  of  steam  on  the  interior,  and  also  withstand  more  sudden 
changes  of  temperature  than  glass,  are  less  fragile,  and  less  attacked  by  solu- 
tions. Casseroles,  Fig.  61,  are  of  a  greater  depth  in  proportion  to  the  diameter 
and  are  provided  with  porcelain  or  wooden  handles  by  which  they  are  more 
comfortably  handled  when  filled  with  hot  liquids. 

Platinum  dishes  are  highly  desirable  for  solutions  of  the  caustic  and  car- 
bonated alkalies,  and  indispensable  for  evaporations  of  liquids  containing 

hydrofluoric  acid.  Though  rather  costly, 
they  last  indefinitely  if  given  proper  care. 
The  edge  of  the  dish  may  be  left  straight 
or  better  curled  outwardly  over  a  plat- 
inum wire  ring,  stiffening  the  dish  and 
making  it  easier  to  handle  when  filled 
with  a  liquid.  Silver  and  nickel  basins 
can  be  used  for  solutions  of  the  caustic 
alkalies. 

jrig.  6i.  A  fluid  evolving  a  gas  on  heating  is 

best  concentrated   in  a  capacious  wide 

mouthed  flask  or  tall  beaker,  and  the  temperature  raised  slowly  and  cautiously. 
A  solution  containing  an  easily  oxidizable  compound  can  hardly  be  concen- 
trated in  a  reasonable  time  without  some  oxidation  taking  place;  it  is  best 
conducted  in  a  flask  through  which  passes  a  current  of  hydrogen  or  carbon 
dioxide. 

Small  volumes  of  liquid  may  be  evaporated  in  vacuo  by  attaching  the  flask  to 
a  filter  pump.* 

Weighing  residues  from  evaporations.  A  bulky  solution  is  first  concentrated 
to  a  small  volume  in  a  beaker  or  dish,  then  transferred  to  a  weighed  capsule 
or  crucible  and  evaporated  to  dryness  on  the  water  bath.  A  liquid  held  in  a 
crucible  is  evaporated  by  laying  the  crucible  as  nearly  horizontal  as  may  be  on 
a  platinum  triangle  which  rests  on  the  ring  of  a  retort-stand;  a  Bunsen 
burner  is  placed  so  that  the  flame  is  several  inches  below  the  edge  of  the 
crucible  and  turned  as  low  as  possible.  Loss  by  spattering  is  guarded  against 
by  directing  a  fine  jet  of  hot  or  cold  air  against  the  surface  of  the  solution; 
evaporation  on  the  water- bath  is  the  safer  plan,  however. 

A  solution  that  tends  to  crystallize  on  the  sides  of  the  dish  and  creep  over 
the  edge  by  capillarity  is  best  evaporated  in  a  watch-glass  resting  on  glass 
fragments  contained  in  a  larger  watch-glass;  any  liquid  passing  over  the  edge 
of  the  smaller  watch-glass  is  absorbed  by  the  broken  glass. 

When  the  residue  is  to  be  dried  at  100  =>  or  thereabouts,  the  most  suitable 
vessel  for  the  evaporation  and  weighing  is  a  dish  with  a  broad  flat  bottom 
rounded  at  its  junction  with  the  sides,  leaving  the  residue  in  the  form  of  a 
film  uniformly  thin  and  readily  dried.  Crystallizing  dishes  would  be  excel- 
lent except  for  the  sharp  corner;  the  best  vessel  for  the  purpose  is  made  by 
cutting  off  a  thin  beaker  about  an  inch  from  the  bottom  (by  filing  a  notch  in 
the  glass  and  leading  around  a  crack  with  the  red  hot  end  of  a  glass  rod) . 
The  edge  may  be  left  sharp,  or  rounded  in  the  Bunsen  flame  and  a  lip  formed 
by  pressing  the  plastic  glass  with  a  cold  glass  rod. 


*  Chera.  News,  1889—2—249  and  Journ.  Amer.  Chem.  Socy.  1895—302. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  61 

If  the  residue  Is  to  be  subsequently  extracted  by  a  volatile  solvent,  the  final  evaporation 
may  be  done  in  a  small  thin  glass  dish  (schaelchen),  afterward  crushed  and  the  fragments 
put  in  the  tube  of  an  extraction  apparatus.  Or  a  disk  of  tin -foil  may  be  bent  up  to  the 
form  of  a  shallow  dish  and  cut  in  pieces  after  the  evaporation. 


Solvent  action  of  liquids,  It  has  long  been  known  that  water  and  aqueous 
solutions  dissolve  appreciable  amounts  of  powdered  glass  and  porcelain,  and 
to  a  less  degree,  corrode  the  surfaces  of  vessels  containing  them.  The  extent 
of  this  action  in  the  case  of  water  and  solutions  of  the  commoner  reagents  has 
been  determined  by  several  investigators.  Comparison  of  their  results  shows 
marked  discordances,  explainable  on  the  ground  that  glasses  and  glazes  of  un- 
like composition  are  unequally  attacked  by  a  liquid,  and  also  that  the  technic 
of  annealing  probably  influences  the  susceptibility  of  a  given  variety.* 

In  general,  acids  (except  sulfuric  and  phosphoric)  have  but  little  effect, 
solutions  of  many  salts  considerable,  and  the  alkalies  and  their  carbonates  an 
energetic  action.  It  is  said  that  the  corrosive  power  is  augmented  by  an  in- 
crease in  the  temperature  of  the  solutions,  and  that  in  presence  of  salts  like 
sodium  carbonate  whose  acid  radicals  form  insoluble  precipitates  with  the  cal- 
cium of  the  glass,  the  action  is  greater  the  more  concentrated  the  solution, 
while  the  reverse  is  true  of  those  forming  soluble  calcium  salts. 

The  following  figures  may  serve  as  illustrations-— 

Pure  water  evaporated  in  chemical  glass  flasks  dissolved  .014  grams  per  liter;  boiled 
for  30  hours  dissolved  .0665  gram,  in  porcelain,  one  liter  dissolved  .0005  gram.  By  30  hours 
boiling,  water  containing  11  per  cent  of  hydrochloric  acid  dissolved  .017  gram  of  glass  and 
.005  gram  of  porcelain;  with  7  per  cent  of  ammonium  chloride,  .015  gram  of  glass,  and 
.004  gram  of  porcelain ;  with  10  per  cent  of  sodium  carbonate,  .450  gram  of  glass,  and  .024 
gram  of  porcelain.  Dilute  sulfuric  acid  has  twice  the  effect  of  water,  and  solutions  of 
sodium  and  ammonium  snlfates  and  phosphates  act  very  strongly.  Platinum  is  not 
affected  by  any  of  the  above. 

Cowper  f  heated  100  cc.  of  water  and  various  aqueous  solutions  in  glass  vessels  for  six 
days  to  100°.    There  was  dissolved  by  pure  water  .009  gram  of  glass;  by  concentrated 
ammonia  of  .880  specific  gravity,  .008  gram;  by  hydrogen  sulflde  water,  .011  gram;  by  am- . 
monia  water  of  .982  specific  gravity,  .035  gram;  by  concentrated  solution  of  ammonium 
sulflde  in  water,  .040  gram ;  and  by  dilute  ammonium  snlflde  solution,  .051  gram. 

A  highly  resisting  composition  for  glassware  is  that  of  an  alkali-calcium 
silicate — the  so-called  Bohemian  glass,  which  is  inferior  in  this  respect  only 
to  the  Jena  ware  of  Schott  and  Genossen,  and  the  e  resistance  glass  »  of  Weber, 
made  after  special  formulae. 

For  liquids  of  a  low  boiling  point,  as  acetone,  ether,  carbon  disulflde,  either 
a  temperature  far  below  100°  is  employed  or  the  solution  is  allowed  to 
evaporate  spontaneously.  On  the  other  hand,  those  boiling  above  this  temper- 
ature may  be  evaporated  on  the  hot-plate  or  in  a  bath  filled  with  glycerol  or 
paraffin  or  a  solution  of  some  inorganic  salt;  if  requiring  a  high  temperature, 
as  sulfuric  acid,  are  heated  directly  over  a  burner,  interposing  a  sheet  of  wire 
gauze  to  distribute  the  heat,  and  dropping  scraps  of  platinum  foil  or  the  like 
into  the  liquid  to  prevent  bumping. 

Boiling  with  succussion.  Some  liquids  are  prone  to  boil  with  a  succession 
of  sudden  bursts  of  vapor,  frequently  splashing  out  a  part  of  the  liquid  or 
endangering  the  vessel.  Especially  is  this  true  of  liquids  covering  heavy  and 
coherent  precipitates ;  manganous  ammonium  phosphate,  for  example,  must  be 
constantly  stirred  while  heating,  even  on  the  water  bath.  Bumping  is  more 


*  Oomey.Dict.  of  Solubilities,  169;  Chem.  News,  1891— 1—82 ;  Wiley,  Agricultural  Analysis, 
1-347. 

t  Journ  Chem.  Socy.  1882—254. 


62  QUANTITATIVE    CHEMICAL,    ANALYSIS. 

apt  to  occur  on  the  hot  plate  or  over  a  direct  flame,  and  may  be  avoided  by 
introducing  a  few  fragments  of  glass  or  pumice  or  a  spiral  of  platinum  wire, 
from  which  the  bubbles  of  vapor  are  quietly  disengaged;  or  by  passing  a 
current  of  steam  through  the  liquid  if  the  introduction  of  water  is  not  an 
objection.  Methyl  alcohol,  one  of  the  most  troublesome  on  this  score,  boils 
quietly  with  a  globule  of  sodium  amalgam,  and  a  solution  of  a  caustic  alkali 
with  bits  of  zinc  or  aluminum,  by  reason  of  the  constant  slight  evolution  of 
hydrogen  gas. 

With  emulsions  and  some  solutions,  such  as  milk,  syrups  and  varnishes, 
a  skin  forms  on  the  surface,  retarding  the  evaporation  or  raising 
the  temperature  to  a  degree  higher  than  is  prudent  in  the  pres- 
ence of  somewhat  volatile  oils,  glycerol,  etc.,  but  if  the  liquid 
before  evaporation  is  spread  over  a  greater  surface,  as  by  imbibition  in 
a  porous  solid,  such  as  infusorial  earth,  blotting  paper  or  purified  wood  saw- 
dust, the  water  is  quickly  and  completely  dissipated  at  a  low  heat,  and  the  res- 
idue, left  in  the  form  of  thin  films,  is  readily  penetrable  by  a  solvent.  In  some 
cases  the  liquid  may  be  *  scaled »  with  advantage. 

The  foam  arising  from  a  liquid  containing  mucilaginous  or  saponaceous  mat- 
ter and  liable  to  overflow  the  beaker  or  dish,  may  be  caused  to  subside  by  the 
addition  of  a  little  alcohol,  tannic  acid,  etc.*  On  boiling  an  aqueous  solution 
covered  with  a  layer  of  oil  or  liquid  fatty  acid,  steam  is  apt  to  be  suddenly  and 
violently  disengaged  and  throw  out  part  of  the  liquid ;  here  the  best  safeguard 
is  the  introduction  of  a  current  of  steam  through  a  narrow  glass  tube  reaching 
to  the  bottom  of  the  dish. 

Protection  from  dust  is  essential  to  the  correctness  of  a  determination  and  is 
especially  to  be  looked  after  in  a  laboratory  occupied  by  several  chemists.  The 
usual  device  for  protecting  a  beaker  or  dish  is  to  cover  it  with  a  watch-glass, 
interposing  a  glass  triangle  to  allow  the  escape  of  the  steam.  Or  a  Meyer's 
funnel  may  be  used,  a  large  glass  funnel  whose  rim  is  curled  inwardly  to  form 
a  gutter  which  at  one  point  opens  into  a  short  glass  tube.  The  funnel  is  hung 
in  an  inverted  position  above  the  dish  by  the  clamp  of  a  retort-stand,  and  the 
water  from  the  steam  condensed  on  the  interior  of  the  funnel  flows  into  the 
gutter  and  is  led  away  through  a  rubber  tube  slipped  over  the  glass  tube. 

DISTILLATION. 

When  a  liquid  is  contained  in  a  closed  vessel  in  vacuo,  evaporation  takes 
place  until  the  pressure  (tension)  of  the  vapor  has  risen  to  a  certain  point  de- 
termined for  any  one  liquid  solely  by  the  temperature ;  in  other  words  for 
each  temperature  there  is  a  specific  vapor-pressure,  assuming  that  some  of  the 
liquid  remains  unevaporated.  Thus  water  at  100°  has  a  tension  of  760  mm. 
(measured  at  zero)  of  mercury,  while  at  zero  the  tension  is  only  4.6  mm.  The 
boiling  point  is  also  related  to  the  vapor-pressure ;  as  water  boils  at  100°  under 
760  mm.  of  mercury,  and  at  zero  when  the  pressure  is  reduced  to  4.6  mm.  If 
instead  of  a  vacuum  another  gas  or  mixture  of  gases  (as  air)  covers  the  liquid, 
evaporation  takes  place  until  the  vapor  at  the  surface  of  the  liquid  has  reached 
a  certain  density  also  fixed  by  the  nature  of  the  vaporable  liquid  and  the  tern  - 
perature. 

Evaporating  a  liquid  from  a  closed  vessel  and  condensing  the  vapor  is  re- 
sorted to  when  the  distillate  is  to  be  weighed  or  measured  or  further  examined ; 
to  recover  an  expensive  or  scarce  solvent  for  future  use,  or  to  avoid  loading 
the  air  of  the  laboratory  with  disagreeable  fumes.  All  apparatus  for  this  pur- 


*  Journ.  Anal.  Chem.  1—116. 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


pose  have  three  essential  parts :  a  still  for  boiling  the  liquid,  a  condenser  for 
liquefying  the  vapor,  and  a  receiver  for  collecting  the  distillate. 

The  distilling  vessel  may  be  the  venerable  glass  retort  either  plain  or  with  a. 
tubulure  closed  with  a  ground  glass  stopper  or  a  cork  carrying  a  thermometer. 
For  liquids  of  high  boiling  point  a  porcelain  retort  is  safer  against  cracking, 
and  where  fluorine  or  hydrofluoric  acid  is  evolved,  one  of  platinum  or  lead. 

At  present,  however,  a  glass  flask  is  more  often  used  than  the  retort,  closed: 
by  a  cork  carrying  an  exit  tube  connected  with  the  condenser,  or  a  side  tube 
may  project  from  the  neck  of  the  flask  near  the  mouth.  To  prevent  any  of  the 
contents  of  the  flask  from  being  carried  over  mechanically  with  the  vaporr 
the  safety  tube  shown  in  Fig.  170,  or  its  equivalent,  may  be  interposed  in  the 
exit  tube. 

The  retort  or  flask  is  heated,  according  to  the  boiling  point  of  the  liquid,  in  a 
water  or  paraffin-bath,  on  the  hot-plate,  or  directly  over  the  flame  of  a  large 
Bunsen.  With  aqueous  liquids  a  current  of  steam  may  also  be  blown  into  the 
liquid,  the  moderate  heat  maintained  avoiding  the  danger  of  decomposing 
certain  organic  bodies  and  also  lessens  the  disagreeable  and  dangerous  bump- 
ing from  sudden  evolution  of  vapor.*  A  copper  or  iron  flask  is  safer  than  one 
of  glass  for  liquids  prone  to  boil  with  succussion. 

It  is  always  advisable  to  introduce  in  the  distilling  vessel  some  solid  nuclei 
such  as  recently  ignited  pumice  stone,  scraps  of  platinum  foil,  zinc  dust,  etc.r 
which  not  only  promote  regularity  of  boiling  but  largely  increase  the  speed  of 
the  distillation.f  Foaming  of  the  liquid  that  may  cause  froth  to  pass  into  the 
condenser  may  be  checked  by  the  addition  of  a  little  alcohol,  tannic  acid,  etc.r 
or  usually  by  previously  heating  the  liquid  to  near  the  boiling  point  for  some 
time  in  a  beaker  or  dish. 

The  condenser  is  generally  of  glass,  either  the  plain  Liebig  (Weigel)  form, 
Fig.  62,  or  a  modification.  Cold  water  entering  at  the  lower  end  of  the  condenser 
streams  through  the  outer  tube  cooling  the  inner  one 
which  condenses  the  vapor  entering  it.  To  extend  the 
surface  of  the  inner  tube  exposed  to  the  vapor,  it  may 
be  coiled  into  a  spiral  or  made  up  of  a  series  of  bulbs 
united  by  short  narrow  tubes.  Since  glass  gives  up 
traces  of  alkalies  to  steam,  a  block- tin,  copper,  or 
platinum  worm,  is  substituted  when  distilling  ammonia 
from  aqueous  solutions. 

The  current  of  cooling  water  should  flow  in  the  op- 
posite direction  to  that  of  the  vapor.  Where  the  con- 
denser is  inverted  or  inclined  so  that  the  condensed 
vapor  returns  to  the  still,  the  rubber  tube  leading  away 
the  waste  water  should  be  higher  at  some  point  than 
the  entrance  tube. 

For  the  condensation  of  a  given  volume  of  vapor  per 
minute  the  size  of  the  condenser  to  be  used,  i.  e.  the  ex- 
tent of  the  cooling  surface  exposed  to  the  vapor,  depends 
on  (a)  the  efficiency  of  the  condenser  —  the  temperature 
of  the  cooling  water,  its  rapidity  of  flow,  the  shape  of 
the  internal  tube  and  its  conductivity  for  heat;  and  (b), 
the  constants  of  the  vapor  —  the  boiling  point  and  latent 
and  specific  heats  of  the  liquid;  e.  g.,  the  conversion  into  vapor  of  one  kilogram 


Fig.  62. 


*  Chenu  News  1897-1-279. 

*  Idem,  1888-1-244;  1892— 1—226. 


64  QUANTITATIVE    CHEMICAL    ANALYSIS. 

of  water  from  zero  requires  636  heat  units,  while  the  same  weight  of  alcohol 
requires  only  250,  and  of  chloroform  81.3  heat  units. 

For  liquids  boiling  at  a  temperature  considerably  above  100° ,  the  condenser 
may  be  simply  a  long  thin  glass  tube,  the  cooling  effect  of  the  air  being  suffi- 
cient to  insure  the  liquefaction  of  the  vapor. 

The  receiver  is  usually  a  glass  flask  of  ample  capacity.  If  the  distillate  is 
practically  not  volatile  at  the  temperature  of  the  air,  the  receiver  may  be  a 
flask  or  cylinder  loosely  closed  by  a  watch-glass,  but  with  more  volatile 
liquids  it  is  connected  to  the  condenser  by  a  perforated  cork,  and  another 
opening  arranged  with  a  capillary  tube  or  a  mercury  seal  to  allow  for  expansion 
of  the  air  within,  yet  prevent  evaporation.  Only  with  highly  volatile  distillates 
is  it  necessary  to  cool  the  receiver  below  the  laboratory  temperature. 

Distillation  in  a  current  of  some  inert  gas  Is  resorted  to  when  the  liquid 
or  vapor  would  be  affected  by  contact  with  the  air.  For  this  purpose  a  tube 
connected  to  a  gas  generator  is  passed  through  the  cork  of  a  distilling  flask, 
and  another  tube  leads  from  the  receiver  to  the  open  air.  By  these  a  slow 
stream  of  gas  (generally  carbon  dioxide  or  hydrogen)  is  transmitted  through 
the  entire  apparatus. 

A  distillation  is  conducted  in  vacuo  when  contact  of  the  liquid  or  vapor  with 
the  air  is  not  allowable  on  account  of  oxidation,  where  the  liquid  or  some 
constituent  is  decomposed  at  the  temperature  of  boiling  under  atmospheric 
pressure,  or  to  promote  the  vaporization  of  some  constituent.  Here  the  con- 
nection of  the  condenser  with  the  receiver  is  made  air-tight  and  the  apparatus 
is  exhausted  by  a  vacuum  pump  connected  to  a  branch  tube  from  the 
receiver. 


The  temperature  of  distillation  of  a  commercial  liquid  is  often  a  criterion  of 
its  purity ;  that  the  thermometer  remains  stationary  or  nearly  so  throughout  the 
distillation  indicates  that  no  other  volatile  body  of  a  higher  or  lower  boiling 
point  is  present  in  any  considerable  proportion,  and  the  range  of  variation  in 
temperature  in  distilling  a  somewhat  impure  sample  is  a  rough  measure  of  the 
amount  of  a  given  impurity.  However,  some  compounds  suffer  partial  decom- 
position at  the  boiling  point,  the  first  part  of  the  distillate  differing  from  the 
last  with  consequent  alteration  in  the  temperature. 

Besides  the  separation  of  a  volatile  constituent  per  se,  distillation  is  largely 
applied  for  the  removal  of  a  gaseous  or  liquid  constituent  from  an  aqueous 
solution  containing  also  non-volatile  bodies.  The  distillate  is  a  dilute  aqueous 
solution  or  an  emulsion  of  the  volatile  constituent.  The  proportion  of  water 
to  be  distilled  in  order  to  carry  over  all  the  volatile  constituent  depends  not 
only  on  the  proportion  of  the  latter  but  on  its  nature  and  boiling  point  as  well  j 
in  some  cases  the  first  small  fraction  of  the  distillate  contains  practically  all 
the  constituent,  in  others  evaporation  to  dryness  at  100°  will  not  accomplish  a 
Complete  separation,  and  repeated  distillations  with  water  are  necessary. 

Should  the  volatile  constituent  be  an  acid  or  alkali,  originally  free  in  the 
mixture  or  liberated  by  a  stronger  acid  or  base,  the  receiver  may  previous  to 
the  distillation  be  charged  with  a  solution  of  an  alkali  or  acid.  If  the  latter  is 
a  standard  volumetric  solution  in  known  volume,  the  determination  may 
be  at  once  proceeded  with  by  a  volumetric  process.  Advantages  of  this 
method  are  that  the  condenser  need  only  be  air  cooled,  and,  provided  the 
vapor  is  brought  into  intimate  contact  with  the  liquid  in  the  receiver,  there  is 
no  danger  of  any  passing  through  uncondensed.  Various  other  absorbents 
that  react  with  the  volatile  compound  may  receive  the  distillate,  such  as  an 
oxidizing  reagent  for  a  reducing  gas  and  vice  versa. 


QUANTITATIVE    CHEMICAL   ANALYSIS, 


65 


A  method  in  frequent  use  Is  that  of  separating  a  volatile  constituent  I  of  a  liquid  L  by 
distillation  into  a  solid  or  liquid  L'  which  reacts  with  I  to  form  a  non-volatile  compound. 
In  some  cases  the  distillation  is  accel- 
erated by  the  addition  to  L  of  a  volatile 
liquid  I'.  To  avoid  the  use  of  the  rela- 
tively large  volume  of  V  required,  the 
distillation  is  several  times  repeated 
with  only  a  moderate  amount.  A  con- 
venient apparatus  for  the  purpose  is 
shown  in  Fig.  62A,  suspended  in  an  in- 
clined powition  from  a  support  A  as 
shown.  The  distillation  of  the  mixture 
of  L  and  I'  is  made  from  the  flask  B 
through  the  condenser  D  into  the  re- 
ceiver C  charged  with  L",  then  the 
apparatus  is  inclined  In  the  opposite 
direction  and  I'  is  distilled  back  into  B, 
leaving  the  compound  of  I  and  L'  re- 
main! ng  in  C.  The  operation  Is  repeated  Fig.  62 A. 
as  often  as  necessary. 

Destructive  distillation.  When  a  dry  organic  substance  is  heated  in  a  closed  retort  to 
a  temperature  above  its  point  of  decomposition,  the  products  are  various  gases  and 
vapors  and  usually  a  residue  of  tarry  matter  or  pitch.  Bituminous  matter  and  animal  or 
•vegetable  bodies  are  subjected  to  this  operation  as  a  check  on  a  similar  or  identical  man- 
ufacturing process. 

Fractional  distillation  is  applied  to  the  separation  of  a  mixture  of  a  number 
of  liquids  each  with  a  specific  boiling  point.  The  apparatus  is,  with  one 
exception,  practically  identical  with  that  for  a  simple  distillation.  Usually, 
though  not  necessarily,  the  constituent  liquid  having  the  lowest  boiling  point 
comes  over  first  and  passes  through  the  condenser  to  the  receiver.  As  the 
distillation  progresses  the  thermometer  rises,  steadily  if  the  boiling  points  of 
the  various  constituents  are  near  together,  but  intermittently  if  they  differ 
considerably,  and  at  an  increment  of  every  few  degrees  the  receiver  is  changed 
giving  a  graded  series  of  distillates  each  of  which  may  be  further  fractionated 
if  desired. 

For  example,  a  distillation  of  500  cubic  centimeters  of  crude  phenols  from 
coal-tar  oils  gave  Pattinson  * :  — 

Water 230  Cc. 

Below  228®  Cent 15  Cc. 

From  228  o  to  235  ° 27  Cc. 

From  2350  to  2500 40  Cc. 

From  250  o  to  270  o 26  Cc. 

From  270°  to  300° 15  Cc. 

In  fractionating  a  series  of  volatile  liquids  in  this  way,  the  vapor  arising  at 
any  moment  is  that  of  the  fraction  boiling  at  the  current  temperature  of  the 
still,  accompanied  by  a'portion  of  those  boiling  at  a  somewhat  higher  temper- 
ature. For  a  sharp  separation,  therefore,  it  is  necessary  to  pass  the  vapors 
before  entering  the  condenser,  through  a  chamber  heated  only  by  the  vapor 
itself  and  remaining  consequently  at  a  slightly  lower  temperature.  In  this 
chamber  there  condense  and  return  to  the  still  a  small  part  of  the  vapor  of 
the  liquid  vaporizing  at  the  current  boiling  point,  and  also  a  large  part  of  the 
vapors  of  the  liquids  of  higher  boiling  points.  The  efficiency  of  this  quasi- 
condenser  determines  the  degree  of  individualization  cf  the  condensed  prod- 
uce. It  is  known  as  the  dephlegmator  or  distilling  tube,  and  is  essentially  a 
wide  glass  tube,  Fig.  63,  clamped  in  a  vertical  position,  connecting  the  still  to 


From  300  ®  to  coking 35  Cc. 

Pitch 72  Cc. 

Loss 40  Cc. 

Total 600  Cc. 


*  Journ.  Socy.  Chem.  Ind.  1883—4 


66 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


tt 


Fig.  63. 


the  condenser  and  cooled  by  the  surrounding  air.  A  thermom- 
eter passes  through  the  cork  closing  the  upper  end,  the  bulb 
situated  near  the  opening  of  the  exit  tube  to  show  the  tempera- 
ture of  the  vapor  entering  the  condenser. 

To  extend  the  surface  exposed,  in  the 
Le  Bel-Henninger  tube,  A,  Fig.  64,  are 
two  or  more  external  branches  connect- 
ing the  bulbs  as  shown;  being  narrow 
tubes  and  of  thin  glass,  the  vapor  enter- 
ing them  readily  condenses,  the  liquid 
flowing  back  into  the  large  tube.  Lin- 
manu's  separating  tube  B  is  a  bulbed 
tube  obstructed  by  several  diaphragms 
of  platinum  gauze  in  which  the  con- 
densed liquid  collects,  washing  the  va- 
por as  it  rises  through  them.  Hart's 
arrangement  is  a  plain  tube  containing 
a  number  of  glass  funnels  with  stems 
bent  to  ISO®.  Various  other  forms  are 
tubes  filled  with  broken  glass,  pebbles, 
etc. 

The  nearest  to  a  complete  partition  is 
afforded  by  a  reversed  condenser  over 


the  still,  the  water  heated  to  the  proper  temperature  to 
condense  all  the  vapors  except  that  of  the  constituent  of 
the  lowest  boiling  point.  Fig.  64. 

The  efficiency  of  the  dephlegmator  is  exemplified  in  the  distillation  of  weak 
alcohol.  By  means  of  a  long  wide  tube  filled  with  glass  beads,  Hempel  recov- 
ered alcohol  of  95  per  cent  from  that  of  18  percent;  distillation  from  a  flask 
directly  would  yield  not  over  90  per  cent  alcohol. 

The  following  table  *  is  a  record  of  the  progress  of  separation  of  a  mixture  of  twenty-  five 
grams  each  of  benzene  (boiling-point  80.40)  and  toluol  (b.  p.  Ill®)  during  six  distillations, 
A  toF,  of  ten  fractions  each  differing  by  3o  Cent,  made  in  the  following  manner;  after  the 
first  distillation  A,  the  retort  was  cleaned  and  the  first  fraction  of  A  introduced  and  dis- 
tilled until  the  thermometer  rose  to  84  o  ;  the  second  fraction  was  then  added  to  the  resi- 
due of  the  first  and  distillation  resumed  until  the  thermometer  again  rose  to  84 o.  The 
receiver,  now  containing  the  first  fraction  of  B,  was  removed,  an  empty  one  substituted 
and  distillation  conducted  up  to  87°  ;  the  third  fraction  of  A  added  and  again  distilled  to 
870  giying  the  second  fraction  of  B.  This  routine  was  continued  throughout. 

VOLUMES  OP  THE  DISTILLATES  IN  CUBIC  CENTIMETERS. 


OGent. 

81-84 

84-87 

87-90 

90-93 

93-96 

96-99 

99-102 

102-105 

105-108 

108-11  1O 

A. 
B. 
C. 
D. 

E. 
P. 

1.0 
9.0 
17.0 
19.5 
21  5 
220 

-10.0 
9.5 
7.0 
5.5 
4.5 
3.5 

14.5 

85 
4.5 
4.0 
3.0 

2.0 

8.0 
5.0 
3.5 
2.5 
1.5 
1.5 

6.0 
3.5 
3.0 
2.0 
2.0 
1.0 

5.5 
3.5 
2.0 
2.5 
1.5 
1.0 

2.5 
2.5 
2.5 
1.5 
15 
1.0 

3.5 
4.0 
2.5 
20 
1.5 
1.0 

3.5 
3.5 
2.5 
1.5 
1.5 
1.5 

3.5 
7.5 
10.5 
13.0 
14.5 
16.0 

For  fractional  distillation  in  a  vacuum,  some  means  must  be  provided  to  pre- 
vent the  entrance  of  air  to  the  still  and  condenser  during  the  operation  of  re- 
moving, emptying,  and  replacing  the  receiver. 
One  apparatus  for  this  purpose  is  shown  in  Fig. 
65.  The  exit  of  the  distilling  tube  A  is  joined 
to  the  inner  tube  of  the  condenser  B,  and  this  in 
turn  with  the  funnel-tube  C.  Terminating  C  is 
a  stopcock  D  fitted  to  the  cork  of  the  receiving 
flask  E.  Both  C  and  E  are  connected  by  tne 
tubes  F  and  G  to  vacuum-pumps. 

The  stopcocks  D  and  H  being  open,  the  pumps 
are  started  and  the  distillation  begun.  As  soon 
Fig.  65.  as  the  first  fraction  has  passed  over,  D  and  H 

*  Lleblg's  Annal.  224-259. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  67 

are  closed  and  E  removed  and  emptied,  the  second  fraction  meanwhile  collect- 
ing in  C.  The  receiver  E  is  replaced  and  H  opened,  and  when  E  becomes  vacu- 
ous D  is  opened,  the  contents  of  C  flowing  down  into  E. 

Distillation  in  a  current  of  steam  is  often  an  advantage,  as  many  organic  bodies  boiling 
in  the  air  considerably  above  100  o  distill  in  steam  at  but  little  above  atmospheric  pressure. 
From  this  it  follows  that  the  volume  and  boiling  points  of  the  several  fractions  distilled 
in  steam  may  differ  widely  from  those  of  an  ordinary  distillation.  Superheated  steam  is 
necessary  for  bodies  of  a  high  boiling  point.* 

As  regards  the  possibility  of  the  separation  of  any  two  mixed  liquids  by  frac- 
tional distillation,  Regnault  divides  the  pairs  into  three  classes ;  those  quite  or 
almost  immiscible,  those  which  mix  to  a  limited  extent,  and  those  mixing  in 
all  proportions. 

1.  With  liquids  totally  insoluble  in  each    other  the  vapor- pressure  at  any 
temperature  is  the  sum  of  the  vapor-pressures  of  the  constituents  at  the  same 
temperature,  that  is  to  say,  the  liquids  evaporate  independently  of  each  other. 
Knowing  the  vapor-pressures  and  vapor-densities  of  the  constituents,  the  boil- 
ing point  can  be  calculated  and  also  the  weights  of  the  two  liquids  in  the  dis- 
tillate.   If  w  is  the  weight  of  one  constituent  in  the   distillate,  p  its  vapor  - 
pressure  and  d  its  vapor-density,  and  w',  p',  and  d',  the  corresponding  values 
of  the  other  constituent,  then  w  :  w'  : :  d.  p  : :  d'.  p'. 

In  all  cases,  the  boiling  point  of  the  mixture  is  lower  than  that  of  either  com- 
ponent and  remains  constant  until  nearly  all  of  one  has  passed  over,  and  the 
composition  of  the  distillate  remains  constant  until  the  quantity  of  one  con- 
stituent of  the  mixture  has  become  very  small. 

2.  No  fixed  laws  can  be  laid  down  for  liquids  mixing  only  to  a  limited  extent, 
as  they  fall  partly  within  the  scope  of  class  1  and  partly  of  class  3,  but  invari- 
ably the  vapor  pressure  of  the  mixture  is  greater  and  the  boiling  point  lower 
than  that  of  either  constituent. 

3.  Mixtures  of  the  third  class  —  those  mutually  dissolving  in  all  propor- 
tions—  behave  quite  differently  according  to  the  nature  of  the  constituents. 
When  the  ratio  of  the  members  is  at  a  certain  numerical  value,  some  pairs 
appear  to  unite  to  a  compound  of  definite  composition  f  which  distills  over  un- 
changed, and  where  a  different  ratio  exists,  the  vaporization  of  this  compound 
is  preceded  or  succeeded  by  that  of  the  component  which  is  in  excess  in 
the  original  mixture.    For  example,  hydrochloric  acid  gas  in  water  loses  either 
gas  or  water,  according  to  the  strength  of  the  mixture,  until  there  remains  a 
solution  containing  at  760  mm.  barometer,  about  20.2  per  cent  of  the  gas, 
which  then  distills  as  such,  while  with  propyl  alcohol  and  water,  first  the  mix- 
ture of  constant  boiling  point  comes  over,  then  the  residue  of  alcohol  or  water 
as  the  case  may  be.    It  is  evident  that  none  of  these  mixtures  can  be  sepa- 
rated by  fractional  distillation. 

Other  pairs  of  liquids  have  throughout  the  distillation  a  vapor-pressure 
and  boiling  point  between  those  of  the  constituents,  and  whatever  the  rela- 
tive proportions  of  the  two  liquids,  no  mixture  of  constant  boiling  point  is 
had.  These  pairs  may  be  parted  more  or  less  perfectly  in  proportion  to 
(1)  the  divergence  of  their  boiling  points,  and  (2)  the  influence  that  the 
presence  in  different  amounts  of  one  component  has  on  the  boiling  point  of 
the  other. 


Sublimation.  This  principle  is  not  often  available  since  comparatively  few 
solid  bodies  volatilize  undecomposed  within  the  range  of  temperature  that 


*  Chem.  NP.WS,  1899— 1—290 ; "1898— 1—25. 
f  Watts  Diet,  of  Chem.  4—187. 


(58  QUANTITATIVE    CHEMICAL    ANALYSIS. 

can  be  employed  for  this  operation.  A  few  alkaloids,  benzoin  and  benzoic 
acid,  camphor,  chloral,  iodine,  iodoform,  etc.,  may  be  sublimed  with  more  or 
less  success  from  less  volatile  bodies,  but  a  loss  by  decomposition  or  oxidiza- 
tion or  through  imperfect  condensation  is  almost  unavoidable.  The  apparatus 
may  be  simply  a  light  tared  funnel  inverted  in  an  evaporating  dish  contain- 
ing the  substance  heated  on  a  sand  bath;  or  a  pair  of  closely  fitting  watch- 
glasses  of  equal  size  may  be  fixed  edge  to  edge  and  the  body  sublimed  from  the 
under  to  the  upper,  interposing  a  perforated  filter  paper  to  prevent  the  subli- 
mate from  dropping  back. 

Bruehl's  apparatus,  Fig.  66,*  is  a  platinum  crucible  A,  surrounded  by  a  copper  box  B, 
through  which  circulates  cold  water.  Upon  the  box  rests  a  light  glass  shade  C.  The  sub- 
limate collects  partly  on  the  upper  plate  of 
the  box,  and  partly  on  the  interior  of  the  glass. 
The  apparatus  may  also  be  arranged  for  sub- 
limation in  vacuo. 

Another  plan  Is  to  place  the  substance  to 
be  analyzed  in  a  porcelain  boat  and  push  the 
boat  to  the  center  of  a  long  wide  glass  tube 
laid   horizontally  In   a  combustion  furnace 
Fig.  66.  (page  296).  One  end  of  the  tube  Is  connected 

with  a  gas  generator  or  air  blast,  the  other  to 

a  bulb  tube  containing  water  or  solution  of  some  reagent.  The  tube  is  heated  to  above 
the  temperature  of  sublimation  while  a  slow  current  of  air  or  gas  passes  through  the  tube 
and  bulbs.  The  volatile  constituent  sublimes  and  Is  partly  condensed  in  the  cooler  part 
of  the  tube  and  partly  in  the  liquid  of  the  bulbs. 

PRECIPITATION. 

The  conversion  of  a  substance  held  in  solution  into  an  insoluble  solid  (or 
liquid)  form  is  called  precipitation,  and  may  be  considered  as  the  continuous 
formation  of  a  super-saturated  solution  which  as  continuously  decomposes. 
Its  purposes  are  to  obtain  a  dissolved  solid  in  a  form  suitable  for  weighing;  as 
a  means  of  separation  from  other  bodies  remaining  in  solution ;  or  to  remove 
matters  from  a  solution  whose  presence  would  be  prejudicial  to  subsequent 
operations. 

Precipitation  of  an  element  or  compound  may  be  induced  either  by  (1)  so 
changing  the  condition  or  combination  that  it  becomes  insoluble ;  as  antimony 
from  an  acid  solution  by  hydrogen  sulflde,  copper  by  the  electric  current,  or 
gelatin  by  formaldehyd;  (2),  reducing  the  solvency  of  the  liquid  by  dilution, 
compounding  with  another  liquid,  etc.;  as  a  resin  in  alcohol  or  barium  sulfate 
in  concentrated  sulfuric  acid  thrown  down  by  dilution  with  water,  globulin 
precipitated  from  urine  by  carbon  dioxide,  or  uric  acid  by  hydrochloric  acid; 
or  (3),  raising  or  lowering  the  temperature  of  the  solution;  as  ferric  hydrate 
precipitated  on  boiling  a  dilute  neutral  solution  of  a  compound  of  iron  with  a 
weak  acid  radical,  paraffin  by  cooling  an  alcoholic  solution  of  petroleum  to 
zero  Cent.,  or  the  separation  of  the  albumin  of  urine  by  boiling. 

Precipitants  may  be  either  general,  forming  precipitates  with  each  of  several 
analogous  bodies,  or  specific,  affording  a  precipitate  with  but  one  or  a  few. 

The  best  form  in  which  to  precipitate  a  body  for  a  determination  or  separa- 
tion —  whether  in  the  elementary  state  or  as  a  particular  one  of  several  pos- 
sible compounds  all  more  or  less  insoluble — is  to  be  decided  by  a  number  of 
considerations,  aiming  for  a  high  degree  of  insolubility,  rapidity  of  collection, 
ease  of  filtration  and  washing,  stability  on  drying  and  ignition,  and  for  com- 
pounds, a  high  molecular  weight.  It  may  also  be  an  advantage  when  the  pre- 


Berichte,  18J9— 238. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  69 

cipitate  is  to  be  weighed,  if  a  specific  precipitant  can  be  used,  avoiding  the 
previous  separation  of  other  bodies  in  the  solution. 

The  general  rule  that  a  body  to  be  precipitated  shall  be  held  in  a  clear 
solution  has  some  exceptions,  as  in  the  case  of  suspended  and  colloidal  matter. 
Some  insoluble  bodies  in  fine  powder  are  transformed  to  other  insoluble  com- 
pounds when  long  digested  with  a  concentrated  solution  of  a  reagent,  entering 
the  solution  for  an  inappreciable  time.  Thus,  certain  organic  bodies  form  in- 
soluble compounds  with  picric  acid,  and  when  one  is  digested  in  the  solid  form 
with  a  nearly  saturated  solution  of  the  acid  it  is  gradually  replaced  by  its  picric 
acid  compound. 

Solubility  of  precipitates.  Since  no  precipitate  is  absolutely  insoluble  in  the 
fluid  from  which  it  separates,  in  every  precipitation  it  is  important  to  know  the 
degree  of  solubility  in  the  liquid  and  the  means  to  reduce  it  to  a  minimum. 

The  proper  volume  of  the  solution  in  which  a  precipitation  is  to  take  place 
depends  on  the  bulk  of  the  precipitate,  the  character  and  amount  of  other 
bodies  present  that  are  liable  to  impurify  it,  and  especially  on  the  solubility. 
For  example,  barium  sulfate  is  so  insoluble  that  a  considerable  dilution  is  not 
prejudicial  to  its  complete  separation,  while  with  the  far  more  soluble  stron- 
tium sulfate,  the  fluid  should  be  as  concentrated  as  possible,  even  at  the  hazard 
of  the  occlusion  of  other  compounds. 

From  a  very  dilutu  solution  precipitation  is  slow  and  often  less  complete  than 
the  solubility  coefficient  would  indicate.  If  evaporation  is  unallowable  for 
any  reason,  a  prompter  and  more  complete  separation  is  had  by  the  addition  to 
the  solution  of  a  known  weight  of  the  body  to  be  precipitated  in  such  a  com- 
bination as  is  most  convenient.  A  deduction  is  made  from  the  weight  of  the 
total  precipitate  for  the  part  inducted,  with  a  correction  for  solubility. 

It  is  sometimes  desirable  to  conduct  a  precipitation  in  a  dilate  solution,  yet  for  sub' 
sequent  determinations  to  keep  the  filtrate  small  in  volume.  To  save  the  time  oi  evapo- 
ration the  solution  is  divided  into  several  equal  parts;  the  first  part  is  diluted  to  the 
proper  concentration,  precipitated  and  decanted  or  filtered;  to  the  decanted  liquid,  with 
more  of  the  precipitant  it  necessary,  is  slowly  added  the  second  part,  this  decanted,  and 
so  on  with  all  the  remaining  parts.  By  this  plan  the  necessary  dilution  is  accomplished 
and  the  final  filtrate  not  greatly  increased  in  volume. 

Where  no  precipitant  is  available  for  a  sparingly  soluble  compound,  the 
dilute  solution  may  be  concentrated  to  a  small  bulk  whereupon  the  greater  part 
of  the  compound  precipitates  or  crystallizes  out.  What  remains  in  solution  is 
calculated  from  the  volume  of  the  concentrated  solution,  and  the  solubility  of 
the  compound  therein,  this  found  by  an  experiment  on  a  solution  similar  in 
composition  to  the  one  in  hand. 

The  solubility  of  a  precipitate  is  sometimes  lessened  by  the  admixture  of 
another  liquid  in  the  solution;  thus,  calcium  malate  is  less  soluble  in  a  dilute 
alcoholic  than  in  an  aqueous  solution;  ether-alcohol  dissolves  less  of  certain 
organic  compounds  than  alcohol  alone ;  etc. 

As  a  rule,  the  solubility  of  a  complex  precipitate  is  lowered  by  the  presence 
of  a  soluble  compound  of  one  of  its  radicals;  thus,  one  part  of  lead  chloride 
dissolves  in  120  parts  of  water  and  in  437  parts  of  a  five  per  cent  solution  of 
sodium  chloride.  But  in  some  cases  should  the  concentration  exceed  a  certain 
proportion  the  solubility  is  increased;  as  lead  chloride  dissolves  in  129  parts  of  a 
concentrated  solution  of  sodium  chloride  —  nearly  as  freely  as  in  water.  On  the 
other  hand  it  is  sometimes  advantageous  to  fully  saturate  the  solution  with  the 
precipitant  or  a  similar  salt. 

An  aqueous  solution  previous  to  precipitation  may  be  saturated  with  the  same 
compound  as  the  precipitate  —  if  a  solution  of  potassium  tartrate  be  compounded 
with  a  slight  excess  of  acetic  acid,  one -half  of  the  potassium  is  abstracted  by 


70  QUANTITATIVE    CHEMICAL    ANALYSIS. 

the  acid  —  K2C4H406  (potassium  tartrate)  -fHC2H302=  KHC4H4O6  (potassium 
bitartrate)  -f-  KC2H302  (potassium  acetate)— and  the  potassium  bitartrate  precip- 
itates; but  only  completely,  since  it  is  somewhat  soluble  in  water,  if  the  solu- 
tion before  acidification  was  saturated  with  potassium  bitartrate  by  shaking  with 
some  of  the  solid  salt  and  filtering  from  the  excess. 

Many  organic  compounds  impede  or  totally  prevent  the  precipitation  of  cer- 
tain bases  by  alkalies,  alkali  phosophates,  etc.*  Ferric  citrate  or  tartrate  is 
not  clouded  by  ammonia,  and  the  presence  of  gallic  acid,  sugar,  glycerol,  and 
the  like  have  a  similar  effect,  so  that  it  is  safer  to  destroy  or  eliminate  organic 
matter  from  a  solution  before  precipitation  by  these  reagents,  or  to  choose 
another  precipitant. 

It  may  happen  that  two  bodies  are  present  in  a  solution  both  forming  pre- 
cipitates with  a  given  reagent  and,  according  to  the  usual  procedure,  must  be 
separated  before  the  reagent  may  be  applied  for  a  determination.  But  a  sep- 
aration can  be  omitted  if  the  precipitates  differ  greatly  in  solubility  and  only 
the  body  forming  the  more  insoluble  precipitate  is  to  be  determined  (as  in  an 
assay),  by  making  the  precipitant  a  saturated  solution  of  the  more  soluble  pre- 
cipitate. Thus,  however  great  may  be  the  proportion  of  lead  nitrate  mixed 
with  silver  nitrate,  only  silver  chloride  falls  when  the  precipitant  is  a  solution 
of  lead  chloride;  similarly  only  quinine  iodosulfate  is  precipitated  by  a  solution 
of  chinoidine  iodosulfate,  from  a  solution  containing  quinine  and  chinoidine. 

When  a  precipitation  is  to  take  place  in  a  strictly  neutral  solution,  a  dilute 
acid  and  alkali  are  alternately  poured  In  with  the  addition  of  some  indicator 
until  a  drop  of  either  reverses  the  reaction;  or  when  it  is  undesirable  to  intro- 
duce any  traces  of  organic  matter,  the  reversal  is  observed  by  spotting  the 
solution  on  litmus  or  turmeric  test-paper.  A  much  easier  way  to  obtain  neu- 
trality is  first  to  either  slightly  acidify  the  solution  if  it  is  alkaline,  or  nearly 
saturate  any  free  acid  by  an  alkali,  and  then  stir  in  an  excess  of  some  solid 
reagent  that  is  insoluble  in  water  but  readily  soluble  in  the  acid,  and  whose 
presence  in  the  solution  and  precipitate  is  unobjectionable;  the  hydrate  or 
oxide  of  the  base  of  the  precipitant  is  often  suitable. 

Thus  In  precipitating  tungstlc  add;  first,  any  free  mineral  acid  In  the  solution  Is  neu- 
tralized by  stirring  In  some  mercuric  oxide,  then  a  solution  of  mercurous  nitrate  Is  added, 
and  the  precipitate  of  mercurous  tungstate,  mixed  with  the  excess  of  the  mercuric  oxide 
Is  filtered;  on  Ignition  all  the  mercury  is  expelled  leaving  pure  tungstic  acid  ready  for 
weighing.  The  same  eifect  is  obtained  without  the  use  of  an  Insoluble  base  by  adding 
to  the  slightly  acid  solution  a  large  excess  of  the  precipitant  followed  by  enough  alkali  to 
more  than  neutralize  the  free  acid,  yet  not  sufficient  to  combine  with  all  of  the  excess  of 
the  precipitant. 

Some  metals  are  precipitated  by  hydrogen  sulfide  only  when  in  combina- 
tion with  a  weak  acid  such  as  acetic,  and  to  convert  the  metal  from  a  com- 
bination with  a  strong  acid  to  an  acetate  it  is  customary  to  add  an  excess 
of  an  alkali  acetate.  Thus,  a  solution  of  zinc  chloride  containing  free 
hydrochloric  acid  on  the  addition  of  sodium  acetate  — 

ZnCla  (zinc  chloride) -f2NaC2H3O2  (sodium  acetate)  =  Zn(C2H3O2)2(zinc 
acetate)  -f  2NaCl  (sodium  chloride). 

HC1  (hydrochloric  acid)  -f  NaC2H3O2  =  NaCl  -f  HC2H3O2  (acetic  acid). 

It  is  said  that  the  excess  of  the  alkali  salt  favors  the  precipitation  by 
lessening  the  ionization  and  consequently  weakening  the  solvent  power  of  the 
organic  acid  set  free  by  the  excess  of  the  mineral  acid  and  the  hydrogen 
sulfide. 

When  precipitating  a  body  from    a   complex  solution  it  must    be  learned 


Oetwald-McGowan,  Foundations  of  Anal.  Ohem.  138. 


QUANTITATIVE   CHEMICAL   ANALYSIS.  71 

•whether  the  associates  are  of  a  nature  and  present  in  a  quantity  likely  to  seri- 
ously impurify  the  precipitate  by  co-precipitation  or  occlusion;  where  this 
is  probable  the  safest  plan  is  to  separate  the  body  before  precipitation. 
There  are  cases,  however,  when  simply  changing  the  combination  of  the 
impurities  to  other  radicals  will  insure  their  remaining  entirely  in  solution. 
For  example,  in  the  precipitation  of  sulfuric  acid  by  barium  chloride  in 
presence  of  ferric  chloride,  by  changing  the  ferric  chloride  to  ferric  oxalate 
any  contamination  of  the  barium  sulfate  with  iron  will  be  prevented. 

The  complete  segregation  of  a  precipitate  from  the  solution  in  which  it  is 
formed  may  take  place  almost  instantaneously  or  not  until  after  a  more  or  less 
protracted  repose,  conforming  to  the  nature  of  the  precipitate  and  the  conditions 
under  which  it  is  formed.  Brisk  agitation  of  the  liquid  after  the  addition  of  the 
precipitant  hastens  deposition,  a  vigorous  stirring  or  shaking  in  a  stoppered 
flask  or  passing  a  rapid  stream  of  air  for  ten  minutes  is  as  effectual  as  an  hour's 
repose.  The  device  for  agitating  the  liquid  shown  in  Fig.  36  is  of  general 
service.* 

Heating  the  liquid  greatly  favors  the  collection  and  should  be  done  whenever 
allowable.  Boiling  has  a  still  more  pronounced  effect  in  causing  a  precipitate 
to  clot  and  settle  well,  and  is  necessary  in  some  cases  for  the  expulsion  of  a 
volatile  compound  in  the  excess  of  which  the  precipitate  is  somewhat  soluble. 


The  precipitant  may  be  either  a  liquid,  gas  or  solid,  but  for  several 
reasons  an  aqueous  solution  is  generally  to  be  preferred,  and  as  a  rule 
a  gas  or  solid  is  introduced  into  the  solution  of  the  body  to  be  precipitated  only 
when  it  is  insoluble  or  sparingly  soluble  in  water  or  other  available  solvent. 
Crystalline  salts  are  dissolved  in  water  to  a  convenient  strength,  alone  or  com- 
pounded with  other  chemicals  if  required  for  solution  or  preservation. 

A  soluble  precipitant  is  introduced  in  the  solid  form  with  the  purpose  of  sat- 
urating the  solution  to  insure  the  entire  precipitation  of  certain  organic  com- 
pounds; to  promote  the  flocculation  of  a  finely  divided  or  slimy  precipitate;  or 
in  cases  where  precipitation  must  take  place  from  a  highly  concentrated  solution 
and  even  the  dilution  caused  by  a  solution  of  the  reagent  is  detrimental. 

An  insoluble  reagent  is  made  the  precipitant  when  it  is  desirable  that  no  ex- 
traneous body  be  brought  into  the  solution  beyond  an  equivalent  of  the  precipi- 
tant. Here  either  the  precipitation  may  be  only  an  apparent  mechanical  absorp- 
tion, as  where  a  coloring  matter  is  withdrawn  from  solution  by  oxycellulose, 
or  a  proteid  by  copper  hydroxide ;  or  it  may  be  the  result  of  a  distinct  chemical 
reaction,  as  when  a  ferrocyanide  is  decomposed  to  form  mercuric  cyanide  and 
iron  oxide  on  boiling  with  water  and  mercuric  oxide. 

Even  an  already  formed  precipitate  may  be  entirely  changed  to  another  insol- 
uble compound  by  suspension  in  water  or  other  liquid,  adding  a  suitable  reagent 
and  heating  the  mixture. 

Frequently  the  insoluble  precipitant  is  an  oxide  or  carbonate  of  a  metal  that 
reacts  in  a  similar  way  to  a  soluble  salt  of  the  metal,  as  when  silver  carbonate 
is  substituted  for  silver  nitrate  for  the  removal  of  hydrogen  sulflde  from  a 
solution;  any  free  acid  in  the  solution  is  neutralized  at  the  same  time.  This 


*  Chem.  News,  1892-1-148;  Journ.  Anal.  Ohem.  4—57;  Wiley,  Agricultural  Anal.  2—144; 
Chem.  Centralb.  18%— 63. 


72 


QUANTITATIVE   CHEMICAL   ANALYSIS. 


plan  is  more  suitable  for  a  separation  than  for  a 
determination ;  for  the  latter  the  necessary  excess 
of  the  precipitant  must  be  entirely  volatile  at  a 
considerably  lower  heat  than  will  affect  the  pre- 
cipitate (e.  g.,  mercuric  oxide),  or  be  otherwise 
removable  in  order  that  the  precipitate  can  be 
weighed. 

When  a  gas  is  so  sparingly  soluble  in  water  that 
it  must  be  carried  into  a  solution  in  the  gaseous 
form  to  avoid  great  dilution,  it  is  best  furnished 
from  a  constant  generator.  Of  these  there  are 
three  types  and  many  modifications.  All  are  so 
.big.  67.  contrived  that  the  generation  of  gas  may  be 

initiated  and  terminated  at  pleasure,  preventing  waste  and  the  nuisance  of  gas 

escaping  into  the  laboratory.* 

As  arranged  for  the  production  of  hydrogen  sulfide  the  three  typical  generators  are  — 
1.  Von  Babo's,  Fig.  67,  consists  of  two  heavy  glass  globes  A  and  B  united  by  the  tube  C, 
and  mounted  in  a  frame  D,  pivoted  in  the  center  to  the  wooden  stand  E,  so  that  they  may 
be  fixed  at  the  angle  shown,  or  inclined  in  the  reverse  position  bringing 
B  higher  than  A.    A  is  filled  with  lumps  of  fused  iron  sulflde  or  barium 
sulfide,  the  opening  Into  O  being  loosely  plugged  with  cotton.    Dilute 
muriatic  acid  is  poured  into  B  until  filled,  the  gas  evolved  passing 
out  at  F,  through  a  wash -bottle  containing  water,  to  the  solution  to  be 
precipitated ;  when  this  is  saturated,  the  frame  D  is  reversed,  the  acid 
running  back  into  B. 

2.  Kipp's,  Fig.  68,f  has  three  glass  globes,  the  upper  one  A,  terminat- 
ing in  the  tube  D,  fitted  to  B  by  a  ground  socket. 

B  holds  lumps  of  iron  sulfide,  and  has  a  tubulure 
near  the  top  from  which  passes  the  exit  tube.  A  is 
filled  with  dilute  acid,  and  when  the  stop -cock  0  is 
opened,  the  acid  rising  into  B  generates  the  gas. 
To  discontinue  the  flow,  C  is  closed  and  the  pres- 
sure of  the  gas  accumulating  in  B  forces  the  acid 
from  thence  back  into  E  and  A. 

3.  Schranche's,  Fig.  69.  J  A  long  glas^  jar  A  con- 
tains the  iron  sulfide,  and  the  bulb  B  dilute  hydro- 
chloric acid;  this  after  percolating  through  A  is 
so  far  weakened  and  impregnated  with  ferrous 
chloride  as  to  be  of  no  further  use,  and  is  allowed 
to  flow  away  by  0  into  a  waste-pipe. 

Other  gases  may  be  generated  in  the  above  de- 
scribed apparatus.  Substituting  zinc  for  iron  sul  ' 
fide  gives  hydrogen;  calcium  hypochlorite,  chlorine;  maganese  and 
barium  peroxides, oxygen ;  calcium  carbide  and  water,  acetylene; 
etc.  A  steady  current  of  hydrochloric  acid  gas  is  furnished  when 
the  strongest  commercial  acid  is  dropped  from  a  funnel  tube  into 
hot,  fairly  concentrated  sulfuric  acid;  and  of  ammonia  by  gently 
heating  the  strongest  liquor  ammoniae  and  passing  the  ammonia  gas 
through  a  desiccating  medium  to  absorb  aqueous  vapor. 

Where  an  aqueous  or  other  solution  Is  to  be  precipitated  by  a  gas  without  being  re- 
duced in  volume  through  the  solvent  being  carried  off  as  vapor,  the  gas  is  previously  sat- 
urated with  the  solvent  by  passing  through  a  wash-bottle  containing  it. 


Fig.  68. 


Fig.  69. 


Beakers  and  Erlenmyer  flasks  serve  well  for  precipitations  as  the  completion 
of  the  reaction  and  the  settling  of  the  precipitate  can  be  plainly  seen,  but  flasks 


*  Chem.  News,  1888-1—213  and  1893-2—52;  Journ.  Amer.  Chem.  Socy.  1898-344. 
t  Journ.  Amer.  Chem.  Socy.  1898—344. 
J  Idem,  1894—868  and  1897—818. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  73 

are  not  suited  to  those  whose  particles  become  firmly  attached  to  the  glass  and 
are  difficult  to  remove ;  this  objection  does  not  hold  when  the  precipitate  is  to 
be  redissolved  for  further  treatment.  On  account  of  the  solubility  of  glass  in 
free  alkalies,  their  carbonates  and  some  other  salts,  porcelain,  platinum  or 
silver  dishes  are  more  suitable  for  use  with  these  reagents. 

Precipitates  of  compounds  containing  silver  must  be  protected  from  actinic 
light  which  causes  decomposition  with  loss  of  weight.  If  precipitation  and 
filtration  are  done  by  gaslight  or  very  subdued  daylight,  and  the  precipitate 
allowed  to  settle  and  dry  after  filtration  In  a  dark  place,  the  decomposition 
will  be  only  superficial  and  may  be  neglected.  Beakers,  flasks  and  watch- 
glasses  of  amber  non- actinic  glass  are  on  the  market. 

A  solution  of  a  base  oxidized  by  the  air  is  manipulated  in  a  closed  flask  in  a  current  of 
some  non-oxidizing  gas  such  as  carbon  dioxide  or  hydrogen.  The  precipitant  is  intro- 
duced by  a  syphon  through  a  funnel  tube  with  a  stop-cock,  and  the  liquid  drawn  off  after 
the  precipitate  settles.  But  on  account  of  the  complexity  of  the  apparatus  some  other 
method  of  analysis  is  generally  chosen. 

Amount  of  precipitant.  A  slight  excess  is  always  added  to  insure  that  a 
sufficiency  has  been  used.  Frequently  this  tends  also  to  decrease  the  solu- 
bility of  the  precipitate  —  in  a  few  instances  a  large  excess  is  necessary  for  this 
purpose  or  to  produce  some  physical  or  chemical  change  in  the  precipitate. 
Yet  in  other  cases  any  excess  whatever  is  objectionable  and  must  be  removed 
before  filtration. 

A  common  error  of  beginners  is  that  of  using  a  great  excess.  This  is  to  be 
avoided  (unless  specially  directed),  as  it  augments  the  necessary  washing  of 
the  precipitate  and  all  subsequent  ones  to  an  undesirable  extent,  and  in  some 
cases  may  cause  decomposition  of  the  precipitate.  Ordinarily  some  indication 
is  given  at  the  point  when  sufficient  has  been  introduced,  such  as  the  cessation 
of  clouding  or  a  reversal  of  the  reaction ;  when  such  evidence  is  doubtful,  the 
amount  of  reagent  required  for  a  given  precipitation  can  be  calculated  from 
the  equation  narrating  the  reaction,  and  a  measured  volume  used. 

Solutions  too  dilute  for  convenient  handling,  or  when  the  compound  to  be 
thrown  down  is  not  highly  insoluble,  are  concentrated  by  evaporation  to  a 
small  bulk  with  precautions  to  prevent  loss  when  dealing  with  bodies  volatile 
at  steam  heat.  After  noting  that  the  liquid  has  the  reaction  directed  by  the 
method  followed,  it  is  heated  to  boiling  (unless  the  precipitation  is  to  take 
place  in  the  cold;  and  from  a  graduated  cylinder  is  run  in  as  much  of  the  solu- 
tion of  the  reagent  as  it  is  presumed  will  suffice.  An  approximation  to  the 
volume  needed  can  be  learned  by  a  calculation  from  the  combining  propor- 
tions. If  an  alkali  or  alkali  carbonate  is  the  precipitant,  a  strip  of  red  litmus 
paper  will  show  when  the  reaction  becomes  alkaline;  sometimes  a  visible 
alteration  in  the  liquid  or  precipitate  will  indicate  an  excess. 

As  a  rule  the  precipitant  is  best  added  slowly,  even  by  drops,  and  with  con- 
stant stirring,  for  the  reason  that  less  of  the  precipitant  may  be  occluded  or 
mechanically  inclosed  by  the  precipitate  than  if  added  at  once  in  full  amount; 
again  the  transformation  of  an  amorphous  to  a  crystalline  (more  easily  fil- 
tered) form  is  favored  by  contact  with  already  formed  crystals.  Exceptions  to 
the  rule  are  where  only  by  a  sudden  admixture  does  the  precipitate  form  in  a 
physical  condition  suitable  for  decantation  or  filtration. 

In  some  determinations  the  solution  to  be  precipitated  is  slowly  added  to 
the  precipitant,  as  here  the  latter  remains  in  excess  throughout  the  formation 
of  the  precipitate.  Certain  precipitates  are  only  to  be  obtained  of  a  definite 
composition  by  proceeding  in  this  way. 

After  mixing  well,  the  precipitate  is  allowed  to  subside  and  a  few  drops  of 


74  QUANTITATIVE    CHEMICAL    ANALYSIS. 

the  reagent  run  down  the  side  of  the  beaker  into  the  supernatant  liquid.  If  no 
cloud  is  formed,  the  whole  is  stirred  vigorously  for  a  few  minutes  and  digested 
for  the  prescribed  time  before  filtering.  When  a  precipitate  is  slow  to  settle, 
the  addition  of  a  few  drops  of  chloroform  and  boiling,  or  intermixing  a  heavy 
powder  like  barium  sulfate  may  hasten  its  deposition;  minute  amounts  of  a 
precipitate  too  finely  divided  to  be  caught  by  a  filter  may  be  entangled  by  stir- 
ring in  some  insoluble  body  of  a  flocculent  or  gelatinous  nature. 

After  allowing  the  liquid  to  stand,  in  the  cold  or  heated  as  directed,  until  the 
precipitate  has  subsided,  the  liquid  is  ready  for  the  process  of  filtration. 

SEPARATION. 

When  two  or  more  elements  or  compounds  are  to  be  determined  in  a  sub- 
stance the  usual  procedure  is  to  part  them  by  a  suitable  process  and  determiae 
each  individually.  Every  method  of  separation  should  comply  with  two  con- 
ditions, namely,  that  the  separation  be  complete  within  reasonable  limits, 
and  that  the  reagents  introduced  into  the  solution  shall  not  interfere  with 
further  operations,  or  if  so,  can  easily  be  removed.  It  is  also  an  advantage  if 
the  separated  compound  is  in  the  solid  form  in  a  combination  suitable  for 
weighing. 

Methods  of  separation  based  on  the  following  principles  are  employed  as 
conditions  indicate.  Some  are  entirely  physical  in  nature,  while  others  are 
preceded  by  a  chemical  reaction.  Unlike  qualitative  analysis,  the  methods  as  a 
rule  separate  not  one  group  from  another  but  each  constituent  in  turn. 

In  some  instances,  as  where  a  substance  is  made  up  of  several  analogues 
they  may  be  separated  by  the  application  of  one  principle  only;  as  the  organic 
constituents  of  the  mineral  asphaltum  dissolved  out  successively  by  ether, 
benzene  and  carbon  disulflde,  or  the  hydrocarbons  of  crude  petroleum  isolated 
by  fractional  distillation.  Usually,  however,  more  than  one  must  be  resorted 
to,  especially  for  complex  mixtures. 

1.  Mechanically.  Alight  powder  may  be  floated  from  a  heavier  by  a  liquid  of 
intermediate  density,  as  gypsum  from  barite  by  a  solution  of  mercuric  potas- 
sium iodide,  their  respective  specific  gravities  being  2.2,  4.5,  and  3.0.    Silt, 
dust,  clay  and  sand  of  different  degrees  of  fineness  and  gravity  are  washed  one 
from  another  by  a  stream  of  water  in  a  special  elutriating  apparatus.    From 
flour,  cold  water  washes  the  light  pulverulent  granules  of  starch  out  of  the 
cohesive  gluten. 

Two  immiscible  liquids  of  different  specific  gravities  can  generally  be  made 
to  stratify  so  that  one  may  be  drawn  off  or  filtered  from  the  other;  as  ether 
after  collecting  an  oil  emulsified  with  water.  The  bibulous  capacity  of  a 
porous  solid  (bone-ash)  is  employed  in  the  cupellation  of  lead  alloys.  Par- 
ticles of  iron,  nickel  and  magnetic  minerals  may  be  drawn  from  a  powder  by  a 
magnet  or  electro-magnet;  and  the  attraction  or  repulsion  of  electrified  par- 
ticles by  static  electricity  has  been  proposed  for  their  separation. 

From  a  solution  a  solid  may  withdraw  by  adsorption  certain  forms  of  dis- 
solved matter  nearly  or  quite  completely.  The  well-known  decolorizing  power 
of  bone-black  and  Fullers  earth  are  examples;  gelatin,  amorphous  silica, 
ferric  oxide,  alumina,  magnesium  carbonate,  and  vegetable  fibre  fix  certain 
organic  bodies,  and  for  an  assay  of  an  impure  dyestuff  the  first  step  may  be  the 
extraction  of  the  dye  from  its  solution  by  raw  silk.* 

Various  other  mechanical  processes  are  employed  for  special  material. 

2.  Solubility.  The  bodies  to  be  separated,  solid,  liquid,  or  gaseous,  may  be 


*  (hem.  News,  1894-1-33  and  43. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  75 

such  that  the  application  of  a  solvent  will  extract  one  from  the  others  directly. 
Thus,  nitre  is  lixiviated  from  gunpowder  by  water  ;  silk  from  wool  by  warm 
hydrochloric  acid;  .strychnia  extracted  from  nux  vomica  by  chloroform;  the 
acid  radical  of  xanthin  sulfate  washed  from  the  xanthin  by  water;  cinchonine, 
cinchonidine,  etc.,  from  quinine  by  washing  with  a  saturated  solution  of  qui- 
nine; copper  from  a  finely  powdered  alloy  of  copper  and  silver  by  a  solution  of 
zinc  chloride;  an  oil  collected  from  an  emulsion  with  water  by  ether;  or  ethy- 
lene  absorbed  by  bromine  from  a  mixture  of  gases. 

Before  applying  the  solvent  some  preliminary  operation  on  the  mixture  may 
be  necessary  to  render  one  or  more  of  the  constituents  soluble  or  insoluble  as 
the  case  may  be,  and  may  be  either  mechanical  or  chemical.  Change  in  va- 
lence, reduction  or  oxidation,  hydrolysis,  saponiflcation,  polymerization,  etc., 
are  the  most  usual  chemical  alterations.  A  semi-organic  compound  may  be 
decomposed  by  an  inorganic  acid  or  base  and  the  liberated  organic  radical 
extracted  by  a  suitable  solvent  from  the  solution;  an  example,  where  both  the 
radicals  are  insoluble  in  water,  is  that  of  napthol-yellow,  the  calcium  salt  of 
dinitronapthol;  on  treatment  with  sulfuric  acid  there  precipitate  calcium  sul- 
fate and  dinitronapthol,  the  latter  soluble  in  ether. 

Some  metallic  sulfldes  are  decomposed  on  treatment  with  a  reagent  evolving 
nascent  hydrogen,  the  constituents  separating.  Thus,  bornite  in  contact  with 
zinc  and  hydrochloric  acid  is  decomposed  to  a  residue  of  metallic  copper,  a  solu- 
tion of  ferrous  chloride,  and  gaseous  hydrogen  sulfide  (CusFeSs  +  2HC1  -f- 
2H2  =  3Cu  +  FeCJ2  +  3H2S) . 

An  extension  of  the  principle  of  separation  by  a  solvent  is  in  the  treatment  of  a 
mixture  of  bodies  by  a  succession  of  different  solvents  each  removing  one  con- 
stituent, and  it  is  often  possible  to  resolve  a  complex  mixture  into  groups  by 
this  process,  each  group  to  be  further  subdivided  by  other  methods.  Thus,  in 
the  analysis  of  a  detonator,  mercury  fulminate  is  extracted  from  antimony 
sulfide  and  potassium  chlorate  by  acetone  saturated  with  ammonia,  then  potas- 
sium chlorate  from  antimony  sulfide  by  water;  in  plant  analysis,  solvents  are 
applied  in  about  the  following  order:  chloroform,  methylated  spirit,  cold  water, 
dilute  sulfuric  acid,  dilute  sodium  hydrate,  bromine  with  ammonia,  and  special 
solvents  like  cupric  oxide  in  ammonia,  etc. 

A  mixture  of  several  analogous  bodies  may  be  roughly  divided  by  fractional 
solution —  treating  the  powder  with  successive  small  portions  of  one  solvent. 
The  most  easily  soluble  of  the  constituents  is  withdrawn  by  the  first  portion, 
the  others  in  the  order  of  their  solubilities.  The  operation  is  best  performed 
in  a  small  percolator,  allowing  each  portion  of  the  solvent  to  digest  with  the 
powder  for  some  time  before  tapping  out.  It  will  readily  be  seen  that  neither 
the  principle  nor  practice  of  this  method  is  compatible  with  more  than  crude 
results,  except  in  the  case  of  binary  mixtures  whose  composition  is  approxi- 
mately known  and  for  which  a  correction  for  solubility  can  be  predetermined 
and  applied.  For  although  a  constituent  be  practically  insoluble  in  the  pure 
solvent  it  may  dissolve  much  more  freely  in  the  solution  of  a  more  soluble 
constituent  especially  when  concentrated,  and  the  process  does  not  admit  of 
the  usual  precaution  of  employing  an  adequate  amount  of  solvent  to  insure  a 
dilute  solution. 

Extraction  of  the  several  constituents  of  a  mixture  of  analogous  bodies  by  means  of  a 
solvent  applied  at  different  temperatures  has  been  proposed  for  the  separation  of  the 
various  tannins  from  tan -wares,  etc. 

In  a  mixture  of  two  salts  both  soluble  In  water  or  other  liquid,  one  may  be  Insoluble 
in  a  concentrated  solution  of  the  other,  and  In  separating  them  by  lixlviation  the  volume 
of  the  solvent  is  kept  as  small  as  possible.  If  one  of  the  salts  is  deliquescent,  the  mixture 
can  be  placed  in  a  paper  filter  and  exposed  for  some  days  to  a  damp  atmosphere,  when 


76  QUANTITATIVE    CHEMICAL    ANALYSIS. 

the  deliquescent  salt  will  have  liquefied  and  passed  through  the  paper;  thus  caesium 
tartrate  from  rubidium  tartrate. 

An  approximate  determination  of  the  proportions  of  two  bodies  admixed, 
the  one  A  much  more  soluble  in  a  given  liquid  than  the  other  B,  may  be  made 
in  the  following  way :  the  mixture  is  treated  with  a  measured  volume  of  the 
solvent,  the  solution  decanted,  evaporated  to  dryness,  and  the  residue 
weighed;  from  the  weight  is  deducted  the  amount  of  B  soluble  in  the  volume 
of  solvent  used.  Thus  if  one  gram  of  a  mixture  of  phenacetin  and  acetanilid 
be  stirred  in  200  cubic  centimeters  of  cold  water,  all  the  acetanilid  goes  into 
solution  together  with  .130  gram  of  phenacetin. 

A  variation  of  the  above  is  as  follows:  after  weighing  the  residue  from  evaporation 
the  original  residue  is  again  treated  as  before,  and  this  process  continued  until  successive 
weights  of  the  residues  show  by  their  again  becoming  equal  that  the  residue  from  the  last 
solution  consists  entirely  of  B.  If  the  solubility- coefficient  of  B  in  the  solvent  is  exactly 
known,  the  calculation  is  simple:  let  r,r',r"  ....  be  the  weights  of  the  residues;  v,  v,f 
v" ,  the  volumes  of  solvent;  and  s,  the  volume  of  solvent  dissolving  one  gram  of  B; 

then  the  weight  of  the  constituent  A  is  r  -\-r'  +r"  +..  — -  -^ —  — :.  The  for- 
mula assumes  that  the  rate  of  solubility  of  B  is  independent  of  the  presence  of  A  in  the 
solution  and  vice  versa;  if  this  is  not  the  case,  a  correction  must  be  found  by  experiment 
and  applied. 

Another  method,  where  the  proportions  of  the  constituents  and  their  solubilities  are 
approximately  known,  is  to  treat  the  weighed  mixture  with  a  volume  of  solvent  sufficient 
to  dissolve  all  of  A  but  not  all  of  B.  The  solution  is  decanted  and  evaporated  and  the 
residue  weighed ;  this  residue  is  then  treated  with  a  somewhat  smaller  volume  of  solvent 
though  still  sufficient  to  dissolve  all  of  A;  the  solution  decanted,  evaporated,  and  the 
residue  weighed.  The  operation  may  be  again  repeated. 

Let  v  and  v'  be  the  respective  volumes  of  the  solvent;  and  r  and  r'  the  weights  of  the 
residues.  Then  r  —  r'  is  the  weight  of  B  soluble  in  v  —  v';  and  the  weight  of  B  dissolving 

r  —  r' 
in  one  cubic  centimeter  of  a  solution  of  A  is :• 


Also      v  —  v'     ls  tne  wel8ht  of  B  in  r-    Hence  the  weight  of  A  in  the  first  extraction  is 

v(r-r') 
r~     r-^7— 

Certain  bodies  dissolve  in  a  mixture  of  two  liquids  to  an  extent  governed  by 
their  proportions  in  the  mixture.  At  times  this  property  affords  an  easy  way 
of  determining  approximately  the  purity  of  a  commercial  article.  The  test 
may  be  applied  by  dissolving  the  substance  in  a  measured  volume  of  one  of  the 
liquids  and  slowly  adding  measured  volumes  of  the  other  until  permanent 
opalescence  or  turbidity  appears.  An  example  is  the  dissolving  of  an  essential 
oil  in  absolute  alcohol  and  adding  water  to  turbidity. 

The  process  of  electro-dissolution  is  employed  in  a  few  cases,  as  in  the  analysis  of  an 
impure  commercial  metal.  On  dissolving  in  hydrochloric  acid  certain  constituents  are 
exposed  to  the  action  of  the  nascent  hydrogen,  evolved  by  the  solution  of  the  metal  in  the 
acid,  and  enter  into  combination  with  it  and  pass  off  as  gases.  But  if  the  metal  be  made 
the  anode  and  a  sheet  of  platinum  the  cathode  of  an  electric  circuit  by  connection  with  a 
battery  of  suitable  strength,  no  free  hydrogen  appears,  an  1  these  constituents  remain 
suspended  in  the  solution  in  the  free  state  or  combine  with  the  acid  and  enter  the  solu- 
tion.* 

In  all  cases,  before  applying  a  solvent  for  the  extraction  of  a  constituent  of 
a  complex  mixture,  it  must  be  learned  whether  any  of  the  following  effects 
supervene  so  far  as  to  impair  the  separation:  (1),  a  chemical  action  between 
the  solvent  and  constituent  to  be  extracted;  (2),  a  solvent  action  of  the  solu- 
tion of  the  soluble  constituent  on  one  or  more  of  the  insoluble  constituents; 


*  Chem    News,  1887-1—62. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  77 

(3),  a  chemical  reaction  between  the  solution  of  the  soluble  constituent  and 
one  of  those  that  are  insoluble;  (4),  a  reaction  between  the  various  insoluble 
constituents  brought  about  by  contact  with  the  solvent;  and  (5),  action  of  the 
oxygen  or  carbon  dioxide  of  the  air,  common  impurities  of  the  solvent,  etc. 


Extraction.  When  a  solid  is  brought  in  contact  with  two  repellant  liquids, 
being  fairly  soluble  in  each,  it  may  dissolve  in  either  according  to  circum- 
stances; but  immediately  a  translation  begins,  terminated  when  it  is  divided 
between  the  liquids  in  a  ratio  determined  by  their  volumes  and  a  specific 
coefficient. 

"  When*  the  molecular  complexity  of  the  dissolved  substance  is  the  same  in 
both  solvents  it  so  distributes  itself  between  them  that  at  any  given  tempera- 
ture there  is  a  definite  ratio  between  the  concentration  of  the  two  solutions 
when  equilibrium  is  attained,  the  ratio  beinz  independent  of  the  amounts  of 
substance  and  solvents  originally  taken.  Thus  iodine  when  shaken  up  with 
carbon  tetrachloride  and  water  is  shared  by  these  solvents  in  such  a  way  that 
the  concentration  of  the  aqueous  solution  produced  is  to  that  of  the  carbon 
tetrachloride  solution  as  1  to  85,  that  being  approximately  the  ratio  of  the 
solubilities  of  iodine  in  the  two  solvents  at  25°  ,  the  temperature  of  the  experi- 
ment. When  the  molecular  complexity  of  the  dissolved  substance  is  not  iden- 
tical in  the  two  solvents,  there  is  no  constant  ratio  of  distribution,  the  ratio  of 
the  concentrations  varying  with  the  original  quantities  present.  There  is, 
however,  in  such  cases  a  somewhat  more  complex  function  which  takes  the 
place  of  the  simple  distribution  ratio.  If  the  molecular  weight  of  the  sub- 
stance in  one  solvent  is  n  times  as  great  as  its  molecular  weight  in  the  other 
solvent,  then  when  equilibrium  is  attained,  the  nth  root  of  the  concentration 
in  the  first  solvent  will  bear  a  constant  ratio  to  the  concentration  in  the  second 
solvent.'* 

Thus,  if  butter-fat  be  agitated  with  a  mixture  of  alcohol  and  carbon  disulflde  and  the 
liquids  allowed  to  separate,  the  lower  stratum  of  carbon  disulfide  will  contain  the  fat,  and 
the  upper  alcoholic  stratum  the  artificial  coloring  matter,  water,  salt,  etc. 

The  principle  is  applied  much  more  frequently  for  the  extraction  of  a  solid  or 
liquid  already  in  solution  than  for  the  direct  solution  of  a  solid  or  liquid  in  the 
mixture  of  two  liquids. 

For  example,  let  it  be  required  to  recover  an  alkaloid  from  its  solution  in 
water.  The  solution  is  sh  iken  up  with  a  moderate  quantity  of  benzin,  then 
allowed  to  stand  until  the  liquids  have  separated.  The  benzin  is  decanted  and 
the  *  shaking  out1  repeated  with  fresh  benzin.  After  a  few  repetitions  prac- 
tically all  of  the  alkaloid  will  have  passed  into  the  several  volumes  of  beuzin, 
the  greater  part  into  the  first  portion,  and  the  least  in  the  last. 

For  separating  an  alkaloid  from  other  bases  a  peculiar  property  is  availed, 
namely  that  many  free  alkaloids  are  comparatively  insoluble  in  water  but  freely 
soluble  in  ether,  benzene  or  chloroform ;  while  on  the  other  hand,  their  salts  are 
insoluble  in  these  liquids,  but  soluble  in  water.  So  if  an  impure  aqueous  solution 
be  acidified  and  mixed  with  chloroform  and  the  liquids  separated,  the  chloro- 
formic  solution  contains  such  of  the  impurities  as  dissolve  in  it,  while  the  salt 
of  the  alkaloid  is  retained  in  the  aqueous  solution.  The  latter  is  made  alkaline 
and  again  shaken  with  chloroform;  the  alkaloid,  now  in  the  free  state,  is  taken 
up,  leaving  the  remainder  of  the  impurities  in  the  aqueous  solution.  Usually 


*  Journ.  Chem.  Socy.  1896—1334. 


78 


QUANTITATIVE   CHEMICAL   ANALYSIS, 


these  separations  have  to  be  repeated  to  isolate  the  alkaloid  in  a  fairly  pure 
state  and  nearly  completely. 

By  extraction  with  suitable  solvents  applied  to  acid,, neutral,  and  alkaline  solutions, 
the  various  organic  constituents  of  a  mixture  may  be  successively  removed.  Thus,  Huse- 
mann  divides  the  ptomaines  into  five  groups:  (1),  extracted  from  an  acid  solution  by  ether; 
(2), from  an  alkaline  solution  by  ether,  (3)  from  an  alkaline  solution  by  chloroform;  (4), 
from  an  alkaline  solution  by  amyl  alcohol ;  (5)»  not  extracted  by  any  of  these  solvents,  but 
sol  able  in  water. 

Up  to  the  present  time,  the  application  of  this  principle  has  been  mainly  re- 
stricted to  organic  bodies.  A  few  inorganic  compounds  in  aqueous  solution 
may  be  separated  by  organic  solvents,  such  as  the  sulfocyanides  of  cobalt  and 
nickel  by  a  mixture  of  ether  and  amyl  alcohol,  and  ferric  from  manganous 
nitrate  by  ether.  But  other  and  more  convenient  and  less  costly  methods  are 
generally  at  hand. 

In  applying  a  succession  of  organic  solvents,  regard  must  be  had  in  exact 
analyses  to  their  solubilities  in  water  or  aqueous  solutions.  While  few  dis- 
solve to  any  great  extent,  yet  enough  of  one  may  enter  the  solution  to  modify 
the  solvent  action  of  the  succeeding  one,  so  that  it  is  advisable  to  remove  the 
former  as  completely  as  may  be  before  applying  the  latter.  By  beginning  the 
sequence  with  gasoline  which  is  practically  insoluble  in  water,  what  dissolves 
of  the  next  solvent,  such  as  ether  or  chloroform,  may  be  removed  from  the 
solution  by  shaking  out  with  gasoline ;  the  latter  after  separation  should  be 
entirely  volatile,  but  if  there  be  any  residue  on  evaporation,  it  is  returned  to 
the  aqueous  solution. 

When  a  liquid  a  passes  from  one  solvent  6  to  another  c  the  volume  of  c  becomes  approxi- 
mately c  +  a,  while  that  of  6  becomes  6  — a.  An  application  of  this  fact  is  for  the  deter- 
mination of  small  amounts  of  amyl  alcohol  in  commercial  spirits. 
A  certain  volume  of  the  spirit,  brought  to  a  definite  gravity,  i» 
shaken  up  with  an  exactly  measured  volume  of  chloroform.  When 
the  chloroform  has  separated,  the  increase  represents  the  volume  of 
amyl  alcohol.  The  process  is  only  approximate  at  best,  the  chief 
error  being  the  contraction  of  a  +  c,  and  requires  strict  attention  to- 
details  of  manipulation. 

An  extraction  is  usually  made  in  a 
separatory  funnel,  Fig.  70,  of  a  size 
adapted  to  the  mixture.  After  stoppering 
the  funnel  the  two  liquids  are  mixed  by 
shaking  (less  or  more  violently  according 
to  the  tendency  of  the  liquids  to  .unite  to 
an  emulsion).  After  standing  until  the 
liquids  have  separated,  the  stopper  is 
removed  and  the  heavier  drawn  out 
through  the  tap. 

An  apparatus  designed  for  the  extrac- 
tion of  an  aqueous  solution  or  an  emul- 
sion with  water  is  shown  in  Fig.  71- 
The  emulsion  is  poured  into  the  funnel 


Fig.  70. 


A  partly  filling  B  and  C,  and  the  ether  or  other  light  solvent 
Into  the  flask  1).  After  capping  A  with  a  condenser  E,  the 
flask  is  heatrd  and  the  vapor  rising  through  Fand  G  into  E, 
is  there  condensed,  and  dropping  into  C  displaces  the  emul- 
sion. From  C  it  ascends  in  drops  through  B  dissolving  out 
the  oil,  and  when  B  is  filled,  the  solution  finally  runs  over 
through  F  into  D. 


Fig.  71. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


79 


Fig.  72. 


The  above  will  not  answer  for  a  solvent  heavier  than  water,  like  chloroform 
or  carbon  disulflde,  and  for  these,  Schiebler's  apparatus  is  arranged  as  shown 

in  Fig.  72.  The  stem  of  the  funnel  A  is  bent  to 
enter  the  flask  C,  while  the  neck  of  A  holds  a  con- 
denser B  and  a  tube  D  passing  to  C.  The  emul- 
sion is  contained  in  A  and  the  solvent  in  C,  and  as 
the  vapors  are  condensed  in  B  the  resulting  liquid 
falls  through  the  emulsion  and  returns  through 
the  stem  to  C. 

Many  bodies  are  more  easily  extractable  if  the 
solution  be  saturated  with  some  inorganic  salt, 
such  as  sodium  chloride,  insoluble  in  the  inorganic 
solvent.    According  to   Ostwald,  only  the   non- 
ionized  portion  of  a  body  passes  from  one  solvent 
to  another,  and  the  original  solution  should  be  as 
concentrated  as  circumstances  will  permit  that  the 
minimum  of    ionization    be   attained,  and   when 
dealing  with  an  acid  of  moderate  strength  there 
ehould  be  added  a  stronger  (mineral)  acid,  and 
with  a  moderately  strong  base,  one  of  the  alkalies. 
The  rate  of  distribution  for  non -ionized  solids  or  liquids  between  water  and 
an  immiscible  organic  solvent  (e.  g.,  for  an  aqueous  solution  agitated  with 
benzin),  may  be  expressed  as 

Weight  passing  into  benzin  .  weight  remaining  in  water  .  .  i  .  g 

Volume  of  benzin  volume  of  water 

K  being  the  distribution  coefficient.    This  may  be  written 
Volume  of  benzin  :  KX  volume  of  water  :   :  weight  in  benzin  :  weight  in 
water. 

Since  IT  and  the  volume  of  water  are  constant,  the  ratio  of  the  amount  of  the 
solid  entering  the  benzin  to  that  remaining  in  the  water  increases  directly  with 
the  volume  of  benzin.  It  is  evident  that  a  single  treatment  will  extract  only 
so  much  of  the  solid  as  corresponds  to  the  respective  rates  of  solubility  in 
water  and  the  organic  solvent,  and  this  ratio  determines  the  number  of  times 
the  agitation  must  be  repeated  with  fresh  portions  of  the  solvent  to  ultimately 
arrive  at  a  reasonably  complete  separation— traces  (or  more)  of  the  solid  will 
inevitably  be  retained  in  the  original  solution. 

II  S  represents  the  original  weight  of  the  solid  in  the  volume  Vot  the  aqueous  solu- 
tion ;  V,  the  volume  of  the  organic  solvent;  K,  the  distribution  coefficient;  S',  the  weight 
of  the  solid  remaining  In  the  aqueous  solution,  and  S  —  S',  the  weight  passing  Into  the 
organic  solvent;  then  for  the  first  extraction, 

SV  KVS 

V  :  KV  :  :  S  —  S'  :  S',  whence  S  —  S'  = 


V  +  KV 


and  8'  = 


V  +  KV 


For  the  second  extraction  with  a  (usually  smaller)  volume  V"  of  the  organic  solvent 
the  volume  of  aqueous  solution  being  practically  the  same  as  before, 

S'V»  KVS' 


S  —  S' 


and  S"  = 


and  so  on. 

If  the  organic  solvent  is  soluble  to  any  extent  in  the  aqueous  solution,  the  solvent  power 
of  the  latter  saturated  with  the  former  must  be  considered.  Ethyl  ether  is  the  only  sol- 
vent in  common  use  that  mixes  with  water  to  a  marked  degree. 

Mixed  gases  may  be  passed  through  a  solution  of  a  reagent  combining  with 
and  retaining  one  or  more  of  the  constituents,  while  the  others  pass  through 
unabsorbed.  Thus,  when  commercial  zinc  is  dissolved  in  a  non-oxidizing  acid, 
any  arsenic  contained  is  converted  into  arsine  and  is  carried  off  with  the 


80  QUANTITATIVE    CHEMICAL    ANALYSIS. 

hydrogen  evolved  by  the  solution  of  the  metal ;  on  conducting  the  mixed  gases 
into  a  solution  of  a  silver  salt  the  arsenic  is  retained  in  the  solution  as  arseni- 
ous  oxide,  through  the  decomposition  of  an  equivalent  of  the  silver  salt. 

Several  of  the  constituent  gases  of  a  mixture  may  be  absorbed  seriatim  in  a 
train  of  absorption  tubes,  each  filled  with  the  special  reagent  to  retain  one  of 
the  members  (page  146). 

3.  Heat.  On  heating  a  solid  to  the  required  temperature  any  volatile  con- 
stituent is  vaporized;  it  may  be  condensed  on  a  cold  surface,  as  iodine 
sublimed  from  earthly  impurities,  or  absorbed  in  a  liquid  or  porous  solid  and 
its  weight  found  by  the  increase ;  or  the  absorbent  is  further  treated  to  obtain 
the  constituent  in  a  form  suitable  for  weighing.  Thus  on  ignition  of  zinc  car- 
bonate the  carbon  dioxide  may  be  passed  through  a  solution  of  barium  hydrate 
contained  in  a  suitable  vessel,  the  increase  in  weight  of  the  latter  equaling  that 
of  the  carbon  dioxide ;  or  the  precipitated  barium  carbonate  may  be  filtered  off 
and  weighed,  and  the  weight  of  the  carbon  dioxide  calculated  therefrom. 

More  usually  the  volatile  element  or  compound  is  allowed  to  escape,  its 
weight  equaling  that  lost  by  the  substance;  thus,  silver  is  volatilized  from 
gold  by  intensely  heating  their  alloy.  Combined  water  and  unimportant 
amounts  of  organic  matter  In  minerals  and  ores  are  often  determined  in  this 
way,  the  results  set  down  as  *f  loss  on  ignition  "  or  "  volatile  at  a  red  heat," 
it  having  been  ascertained  that  the  residue  is  of  such  a  composition  as  not  to 
be  altered  in  weight  by  oxidation,  reduction,  or  through  inter- reactions  initiated 
at  a  high  temperature. 

The  metals  of  certain  alloys  can  be  separated  by  heating  the  powder  in  a 
current  of  some  gas  which  combines  with  one  metal  to  form  a  volatile  com- 
pound while  the  other  metals  are  either  unaffected  or  the  products  are  not 
volatile  at  the  maximum  temperature  employed.  Dry  chlorine  and  gaseous 
hydrochloric  acids  are  the  usual  reagents  for  the  purpose,  as  many  metallic 
chlorides  are  vaporable  at  or  below  a  red  heat.  Many  alloys  that  are  insoluble 
in  all  acids  are  readily  decomposed  by  ignition  in  chlorine,  the  volatile  chlo- 
rides passing  over,  and  the  fixed  chlorides  remaining  in  a  crystalline  form; 
metallic  oxides  are  previously  reduced  to  the  metallic  state  by  heating  in  a 
current  of  hydrogen  or  formic  acid  vapor.  Native  sul  fides  can  be  opened  up 
by  heating  in  dry  air  loaded  with  the  vapors  of  bromine,*  and  some  alloys  con- 
verted to  volatile  or  fixed  sulfldes  by  ignition  in  the  vapor  of  sulfur. f 

The  vapor  of  a  halogen  traversing  a  compound  of  a  metal  with  a  weaker 
halogen  displaces  the  latter;  as  when  silver  bromide  is  heated  in  chlorine,  the 
bromine  is  displaced  and  may  be  allowed  to  escape  and  its  weight  calculated 
from  the  decrease  in  weight  of  the  residue  through  the  conversion  of  silver 
bromide  (molecular  weight  187.87)  to  silver  chloride  (molecular  weight  143.37); 
or  tne  bromine  may  be  received  in  an  absorbent  and  determined  as  usual.  The 
operation  is  conducted  in  a  combustion  tube,  the  bromide  contained  in  a  porce- 
lain boat. 

After  the  evaporation  of  a  solution  to  dryness,  as  a  rule  the  residue  may  be 
redissolved  by  the  same  menstruum  as  formerly  held  it,  but  some  compounds 
are  so  altered  or  decomposed  during  this  operation,  or  the  residue  by  its  expo- 
sure to  the  temperature  of  100  o ,  as  to  remain  insoluble,  and  the  other  soluble 
constituents  be  separable  from  it  by  lixiviation.  Familiar  examples  of  this 
transformation  are  silicic  and  tungstic  acids.  Where  practicable,  the  above 
separation  is  usually  the  first  step  in  an  analysis  of  a  complex  mixture. 

One  of  a  mixture  of  several  bodies  may  be  decomposed  at  a  temperature 


*  Them.  News,  1890— 1— 114. 

f  Idem,  189^-1— -28;  Journ.  Amer.  Chem.  Socy.  1898—797. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


81 


above  100°  becoming  soluble  or  insoluble  as  the  case  may  be.  Thus  if  a  solu- 
tion of  the  chromates  of  cerium  and  lanthanum  be  evaporated  and  the  residue 
heated  to  110°,  the  cerium  chromate  is  decomposed  into  insoluble  cerium 
oxide  and  soluble  chromic  acid,  while  the  lanthanum  chromate  is  unaffected 
and  may  be  lixiviated  from  the  cerium  oxide  by  water ;  on  moderately  heating 
a  powdered  mixture  of  lanthanum  nitrate  and  didymium  nitrate,  only  the  latter 
is  converted  into  the  insoluble  subnitrate;  etc. 

One  of  the  constituents  of  a  mixture  may  combine  with  a  given  reagent  only 
at  a  temperature  much  higher  than  will  the  other  constituents;  or  on  raising 
the  temperature  of  a  solution,  one  of  the  dissolved  salts  may  decompose  and 
precipitate  at  a  point  much  lower  than  the  others.  Thus,  a  solution  of  copper 
and  silver  nitrates  mixed  with  magnesium  carbonate  and  heated  to  105  °  - 
122®  Fahr.,  the  copper  precipitates  at  once,  the  silver  only  after  standing  for 
some  time. 

Distillation.  The  separation  of  a  liquid  from  a  non-volatile  substance,  dis- 
solved or  diffused,  is  accomplished  by  simple  evaporation,  OP  by  distillation  if 
the  liquid  is  to  be  preserved.  A  mixture  of  two  immiscible  liquids  may  be  dis- 
tilled and  separated  in  the  distillate  by  decantation^  offering,  for  example,  a 
means  of  resolving  an  emulsion  obstinately  resisting  simpler  methods  of  sepa- 
ration. And  with  a  homogeneous  mixture  of  volatile  liquids  a  more  or  less 
complete  separation  may  be  had  by  distilling  —  a  familiar  example  is  the  part- 
ing of  ethyl  alcohol  (boiling  at  78.4  ®)  from  water  in  the  analysis  of  wine  and 
spirits.  It  is  of  course  essential  that  the  boiling  points  of  the  members  differ 
to  a  considerable  decree  in  order  to  separate  them  in  this  way  —  the  distillate 
of  alcohol  in  the  above  example  is  always  accompanied  by  water,  though  the 
proportion  may  be  lessened,  up  to  a  certain  point,  by  redistillation. 

The  coefficient  of  solubility  of  a  gas   in  a  liquid  varies  inversely  with  the 
temperature  of  the  latter,  and  if  the  solution  be  raised  to  the  boiling  point  and 
kept  in  ebullition  for  a  time,  the  gas  will  be  expelled 
more  or  less  completely  and  may  be  collected  in  a  gaso- 
meter or  absorbed  by  a  solution  of  some^reagent. 

An  apparatus  for  the  purpose  is  •shown  in  Fig.  73. 
The  solution  of  the  gas  is  placed  in  the  flask  A  which  is 
connected  to  the  flask  B  containing  a  solution  of  the 
absorbent.  A  guard-tube  C  is  filled  with  broken  glass 
saturated  with  the  absorbing  solution.  To  assist  in  the 
evolution  of  the  gas,  a  current  of  air  or  another  gas 
whose  admixture  with  the  original  is  not  objectionable, 
is  drawn  through  the  liquid  in  A  entering  at  the  funnel 
tube.  Or  a  gas  may  be  generated  within  the  solution 
by  the  introduction  of  some  reagent.  Thus,  a  metallic 
sulflde  in  A  may  be  decomposed  by  hydrochloric  acid 
and  the  liberated  hydrogen  sulflde  bubbled  through  a 
solution  of  silver  nitrate  in  B,  precipitating  argentic  sulflde  to  be  further  dealt 
with.  If  when  the  reaction  is  over,  a  solution  of  sodium  bicarbonate  is  poured 
into  A  it  will  be  dissolved  by  the  excess  of  acid  and  generate  a  large  volume 
of  carbon  dioxide  gas,  which  passing  out  through  B,  carries  with  it  the  hydro- 
gen sulflde  that  remains  dissolved  in  the  solution  in  A  or  fills  the  space  in  the 
flask  above  the  liquid. 

A  method  that  is  occasionally  of  service  is  that  of  distilling  a  complex  mixture  with 
some  reagent  that  will  mechanically  or  by  chemical  action,  retain  one  or  more  volatile 
constituents  in  the  still;  thus  In  distilling  a  mineral  oil  with  sodium  bicarbonate,  the 
sulfur  is  fixed  by  the  alkali. 


Fig.  73. 


82  QUANTITATIVE    CHEMICAL   ANALYSIS. 

Fractional  distillation  (page  65)  serves  to  approximately  separate  a  number 
of  allied  volatile  organic  bodies  from  each  other.  A  modification  applied  to 
organic  acids  is  known  as  '  fractional  saturation,'  in  which  one  or  more  of  the 
members  is  united  with  a  base  to  form  a  non- volatile  compound  from  which 
the  other  acids  are  distilled.  The  acid  with  the  greatest  percentage  of  carbon 
in  the  molecule  and  the  highest  melting  point  unites  with  the  base  in  prefer- 
ence to  an  acid  of  lower  carbon  and  melting  point;  an  exception  is  acetic  acid. 
The  process  depends  on  the  relative  affinity  for  the  acids  to  the  base,  both  in 
the  cold  and  at  the  temperature  of  distillation. 

For  example,  both  butyric  and  valeric  acids  readily  pass  over  when  their  solution  is 
distilled,  while  their  alkali  salts  are  not  volatile  at  moderate  temperatures.  For  separat- 
ing them,  to  the  aqueous  solution  of  the  mixed  acids  is  added  so  much  sodium  hydrate  as 
will  surely  be  sufficient  to  combine  with  all  the  butyric  acid,  yet  leave  part  of  the  valeric 
acid  unneutralized.  On  distillation  pure  valeric  acid  passes  over  into  the  receiver.  The 
residue  of  sodium  bntyrate  and  valerate  in  the  still  is  treated  with  a  little  sulfuric  acid  to 
liberate  more  of  the  valeric  acid,  and  the  liquid  again  distilled.  The  acidification  and  dis- 
tillation are  repeated  until  butyric  acid  begins  to  come  over,  showing  that  all  the  valeric 
acid  is  in  the  receiver. 

A  similar  plan  may  be  adopted  with  a  mixture  of  propionic,  butyric,  caproic,  and  cap- 
rylic  acids.  Practically,  however,  the  separation  is  never  as  sharp  as  could  be  desired. 

4.  Precipitation.  Where  a  choice  of  methods  is  allowed,  that  of  precipita- 
tion is  generally  given  the  preference,  for  the  reasons  that  the  separation  is, 
as  a  rule,  complete  enough  for  practical  purposes,  the  manipulations  simple, 
and  the  separated  compound  usually  left  in  a  form  suitable  for  weighing. 

The  solvency  of  a  liquid  may  be  so  far  reduced  that  one  constituent  in  solu- 
tion separates  unaltered.  Alcohol  precipitates  dextrin  from  an  aqueous  solu- 
tion; potassium  chloride  in  concentrated  solution  is  precipitated  by  hydro- 
chloric acid;  paraffin  in  mineral  oil  is  thrown  down  by  the  addition  of  twenty 
volumes  of  glacial  acetic  acid;  skatole  is  separated  from  indole  when  the  mix- 
ture is  dissolved  in  the  least  quantity  of  absolute  alcohol  and  diluted  with, 
water;  etc. 

Many  organic  bodies  are  precipitated  from  an  aqueous  solution  on  the  addi- 
tion of  a  considerable  amount  of  some  inorganic  salt  (*  salting  out'),  more 
completely  if  the  solution  be  fully  saturated  by  stirring  with  an  excess  of  the 
powdered  salt.  Should  the  precipitate  be  a  salt  of  a  metal  precipitable  by 
hydrogen  sulfide  in  a  neutral  solution,  the  filtered  and  washed  precipitate  may 
be  suspended  in  water  and  the  metal  precipitated  as  sulfide;  after  filtering, 
the  excess  of  hydrogen  sulfide  is  removed  from  the  regenerated  organic  body 
by  boiling,  fixation  by  an  insoluble  metallic  compound,  or  transmitting  a  cur- 
rent of  air  or  a  gas. 

On  saturating  a  solution  of  several  analogous  bodies  with  a  salt,  some  will 
precipitate  only  at  specific  elevated  temperatures,  and  by  slowly  heating  the 
solution  and  filtering  at  the  proper  temperatures,  a  fair  separation  is  possible. 

Two  bodies  may  react  with  a  third  but  one  much  more  slowly  than  the  other, 
so  that  by  limiting  the  time  of  contact  previous  to  filtration,  one  may  be 
practically  unaffected,  the  other  completely  transformed,  and  the  two  be 
separable  by  filtration  or  otherwise.  Similarly  the  reaction  with  one  body 
may  be  greatly  hastened  or  retarded  by  raising  or  lowering  the  temperature 
or  by  other  means. 

When  a  mixture  of  liquids  of  different  congealing  points  is  cooled,  the  liquid 
of  the  highest  melting  point  first  solidifies  and  can  be  collected  by  expression 
through  a  cloth.  This  mode  of  separation  is  applied  to  the  fats  to  remove 
stearin,  to  mineral  oils  for  paraffin,  etc.,  but  the  results  are  only  approximate 
at  best. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  83 

Crystallization.  Many  salts  crystallize  out  more  or  less  completely  when  their 
solutions  are  cooled,  but  the  ratios  of  decrease  of  solubility  to  decrease  of 
temperature  are  seldom  so  divergent  as  to  allow  of  more  than  an  approximate 
separation. 

In  fractional  crystallization,  the  solution  of  several  analogous  salts  is  con- 
centrated by  evaporation  and  allowed  to  cool,  whereupon  the  salt  least  soluble 
crystallizes,  and  as  the  mother-liquor  is  further  evaporated  another  separates, 
and  so  on.  Thus,  the  barium  salts  of  the  homologues  of  acetic  acid  crystallize 
in  the  order  of  the  caprate,  pelargonate,  capryiate,  oenanthylate,  and  caproate. 
This  process,  however,  is  but  a  beginning,  as  each  crop  of  crystals  is  far  from 
being  unmixed  with  tfoe  others  and  must  again  be  put  through  the  same  routine 
and  its  products  also,  so  that  the  great  number  of  repetitions  demanded  is 
wearying  and  the  method  resorted  to  only  where  no  substitute  can  be  found. 
The  process  is  used  in  the  separation  of  some  of  the  rare  earths,  and  for  dis- 
tinguishing the  dextro-  and  laevo-rotary  varieties  of  certain  optically-neutral 
salts. 

A  precipitant  may  react  with  but  one  element  or  compound  of  a  mixture  to 
form  a  precipitate,  or  at  the  same  time  others  be  brought  to  an  insoluble  state 
but  redissolved  by  the  excess  of  the  reagent.  The  separation  in  the  latter  case 
is  seldom  as  complete  as  in  the  former  since  the  permanent  precipitate  tends 
to  prevent  entire  resolution,  usually  by  inclosure  or  occlusion,  though  in  some 
cases,  undoubtedly,  a  true  chemical  union  is  established.  As  might  be  pre- 
sumed, a  considerable  excess  of  the  precipitant  favors  the  resolution  of  the 
soluble  part  of  the  precipitate,  sometimes  to  a  remarkable  extent,  e.  g.,  the 
separation  of  a  ferric  salt  from  salts  of  copper,  chromium,  etc.,  by  precipita- 
tion with  ammonia  is  only  successful  with  a  large  excess  of  the  alkali. 

The  completeness  of  a  separation  depends  mainly  upon  the  degree  of  insolu- 
bility of  the  precipitate  and  the  solubility  of  the  other  constituents  in  presence 
of  the  excess  of  the  precipitant;  yet  the  tendency  of  the  precipitate  to  occlude 
soluble  matters,  and  their  co-precipitation  should  be  guarded  against. 
With  many  elements  a  precipitation  only  may  take  place  under  widely  varying 
conditions  of  concentration,  temperature,  acidity,  etc.,  without  affecting  either 
the  composition  of  the  precipitate  or  lessening  its  insolubility.  In  a  separa- 
tion, however,  these  matters  must  be  regulated  more  closely  lest  the  precipi- 
tate retain  a  portion  of  the  other  constituents  or  itself  partly  remain  in  solution. 
When  the  bulk  of  the  precipitate  is  relatively  small  and  its  aggregation  granu- 
lar, one  precipitation  may  be  sufficient,  but  when  considerable  in  volume  and 
gelatinous  or  flocculent,  it  is  safer  to  dissolve  it  after  filtration,  and  reprecipi- 
tate  —  provided  that  It  is  so  insoluble  that  no  considerable  loss  will  be 
incurred. 

Occasionally  an  insoluble  precipitant  is  used  for  separations  where  it  is 
necessary  or  desirable  that  no  bodies  be  introduced  into  the  solution  other  than 
are  already  present,  or  for  other  reasons.  An  example  is  the  withdrawal  of 
phosphoric  acid  in  solution  in  strong  nitric  acid  by  metastannic  acid  formed 
in  the  solution  by  introducing  metallic  tin  which  is  immediately  oxidized  by  the 
nitric  acid.  Another  is  the  indirect  determination  of  a  mixture  of  free  formic  and 
acetic  acids  by  boiling  the  aqueous  solution  with  mercuric  oxide.  The  acetic 
acid  dissolves  an  equivalent  of  the  oxide,  mercuric  acetate  passing  into  solu- 
tion, while  the  formic  acid  breaks  up  to  carbonic  acid  and  water  reducing  an 
equivalent  of  the  oxide  to  metallic  mercury.  The  residue  is  treated  with  dilute 
hydrochloric  acid  which  dissolves  the  excess  of  mercuric  oxide  and  leaves 
metallic  mercury  ready  for  weighing. 

Fractional  precipitation  presupposes  a  solution  of  two  or  more  analogous 


84  QUANTITATIVE   CHEMICAL   ANALYSIS. 

elements  or  compounds  a,  6,  c, each    precipitable  by   a  reagent  r.    Of 

the  reagent  there  is  introduced  somewhat  more  than  it  is  judged  will  saturate 
a,  forming  a  precipitate  of  indefinite  amounts  of  ar,  6r,  cr, ;  but  on  stand- 
ing for  a  time  a  replacement  occurs,  and  the  precipitate  finally  contains  all  of 

a  as  ar  with  more  or  less  of  br.     After  filtering,  the  remainder  of  6,  c, are 

precipitated  in  the  same  way.  Obviously  the  separation  is  never  exact  even 
though  each  fraction  be  repeatedly  redissolved  and  fractionally  precipitated  .* 

For  example,  if  a  solution  of  the  nitrates  of  lanthanum,  samarium  and  didymium  be 
fractionally  precipitated  by  ammonia  the  first  precipitate  will  be  rich  in  samarium  but 
also  contain  much  didymium;  the  second  precipitate  is  mainly  didymium  mixed  with  some 
lanthanum ;  and  the  third  almost  wholly  lanthanum.  Again,  in  the  analysis  of  meat 
extracts,  the  gelatin  precipitates  when  alcohol  is  added  to  the  aqueous  solution  to  the 
extent  of  40  per  cent  by  volume,  albumose  at  80  per  cent,  and  peptones  at  94  per  cent. 

Electrolysis  is  employed  to  dissociate  a  metal  from  an  acid  rest,  and  as  a 
means  of  separation  from  metals  not  deposited  by  a  current  of  moderate 
strength  and  from  non-electrolytes.  It  is  a  neat  and  accurate  method  where 
available.  Two  metals  can  be  separated  at  one  time  where  one  deposits  on  the 
cathode  in  the  metallic  state,  the  other  on  the  anode  as  an  oxide;  or  succes- 
sively when  their  electrolytes  are  decomposable  only  under  widely  differing 
conditions  (page  (286). 

5.  Dialysis.  This  principle  has  some  applications  in  organic  analysis  and 
toxicology.  It  utilizes  the  power  possessed  by  the  bodies  known  as  « crystal- 
loids '  of  penetrating  through  a  porous  septum  (e.  g.,  an  animal  membrane), 
while  other  bodies  known  as  '  colloids  '  do  not  pass  through.  To  the  former 
class  belong  the  crystalline  salts,  and  to  the  latter  the  proteids,  gums,  starches, 
gelatin,  etc.  The  process  can  be  used  for  the  separation  of  inorganic  colloidal 
compounds,  such  as  silicic,  molybdic,  and  tungstic  acids,  from  crystalline 
salts,  and  to  separate  alkaloids  from  organic  impurities  or  animal  extractives. 
The  same  principle  is  occasionally  applied  to  the  separation  of  gases. 

In  one  form  of  dialyzer  a  small  glass  hoop  has  a  sheet  of  parchment  stretched 
over  the  bottom  forming  a  water-tight  dish  in  which  is  contained  the  liquid  to 
be  dialyzed.  It  is  half-immersed  in  a  vessel  of  water,  and  after  the  lapse  of 
some  time,  often  several  days,  and  with  frequent  changes  of  the  water,  the 
crystalloid  will  have  passed  out  into  the  water  leaving  the  colloid,  although  the 
separation  is  never  quite  complete.  Other  septa  for  the  purpose  are  bisque- 
clay  jars,  and  parchment  paper  made  into  tubes  or  thimbles. 

A  simple  modification  Is  due  to  Bauer. t  A  funnel  is  cut  off  a  short  distance  above  the 
stem,  and  a  parchment  filter  fitted  so  as  to  project  below  the  glass.  The  funnel  is  filled 
with  the  solution  to  be  dialyzed  and  supported  in  r,  beaker  of  water.  After  severa 
changes  of  water  the  greater  part  of  the  crystalloids  will  have  transpired. 

As  the  rapidity  of  the  diffusion  is  greater  in  proportion  to  the  difference  in  concentra- 
tion of  the  two  liquids,  time  is  saved  by  an  arrangement  to  continuously  remove  the  dif- 
f usate  and  supply  fresh  water.  The  stem  of  a  funnel  is  closed  by  a  rubber  tube  and  screw 
pinch-cock;  the  solution  to  be  dialyzed  is  held  in  a  plaited  filter  of  parchment  paper,  and 
the  water  conducted  to  the  space  between  the  paper  and  funnel  by  a  rubber  tube.  The 
pinch-cock  is  opened  so  far  that  the  water  flows  out  in  drops,  and  the  water  in  the  funnel 
is  kept  at  the  proper  level  by  some  automatic  contrivance  such  as  a  Mariotte's  bottle. 

In  general,  a  separation  is  most  successful  when  the  constituents  bear  to  each 
other  a  comparatively  small  ratio  by  weight.  When  one  of  a  mixture  largely 
predominates  and  is  precipitated  or  left  insoluble,  the  others  will  often  be 
occluded  or  mechanically  held  in  part;  and  precipitation  of  a  relatively  small 
amount  of  one  body  in  presence  of  a  large  amount  of  another  is  slower  and 
may  be  less  complete  than  if  alone. 


*  Speyers,  Text  book  of  Physical  Chem  118. 
t  Chem.  News,  1890-1—193. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  85 

The  majority  of  analyses  made  for  practical  ends  —  of  ores,  commercial 
metals,  pigments,  oils,  dyes,  crude  chemicals,  etc.  — call  for  the  separation  of 
the  minor  constituents  or  impurities  from  the  greatly  preponderating  principal. 
Should  a  specific  solvent  or  precipitant  for  each  of  the  former  be  available, 
the  separation  presents  no  great  difficulties;  otherwise,  a  partial  separation 
can  often  be  made  with  advantage,  dividing  the  original  mixture  into  (1)>  the 
greater  part  of  the  major  constituent  in  a  pure  state,  and  (2),  all  of  the  minor 
constituents  mixed  with  the  remainder  of  the  major.  Further  separation  of 
(2)  is  proceeded  with  by  suitable  methods,  facilitated  by  the  presence  of  oaly 
a  relatively  small  proportion  of  the  major  constituent.  Such  a  preliminary 
separation  may  be  done  in  various  ways. 

1.  A  partial  solution  of  the  major  constituent  may  be  effected  by  a  suitable 
solvent,  provided  always  that  the  minor  constituents  or  impurities  are  practi- 
cally insoluble  both  in  the  solvent  and  its  solution  of  the  principal  constituent. 
If  not  highly  insoluble  and  a  correction  for  solubility  is  attempted,  that  the 
solution  of  the  principal  constituent  is  jquite  concentrated  in  regard  to  the 
latter,  must  not  be  forgotten.    The  principal  constituent  must  dissolve  freely 
in  the  solvent,  as  if  only  moderately  soluble  the  separation  is  less  exact. 

When  the  greater  part  of  a  commercial  metal  is  dissolved  by  an  acid  the  more  electro- 
positive impurities  remain  insoluble,  as  when  bar-lead  is  nearly  dissolved  in  dilate  nitric 
acid,  the  silver  it  contains  is  concentrated  in  what  remains. 

Or  a  solvent  may  be  chosen  in  which  the  main  constituent  remains  almost  entirely  un. 
dissolved.  For  powdered  commercial  metals,  alloys  and  amalgams,  the  solvent  may  be  a 
solution  or  suspension  of  a  salt  of  the  major  constituent,  the  base  of  the  salt  replacing 
the  minor  constituents  which  pass  into  solution.  The  method  is  more  successful  where 
the  latter  are  in  separate  grains  or  crystals  mixed  with  those  of  the  former  and  the  sub- 
stance finely  powdered  than  with  alloys. 

2.  Partial  crystallization.  Where  the    minor    constituents  are    much  less 
soluble  than  the  major,  perhaps  the  most  successful  plan  is  to  prepare  a  con- 
centrated solution,  evaporate  until  crystals  begin  to  form,  and  filter  while  the 
solution  is  hot.    As  soon  as  a  small  amount  of  crystals  form  in  the  filtrate 
through  its  slow  cooling,  it  is  again  filtered,  and  this  process  is  continued  until 
from  the  color,  shape,  or  other  appearance  of  the  crystals  or  by  qualitative 
tests,  it  is  seen  that  the  mother  liquor  is  free  from  the  minor  constituents. 
The  different  crops  of  crystals  are  united  and  the  further  separation  proceeded 
with  by  a  different  method. 

Sometimes  the  members  of  a  mixture  of  crystal lizable  acids  or  bases  may 
not  differ  in  solubility  so  greatly  as  to  make  possible  a  partial  separation  by  the 
above  method,  but  by  combination  to  salts  with  a  suitable  base  or  acid  radical, 
the  requisite  difference  will  be  obtained. 

Should  the  major  constituent  be  less  soluble  than  the  minor  ones,  as  much 
as  possible  of  the  former  is  crystallized  out,  the  crystals  redissolved  and  the 
operation  repeated  once  or  of tener.  The  mother  liquors  are  united  and  further 
treated.  Possibly  a  process  analogous  to  that  of  fractional  distillation  (page 
65)  may  repay  the  time  and  labor  required. 

3.  Partial  precipitation.    A  precipitant   that  has  a  common  but  selective 
action  for  the  various  constituents  is  added  to  their  solution  in  quantity  only 
sufficient  to  throw  down  either  the  greater  part  of  the  major  constituent  alone, 
or  all  the  minor  constituents  mixed  with  some  of  the  major,  the  amount  greater 
or  less  as  the  composition  of  the  original  mixture  was  known  and  the  propor- 
tion of  the  precipitant  limited  accordingly.    For  example,  from  a  solution  of  a 
manganous  salt  containing  a  little  ferric  salt,  ammonium  sulfide  precipitates 
first  the  iron,  later  the*manganese.    The  contrast  in  color  between  the  iron  and 
manganese  sulfides  is  so  marked  as  to  indicate  when  all  the  iron  has  been  pre- 


86  QUANTITATIVE   CHEMICAL   ANALYSIS. 

cipitated.  However,  this  method  is  not  to  be  depended  on  where  the  minor 
constituents  of  the  mixture  bear  less  than  a  certain  proportion  to  the  major. 

Another  method  is  to  react  with  the  minor  constituent  by  introducing  into 
the  solution  some  sparingly  soluble  compound  of  the  major  constituent.  Thus, 
to  a  solution  of  much  potassium  chloride  and  a  little  rubidium  chloride  is 
added  the  slightly  soluble  compound  potassium  platinchloride;  this  reacts  only 
with  the  rubidium,  precipitating  rubidium  platinchloride,  an  equivalent  of 
potassium  passing  into  solution. 

4.  Other  principles  may  be  employed  as  the  nature  of  the  mixture  indicates. 

DECANTATION  —  FILTRATION  -  WASHING. 

The  separation  of  a  precipitate  from  the  liquid  in  which  it  has  been  formed  is 
an  operation  common  to  most  analyses ;  and  frequently  the  collection  of  a  resi- 
due left  after  partial  solution  of  a  solid,  or  the  parting  of  two  immiscible 
liquids  is  called  for. 

Obviously  the  most  direct  procedure*  is  to  allow  the  solid  to  collect  and  pour 
off  the  supernatant  liquid,  but  in  practice  this  simple  and  convenient  means  is 
restricted  to  residues  and  precipitates  of  the  heavier  metals  or  acid  radicals,  or 
those  formed  in  light  liquids,  and  to  some  organic  bodies  that  form  a  viscid 
mass  or  adhere  closely  to  the  interior  of  the  vessel. 

In  dealing  with  a  metal  precipitated  as  a  spongy  mass  or  left  as  a  powder 
after  extraction  of  another  metal  alloyed  with  it,  the  liquid  is  cautiously  de- 
canted, and  the  remaining  small  amount  displaced  by  agitating  with  water  and 
pouring  off  until  the  metal  has  been  washed  practically  clean.  Both  the  decant- 
ations  and  washings  are  examined  after  standing  for  a  time  that  any  particles 
of  metal  carried  over  may  be  detected  and  recovered.  The  metal  is  washed 
into  a  light  tared  dished  or  flask,  as  much  of  the  water  decanted  as  possible, 
then  rinsed  once  or  twice  with  strong  alcohol,  dried  at  a  gentle  heat,  and 
weighed.  The  final  drying  is  done  in  a  current  of  some  reducing  gas  should 
the  metal  be  readily  oxidizable  by  the  air. 

Since  the  residue  or  precipitate  should  be  disturbed  as  little  as  possible  dur- 
ing the  decantations  and  washings,  the  operation  is  best  performed  in  a  tall 
glass  vessel  shaped  like  the  •  precipitating  jar ',  Fig.  74,  the  body 
merging  abruptly  into  the  bottom.  The  powder  collects  in  the 
angle  as  the  jar  is  inclined,  allowing  nearly  all  of  the  liquid  to  be 
poured  off,  and  the  conical  shape  also  tends  to  the  collection  of 
the  precipitate  at  the  bottom.  Where  the  volume  of  the  super- 
natant liquid  is  greater  than  can  be  conveniently  decanted  it  is 
drawn  off  by  a  narrow  glass  syphon  or  a  large  pipette. 

Precipitates,  residues  from  partial  solution,  and    suspended 
matter  in  general,  if  small  in  bulk  and  held  in  a  moderate  volume 
Fig.  74.       Qf  liquid^  can  be  made  to  deposit  quickly  and  in  a  compact  and 
coherent  mass  by  the  aid  of  centrifugal  force. 

The  turbid  liquid  is  transferred  to  a  thick-walled  test-tube,  best  having  the 
bottom  drawn  into  a  conical  shape;  small  flasks  or  beakers  of  heavy  glass  may 
also  be  used  in  special  machines. 

The  centrifugal  machine,  '  centrifuge  »  or  *  whirl »  is  essentially  a  vertical 
iron  shaft  provided  with  a  mechanism  at  the  bottom  by  which  it  can  be 
rotated  at  a  high  speed.  The  power  may  be  a  geared  hand -crank  or  a  small 
electric  or  water  motor.  From  the  upp^r  end  of  the  shaft  project  a  number  of 
horizontal  radial  arms  symmetrically  disposed;  to  the  outer  end  of  each  arm  is 
hinged  a  holder,  a  metal  cup  of  such  a  size  and  shape  that  the  test-tube  fits 


QUANTITATIVE    CHEMICAL    ANALYSIS,  87 

snugly  wUhin.  Being  hinged,  the  holders  hang  vertically  when  the  machine  is 
at  rest,  but  when  the  shaft  attains  a  certain  speed  they  rise  to  a  horizontal 
position,  the  mouth  of  the  test-tube  inward. 

Through  the  centrifugal  force  imparted  by  the  rapid  circular  motion,  the 
liquid  is  held  in  the  test-tube,  and  a  precipitate  of  a  specific  gravity  greater 
than  the  liquid  is  thrown  to  the  bottom  of  the  test-tube  where  it  eventually 
coheres  to  a  more  or  less  compact  layer  leaving  the  liquid  perfectly  clear. 
Frequently  the  cohesion  of  the  particles  is  so  marked  that  the  liquid  can  be 
entirely  decanted  without  rousing  the  precipitate. 

The  time  required  for  the  deposition  and  the  compactness  of  the  deposit 
depends  on  the*  speed  of  the  machine.  From  five  to  ten  minutes  rotation  at 
from  1,000  to  6,000  revolutions  per  minute  will 
usually  be  sufficient  for  the  purpose. 

Purdy's  machine,  Fig.  75,  is  specially  designed  for  the 
analysis  of  urine.  Within  the  base  is  a  small  motor 
wound  (or  an  ordinary  incandescent  light  current  and 
is  connected  to  a  socket  by  flexible  conducting  wires. 
The  two  sheet-metal  holders  at  the  extremities  of  the 
cross-bar  are  hung  to  it  by  pivots  and  have  the  shape  of 
the  test-tube  contained. 

Gaertner's  centrifuge  has  a  central  shaft  supported  on 
pointed  bearings  at  the  extremities.  Fixed  near  the  bot- 
tom is  a  disk  sloping  outward  at  an  angle  of  about  30  o 
from  the  horizontal.  On  the  disk  are  a  number  of  spring 
clamps  for  holding  the  test-tubes,  and  a  cover  screws 
down  over  it.  A  long  cord  is  wound  around  the  shaft 
and  the  free  end  forcibly  pulled  away.  As  the  cord  un- 
winds, the  shaft  ie  impelled  to  rotate  for  several  minutes 

at  an  initial  high  speed. 

.big.  76. 

The  centrifuge  is  found  very  useful  in  technical  work,  and  many  of  tbe  quick 
and  fairly  accurate  tests  of  urine,  milk,  sugar,  etc.,  are  due  to  its  aid. 

The  thin  coating  of  a  metal  deposited  by  electrolysis  on  a  smooth  platinum 
sheet  is  so  compact  and  firmly  adherent  that  decantation  of  the  solution  and 
washing  can  be  done  with  the  greatest  facility,  but  with  some  metals  and  solu- 
tions it  is  required  that  the  solution  be  poured  off  and  the  deposit  washed  while 
the  electric  current  continues  to  pass  between  the  electrodes,  as  if  interrupted, 
the  surface  of  the  deposit  would  be  redissolved.  For  this  purpose,  a  syphon 
made  of  a  narrow  glass  tube  is  filled  with  water  and  the  lower  limb  closed  with 
the  finger,  then  the  shorter  limb  is  lowered  into  the  solution.  As  the  liquid  is 
drawn  up  from  the  bottom  of  the  vessel,  water  is  gently  poured  in  against  tiie 
side,  the  greater  density  of  the  solution  preventing  their  mixing  to  any  great 
extent. 

Some  organic  bodies  are  thrown  out  of  solution  in  the  form  of  clots  that  on 
stirring  the  solution  become  so  tenaciously  attached  to  the  sides  of  the  vessel 
that  the  liquid  may  be  poured  off  perfectly  clear.  In  this  case  the  vessel  Is 
commonly  a  light  tared  Erlenmyer  flask,  and  after  decanting  the  solution  and 
-washing  the  precipitate,  the  latter  is  dried  by  blowing  in  a  current  of  dry  air, 
the  flask  heated  gently  meanwhile  provided  this  is  not  contraindicated.  The 
increase  in  weight  of  the  flask  gives  the  weight  of  the  precipitate. 

It  often  happens  in  technical  analysis  that  some  element  or  body  whose 
determination  is  not  required,  is  to  be  separated  by  precipitation.  To  save  the 
time  of  filtration  and  washing  or  for  other  reasons,  the  clear  cold  solution  may 
be  poured  into  a  measuring  flask  followed  by  the  precipitant.  After  mixing 
well,  the  turbid  liquid  is  made  up  to  the  mark  with  water,  again  well  shaken, 
and  allowed  to  settle.  The  greater  part  of  the  clear  supernatant  liquid  .is 


88 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


poured  off,  and  measured  portions,  aliquot  parts  of  the  volume  held  by  the 
measuring  fla«k,  are  used  for  the  necessary  determinations  as  though  each  was 
the  entire  filtrate.  The  liquid  may  be  conveniently  drawn  out  by  a  small 
burette  from  which  several  fractional  parts  may  be  tapped  for  separate  exami- 
nation. 

The  space  occupied  by  a  precipitate  or  residue  of  apparent  moderate  bulk  is 
often  so  small  as  to  be  neglected  with  safety,  for  many  seemingly  voluminous 
precipitates  are  so  attenuated  that  they  in  reality  displace  but  little  of  the 
liquid.  If  the  bulk  is  considerable  in  relation  to  the  volume  of  the  supernatant 
solution,  a  correction  must  be  found  and  applied ;  it  may  be  roughly  deter- 
mined by  mixing  measured  volumes  of  the  clear  solution  and  precipitant  in  a 
tall  measuring  jar  and  noting  to  what  extent  their  united  volume  is  exaggerated 
by  the  precipitate  produced. 

A  more  accurate  plan  is  as  follows,  on  the  principle  that  the  more  dilute  the  superna- 
tant fluid,  the  less  is  any  property  thereof  altered  by  the  space  occupied  by  a  precipitate ; 
in  other  words,  the  concentration  of  the  supernatant  fluid  varies  in  a  direct  ratio  with  the 
volume  of  the  precipitate,  the  ratio  of  increase  being  less,  the  more  dilute  the  fluid.  One 
method  is  to  make  up  the  turbid  liquid  to  a  certain  volume  Fand  after  settling  to  draw 
off  an  aliquot  part  V  and  determine  therein  the  value  of  some  constant  a.  The  remainder 
of  the  liquid  in  the  flask  is  again  diluted  to  V,  the  volume  V  withdrawn,  and  the  same 
constant  again  determined  giving  b.  Then  if  m  is  the  concentration  of  the  original  solu- 

m  V 
tlon,  and  a;  the  volume  of  the  precipitate,     v—x    =  a*  an(*a  :  "* :  :  ^:  m~  °"  Whence  x  = 

v         av' 
'     a-  b 

The  datum  to  be  secured  may  be  either  a  physical  constant  or  the  proportion  of  some 
one  or  more  constituents  —  the  weight  of  the  residue  left  on  evaporation  of  the  solution  is 
often  convenient.  It  must  be  known,  however,  that  the  value  of  the  physical  constant 
determined  is  not  altered  beyond  the  normal  by  dilution. 

For  technical  work  on  special  commercial  liquids,  vegetable  extracts,  saps, 
etc.,  flasks  may  be  purchased  that  are  calibrated  to  hold  a  standard  volume  of 
the  liquid  plus  the  volume  of  a  certain  precipitate  formed  therein  or  an  in- 
soluble residue,  obviating  the  necessity  of  a  correction  for  the  latter. 

With  precipitates  that  are  slow  to  settle,  instead  of  decantation, 
the  bulk  of  the  liquid  can  be  filtered  through  a  dry  paper.  Excep- 
tions are  highly  volatile  liquids,  where  the  unavoidable  evaporation 
would  increase  their  concentration.  But  pre- 
cipitates settle  more  readily  and  can  be  decanted 
from  more  easily  when  formed  in  a  light  volatile 
liquid  than  in  an  aqueous  solution. 

To  part  two  immiscible  liquids,  as  an  oil  from 
a  watery  solution,  the  mixture  is  poured  into  a 
separatory  funnel,  Fig.  70.  The  stem  should 
be  short  and  wide,  and  cut  off  obliquely  that 
it  may  readily  empty,  and  tbe  funnel  have  a 
narrow  bore  for  a  short  distance  above  the 
stopcock.  The  lower  stratum  is  drawn  by 
the  tap  from  the  upper  layer,  and  if  necessary, 
the  latter  washed  by  alternately  agitating  with 
water  or  some  solution  and  drawing  off  when 
the  layers  have  formed.  The  traces  of  oil 
Fig.  76.  always  carried  off  in  an  aqueous  solution  are 

recovered  by  shaking  the  latter  with  an  organic  solvent,  then  sepa- 
rating as  above.  FigTW. 
A  simpler  form  of  separator  without  a  stop -cock  is  shown  in  Fig.  76.    In  the  wide  stem 


QUANTITATIVE    CHEMICAL    ANALYSIS.  89 

of  a  funnel  is  a  tighly  fitting  cork  A,  through  it  passing  a  glass  tube  B.  The  top  of  the 
tube  has  been  sealed,  and  perforated  at  C  with  a  small  hole  by  heating  the  tube  at  that 
point  with  a  fine  blowpipe  flame  and  applying  suction  at  the  open  end.  The  tube  is 
inserted  in  the  cork  only  so  far  that  C  is  below  the  top  of  the  cork.  The  mixture  of  liquids 
Is  poured  into  the  funnel,  and  when  stratification  has  taken  place  the  tube  is  pushed  up 
until  the  hole  just  appears  above  the  cork,  retracting  the  tube  as  soon  as  the  lower  layer 
has  run  out  through  B. 

In  another  contrivance  the  liquids  are  held  in  a  test-tube  closed  by  a  loosely  fitting 
cork  through  which  passes  a  pipette,  Fjg.  77.  After  the  liquids*  have  separated,  the 
pipette  is  adjusted  until  the  lower  orifice  nearly  reaches  the  burface  of  the  lower  layer; 
the  upper  layer  is  then  withdrawn  by  suction.  A  long-rubber  tube  passed  over  the  top 
of  the  pipette  will  facilitate  the  operation. 

FILTRATION. 

But  it  is  the  exception  that  a  precipitate  or  residue  is  so  dense,  heavy  and 
coherent  that  the  liquid  may  be  decanted  or  syphoned  perfectly  clear,  so  that  a 
medium  must  be  interposed  whose  pores  are  so  minute  as  to  retain  solids,  even 
in  fine  powder,  yet  not  obstruct  the  rapid  passage  of  a  liquid. 

The  nature  of  both  the  solid  and  the  liquid  must  be  considered  in  the  selec- 
tion of  a  filtering  medium.  The  precipitate  may  be  curdy,  like  silver  chloride; 
gelatinous,  like  aluminum  hydrate;  crystalline,  as  ammonium  manganous 
phosphate;  or  pulverulent,  as  barium  sulfate;  frequently  shrinking  or  crystal- 
lizing on  standing  from  a  voluminous  flocculent  condition  to  a  dense  pulver- 
ulent form ;  while  a  residue  left  after  the  partial  solution  of  a  substance  may 
resemble  any  of  the  above,  or  if  complex  in  character,  a  mixture  of  two  or 
more.  The  supernatant  liquid  may  be  acid,  neutral  or  alkaline,  dilute  or  con- 
centrated, hot  or  cold.  To  meet  these  widely  different  conditions,  various 
media  for  filtration  are  in  use. 

Paper.  We  have  in  unglazed  paper  a  material  well  suited  to  the  majority  of 
nitrations.  For  quantitative  analysis  it  is  prepared  from  linen  or  a  mixture  of 
linen  and  cotton,  undergoing  an  elaborate  treatment  during  its  manufacture  to 
eliminate  foreign  matter  and  give  a  product  of  almost  pure  cellulose.  Aa 
analysis  of  a  German  paper  showed :  — 

Moistureat  100° 5.36 

Ash 37 

Hydrocellulose,  soluble  in  alcohol 73 

Lignin,  etc None 

Cellulose 93.69 

Much  of  the  inorganic  matter  or  ash  (chiefly  silicates  of  calcium  and  iron) 
may  be  dissolved  out  of  the  paper,  or  the  pulp  during  the  manufacture,  by 
hydrochloric  acid,  and  practically  all  the  remainder  by  hydrofluoric  acid. 
Through  the  previous  elimination  of  the  ash,  its  introduction  into  an  acid  fluid 
during  filtration  is  avoided,  and  when  a  filter  is  burned  with  a  precipitate,  this 
"  extracted  "  paper  leaves  so  small  a  proportion  of  ash  that  no  account  need 
be  taken  of  its  weight  even  from  the  larger  sizes  of  filters. 

All  grades  of  paper  can  be  purchased  in  large  sheets  of  different  thickness 
and  closeness  of  fiber,  or  ready  cut  in  circles  of  various  sizes,  either  extracted 
or  untreated.  The  best  known  manufacturers  are  J.  H.  Munktell,  of  Gryksbro, 
Sweden;  Carl  Schleicher  &  Schuell,  of  Duren,  Rhenish  Prussia;  and  Max 
Dreverhoff,  of  Dresden ;  their  products  are  also  acid  extracted  by  several 
European  and  American  firms  who  market  the  washed  paper  under  their  own 
trade -marks. 

Other  conditions  being  the  same,  the  rapidity  of  a  filtration  is  governed  more 
by  the  compactness  of  the  fiber  than  the  thickness  of  the  paper,  a  thinner 


90 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


variety  being  often  less  readily  permeable  than  a  heavier  one.  For  quantita- 
tive analysis  it  is  well  to  keep  in  stock  several  grades  of  paper  —  one  for  gen- 
eral work,  one  for  finely  divided  precipitates,  and  one  for  precipitates  that  have 
a  tendency  to  clog  the  paper.  For  general  work  may  be  recommended  the 
S.  &  S.  Nos.  595,  597  and  598,  the  Munktell  Nos.  2  and  3,  and  the  Dreverhoff 
No.  480;  and  the  extracted  paper  S.  &  S.  Nos.  589  and  590,  Munktell  Nos.  0 
and  00,  and  Dreverhoff  Nos.  400  and  418.  To  retain  the  finest  precipitates 
S.  &  S.  No.  575,  602,  and  689  "  blue  ribbon  "  or  Munktell  No.  100,  and  for  oils, 
juices,  etc.,  S.  &  S.  No.  584  or  591,  or  Dreverhoff  No.  260.  The  sizes  mostly 
used  in  analysis  are  9, 11  and  12.5  cm.  in  diameter.* 

Or  the  chemist  may  purchase  the  paper  in  sheets  and  extract  it  with  acid. 
The  bottom  of  a  bottle  is  cut  off  and  a  rubber  tube  with  a  pinch-cock  joined  to 
a  glass  tube  passed  through  a  cork  in  the  neck.  The  paper  is  cut  into  circles 
of  the  proper  sizes  and  digested  over  night  with  dilute  hydrochloric  acid ;  then 
washed  with  distilled  water,  thoroughly,  since  if  a  trace  of  acid  remains,  the 
dried  paper  will  be  found  brittle  and  worthless. 

Many  specially  prepared  papers  are  now  manufactured,  or  have  been  suggested,  designed 
to  meet  the  requirements  of  the  technical  chemist.  Starch-free  and  fat-free  papers  are 
on  the  market  for  use  In  the  analysis  of  materials  containing  these  bodies,  in  the  form  of 
circles,  strips  for  coiling,  and  thimble -shapes  for  extraction.  It  has  been  proposed  to 
utilize  the  decolorizing  property  of  animal  charcoal,  and  the  absorptive  power  of  hide- 
powder  for  tannin  and  the  like,  by  mixing  a  certain  proportion  of  these  in  the  paper-pulp. 
The  paper  may  be  hardened  or  toughened  sufficiently  to  resist  the  pressure  of  the  filter 
pump  by  treatment  with  nitric  acid,  or  the  pulp  partially  or  entirely  converted  into  nitro- 
cellulose which  is  said  to  filter  more  rapidly  as  well  as  burn  more  quickly  than  the 
untreated. 

To  support  the  paper  during  filtration,  funnels  made  of  chemical  glassware 
are  almost  invariably  employed.  The  interior  should  be 
a  true  cone  at  an  augle  of  60  °  or  nearly  so,  and  the  stem 
narrow  and  uniform  in  bore  that  it  may  fill  with  the 
liquid  during  filtration.  They  are  held  in  a  wooden 
stand,  Fig.  78,  of  which  the  best  form  has  the  supporting 
arm  of  sufficient  breadth  to  cover  the  beaker  receiving 
the  filtrate,  protecting  it  from  dust;  if  too  narrow  for 
this  purpose  the  beaker,  as  well  as  the  funnel,  is  covered 
Fig.  78.  Vis  by  a  watch-glass. 

It  is  sometimes  necessary  to  keep  the  contents  of  the  funnel  hot  during  a  filtration  — 
as  when  a  melted  fat  is  to  be  clarified  to  avoid  the  danger  of  the  less  fusible  stearin 
solidifying  and  remaining  on  the  filter  while  the  liquid olein  passes  through,  or  to  prevent 
a  solution  of  a  soap  from  setting  to  a  jelly.  Here  the  funnel  is  encased  in  a  double  sheet- 
metal  jacket  containing  water  or  a  salt  solution  kept  boiling  by  a  burner  beneath  a  pro- 
jecting arm.f  If,  on  the  other  hand,  the  contents  must  be  maintained  at  a  lower  tempera- 
ture than  that  of  the  laboratory,  as  in  dealing  with  a  precipitate  whose  solubility  decreases 
with  the  temperature,  the  funnel  is  surrounded  by  a  coil  of  metal  pipe  through  which 
circulates  ice-water  or  chilled  brine. 

Porcelain,  hard-rubber,  and  stone 
ware  funnels  have  an  occasional  use 
in  special  technical  analyses. 

Filtration.  A  paper  is  chosen 
so  large  that  the  precipitate 
will  not  more  than  half  fill  it 
and  the  circle  folded  into  a 
quadrant  as  at  A,  Fig.  79.  A 


Fig.  79, 


*  "  These  filters  must  satisfy  the  highest  pretensions  of  the  most  painful  analytical 

ch.-mist." 

t  Chem.  News,  1894—1—60. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


91 


glass  funnel  is  selected  whose  slant  height  is  at  least  one -quarter  of  an  inch 
greater  than  the  radius  of  the  flter;  if  it  be  of  an  angle  of  exactly  60°  the 
filter  will  fit  snugly  throughout,  but  as  few  funnels  can  be  found  that  are 
not  of  a  greater  or  less  angle,  the  paper  must  be  folded  accordingly.  After 
moistening  with  water,  any  air-bubbles  beneath  the  paper  are  pressed  out 
by  the  fingers.  The  funnel  is  inverted  and  the  stem  filled  with  water  from 
the  wash -bottle. 

Other  ways  of  folding  are  shown  at  C,  and  D  which  allow  a  more  rapid  fil- 
tration—  the  re-entrant  folds  of  D  are  held  to  place  by  a  bent  glass  rod.*  It 
is  well  for  future  use  to  make  two  file  marks  on  the  edge  of  each  funnel  show- 
ing how  far  one  fold  of  the  paper  should  overlap  at  the  edge  to  insure  a  perfect 
fitting  filter. 

Much  time  will  be  saved  by  allowing  the  precipitate  or  insoluble  matter  to 
settle  before  filtration,  and  retaining  it  in  the  beaker  as  far  as  can  be  done 
until  the  liquid  has  passed  through  the  filter.  To  prevent  splashing,  a  glass 
rod  with  rounded  ends  will  assist  in  directing  the  stream  against  the  paper 
near  its  edge;  and  the  orifice  of  the  stem  of  the  funnel  should  touch  the  side 
of  the  receiving  vessel.  When  as  much  as  possible  of  the  liquid  has  been  run 
through,  the  precipitate  may  be  washed  by  decantatioa  or  brought  directly 
on  the  filter,  flushing  out  the  last  portion  by  holding  the  beaker  in  an  inclined 
position  over  the  funnel  and  directing  a  stream  of  water  from  the  wash  bottle 
around  the  interior. 

Usually  the  precipitate  is  heavier  than  the  liquid  surrounding  It  and  sinks  to  the 
bottom ;  In  the  few  Instances  where  the  liquid  is  the  denser,  the  precipitate  may  be  caused 
to  sink  by  dilution  with  water. 

Strainers  of  muslin  or  linen  cloth  or  chamois  skin  and  conical  bags  of  felt  are  of  occa- 
sional use  for  removing  gross  particles  from  a  vegetable  extract  or  similar  liquid,  and  a 
small  filter-press  may  serve  to  hasten  the  operation.  From  the  large  amount  of  ma- 
terial used  In  examinations  of  this  kind,  small  mechanical  losses  are  not  of  so  much 
consequence. 

A  plug  of  cotton-wool,  best  of  the  kind  sold  for  surgical  purposes  as  "  absorbent  cot- 
ton "  is  used  to  close  a  funnel  stem  or  percolator.  It  is  only  suitable  for  straining  out 
coarse  particles,  as  if  compacted  enough  to  retain  fine  powders  the  filtration  becomes 
very  slow  on  account  of  the  small  surface  exposed. t 

Other  filtering  mediums.  Concentrated  mineral  acids, 
strong  alkali  solutions,  and  powerful  oxidizers  like  chromic 
acid  and  the  permanganates  destroy  or  are  acted  on  by 
paper  and  other  organic  filters,  and  for  these  liquids  there  is 
chosen  some  inorganic  substance  that  is  unaffected.  A 
medium  free  from  carbon  is  used  for  organic  residues  and 
precipitates  that  are  to  be  afterward  subjected  to  an  ulti- 
mate analysis  without  removing  from  the  filter. 

The  mineral  asbestos  (actinolite)  is  practically  unaltered 
by  contact  with  chemical  solutions  including  the  acids 
(hydrofluoric  excepted),  though  long  digestion  with  strong 
acids  will  dissolve  small  amounts  of  the  constituents  of 
the  pure  mineral.  It  is  inf usble  up  to  a  white  heat.  Un- 
like those  of  vegetable  origin,  the  filaments  cannot  be 
felted  into  thin  sheets,  so  that  a  filter  is  prepared  by  cut- 
ting the  asbestos  across  the  fiber  into  short  lengths  and 
boiling  with  strong  hydrochloric  acid  to  extract  any  oxide 
of  iron,  soluble  silicates,  etc.,  that  may  accompany  -the  Fig.  80. 

mineral.    The  disintegrated  fibers  are  then  stirred  up  with  water. 


*  Chem.  News,  1889—2—102. 
7  '  '-ookes'  Select  Methods,  671. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


In  a  funnel  or  carbon  tube  is  laid  a  disk  of  platinum  gauze  or  a  circular 
plate  of  glass  or  porcelain  perforated  with  numerous  flue  holes,  of  such  a 
diameter  as  to  rest  a  short  distance  above  the  funnel  stem.  As  the  disk  is 
apt  to  be  displaced  during  filtration,  it  is  safer  to  substitute  for  it  a  flat  closely 
rolled  coil  of  heavy  platinum  wire,  Fig.  80,  the  inner  end  bent  at  a  right  angle 
to  the  coil  and  extending  through  the  funnel  stem,  thus  preventing  the  coil  from 
being  more  than  slightly  tilted. 

The  asbestos  pulp  is  poured  In  the  funnel  until,  when  the  water  has  drained 
away,  there  is  left  a  layer  of  fibers  one-eighth  inch  or  more  in  thickness. 
Water  holding  in  suspension  the  finest  particles  is  then  poured  in  until  the 
layer  is  covered  by  a  thin  close  film  that  will  retain  a  finely  divided  precipitate. 
During  the  filtration  the  liquid  and  wash-water  must  be  run  into  the  funnel 
carefully  in  order  that  the  asbestos  may  not  be  disturbed.  Usually  the  funnel 
is  a  part  of  the  vacuum  filtering  apparatus  (page  93),  and  in  this  case  the  pre- 
caution is  of  less  import,  as  the  asbestos  is  held  to  a  more  compact  condition 
by  the  pressure  of  the  air. 

The  Gooch  crucible,  Fig.  81,  is  made  of  platinum,  sometimes 
of  porcelain,  and  has  a  flat  bottom  perforated  with  many  fine 
boles.  During  a  filtration  the  crucible  is  held  in  a  funnel  by 
the  aid  of  a  wide  rubber  band  drawn  over  the  edge.  The  fun- 
nel stem  is  Inserted  in  the  cork  of  a  filtering-flask  or  bell -jar 
connected  with  a  vacuum-pump,  and  the  bottom  of  the  crucible 
covered  with  a  layer  of  asbestos  as  described  above,  of  a  thick- 
ness of  from  one  to  five  millimeters  according  to  the  texture 
of  the  asbestos  and  the  fineness  of  the  precipitate  to  be  held. 
After  preparing  the  crucible  it  is  ignited  and  weighed  before 
filtration.  Fig.  81. 

For  filtering  an  organic  residue  or  precipitate  that  is  to  bo  submitted  to  an  olomqntary 
analysis,  a  platinum  boat  with  perforated  bottom  and  held  In  a  funnel  of  the  same  shape 
is  prepared  In  the  same  way  as  a  Goooh  crucible.  After  filtration  and  washing  the  boat 
can  be  Inserted  bodily  Into  a  combustion  tube  without  need  of  transferring  the  contents. 

As  substitutes  for  asbestos  there  have  been  proposed  anthra- 
cene precipitated  In  flocoose  tufts,  It  being  Insoluble  In  most 
reagents  except  the  strong  acids  and  some  organic  liquids,  and 
in  entirely  volatile  at  a  moderate  heat;  and  a  metallic  felt,  such 
as  spongy  platinum,  a  layer  formed  in  the  crucible  by  packing 
in  ammonium  platlnlo  chloride,  then  driving  off  the  ammonium 
and  chlorine  by  heating  to  redness. 

Glass-wool  (matted  filaments  of  Bohemian  glass)  gives  a 
filter  so  open  in  texture  that  it  is  adapted  only  to  the  straining 
of  precipitates  of  the  coarsest  nature.  For  certain  purposes  a 
layer  over  or  under  an  asbestos  f  elc  affords  a  quick  filtration  and 
a  clear  filtrate.  A  thin  jar  of  nnglazed  clay  or  porcelain  retains 
the  most  finely  divided  matter  (even  bacteria  are  held)  but  is 
mostly  used  for  clarification  only,  or  where  corrosive  liquids 
are  to  be  filtered. 

Sand  or  powdered  glass  or  quartz  Is  the  medium  In  a  few 
flltratlons  where  the  precipitate  is  fairly  coarse  and  is  to  be 
redlssolved  after  filtration  and  washing.  A  glass  rod  A,  Fig. 
82,  greater  in  diameter  than  the  bore  of  the  stem  of  the  funnel 
has  its  lower  end  drawn  out  Into  a  conical  point  B  which 
closes  the  apex  so  nearly  that  while  a  fluid  readily  passes,  the 
particles  of  sand  heaped  around  It  are  retained.  After  ultra 
Ki'_r.  82.  tion  and  washing,  the  rod  is  withdrawn  and  the  precipitate 

and  sand  Unshed  with  water  through  the  stem  into  a  beaker.    For  volumetric  analysis 
tho  admixture  of  sand  with  the  precipitate  Is  of  no  consequence. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


93 


Rapid  nitration.  Even  under  conditions  the  most  favorable,  a  filtration  takes 
considerable  time  and  attention  and  is  especially  tedious  when  dealing  with 
gelatinous  and  slimy  precipitates,  or  with  liquids  slowly 
evolving  gases.    A  number  of  devices  have  been  proposed 
to  hasten  the  passage  of  the  liquid  through  the  filter,  such 
as  applying  centrifugal  force  in  a  centrifuge,  increasing  the 
pressure  of  the  air  on  the  liquid  in  the  funnel,  etc.    But 
two  of  these  are  in  common  use.    In  the  first,  the  funnel 
stem  is  lengthened  to  the  extent  of  a  foot  or  more  by  attach- 
ing a  narrow  glass  tube  with  a  loop  near  the  middle;  as  the    Fig.  83.    1  to  '/» 
tube  fills  with  the  filtrate,  the  column  exerts  a  tension  pro- 
portionate to  its  length. 

The  second  is  the  well  known  invention  of  Bunsen,  utilizing  the  pressure  of 
the  atmosphere  on  the  surface  of  the  liquid  in  the  funnel  by  maintaining  a  more 
or  less  complete  vacuum  in  the  funnel  stem.  It  has  three  essential  parts;  a 
funnel  and  cone  for  the  filter,  an  air-tight  receiver  for  the  filtrate,  and  a  pump 
to  exhaust  the  air. 

The  funnel  should  be  very  nearly  of  an  angle  of  60°  since  the  apex  of  the 
moist  paper  is  too  weak  to  withstand  the  increased  pressure  and  must  be  sup- 
ported by  a  perforated  cone  of  that  angle  made  of  sheet  platinum,  Fig.  83,  and 
the  filter  folded  to  fit  both.  Or  the  cone  may  be  nearly  as  large  as  the  interior 
of  the  body  of  the  funnel,  a  fiange  from  the  edge  resting  on  the  edge  of  the 
funnel  with  a  soft  rubber  gasket  Interposed  to  make  the  junction  air-tight. 

Another  device  Is  a  flat  circular  plate  of  slightly  less  diameter  than  the  funnel  rim  and 
profusely  perforated  with  small  holes.  The  plate  lays  horizontally  in  the  funnel  a  short 
distance  below  the  rim,  and  on  It  Is  laid  a  circle  of  filter  paper  of  slightly  greater  diam- 
eter. In  an  improvement  the  edge  of  the  plate  has  a  peripheral  U-shaped  groove  in  which 
is  held  a  rubber  ring  to  make  an  air-tight  Junction  between  the  plate  and  funnel. 

The  funnel-stem  passes  through  a  cork  or  rubber  stopper  inserted  in  the  neck 
of  a  flask  of  thick  glass  with  a  side  tube  connecting  with  the  pump,  or  of  a 
tubulated  bell-jar  resting  on  a  ground-glass  plate  and 
inclosing  a  beaker,  Fig.  84.  The  filtrate  passes  through 
the  funnel  stem  in  alternate  bands  of  liquid  and  bubbles 
of  air,  and  as  the  latter  emerge 
from  the  orifice,  tend  to  spat- 
ter the  liquid  in  all  direc- 
tions; for  this  reason  the 
stem  should  be  of  sufficient 
length  to  reach  well  into  the 
beaker,  passing  through  a 
small  hole  in  a  watch  glass 
covering  it.  Or  a  small  in- 
Fig.  84.  verted  funnel  may  be  closely 

attached  to  the  stem  by  rubber  tubing. 

In  the  modification  of  the  filter  flask  described  by  Wal- 
ther,*  the  neck  is  expanded  to  a  conical  shape  conforming 
to  the  usual  angle  of  glass  funnels.  Between  the  expan- 
sion and  the  body  of  the  funnel  is  placed  a  rubber  ring. 
Advantages  are  that  no  cork  is  needed  and  funnels  of 
different  sized  stems  are  accommodated  equally  well. 

The  pump  for  exhausting  the  air  is  made  of  glass 
or  brass  on  the  principle  of  the  tromp6  or  Sprengel's  Fig.  86. 


*  Analyst,  1898-  306. 


94 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


mercury  pump.  Fig.  85  shows  in  section  an  efficient  and  rapid  working  brass 
pump  designed  by  Richards.  The  inlet  is  threaded  for  screwing  to  a  faucet 
delivering  water  under  pressure.  The  water  enters  at  A  and  as  it  passes  the 
contraction  in  the  tube  at  B  drags  air  from  C  with  it,  the  foam  flowing  out 
through  D.  C  is  joined  by  rubber  tubing  to  the  filtering  flask;  an  empty  wash- 
bottle  may  be  interposed  between  C  and  the  filtering  fla^k  for  the  purpose  of 
intercepting  and  retaining  any  water  drawn  back  from  the  waste-pipe  below 
D,  which  may  happen  when  the  water  supply  is  suddenly  checked. 

Chapman's  filter  pump  is  somewhat  slower  in  action  than  Richards',  but  gives 
eventually  a  remarkably  complete  exhaustion  under  favorable  conditions.  A 
rubber  valve  in  the  pump  serves  the  purpose  of  the  empty  wash-bottle. 

A  blast  of  steam  or  highly  compressed  air  will  answer  as  a  substitute  for  the 
stream  of  water. 

The  tension  of  the  vacuum  obtainable  with  a  pump  of  the  above  description 
is  governed  mainly  by  the  rapidity  with  which  the  water  flows  past  B,  this 
depending  on  the  initial  pressure  and  the  vertical  length  of  the  waste-pipe.  In 
practice,  by  reason  of  leakage  of  air  through  and  around  the  filter  and  else- 
where, the  tension  in  the  filtering  flask  is  seldom  reduced  below  half  an  atmos- 
phere; however,  a  higher  exhaustion  is  not  often  required  in  ordinary 
analytical  work. 

Various  devices  may  be  adopted  to  obtain  a  moderate  vacuum  where  a  flow  of  water  i& 
not  available,  and  for  a  description  of  these  reference  may  be  had  to  the  original  papers. 

A  filter  paper  is  folded  to  exactly  fit  the  funnel,  inserted  and  moistened  with 
water.  The  pump  is  then  started  with  a  stream  so  slow  as  to  give  only  a  mod- 
erate vacuum  —  a  higher  one  is  apt  to  tear  the  paper  or  clog  it  with  the  finer 
floating  particles  of  the  precipitate.  When  all  the  precipitate  has  been  trans- 
ferred to  the  filter  and  before  it  has  shrunken,  the  washing  fluid  is  poured 
on  from  a  beaker.  The  vacuum  may  be  then  increased  to  the  full  capacity 
of  the  pump,  when  two  or  three  additional  washings  will  suffice,  since  the 
atmospheric  pressure  drives  the  wash-water  through  so  rapidly  that  there  is 
but  little  diffusion  of  the  adhering  part  of  one  washing  into  the  following 
one.  The  pressure  compacts  the  precipitate  to  only  a  small  fraction  of  the 
apparent  original  volume. 

Advantages.  As  the  great  merit  of  this  device  lies  in  the  rapidity  with  which 
precipitates  may  be  filtered  and  the  facility  of  their  washing,  it  is  especially 
acceptable  in  technical  work  where  time  is  so  important  a  consideration.  Yet 
instances  are  not  few  where  its  employment  is  certainly  unadvisable  —  the 
tendency  of  some  slimy  precipitates  to  compact  to  an  almost  impervious  varnish 
against  the  filter  increases  with  the  pressure,  as  does  that  of  finely  divided 

ones  to  run  through  a  paper  of  ordinary 
porosity;  moreover  the  precipitate  cannot  so 
well  be  protected  from  dust  and  laboratory 
fumes  by  covering  the  funnel  as  in  the  ordinary 
method  of  filtration.  On  the  whole  the  student 
will  do  well  to  adopt  the  older  plan  until  he  has 
become  familiar  with  the  behavior  of  precip- 
itates of  different  aggregation  and  the  best  way 
of  dealing  with  them. 

In  Carmichael's  device,  Fig.  86,*  for  upward  filtratiom 
a  long  glass  tube  A  B  is  twice  bent  at  right  angles.  One 
limb  passes  at  B  through  a  T-tube  0  D.  Terminating 
the  other  limb  is  a  small  rose  or  bulb  A  flattened  be- 
neath to  a  circle  about  an  Inch  In  diameter  which  is  per- 


Fig.  86. 


*  Crookes,  Select  Methods,  665. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  95 

forated  by  many  fine  holes.  The  receiving  vessel  is  a  beaker  with  edge  ground  flat, 
closed  by  a  glass  plate  E  F,  and  exhausted  through  a  connection  of  the  branch  D  of  the 
T-tube  with  a  vacuum-pump,  air-tightness  being  secured  by  rubber  tubing  and  sheeting. 

A  moistened  filter  paper  of  the  same  diameter  as  the  bulb  is  pressed  against  it,  the 
pump  Is  started,  and  the  bulb  lowered  into  the  dish  containing  the  solution  to  be  filtered. 
After  filtration  the  precipitate  is  washed  by  pouring  water  into  the  dish. 

The  apparatus  of  the  size  described  is  only  suited  to  small  precipitates.  It  Is 
recommended  that  the  precipitation  be  done  in  a  small  tared  platinum  dish,  and  after 
washing,  the  paper  and  adhering  precipitate  blown  back  into  the  dish  ready  for  drying  or 
ignition  and  weighing. 

Filtration  of  liquids  and  gases.  Two  immiscible  liquids  can  be  parted  by 
filtration  through  paper  or  other  medium.  Before  pouring  in  the  mixture  the 
filter  is  wetted  by  the  same  liquid  as  the  heavier  of  the  two,  and  throughout 
the  filtration  it  should  always  contain  some  of  the  heavier  liquid,  as  otherwise 
part  of  the  lighter  may  also  pass  through. 

The  filtration  of  a  fat  or  fatty  acid  from  an  aqueous  solution  may  also  be  done  by  cool- 
ing the  mixture  to  the  point  of  solidification  of  the  former,  with  constant  stirring.  The 
disintegrated  particles  are  readily  filtered  through  paper  by  a  light  suction,  and  on 
washing  with  cold  water  show  no  tendency  to  pass  through. 

Gases  are  freed  from  suspended  soot  or  fume  by  passage  through  a  scrubber,  usually 
a  long  U-tube  packed  with  moist  cotton  or  asbestos. 


A  pulverulent  precipitate,  especially  when  but  recently  formed,  may  be  so 
finely  divided  as  to  pass  through  the  pores  of  even  a  thick  or  specially  prepared 
paper.  It  may  save  a  refiltration  to  replace  the  beaker  receiving  the  filtrate  by 
another,  after  decanting  the  clear  liquid  but  before  transferring  the  precipi- 
tate to  the  filter,  and  should  the  filtrate  or  washings  be  turbid,  to  return  it  to 
the  filter  until  what  it  passes  is  perfectly  clear. 

When  it  is  anticipated  that  a  precipitate  is  so  finely  divided  that  it  will  pass 
the  paper:  (1),  two  filters  should  be  used,  inclosing  one  within  the  other 
with  the  plications  counterposed  so  as  to  bring  the  single  thickness  of  the  in- 
terior against  the  triple  fold  of  the  exterior.  Sometimes  a  triple  filter  of  the 
ordinary  quality  is  safer,  although  one  paper  of  a  specially  close  texture  (such 
as  the  Dreverhoff's  No.  400),  or  paper  that  has  been  indurated  may  answer; 
(2),  after  decanting  the  clear  fluid,  the  precipitate  may  be  stirred  up  with 
some  inert  fibrous  or  gelatinous  substance  to  entangle  the  fine  powder,  such 
as  cellulose  (prepared  by  beating  up  filter  paper  with  strong  hydrochloric  acid 
and  washing  the  residue);  recently  precipitated  aluminum  hydrate;  albumin, 
to  be  subsequently  coagulated  by  boiling  the  solution;  or  a  few  drops  of 
collodion,  water  precipitating  the  pyroxylin  from  its  alcohol  ether  solution; 
(3),  heating  the  fluid  before  or  after  precipitation  causes  the  finer  particles 
to  coalesce  to  larger  ones;  moreover  a  hot  liquid  always  passes  through  paper 
more  rapidly  than  when  cold  by  reason  of  its  lessened  viscosity;  (4),  col- 
loidal precipitates  filter  clear  when  a  certain  proportion  of  an  inorganic  salt  is 
dissolved  in  the  supernatant  liquid.  The  character  of  the  precipitate  and  the 
after  treatment  intended  will  determine  which,  if  any,  of  the  above  may  be 
adopted. 

On  the  other  hand,  many  precipitates  and  residues  are  of  a  nature  tending  to 
clog  the  pores  of  the  paper  and  retard  the  filtration  to  an  annoying  extent. 
Ribbed  or  corrugated  filters,  funnels  internally  fluted,  and  other  devices  to  in- 
terrupt contact  of  the  paper  with  the  glass,  while  materially  hastening  filtra- 
tion, are,  from  the  difficulty  or  impossibility  of  thoroughly  washing  the 
precipitate  and  filter,  limited  to  such  flltrations  as  require  only  the  examina- 
tion of  an  aliquot  part  of  the  filtrate. 


96  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Generally,  such  precipitates  are  best  handled  by  heating  the  fluid  before 
precipitation,  afterwards  boiling  for  a  short  time  —  long  boiling  is  apt  to  cause 
the  precipitate  to  become  slimy.  The  washing  is  done  by  decantation  keeping 
the  precipitate  in  the  beaker,  as  far  as  may  be,  until  the  washing  is  nearly 
completed.  As  in  the  case  of  a  pulverulent  precipitate,  a  vehicle  may  be 
intermixed  with  the  liquid,  but  here  a  dense  powder  like  silica,  kaolin, 
magnesia  or  calcium  carbonate,  though  it  must  always  be  predetermined  that 
no  soluble  matter  is  absorbed  by  the  powder  from  the  solution. 

When  a  liquid  or  precipitate  must  be  shielded  from  the  oxygen  of  the  air 
during  filtration,  a  device  similar  to  the  Carmichael  filter  (page  94)  may  be 
fitted  to  a  wide-mouth  bottle  through  the  cork  of  which  passes  also  a  tube 
conveying  a  current  of  some  non-oxidizing  gas,  and  a  funnel- tube  to  introduce 
the  precipitant  and  wash  water.  In  filtering  dilute  solutions  of  the  caustic 
alkalies  or  the  earths,  a  jet  of  hydrogen  played  over  the  surface  of  the  liquid  in 
the  filter  is  a  sufficient  protection  from  the  carbon  dioxide  of  the  air. 

WASHING  OF   PRECIPITATES. 

After  filtration,  the  impurities  from  which  a  precipitate  or  residue  must  be 
freed  before  it  is  weighed  maybe:  — 

1.  Mechanically  adherent  or  entangled;  as  a  solution  of  sodium  phosphate  to 
a  precipitate  of  calcium  phosphate,  or  inclosed  in  a  curdy  or  gelatinous  one, 
as  sodium  chloride  in  chromium  hydrate. 

2.  Suspended  matter  or  colloidal  bodies  carried  down  by  the  precipitate. 

3.  Co-precipitated  by  some  secondary  reaction;  as  in  the  separation  of  ferric 
chloride  from  manganous  chloride  by  ammonia,  where  the  ferric  hydrate  always 
contains  some  manganic  hydrate  formed  by  the  decomposition  of  the  soluble 
ammonium  manganous  chloride  by  absorption  of  oxygen  from  the  air;   sulfur 
accompanying  a  metallic  sulfide;  etc. 

4.  Chemically  combined  in  the  form  of  a  double  salt  or  other  complex ;  as 
when  lead  acetate  is  compounded  with  potassium  sulfate,  there  is  precipitated 
potassium  lead  sulfate  which  is  but  slowly  decomposed  by  water  into  the  simple 
sulfates. 

The  impurities  of  (1)  can  usually  be  removed  by  sufficient  washing  with 
water  or  other  fluid,  though  sometimes  only  completely  after  a  structural 
change  in  the  precipitate ;  those  of  (2)  and  (3)  usually  only  by  solution  of  the 
precipitate  and  reprecipitation ;  and  of  (4)  by  one  or  the  other  of  these  accord- 
ing to  circumstances. 

Washing.  As  a  rule,  the  smaller  the  proportion  of  any  body  in  solution,  the 
less  it  tends  to  contaminate  a  precipitate  formed  therein,  so  that,  other  condi- 
tions being  the  same,  a  precipitate  will  be  purer  the  greater  the  volume  of  the 
solution  from  which  it  falls ;  this  in  addition  to  the  influence  of  high  dilution 
toward  the  slow  collection  of  a  precipitate,  a  condition  favorable  to  purity. 
But  we  are  restrained  from  any  considerable  dilution  of  the  liquid  on  account 
of  the  loss  incurred  through  the  greater  or  less  solubility  of  all  precipitates, 
not  to  speak  of  the  time  lost  in  concentrating  the  unwieldy  filtrate  previous  to 
further  treatment,  and  so  are  restricted  to  dilution  of  the  fluid  immediately 
surrounding  the  precipitate  after  the  major  part  of  the  liquid  has  been  decanted 
through  a  filter.  By  successive  dilutions  and  decantations  the  soluble  impuri- 
ties are  rapidly  reduced  to  the  point  of  practical,  though  never  entire,  elimina- 
tion. Tables  have  been  published  showing  the  rate  of  this  reduction  when 
the  bulk  of  the  precipitate  and  volume  of  water  used  are  approximately  known, 
and  the  number  of  times  a  given  precipitate  must  be  washed  with  given  vol- 
umes of  water  to  practically  free  it  from  impurities;  but  owing  to  diff  rences 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


97 


in  the  structure  and  agglomeration  of  precipitates  formed  under  varying  con- 
ditions, and  other  factors,  they  are  of  little  practical  value. 

Water  is  used  for 'the  detersion  of  a  precipitate  formed  in  an  aqueous  solu- 
tion, unless  debarred  (1),  by  its  solvent  effect,  when  another  fluid  or  a  solution 
is  substituted,  as  alcohol  for  calcium  sulfate,  ether  for  some  alkaloids,  or  a 
saturated  solution  of  the  same  compound  as  the  (sparingly  soluble)  precipitate 
to  remove  impurities  from  an  alkaloidal  salt;  (2),  by  allowing  oxidation  —  many 
metallic  sulfldes  must  be  shielded  from  the  air  by  saturating  the  wash  water 
with  hydrogen  sulfide;  (3),  by  its  action  to  hydrolize  or  decompose  the  pre- 
cipitate, as  barium  stearate  resolved  by  water  into  baryta  and  stearic  acid;  or 
(4),  by  its  tendency  to  disintegrate  colloidal  precipitates  which  lose  their  orig- 
inal coherency  and  incline  to  run  through  the  filter  —  with  these  must  be  used 
a  dilute  solution  of  some  salt,  preferably  one  of  ammonium  if  the  precipitate  is 
to  be  afterward  ignited  and  weighed. 

Certain  imparities,  soluble  bat  firmly  adhering  to  the  precipitate,  may  be  transformed 
by  a  suitable  reagent  in  the  washing  flaid  to  another  soluble  combination  and  pass  entirely 
into  the  washings.  The  purification  of  the  precipitate  is  to  be  credited  rather  to  the 
effect  of  a  physical  change  destroying  the  adhesion  than  to  the  superior  solubility  of 
the  new  compound.  Even  insoluble  impurities,  if  In  small  amount,  may  be  changed  to 
soluble  forms  and  washed  out,  though  the  chances  of  failure  are  greater  than  in  the  former 
case.  Similarly,  a  washing  fluid  may  be  so  compounded  as  to  Induce  a  physical  change  in 
some  precipitates  —  as  from  flocculent  to  pulverulent,  or  amorphous  to  crystalline  —  and 
free  occluded  or  adherent  Imparities. 

An  alternation  of  two  different  washing  fluids  is  sometimes  of  advantage.  One  of  the 
impurities  to  be  washed  out  may  be  freely  soluble  in  the  first  and  but  sparingly  in  the 
second,  another  the  reverse;  an  impurity  may  be  present  that  tends  to  decompose  with 
the  first  to  some  insoluble  form  and  must  be  brought  back  to 
its  original  soluble  combination  by  the  second;  an  undesir- 
able physical  change  in  the  precipitate  may  be  wrought  by 
the  first  and  the  original  condition  restored  by  the  second; 
etc. 

Wash-bottle.  A  stream  of  water  is  furnished  by  a 

wash-bottle,  Fig.  87.    It  is  a  flat -bottomed    glass 

flask,  thin  if  to  withstand  heating,  closed  by  a  doubly 

perforated  cork.    Through  one  hole  passes  a  glass 

tube  A  reaching  nearly  to  the  bottom  of  the  flask  and 

beet  above  to  an  angle  of  about  135  ° .  Joined  to  the 
end  of  the  bent  limb  by  rubber 
tubing  is  a  short  tube  B  drawn  down 
to  a  fine  orifice,  the  flexible  joint 
allowing  the  stream  to  be  thrown  in 

any  direction.  Through  the  other  perforation  a  second  tube 
C  reaches  just  below  the  cork,  and  is  bent  to  about  45  °  from 
the  vertical.  On  blowing  into  D  a  fine  jet  of  water  is 
ejected  from  B. 

Two  wash- bottles  should  be  provided  for  general  work, 
one  for  cold  and  one  for  hot  distilled  water.  Others  of 
smaller  capacity  for  dilute  hydrochloric  acid,  dilute  ammonia, 
and  a  small  one  for  the  occasional  use  of  the  less  common 
washing  fluids,  are  not  infrequently  needed.  To  protect  the 
Fig.  88.  hand  the  neck  Qf  tne  Q0t  water  flagk  ig  covere(j  with  some 

non-conductor  like  twine,  chamois -skin,  or  cork.  For  dilute  ammonia  hydro- 
gen sulflde  and  the  like,  the  air  pressure  may  be  produced  more  pleasantly 
to  the  operator  by  the  compression  of  a  rubber  bulb  slipped  over  D  or,  as 
In  Fig.  88,  C  may  be  terminated  by  a  rubber  tube  the  bottom  of  which  is 


Fig.  87. 


98  QUANTITATIVE    CHEMICAL    ANALYSIS. 

plugged  by  a  short  glass  rod;  a  longitudinal  slit  F  between  the  rod  and  C 
forms  a  valve  closing  against  external  pressure;  a  third  glass  tube  G,  open 
at  both  ends,  extends  through  the  cork,  and  when  the  upper  end  is  stopped 
by  the  finger  and  the  air  in  the  flask  compressed  by  blowing  into  C,  the 
stream  of  water  continues  until  the  finger  is  lifted. 

For  highly  volatile  liquids  the  tubes  may  be  provided  with  stop-cocks  or  otherwise  ar- 
ranged to  prevent  evaporation.  Corrosive  or  fuming  liquids  will  soon  destroy  a  cork  or  rub- 
ber stopper,  and  for  these  a  bottle  may  be  had  made  entirely  of  glass,  the  stopper  ground 
in.  Another  form  of  wash  bottle  is  simply  a  small  flask  with  a  cork  through  which  passes 
a  single  tube  drawn  to  a  fine  orifice;  on  grasping  the  bottle  firmly  and  inverting,  the  air 
within,  expanded  by  the  heat  of  the  hand,  forces  out  a  part  of  the  liquid. 

Where  many  precipitates  are  to  be  washed,  a  large  bottle  of  water  is  set  on  a  high  shelf 
at  the  rear  of  the  work  table,  and  a  syphon  Introduced,  the  outer  limb  entering  a  long 
rubber  tube  bearing  a  spring  pinch-cock  and  terminating  in  a  short  piece  of  glass  tubing 
with  a  small  orifice.  An  elevation  of  the  bottle  three  or  four  feet  above  the  filter  stand  Is 
sufficient  to  eject  the  water  with  considerable  force.* 

There  are  a  number  of  devices  for  the  automatic  continuous  washing  of  precipitates  on 
the  principle  of  the  Mariotte  bottle,  as  in  Fig.  45.  Generally  speaking  this  method  of 
percolation  cannot  be  recommended  for  the  reason  that  the  precipitate  receives  no  stirring 
and  parts  become  less  compacted  through  which  the  water  descends  preferentially,  to  the 
privation  of  the  remainder.! 

The  operation  of  washing  may  be  conducted  either  entirely  by  decantation  as 
described,  or  after  transferring  the  precipitate  to  the  filter.  Each  way  has  cer- 
tain merits  and  a  combination  of  the  two  is  usually  resorted  to  in  this  manner; 
after  filtering  the  clear  supernatant  liquid,  the  precipitate  is  roused  with  water 
(or  other  washing  fluid),  allowed  to  settle,  and  the  liquid  poured  on  the  filter? 
repeating  this  one  or  more  times,  the  precipitate  is  transferred  by  inverting  the 
beaker  over  the  funnel  and  washing  out  the  sediment  by  a  stream  from  the 
wash-bottle.  Any  particles  adhering  to  the  beaker  so  firmly  that  the  stream 
will  not  remove  them  are  dislodged  by  a  short  piece  of  black  rubber  tubing 
drawn  over  the  end  of  a  glass  rod,  or  a  conical  tip  of  soft  rubber  fixed  to  a 
vulcanite  rod  (a  " policeman"). 

The  stream  should  be  directed  first  around  the  funnel  above  the  filter,  then 
more  forcibly  into  the  precipitate  to  break  up  any  channels  formed,  and  at  the 
final  addition  the  precipitate  is  brought  well  down  into  the  apex  of  the  filter. 
It  is  obvious,  considering  the  process  as  a  course  of  successive  dilution,  that 
with  a  given  amount  of  water  the  smaller  the  volume  introduced  at  each 
washing  the  more  thoroughly  will  the  precipitate  be  cleansed,  and  for  this 
reason  each  addition  is  made  after  the  preceding  one  has  quite  run  through. 
With  a  bulky  precipitate  it  is  well  to  allow  a  short  time  between  successive 
washings  that  the  adhering  mother  liquor  may  by  diffusion  become  equally 
distributed  throughout  the  precipitate. 

Some  precipitates  have  a  fashion  of  creeping  above  the  edge  of  the  filter 
during  the  washing,  then  running  down  the  exterior  at  its  crease.  This  is 
prevented  by  slightly  greasing  the  edge  with  pure  vaseline;  a  funnel  roughened 
interiorly  by  grinding  is  said  to  obviate  this  tendency. 

Owing  to  evaporation  the  upper  part  of  the  filter  becomes  more  charged  with 
the  soluble  matter  and  care  must  be  taken  that  it  receives  a  due  share  of  the 
wash  water.  If  it  is  desired  to  limit  the  amount  of  wash  water,  after  washing 
a  few  times  the  upper  edge  of  the  filter  may  be  cut  off,  cut  into  pieces,  and 
thrown  in  with  the  precipitate,  and  the  washing  completed. 

The  washings  are  usually  allowed  to  flow  into  the  beaker  containing  the 
filtrate.  But  should  the  precipitate  be  of  such  a  nature  that  it  tends  to  run 


*  Journ.  Anal.  App.  Chem.  1893—126. 
f  Chem.  News,  1892—2—55. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  99 

through  the  paper  on  washing,  or  if  for  any  reason  it  is  not  judicious  to  expose 
the  filtrate  to  the  heat  of  evaporation  in  a  subsequent  concentration,  the 
washings  are  collected  separately  from  the  filtrate. 

A  liquid  residue  or  precipitate  is  washed  in  the  same  manner  as  a  solid, 
but  the  filter  is  always  to  be  kept  partly  filled  with  the  wash  water  to  prevent 
any  of  the  liquid  running  through. 

Successive  portions  of  wash  water  are  run  through  until  an  appropriate 
test  shows  that  the  attenuation  has  reached  a  point  where  the  weight  of  the 
precipitate  will  not  be  sensibly  augmented  by  the  non- volatile  impurities 
remaining,  or  that  the  chemical  action  of  the  remaining  impurities  that  are 
volatilized  on  the  ignition  of  the  precipitate  may  be  neglected. 


The  testing  of  the  last  washing  may  be  done  in  several  ways  according  to 
the  nature  of  the  impurities.  As  the  final  washing  represents  an  extremely 
dilute  solution  it  is  always  well  to  apply  the  test  to  a  volume  of  several  cubic 
centimeters  rather  than  the  few  drops  sometimes  directed,  as  the  indications 
are  more  apparent. 

1.  By  observing  whether  a  sensible  residue  is  left  on  evaporation.    This  pro- 
ceedure  is  of  course  invalid  where  the  precipitate  is  not  highly  insoluble,  or 
where  the  impurities  are  volatile  on  evaporation. 

2.  By  the  formation  of  a  precipitate  or  coloration  with  a  reagent.    A  little  of 
the  final  washing  is  caught  in  a  test-tube  and  examined  for  the  precipitant  or 
that  compound  that  is  present  in  the  solution  in  the  largest  amount  and  pre- 
sumably retained  in  the  greatest  proportion.    Here  it  is  essential  that  the  quali- 
tative test  be  a  very  delicate  one,  and  it  is  best  to  view  the  mixed  liquids 
vertically  in  a  test  tube  as  a  change  in  color  or  a  turbidity  will  be  more  plainly 
shown. 

3.  By  some  physical  property,  best  observed  on  the  predominating  constitu- 
ent of  the  solution.    The  rate  of  removal  of  a  highly  colored  solution  may  be 
traced  by  the  gradual  lightening  of  the  washings  and  also  of  the  precipitate  and 
filter  paper,  but  it  is  always  a  safer  plan  to  continue  washing  for  a  few  times 
after  the  color  has  entirely  disappeared.     Some  intensely  sweet  or  bitter  or- 
ganic principles  may  be  detected  in  highly  dilute  solutions  by  taste  far  more 
certainly  than  in  any  other  way,  but  caution  should  be  observed  in  tasting  even 
dilute  solutions  of  highly  poisonous  alkaloids  or  other  bodies.    And  the  re- 
moval of  a  substance  having  a  pronounced  or  persistent  odor  is  known  by  the 
washings  becoming  nearly  odorless. 

Occasionally  a  precipitate  is  met  with  that  is  stable  and  insoluble  in  the  pres- 
ence of  some  constituent  of  the  solution  in  which  it  has  been  formed,  but  be- 
gins to  decompose  or  dissolve  when  the  constituent  has  been  removed,  or 
nearly  so,  by  washing.  The  disappearance  of  the  constituent  in  question  from 
the  washings  or  the  appearance  of  the  precipitate  or  a  dissociation  product 
therein  is  a  warning  to  terminate  the  process;  unless,  of  course,  it  is  allow- 
able that  the  constituent,  another  of  the  same  character,  or  a  dissociation 
product  may  be  dissolved  in  the  wash  water. 

When  a  precipitate  on  a  filter  is  to  be  redissolved,  an  ample  quantity  of  the 
solvent  may  be  poured  on  it  and  returned  when  run  through,  until  all  is  dis- 
solved, after  which  the  solution  adhering  to  the  filter  is  washed  away.  If  the 
precipitate  is  bulky  and  one  wishes  to  limit  the  amount  of  solvent,  it  is  trans- 
ferred to  a  beaker  by  a  horn  or  platinum  spatula,  or,  holding  the  funnel  hori- 
zontally, washed  out  by  a  stream  from  the  wash  bottle;  what  remains  adhering 
to  the  filter  is  dissolved  by  washing  with  tbe  solvent,  then  with  water.  When 
i he  precipitate  is  to  be  dissolved  in  a  moderately  strong  acid  or  alkali  solution, 


ItiO  QUANTITATIVE    CHEMICAL    ANALYSIS. 

only  the  latter  plan  is  feasible,  as  the  paper  would  be  broken  were  the  solvent 
poured  directly  into  the  filter. 

IGNITION  —  ROASTING. 

Before  proceeding  to  weigh  a  precipitate  or  residue  it  must  be  freed  from 
moisture  and  brought  to  the  state  of  a  definite  chemical  compound  if  not 
already  so.  The  water  is  removed  by  drying;  the  heat  limited  to  100°  or  be- 
low where  volatile  matter  is  contained  that  is  not  to  be  eliminated,  otherwise 
by  the  temperature  of  melting  or  decomposition;  though  a  volatile  constituent 
m  y  be  purposely  expelled  by  heating  should  the  remainder  be  more  stable  or 
have  a  less  questionable  composition  than  the  original. 

Drying.  Precipitates  wholly  or  in  part  decomposed  or  volatile  at  tempera- 
tures above  100°  Cent,  are  dried  in  the  water -oven.  For  holding  the  filter 
during  the  drying  and  weighing  there  can  be  used  two  watch-glasses  of  the 
same  diameter  bound  together  by  a  brass  spring-clip,  Fig.  31.  The  filter  is 
folded  and  dried  on  one  glass,  then  the  other  clamped  to  it  and  the  whole 
cooled  in  the  desiccator  and  weighed.  A  weighing-bottle  of  light  glass,  Fig. 
34,  is  a  convenient  substitute  for  the  watch-glasses.  After  the  filtration  and 
washing,  the  filter  with  the  precipitate  is  dried,  cooled,  and  weighed  In  the 
same  way.  The  drying  should  be  repeated  until  there  is  no  further  loss  in 
weight.*  For  the  comparatively  few  precipitates  that  are  altered  at  a  tempera- 
ture below  100  ° ,  the  heat  is  limited  to  a  temperature  as  far  below  that  where 
decomposition  or  volatilization  begins  as  prudence  dictates. 

If  it  be  feared  that  the  fluid  passing  through  will  slightly  dissolve  and 
lessen  the  weight  of  the  paper,  or  increase  it  (e.  g.,  barium  hydrate  is  retained 
to  a  slight  extent  in  spite  of  protracted  washing),  two  filters  from  the  same 
sheet  are  balanced  by  clipping  their  edges,  put  in  funnels,  and  the  filtrate 
from  one  passed  through  the  other.  After  washing  each  separately  and 
drying,  one  is  placed  on  each  pan  of  the  balance,  the  difference  in  weight 
being  that  of  the  precipitate. 

It  is  seldom  necessary  to  protect  a  precipitate  from  oxidation  or  absorption 
of  carbonic  acid  from  the  air  during  the  desiccation,  but  if  so,  the  drying 
tube,  described  on  page  28,  may  be  the  container. 

Ignition.  The  majority  of  inorganic  precipitates  can  be  heated  to  dull  red- 
ness without  fear  of  decomposition,  fusion,  or  oxidation  by  the  air.  For 
practical  reasons,  ignition  of  a  precipitate  is  usually  preferred  to  the  process 
of  drying  as  described  above,  a  choice  of  the  two  being  allowed.  The  filter 
is  burned  and  the  precipitate  heated  to  the  proper  temperature  in  a  small 
crucible  of  metal  or  porcelain  provided  with  a  loosely  fitting  lid,  and  weighed 
therein. 

Crucibles.  Those  made  of  platinum  are  rightly  held  in  high  esteem  by  the 
chemist  for  their  many  desirable  qualities,  and  are  always  used  in  preference  to 
those  of  other  materials  unless  there  are  good  reasons  to  the  contrary.  They 
may  be  heated  and  cooled  rapidly  without  danger  of  fracture,  withstand  even  a 
white  heat  for  an  indefinite  time  without  softening  or  oxidation  or  any  consid- 
erable alteration  in  weight,  resist  the  solvent  action  of  most  chemicals,  and  are 
not  brittle  or  easily  injured.  But  certain  reagents  and  fluxes  cannot  be  heated 
in  platinum  without  danger  of  corroding  it  and  introducing  platinum  into  the 
material  contained;  such  are  the  sulfldes  of  easily  fusible  metals,  and  their 
oxides,  carbonates,  sulfates,  phosphates  and  arseniates  when  associated  with 
carbon  or  organic  compounds;  free  sulfur;  baryta  and  the  fixed  caustic  alkalies 
and  their  nitrates ;  and  liquids  containing  or  generating  free  chlorine  or  bro- 


*  Chcm.  Nowrs,  1892-2—25. 


QUANTITATIVE   CHEMICAL   ANALYSIS,./    ,   ; 


mine.  Metals  fusible  at  the  heat  applied  to  the  crucible  will  alloy  with  the 
platinum  and  perforate  it. 

Two  shapes  of  platinum  crucibles,  Fig.  89,  are  manufactured,  known  as  the 
tall  and  broad  forms,  the  former  being  the  standard  shape  and  in  general  use. 
They  range  in  capacity  from  10  to  100  cubic 
centimeters,  and  weigh,  including  the 
cover,  about  as  many  grams  as  the  volumes 
of  water  held.  A  crucible  is  made  from  a 
circle  of  heavy  platinum  foil  spun  into  shape 
over  a  metal  form  leaving  the  crucible 
thicker  at  the  bottom  than  at  the  edge. 
Some  makers  supplement  the  spinning  by 
hammering  the  metal,  which  is  claimed  to  I 

compact  it  and  diminish  the* tendency  to    gjj&j-  •- ..  ,   .  .... •- 

superficial  loosening  or  "  blistering"  under 

prolonged  heating.    The  platinum  used  is  Fig.  89.     1/2~l/3 

the  pure  metal;  an  iridio-platinum  alloy  has  been  on  the  market  for  some 
years,  much  harder  and  stiffer  than  pure  platinum  and  less  liable  to  mechanical 
injury,  and  the  manufacturers  claim  to  have  overcome  its  great  defect,  a  tend- 
ency to  crack  during  service. 

A  light  crucible  is  quite  as  serviceable  and  long  lived  as  a  heavier  one  of  the 
same  size,  given  proper  treatment.  It  has  been  advised  that  before  putting  a 
crucible  into  use,  a  mould  be  made  by  filling  it  with  potassium  bisulfate  and 
melting  over  a  Bunsen  burner,  that  the  original  form  can  be  restored  if  bent 
or  bruised.  However,  the  only  occasion  for  distorting  a  crucible  is  for  the 
removal  of  an  obstinately  adhering  melt,  and  this  difficulty  may  nearly  always 
be  obviated  by  certain  precautions*  (page  55). 

Platinum  vessels  are  cleansed  after  use  by  rubbing  with  moist,  water-worn 
sand  or  precipitated  silica.  Refractory  stains  can  be  removed  by  digestion  in 
strong  hydrochloric  acid  or  rubbing  with  sodium  amalgam  followed  by  water, 
or  by  melting  sodium  carbonate  or  potassium  bisulfate  in  the  crucible  ;  these 
failing,  as  a  last  resort,  treatment  with  boiling  aqua  regia. 

A  crucible  of  the  usual  tall  form  of  a  size  holding  about  fifteen  cubic  centi- 
meters of  water  will  be  found  sufficiently  large  for  most  ordinary  analyses. 

A  porcelain  crucible,  Fig.  90,  is  substituted  where  platinum  would  be  injured 
by  the  substance  heated  therein,  or  a  precipitate  be  reduced  by  the  permeation 
of  gases  from  the  burner.  They  are  made  at  the  Royal 
Berlin  and  Meissen  potteries  of  the  finest  grade  of 
China  clay,  glazed  inside  and  out.  The  sizes  run  from 
5  to  300  cubic  centimeters  capacity.  Their  faults  are 
fragility  and  liability  to  crack  with  sudden  changes  of 
temperature,  and  the  difficulty  of  cleaning  them  after 
use,  other  than  by  hydrofluoric  acid.  The  sizes  best 
suited  for  general  analytical  work  are  Numbers  0  to  3. 

Silver  and  gold  or  gold  linedf  silver  crucibles  are  used 
Tf-a    QO     i/     i/          exclusively  for  fusions  in  which  a  caustic  alkali  or  baryta 
!g-      •     l\-  U        ig  the  flux.  tnese  metais  are  but  little  attacked  by  these 
chemicals  as  compared  with  platinum  —  gold  less  than  silver  —  but  have  the 
disadvantage  of  softening  and  melting  at  much  lower  temperatures  than  plat- 
inum.   Bunsen  proposed  to  line  the  interior  of  a  platinum  crucible  with  a 
layer  of  mercuric  oxide  to   prevent  the  corrosion  following  the  ignition  of 
certain  sulphides. 


*  Crookes'  Select  Methods,  461. 
t  Chem.  News,  1891-2-146. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


Nickel,  so  exploited  some  years  ago  as  a  material  for  crucibles,  has  not  ful- 
filled expectations,  proving  suitable  only  for  oxidizations  and  ignitions  at  low 

temperatures  and  for  fusions  with  al- 
kalies.* 

Appliances  for  ignition.  The  cru- 
cible is  supported  on  a  triangle  made 
of  platinum  wire  or  iron  wire  wrapped 
with  platinum  foil  or  surrounded  by 
pieces  of  pipe  stem  or  clay  tubes,  Fig. 
91,  resting  on  the  rirg  of  an  iron 
retort  stand.  The  most  convenient 
source  of  heat  is  the  ordinary  Bunsen 
burner,  the  air  supply  adjusted  to  give 
a  perfectly  non-luminous  flame  with 
the  interior  blue  cone  sharply  defined. 
The  bottom  of  the  crucible  should  al- 
ways be  above  the  cone,  since  contact 
with  it  opens  the  polished  surface  of 

Fig.  91.     J/2  platinum  to  a  canescent  coating,  also 

formed  when  the  burner  through  faulty  construction  yields  a  large  straggling 
flame.  Should-  the  flame  be  larger  than  is  desired,  a  short  glass  tube  fitting 
snugly  in  the  tube  and  having  the  upper  end  slightly  expanded  may  be  inserted. 
The  burner  is  shielded  from  draughts  by  surrounding  it  with  a  cylinder  of  fire 
clay  about  four  inches  in  diameter,  supported  on  three  iron  legs,  the  wire 
triangle  resting  on  the  upper  edge  of  the  cylinder. 

Instead  of  a  Bunsen  burner,  some  prefer  to  conduct  ignitions  In  a  muffle  heated  to  red- 
ness In  a  gas  or  coke  furnace;  a  miniature  muffle  of  platinum  foil  heated  by  Bunsen 
burners  has  been  recommended  for  some  technical  work.  The  open  crucibles  are  set 
just  inside  the  front  until  the  paper  is  burned,  then  gradually  retreated  to  the  hotter  in- 
terior. From  the  radiated  heat  and  free  access  of  air,  difficultly  combustible  forms  of 
carbon,  like  graphite  and  dense  coke,  are  more  rapidly  burned  than  over  a  free  flame,  and 
where  a  precipitate  would  be  affected  by  reducing  gases  from  a  gas  flame  permeating  the 
hot  platinum,  or  would  absorb  sulfur  gases  therefrom,  a  muffle  is  more  suitable. 

Blast- lamp.  A  temperature  higher  than  that  of  a  Bunsen  is  afforded  by  the 
blast-lamp,  shown  in  Fig.  92,  through  the  injection  of  a  jet  of  air  into  the 
blaze  at  A.  The  gas  enters  at  B,  and  the  air  at  C.  An  extension  sleeve  D 
slides  on  E,  and,  when  used  in  conjunction  with  the  proper  sized  tip  inserted 
in  F,  serves  to  adjust  the  flame  from  a  fine  point  to  a  wide  brush. 

The  compressed  air  is  furnished  by  a  foot  bellows  or  by  a  fan  driven  by  a 
water  or  electric  motor.  A  water  air-pump  is  shown  in  Fig.  93  delivering  the 
air  drawn  in  by  a  vacuum-pump  (page  93).  The  waste- 
pipe  from  the  pump  A  enters  a  metal  cylinder  B  in 
which  the  air  imprisoned  in  the  waste-water  is  liber- 
ated, the  water  flowing  away  through  the  trap  C.  The 
air  accumulating  in  B  gives  a  moderate  pressure  and 
is  connected  at  D  by  rubber  tubing  to  C,  Fig.  92. 

For  analytical  purposes  the  blast-lamp  is  seldom 
needed,  a  powerful  Bunsen  burner  meeting  all  the 
demands  of  ordinary  work  with  a  few  exceptions. 


Igniting  precipitates.  The  precipitate  may  be  re- 
moved from  the  filter  previous  to  ignition,  but  as  this 
is  not  easily  done  without  loss,  it  is  better,  unless 


Fig.  92.    V* 


*  Chem.  News,  1887—1—11  et  seq. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


103 


there  are  reasons  to  the  contrary,  to  burn  the  paper  with 
the  precipitate  inclosed.  Most  precipitates  are  not  altered 
on  ignition  in  contact  with  carbon,  or  but  transiently;  with 
these  the  filter  and  its  contents  are  partly  dried  by  opening  it 
on  several  sheets  of  filter  paper  or  a  bisque  clay  plate.  If 
the  filter  pump  has  been  used  in  the  filtration  the  effect  of  the 
pressure  on  the  precipitate  leaves  it  in  a  shrunken  compact 
condition,  so  free  from  adhering  water  that  it  can  be  at 
once  ignited  without  further  drying,  there  being  no  danger 
of  a  projection  of  the  precipitate  or  of  cracking  a  porcelain 
crucible. 

The  filter  is  then  folded  around  its  contents  and  pressed 
into  the  weighed  crucible.  If  a  ring  of  precipitate  is 
left  in  the  funnel  when  the  filter  is  withdrawn  it  is  wiped 

out  by  a  slip  of  FiS-  93- 
filter  paper  also  put  into  the  cruci- 
ble. The  covered  crucible  is 
heated  over  a  very  low  flame  until 
no  more  smoke  escapes,  then  turned 
on  its  side  and  the  cover  supported 
against  the  upper  edge,  Fig.  94;  a 
piece  of  platinum  foil  laid  under 
the  front  directs  a  current  of  air 
into  the  interior.  When  the  char 
has  burned  (if  necessary  turned 
over  occasionally  by  a  thin  platinum 
wire),  the  incrustation  of  carbon 

on  the  cover  is  dissipated  by  heating  to  redness;  the  crucible  is  turned  upright, 
covered,  and  heated  at  the  temperature  and  for  the  length  of  time  specified  in 
the  method;  transferred  to  a  desiccator,  and  when  quite  cold  is  ready  for  weigh- 
ing. If  the  crucible  is  weighed  as  soon  as  it  has  cooled,  inclosure  in  a  desic- 
cator is  not  imperative,  since  only  unweighable  traces  of  moisture  can  enter 
the  covered  crucible  in  this  short  Interval,  though  of  course  it  is  the  safer  pro- 
cedure with  compounds  at  all  hygroscopic. 

The  above  directions,  however,  will  not  apply  to  easily  reducible  metallic 
compounds,  as  a  considerable  weight  may  be  lost  from  reduction  or  volatiliza- 
tion on  ignition  with  carbon  or  reduc- 
ing gases.    With  one  of  this  character 
it  is  well  to  dry  the  filter  by  standing 
the  funnel  in  the  water-oven,  and  then 
remove  the  precipitate  from  the  paper 
as  completely  as  possible  by  rubbing 
the  sides  together    over   a   sheet  of 

glazed  paper.    After  burning  the  filter  Flg<  95-      '« 

to  an  ash  in  the  tared  crucible,  the  small  amount  of  precipitate  that  has  re- 
mained adhering  to  it  is  restored  to  the  original  composition  by  appropriate 
reagents  (e.  g.,  when  lead  sulfate  has  been  reduced  to  the  sulflde  or  metal,  a 
few  drops  of  dilute  nitric  acid  dissolves  the  residue  to  the  nitrate,  and  a  drop 
of  sulfuric  reprecipitates  it;  on  evaporating  the  water  and  excess  of  acids, 
lead  sulfate  remains) .  The  major  portion  of  the  precipitate  is  brushed  into 
the  crucible  which  is  then  heated  to  the  proper  temperature,  cooled,  and 
weighed. 
The  crucible  should  be  weighed  empty  before  use  as  traces  of  platinum  may 


104  QUANTITATIVE    CHEMICAL    ANALYSIS. 

be  dissolved  when  the  precipitate  is  removed  by  a  solvent;  and  if  ignited  over 
a  blast- lamp,  the  weight  should  be  taken  both  before  and  after  the  operation.* 

Another  way  to  burn  the  filter  is  to  fold  it  compactly  and  wind  around  it  the  end  of  a 
thin  platinum  wire ;  the  roll  is  kindled  and  held  over  the  crucible,  and  when  the  flame  has 
entirely  died  out,  the  char  is  burned  to  an  ash  by  bringing  a  Bunsen  flame  near  it,  allow- 
ing the  ashes  to  drop  into  the  crucible.  But  slight  draughts  of  air  may  easily  cause 
mechanical  losses. 

The  reduction  of  metallic  compounds  by  carbon  begins  only  at  a  fairly  high 
temperature,  hence  if  the  heat  of  combustion  be  kept  as  low  as  possible,  ordi- 
narily but  little  or  no  reduction  will  take  place.  With  some  precipitates  it 
may  be  the  better  plan  to  treat  the  washed  precipitate  on  the  filter  with  some 
volatile  solvent  and  catch  the  solution  running  through  in  a  tared  platinum 
dish,  then,  after  washing  the  Alter,  evaporate  the  solution  plus  washings,  dry 
or  ignite  the  residue  and  weigh.  The  solvent  may  be  chosen  to  effect  a  simple 
solution  only,  the  composition  of  the  residue  on  evaporation  being  the  same  as 
that  of  the  precipitate;  or  one  that  will  change  it  to  another  definite  combina- 
tion, stable  on  evaporation  and  drying  or  ignition. 

After  filtration  through  a  Gooch  crucible  by  the  aid  of  the  vacuum  pump,  the 
precipitate  is  dried  or  ignited  at  once  without  transference.  The  Gooch  is 
particularly  well  adapted  for  compounds  of  metals  that  are  affected  by  carbon 
on  ignition.  For  greater  security  against  loss  during  ignition  by  reason  of  the 
finer  particles  of  asbestos  or  the  precipitate  sifting  through  the  holes  in  the 
crucible,  a  movable  shoe,  Fig.  81,  may  be  fitted  to  the  bottom,  it  being  weighed 
as  part  of  the  crucible.  The  crucible  and  asbestos  felt  are  to  be  heated 
before  filtration  up  to  the  same  temperature  as  will  be  employed  in  the  sub- 
sequent ignition. 

Burning  filter  paper.  The  physical  character  of  the  carbonaceous  residue 
from  cellulose  heated  in  a  closed  crucible  is  determined  by  the  degree  of  heat 
employed,  varying  from  dull  black,  loose,  and  easily  burned,  to  glossy,  dense 
and  refractory,  and  therefore  the  temperature  in  charring  a  filter  should  never 
rise  so  high  that  the  escaping  smoke  will  burn  when  touched  with  aflame.  For 
incinerating  the  char,  the  under  side  of  the  crucible  should  be  only  dull  red, 
air  having  free  access  to  the  interior,  as  at  this  moderate  temperature  the 
extent  of  the  action  of  carbon  on  the  precipitate  is  a  minimum. 

When  a  precipitate  is  slightly  soluble  in  the  fluid  used  for  washing  it,  the 
pores  of  the  paper  remain  impregnated  with  the  solution,  and  the  combustion 
is  somewhat  retarded,  and  the  same  effect  will  be  noticed  when  the  precipitate 
is  of  such  a  nature  that  during  its  ignition  there  is  evolved  a  gas  which  is  a 
non-supporter  of  combustion. 

Filter  ash.  The  weight  of  the  ash  of  the  filter  is  deducted  from  the  total 
weight  of  the  contents  of  the  crucible.  By  burning  a  number  of  filters,  say 
ten,  in  a  crucible  and  weighing  the  total  ash  it  is  easy  to  compute  the  weight 
corresponding  to  a  square  centimeter  of  paper  and  to  a  filter  of  any  given 
diameter.  No  deduction,  however,  need  be  made  for  paper  which  has  previously 
been  extracted  by  hydrochloric  acid  (page  90)  or  when  an  acid  solution  has 
been  filtered  through  it,  as  the  ash  weight  is  here  inconsiderable. 

A  liquid  to  be  evaporated  to  dryness  to  obtain  the  weight  of  the  solids  con- 
tained is  concentrated  in  a  large  dish  to  a  small  bulk,  then  transferred  to  a 
small  tared  capsule  of  platinum  or  porcelain  and  the  evaporation  completed. 
For  evaporating  concentrated  solutions  that  are  apt  to  spatter  when  heated 
directly  over  a  Bunsen  burner,  Rogers  has  devised  a  special  burner  in  which 


*  Caldwell,  Chemical  Anal.  123. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  105 

several  small  jets  of  flame  are  directed  inwardly  from  a  ring  tube  toward  the 
upper  part  of  a  dish  or  crucible  supported  within  the  ring. 

When  a  compound  of  gold,  silver,  or  platinum  with  an  organic  radical  is 
heated  with  free  access  of  air,  there  is  left  a  residue  of  the  metal;  organic 
compounds  of  barium, strontium,  potassium  and  sodium  leave  carbonates;  and 
other  non-volatile  metals  remain  as  oxides.  Ignition  of  such  compounds 
should  be  carefully  performed  as  the  loose  powder  is  apt  to  be  carried 
away  in  the  escaping  gases.  Salts  which  decrepitate  on  heating  are  first 
thoroughly  dried  on  the  water-bath  and  then  exposed  in  a  covered  crucible  to 
a  very  gentle  heat  over  the  burner  until  crackling  ceases.  It  is  advisable  to 
inclose  the  crucible  in  a  larger  one,  both  covered,  and  weigh  them  together. 

To  convert  into  oxide  a  compound  of  an  easily  reducible  metal  with  an 
organic  radical,  the  compound  is  ignited  with  free  access  of  air,  the  residue 
treated  with  a  few  drops  of  nitric  or  fuming  nitric  acid,  evaporated,  and  again 
ignited.  On  heating  the  nitrates  or  chlorides  of  some  metals,  a  small  amount 
of  the  base  is  carried  off  in  the  escaping  vapors ;  this  does  not  occur  with  the 
sulfates,  therefore  it  is  well  to  add  a  slight  excess  of  this  acid  to  the  solution 
before  evaporation  to  dryness.  The  residue  is  heated  either  gently  and  weighed 
as  sulphate,  or  more  strongly  to  the  oxide,  according  to  the  metal  in  com- 
bination. 

Vegetable  bodies  contain  alkali  salts  which  tend  to  fuse  and  encyst  the  car- 
bon and  delay  or  entirely  prevent  complete  combustion ;  moreover,  if  the  heat 
exceed  dull  redness,  or  if  a  jet  of  oxygen  be  played  over  the  surface  to  hasten 
the  burning,  there  will  be  a  liability  of  the  volatilization  of  part  of  the 
alkalies.  To  meet  these  difficulties  various  plans  of  calcination  have  been  pro- 
posed. Perhaps  the  one  most  used  is  that  of  first  charring  the  substance  at  a 
very  low  heat,  then  lixiviating  the  soluble  salts  with  water;  the  residual  carbon 
burns  easily,  and  to  the  inorganic  residue  is  returned  the  aqueous  solution, 
the  whole  evaporated  to  dryness,  gently  ignited,  and  weighed. 

Another  way  of  calcining  vegetable  substances  Is  that  of  Flucklnger,*  who,  to  prevent 
the  material  from  burning  with  a  flame,  would  calcine  It  in  a  platinum  dish  covered  by  a 
sheet  of  platinum  gauze.  When  the  volatile  matter  Is  expelled,  the  residue  is  treated 
with  water,  evaporated,  and  burned  to  an  ash,  this  repeated  If  particles  of  carbon 
remain.  Stone  and  Dickson.j  in  the  determination  of  the  ash  of  sugar  syrups,  aim  to  pre- 
vent the  great  swelling  up  in  carbonization,  unmanageable  with  large  weights  of  syrup. 
They  fill  a  small  pipette  with  the  syrup,  weigh  and  bang  over  a  small  platinum  dish  kept 
at  a  red  heat.  The  syrup  is  allowed  to  fall  drop  by  drop' into  the  dish,  each  drop  carbon- 
izing before  the  succeeding  one  falls.  When  a  sufficient  quantity  has  been  withdrawn 
the  pipette  is  reweighed.  The  char  is  extracted  by  water  and  proceeded  with  as  usual. 

As  the  rapidity  with  which  carbon  is  consumed  depends  largely  on  a  free 
access  of  oxygen,  in  certain  cases  the  particles  of  an  organic  substance  in  pow- 
der may  be  dispersed  by  the  admixture  of  several  times  its  bulk  of  some  in- 
fusible inorganic  powder,  allowing  the  air  to  easily  permeate  the  blend.  The 
usual  diluents  for  the  purpose  are  magnesia,  silica,  precipitated  silver,  or  some 
easily  reducible  metallic  oxide;  as  all  of  these  are  insoluble  in  water,  the 
soluble  constituents  of  the  ash  may  be  lixiviated.  For  the  absorption  of  cer- 
tain products  of  the  combustion  of  bodies  containing  acidogens,  there  may  be 
added  an  alkali  carbonate.  This  plan  can  be  followed  with  safety  in  the  deter- 
mination of  the  ash  of  bodies  that  explode  on  heating. 

Vegetable  matter  leaving  a  compact  coke  on  strong  heating  may  be  diluted  with  one  of 
these  powders  to  advantage.  ShuttleworthJ  would  mix  vegetable  matter  with  a  measured 


*  Zeits.  Anal.  27—637. 

t  Journ.  Anal.  Appl.  Chem.  1893—319. 

J  Analyst,  1899—271. 


106  QUANTITATIVE    CHEMICAL    ANALYSIS. 

volume  of  a  solution  of  calcium  acetate  of  known  concentration,  then  evaporate  to  dry  - 
ness;  the  calcium  oxide,  formed  from  the  acetate  when  the  mixture  is  Ignited,  not  only 
Jhastens  combustion  but  makes  the  ash  more  refractory.  The.  weight  of  extrinsic  lime  in 
the  ash  is  calculated  from  the  volume  of  the  solution  of  calcium  acetate. 

Another  plan  Is  that  of  moistening  with  a  solution  of  some  organic  compound  that  is 
volatile  at  a  moderate  heat,  such  as  benzoic  acid;  on  evaporation  and  ignition  the  vapors 
evolved  cause  the  residue  to  swell  and  become  spongy  and  easier  of  combustion. 

Some  organic  bodies  burn  readily  and  completely  when  moistened  with  strong  nitric 
acid,  dried,  and  ignited,  at  first  gently,  then  to  full  redness.  A  probable  alteration  of  the 
composition  of  the  ash  must  be  considered,  however. 

On  igniting  a  complex  substance  in  the  air,  the  inorganic  residue  may  not  be 
left  of  the  same  composition  as  in  the  original,  being  decomposed  by  heat 
alone,  oxidation  by  the  air,  or  reduction  by  carbon,  or  by  two  or  all  of  these. 

For  example,  the  ash  of  vegetable  matter  is  largely  in  the  form  of  carbonates  of  the 
alkalies  and  earths,  resulting  from  the  combustion  of  the  original  organic  salts.  In  re- 
porting results  some  analysts  deduct  the  carbon  dioxide  and  return  the  remainder  (oxides) 
as  the  ash;  others  attempt,  often  with  doubtful  warrant,  to  compute  the  original  composi- 
tion from  an  analysis  of  the  ash.  Where  the  inorganic  bases  have  been  converted  to  sul- 
iates  by  evaporation  with  sulfuric  acid,  the  result  is  reported  as  "  sulfated  ash,"  or  a  con- 
ventional deduction  is  made  for  the  sulfuryl.  Sometimes  the  residue  may  be  reconverted 
to  the  original  composition  by  treatment  with  appropriate  reagents,  evaporation,  and 
drying  at  a  low  temperature. 


Although  it  may  be  said  that  the  majority  of  precipitates  are  weighed  in  the 
same  chemical  combination  as  thrown  down  from  solutions  yet  exceptions  are 
numerous.  The  precipitate  may  be  of  a  somewhat  indefinite  composition  or 
contain  another  body  admixed  in  indefinite  proportion;  or  the  composition  is 
changed  in  part  or  entirely  to  another  by  the  action  of  the  heat  of  drying  or 
ignition  to  volatilize  some  constituent  or  produce  an  inter-reaction,  by  the  re- 
ducing effect  of  carbon  or  the  pyrogens  of  the  filter  paper,  by  oxidation  by  the 
air,  or  from  the  action  of  aqueous  vapor  or  carbon  dioxide.  Precipitates  or 
residues  from  evaporation  or  partial  solution,  of  the  nature  described  above,  are 
brought  to  a  combination  suitable  for  weighing  in  the  following  ways: — 

1.  By  heat  alone.  Some  one  volatile  constituent  is  expelled,  proportionally 
reducing  the  molecular  weight,  or  extrinsic  volatile  matter  mixed  with  the 
precipitate  is  driven  out.  The  change  in  composition  is  usually  indicated  by 
some  physical  alteration  of  the  precipitate.  That  a  definite  compound  results 
is  more  certain  if  the  change  in  composition  is  induced  only  at  a  high  tempera- 
ture; if  at  one  more  moderate,  it  is  essential  that  no  further  alteration  will 
occur  should  the  heat  be  increased. 

By  ignition,  a  higher  oxide  or  a  mixture  of  several  higher  oxides  may  be 
brought  to  one  definite  stable  lower  oxide,  oxygen  escaping;  some  per-salts 
are  changed  to  proto-salts ;  ammonium  salts  with  volatile  radicals,  loose-bound 
halogens,  free  sulfur,  and  carbon  dioxide  pass  off  as  such;  inorganic  bases 
combined  with  organic  radicals,  and  many  sulfates  and  nitrates  are  decomposed, 
leaving  residues  of  carbonates  or  oxides;  etc.  In  most  cases  the  original 
composition  can  be  restored  by  moistening  the  residue  with  a  solution  of  a 
reagent  wholly  volatile  containing  the  element  or  radical  driven  off,  then 
gently  igniting;  e.  g.,  calcium  carbonate  on  intense  ignition  passes  to  calcium 
oxide,  but  on  moistening  the  oxide  with  a  saturated  solution  of  ammonium 
carbonate,  it  is  reconverted  to  the  carbonate. 

The  water  of  constitution  of  stable  bodies  is  usually  determined  by  igniting 
the  substance  in  a  closed  platinum  crucible.  The  loss  in  weight  is  assumed  to 
be  combined  water  only,  but  may  include  other  volatile  constituents  and  easily 


QUANTITATIVE    CHEMICAL    ANALYSIS.  107 

combustible  carbonaceous  matter;  moreover,  the  composition  of  the  residue 
may  have  undergone  a  change  by  internal  reactions,  oxidation  by  the  air  or 
aqueous  vapor,  etc. 

A  direct  determination  of  combined  water  is  always  advisable  where  the 
composition  of  the  material  in  hand  is  not  known  with  certainty.  The  ap- 
paratus is  a  long  porcelain  combustion  tube  laid  horizontally  in  a  combustion 
furnace  (page  296)  and  connected  at  one  end  to  a  source  of  dry  air,  and  at  the 
other  to  a  weighed  U-tube  containing  dried  calcium  chloride.  The  substance 
is  dried  at  100  °  and  a  suitable  amount  weighed  in  a  porcelain  boat;  the  boat 
is  inserted  midway  in  the  tube  and  the  latter  connected  up  air-tight  with  the 
U-tube.  A  slow  current  of  dried  air  is  passed  through  the  train  while  the 
porcelain  tube  is  heated  about  the  middle  to  bright  redness.  The  U-tube  is 
detached,  cooled  and  weighed,  the  increase  being  the  water  expelled  from  the 
substance.  Hygroscopic  moisture  may  also  be  determined  in  the  same  way 
by  subjecting  the  undried  substance  to  the  above  process  and  taking  the  differ- 
ence between  the  two  results. 

For  a  direct  determination  of  the  water  of  minerals,  Brush  and  Penfield  *  prepare  a 
tube  of  hard  glass  with  a  bulb  A  at  one  end  and  two  bulbs  BB  about  the  middle.  The  min- 
eral is  placed  in  A,  and  BB  cooled  by  wet  paper.  On  heating  the  mineral,  the  water  distills 
and  condenses  in  BB ;  the  tube  is  then  drawn  off  by  a  blowpipe  flame  near  A,  cooled  and 
weighed.  The  water  is  poured  out,  the  tube  dried  and  reweighed.  A  short  rubber  tube 
drawn  over  the  end  of  a  glass  rod  serves  as  a  stopper  for  the  tube  during  the  weighings. 

2.  By  oxidation  to  a  higher  compound.    Various  suboxides  on  ignition  in 
the  air  pass  to  a  stable  higher  oxide,  though  it  is  the  safer  plan  to  supplement 
the  operation  by  treatment  with  a  more  energetic  oxidizer  than   the  air,  such 
as  pure  oxygen,  nitric  acid,  or  bromine  water.    A  fairly  large  platinum  crucible 
will  answer,  but  to  maintain  a  uniform  heat  throughout  the  mass,  it  is  better 
that  the  crucible  be  inclosed  or  suspended  within  a  larger  one  of  platinum  or 
porcelain,  thus  heating  the  inner  one  by  radiation  only. 

Most  metallic  sulfldes  and  sulfo-salts  on  ignition  in  a  current  of  air  are 
transformed  eventually  to  oxides,  sulphur  dioxide  escaping.  Moderate  heat- 
ing at  the  beginning  of  the  process  is  here  of  prime  importance,  since  the 
sulflde  may  sinter  or  even  fuse  at  a  dull  red  heat,  and  the  oxidation  proceed 
very  slowly.  After  partial  conversion  at  the  lowest  temperature  practicable, 
the  flame  is  cautiously  raised,  finally  heating  to  bright  redness.  Mercuric 
oxide,  volatile  at  a  red  heat,  may  be  mixed  with  the  sulflde  to  assist  oxidation, 
or  a  weighed  amount  of  some  stable  metallic  oxide  in  fine  powder  to  act  as  a 
carrier  of  oxygen. 

3.  By  reduction  to  a  lower  oxide  or  other  condition.    The  reduction  may  be 
done  by  means  of  some  reducing  gas,  usually  hydrogen,  carbon  monoxide,  or 
the  vapor  of  formic  acid,f  in  a  porcelain  or  platinum  crucible.    The  gas  is 
passed  in  through  a  bent  porcelain  tube,  bearing  a  shield  to  serve  as  a  loose 
cover.    The  current  of  gas  should  be  slow,  and  it  is  safer  to  interpose  a  disk 
of  perforated  platinum  between  the  precipitate  and  the  end  of  the  tube  to  pre- 
vent mechanical  loss.    The  apparatus  is  also  of  use  in  ignitions  where  it  is 
desired  to  protect  sulfldes  from  oxidation  by  the  air.     Hydrogen  and  other 
gases  highly  compressed  in  steel  or  copper  tanks  or  steel  cylinders  are  now  on 
the  market,  the  gases  guaranteed  of  a  purity  sufficient    for  this  and  like 
purposes. 

4.  By  transformation  to  another  combination;  as  where  the  acid  radical  is 
exchanged  for  another.    The  most  common  is  the  conversion  to  the  sulfate  of 


*  Amer.  Journ.  Science,  1894 — 31. 
f  Analyst,  1898—16. 


108  QUANTITATIVE    CHEMICAL    ANALYSIS. 

a  compound  of  a  base  with  a  volatile  acid  radical  by  evaporation  with  a  slight 
excess  of  sulfuric  acid  followed  by  gentle  ignition.  Less  often  is  a  compound 
converted  to  the  chloride.  If  the  freed  radical  is  not  volatile  and  the  sulfate 
or  chloride  is  insoluble,  the  former  may  be  removed  by  lixiviation,  but  in  thi& 
case  it  is  the  better  plan  to  redissolve  the  precipitate  before  drying  and  repre- 
cipitate  in  a  weighable  combination. 

Sometimes  a  precipitate  or  residue  is  obtained  in  analysis  consisting  of  a  definite  com- 
pound a  6  mixed  with  an  indeterminate  amount  of  the  same  base  or  acid  b  that  exists  in 
the  compound  —  thus  magnesium  borate  with  magnesia.  Usually  the  simplest  procedure 
Is  to  find  the  weight  of  the  mixture  and  then  determine  the  total  base  or  acid  radical, 
easiest  by  conversion  of  the  entire  base  or  acid  to  a  definite  compound ;  the  difference  is  a 
from  which  the  base  or  radical  combined  with  It  may  be  calculated. 


That  a  dried  or  ignited  precipitate  contains  impurities  to  a  ponderable  ex- 
tent may  be  evidenced  by  an  abnormal  color  or  agglomeration,  fusibility  at  a 
lower  heat  than  should  melt  the  pure  compound,  an  escape  of  fume  on  heating, 
or  condensation  of  sublimed  matter  on  the  bottom  of  the  crucible  lid,  and  other 
characteristics. 

In  all  cases  it  is  the  part  of  prudence  to  test  the  weighed  precipitate  to  make 
sure  that  it  is  one  definite  chemical  compound  and  free  from  other  bodies. 
The  method  of  examination  is  decided  by  the  nature  of  the  compound  and  the 
impurities  likely  to  contaminate  it. 

1.  Lixiviation  with  water  or  other  liquid  in  which  the  precipitate  is  insol- 
uble, followed  by  evaporation  or  precipitation.     Soluble  impurities  are  de- 
tected in  this  way,  sometimes  apparent  in  the  lixiviation  by  color  or  taste. 
But  often  the  impurities  are,  in  large  part  or  wholly,  so  inclosed  in  or  attached 
to  the  precipitate  as  to  resist  solution;  in  this  case  they  may  usually  be  freed 
by  effecting  some  structural  or  chemical  change  in  the  precipitate,  e.  g.,  an 
oxide  reduced  to  metallic  powder  by  ignition  in  a  reducing  gas,  or  a  metal 
oxidized  by  ignition  in  oxygen. 

2.  The  precipitate  may  itself  be  dissolved,  leaving  the  impurities  as  such  or 
changed  to  an  insoluble  form  by  reaction  with  the  solvent.     This  scheme, 
though  less  often  available,  is  generally  preferable  to  the  above,  as  the  impuri- 
ties, forming  only  a  small  fraction  of  the  precipitate,  are  left  reasonably  pure, 
and  may  be  weighed  and  deducted  from  the  combined  weight. 

3.  Both  the  precipitate  and  the  impurities  may  be  dissolved  by  a  suitable 
solvent,  and  a  reagent  introduced  in  the  solution  that  will  precipitate  only  the 
former ;  the  impurities  can  then  be  tested  for  in  the  filtrate,  or  may  sometimes 
be  evidenced  by  the  color  of  the  liquid. 

4.  Both  the  precipitate  and  impurities  may  be  transformed  to  another  com- 
bination and  the  resulting  mixture  weighed;  this  weight  should  equal  that 
calculated  by  stoichiometrical  rules,  any  discrepancy  being  credited   to  the 
presence  of  impurities.    For  example,  calcium  oxide  on  evaporation  with 
sulfuric  acid  leaves  calcium  sulfate  weighing  2.43  times  that  of  the  oxide. 
Any  associated  barium  carbonate  would  pass  to  barium  sulfate  weighing  only 
1.18  times  that   of   the  carbonate;   and  any  silica  present  would  retain  its 
original  weight. 

Similarly,  a  precipitate  may  be  evaporated  with  the  same  volatile  acid  as 
forms  the  radicil  of  the  compound,  when,  of  course,  the  weight  of  the  pure 
precipitate  will  remain  unchanged,  but  that  of  the  impurities  of  a  different 
composition  may  be  more  or  less  altered.  However,  these  methods  are  only  of 


QUANTITATIVE    CHKMICAL    ANALYSIS.  109 

use  in  a  few  special  cases,  as  the  variation  in  the  total  weight  is  usually  so 
small  as  to  make  it  doubtful  whether  it  should  not  be  ascribed  to  other  causes 
5.  Various   special  tests  will  be  suggested  to  the  operator  during  analysis, 
equal  or  superior  to  those  outlined  above. 


110  QUANTITATIVE    CHEMICAL   ANALYSIS. 


CHAPTER  5. 

VOLUMETRIC  ANALYSIS. 

In  volumetric  analysis  the  weight  of  a  body  is  determined  by  finding  what 
weight  of  a  given  reagent  is  requisite  to  exactly  fulfill  the  reaction  taking 
place  between  them.  In  practice  there  is  used  a  « standard  solution '  of  the 
reagent  —  an  aqueous  solution  whose  concentration  has  been  accurately  ascer- 
tained. The  usual  routine  is  to  weigh  a  suitable  amount  of  the  substance  in 
which  the  proportion  of  the  constituent  body  is  to  be  determined,  dissolve  in 
water  or  other  solvent  (forming  the  titrate),  and  gradually  run  in  the  standard 
solution  (the  titrand)  until  the  reaction  is  complete,  this  point  being  manifested 
by  the  incipience  of  a  secondary  reaction  or  otherwise.  The  weight  of  the 
substance,  the  volume  of  the  standard  solution,  and  the  combining  weights 
as  shown  by  the  equation  describing  the  reaction  are  the  data  for  comput- 
ing the  result. 

The  reactions  on  which  depend  the  methods  of  volumetry  may  be  classified 
as  follows :  —  * 

A.  The  combination  of  acids  with  alkalies  or  earths :   as  sulf uric  acid  neu- 
tralized by  ammonia;  calcium  hydrate  by  oxalic  acid. 

B.  An  increase  or  reduction   in  the   number  of    atoms  in  a  molecule: 'as 
stannous  chloride    perduced  to  stannic    chloride  by   ferric  chloride;    ferric 
chloride  reduced  to  ferrous  chloride  by  stannous  chloride. 

C.  A  change  in  the  acid  radical  combined  with  a  base  or  vice  versa,  the  new 
compound  usually  insoluble:  as  barium  nitrate  converted  to  barium  sulfate  by 
sulf  uric  acid;  sodium  chloride  to  silver  chloride  by  silver  nitrate. 

D.  A  direct  union  of  the  molecules  of  the  titrand  and  titrate :  as  anilin  with 
chloroplatinic  acid,  giving  anilin  chloroplatinate. 

E.  A  few  reactions  of  special  application  are  not  included  in  the  above. 


Apparatus  for  measuring.  As  gravimetric  analysis  is  founded  on  the  deter- 
mination of  mass  by  exact  weighing,  so  the  basis  of  the  practice  of  volumetric 
analysis  is  the  accurate  measurement  of  liquids.  For  this  purpose  there  are 
employed  glass  instruments  of  four  varieties,  namely  measuring  or  volumetric 
flasks,  and  graduated  cylinders  or  measuring  jars  to  contain,  and  burettes  and 
pipettes  to  deliver  certain  volumes  of  liquids.  The  height  of  the  surface  of  a 
liquid  of  a  given  volume  is  indicated  by  a  horizontal  line  etched  in  the  glass. 

The  unit  of  capacity  under  the  Metric  system  is  the  cubic  centimeter  —  the 
volume  of  one  gram  of  pure  water  weighed  in  vacuo  at  the  sea-level  and  at  the 
temperature  of  its  greatest  density,  4°  Cent.  —  therefore,  following  Fresenius, 
all  the  vessels  intended  for  volumetric  purposes  should  contain  or  deliver,  as 
the  case  may  be,  the  specified  number  of  cubic  centimeters  weighed  at  this  tem- 
perature. 

But  Mohr  advanced  the  objection  to  this  rule  that  at  the  ordinary  tempera- 
ture of  the  laboratory  it  is  impossible  to  graduate  or  confirm  the  graduations  by 
weighing,  on  account  of  the  deposition  of  dew  on  the  exterior  of  vessels  con- 


QUANTITATIVE    CHEMICAL    ANALYSIS.  11  L 

taining  water  at  4  ® ,  and  proposed  that  the  volume  of  one  gram  of  water 
weighed  at  17.5°  should  constitute  the  unit  for  the  purpose.  The  precedent 
has  encouraged  the  advocacy  of  other  arbitrary  temperatures,  such  as  15  <=>, 
16°,  20  o,  and  25°,  resulting  in  a  multiplicity  of  standards,  with  attendant 
confusion  and  liability  to  error.  While  the  actual  content  of  the  different 
instruments  one  has  in  use  is  seldom  a  matter  of  consequence  provided  all 
are  based  on  one  standard,  the  proposal  to  depart  from  the  rational  standard 
of  4  °  was  unfortunate  since  neither  accuracy  nor  convenience  (the  coefficients 
of  the  expansion  of  water  by  heat  being  exactly  known)  is  enhanced,  and  the 
significance  of  the  term  l  cubic  centimeter',  which  should  be  reserved  for  a 
definite  invariable  volume,  and  any  other  volume  given  a  distinguishing  name, 
is  made  ambiguous. 

In  selecting  an  outfit  of  volumetric  ware  one  must  make  sure  that  the  tem- 
perature at  which  they  were  graduated  was  the  same  for  all.  The  importance 
of  this  provision  will  be  seen  when  it  is  considered  that  1000  cubic  centimeters 
of  water  at  4°  Centigrade  expands  to  1000.85  Cc.  at  15  ® ,  to  1001.73  Cc.  at 
20°,  and  to  1002.87  Cc.  at  25®  .  In  the  United  States  the  temperature  of  15° 
has  been  generally  adopted. 

The"Mohr"of  DeKoninck  is  1000  "fluid  grams",  a  flnid  gram  or  "  millimohr  "  being 
the  volume  of  a  quantity  of  water  at  15 o  of  which  the  weight  determined  in  the  air  at 
15®  and  under  a  pressure  of  760  millimeters  of  mercury,  by  means  of  brass  weights,  is 
one  gram. 

The  burette  is  an  open  glass  tube  supported  vertically,  from  which  any  desired 
volume  of  liquid  within  its  capacity  may  be  poured,  or,  in  the  more  recent 
forms,  drawn  from  the  bottom  through  a  tap  with  a  small  orifice.  The  usual 
sizes  are  from  one  -fourth  to  three-fourths  of  an  inch  in  internal  diameter,  de- 
livering a  total  volume  of  30,  50,  or  100  cubic  centimeters,  graduated  into  cubic 
centimeters  and  fifths  or  tenths  of  each. 

Various  devices  to  control  the  outflow  are  in  use,  a  choice  depending 
largely  on  personal  fancy  and  all  having  some  objectionable  features  for  prac- 
tical work. 

Of  the  styles  delivering  their  contents  from  the  top,  Binks',  Gay-Lussac's,  and  Casa- 
major's,  are  simply  different  forms  of  a  slender  measuring-jar  set  in  a  broad  metal  or 
wooden  foot,  the  solution  poured  out  in  a  stream  or  by  drops  as  desired.  In  Casamajor's 
the  tube  is  held  nearly  horizontal,  the  base  resting  on  a  block,  and  rotated  so  far  to  allow 
the  solution  to  flow  from  a  curved  spout;  Gay-Lussac's  is  fitted  up  with  tubes  like  a  wash- 
bottle,  an  improved  form  having  the  longer  branch  of  the  exit  tube  inclosed  in  the  bu- 
rette. Several  defects  are  inherent  to  all  burettes  of  this  style  and  they  have  been  very 
largely  supplanted  by  others  such  as  are  described  below. 

Among  the  forms  of  tap  that  have  been  invented  for  burettes  delivering  the 
liquid  from  below,  that  of  Mohr,  Fig.  96,  is  in  common  use.  The  lower  end 
of  the  burette  is  contracted  to  a  smaller  bore  and  joined 
by  rubber  tubing  to  a  narrow  glass  tube  drawn  to  a  small 
orifice.  A  spring  pinchcock  compresses  the  rubber  tube 
so  tightly  that  no  liquid  can  pass,  and  can  be  opened  at 
will  by  pressing  the  disks,  allowing  a  stream  of  any  de- 
sired speed  to  flow.  Instead  of  the  pinchcock,  a  short  glass 
rod  of  the  same  diameter  as  the  interior  of  the  rubber 
tube  may  be  held  within  it;  when  the  rubber  tube  is  com- 
pressed between  the  thumb  and  finger,  two  narrow  chan- 
nels are  opened.  The  burette  is  partly  filled  with  the 
titrand  by  first  inserting  the  orifice  in  the  liquid,  opening 
the  pinchcock,  and  applying  suction  at  the  top  until  the 
liquid  has  risen  above  the  tap;  after  this  the  titrand  is 
poured  into  the  burette.  Proceeding  in  this  way  avoids  Fig.  96. 

the  danger  of  entrapping  air  bubbles  in  the  tap. 


112 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


In  Koenig's  design,  Fig.  97A,  the  lower  end  is  contracted  and  cut  off  obliquely.  The 
orifice  is  closed  by  a  platinum  or  rubber  plate  A  ground  to  fit,  which  is  pivoted  to  a  brass 
spring  S  that  presses  the  plate  against  the  glass.  The  pressure  of  the  spring  is  released 
on  turning  the  screw  T. 

Winkler's  burette  is  shown  in  Fig.  97B.  Within  the  burette  and  extending  to  the 
bottom  Is  a  rod  of  heavy  glass,  the  lower  end  drawn  to  a  cone  and  ground  to  make  a 

water-tight  valve  with  the  contraction  of  the 
burette  at  that  point.  The  upper  end  is  sup- 
ported in  a  frame  and  slightly  lifted  from 
its  seat  by  a  simple  mechanism  under  easy 
control  of  the  operator. 

Oarbutt  describes  a  peculiar  design  of 
burette  for  which  several  advantages  are 
claimed.  The  reservoir  of  the  titrand  is  a 
closed  flask  from  which  the  liquid  is  drawn 
by  a  syphon  tube,  and  as  the  titrand  leaves 
the  flask  an  equal  volume  of  air  must  enter. 
Not  the  volume  of  liquid  withdrawn  Is 
measured,  but  that  of  the  air  replacing  it, 
this  ascertained  by  a  water -manometer  con- 
nected with  the  flask. 

All  things  considered,  probably  the  most  generally  serviceable  form  is  that 
shown  in  C,  the  tap  a  ground-in  glass  stopcock,  it  having  tho  advantage,  among 
others,  that  solutions  slowly  decomposed  by  rubber  can  be  used  in  any  con- 
centration. The  plug  of  the  stopcock  should  be  well  fitted  so  as  to  turn 
smoothly  without  sticking  at  any  point,  and  when  slightly  lubricated  with 
vaseline,*  allow  no  leakage.  The  oblique-bore  stopcock  D,  introduced  by 
Greiner,  is  an  improvement  on  the  older  form,  as  in  the  latter  a  groove  may  be 
worn  in  the  socket  by  the  edges  of  the  perforation,  allowing  liquid  to  pass. 

It  is  much  more  convenient  for  most  ope  rators  that  the  handle  of  the  plug  be 
situated  at  the  left  hand  when  facing  the  graduations  than  at  the  right  as  is 
usual.  The  plug  should  be  withdrawn  and  tied  to  th«  burette  before  putting 
away  the  latter;  if  this  is  not  done  the  plug  may  later  be  found  cemented  in  the 
socket,  especially  after  the  use  of  caustic  solutions.  In  cold  weather  the  stop- 
cock should  not  be  left  closed  over  night,  as  the  plug  is  invariably  split  when 
the  liquid  in  the  bore  freezes. 

When  a  number  of  titrations  are  to  be  made  with  the  same  solution,  the  burette  can  be 
arranged  to  connect  by  glass  and  rubber  tubing  with  an  elevated  reservoir,  so  that  by 
simply  opening  a  pinch -cock  the  burette  fills.  The  tubing  should  not 
be  of  rubber  for  titrands  affected  by  this  material,  nor,  according  to 
Greiner,  for  reducing  solutions,  as  oxygen  from  the  air  will  pass  through 
and  oxidize  the  solution.  Another  scheme  is  that  of  substituting  an 
oblique  three-way  stopcock  for  the  straight-way  plug;  when  the  plug 
is  turned  180®  from  the  position  shown  In  Fig.  98,  the  solution  enters 
the  burette  from  a  rubber  tube  slipped  over  the  side  tube,  while  at  90® 
both  communications  are  closed. 

In  a  device  described  by  Greiner,  Fig.  99,  the  lower  end  of  a  narrow 
tube  A  enters  a  Woolf's  bottle  containing  the  titrand.  By  suction  on  a 
rubber  tube  connected  to  B  the  titrand  is  drawn  up  through  A  Into  the 
burette  to  above  the  zero  mark ;  on  allowing  air  to  enter  B  there  re- 
cedes into  the  bottle  all  above  the  orifice  of  A.  As  this  point  Is  the  zero 
of  the  graduation,  the  titration  may  be  at  once  proceeded  with. 


Fig.  <J8. 


Whatever  style  of  burette  be  preferred,  a  perfect  instrument 
must  meet  two  important  requirements,  namely  that  the  out- 
flow be  under  perfect  control  of  the  operator,  that  he  may  be  able  to  withdraw 
the  titrand  in  a  full  stream  or  by  drops,  as  desired,  and  that  when  the  tap  is 
closed  there  is  no  leakage  whatever. 


*  Journ.  Amor.  Chcm.  Socy.  1898—679, 


QUANTITATIVE    CHEMICAL,    ANALYSIS. 


113 


Fig.  99. 


As  a  rule,  the  taps  of  burettes  found  on  the  market  have  too  large  orifices, 
making  it  difficult  to  withdraw  a  small  drop;  moreover,  no  time  is  saved  in  a 
titration  by  the  larger  stream,  since  a  proportionately 
longer  period  must  be  allowed  for  the  liquid  to  collect  from 
the  sides  of  the  burette  before  reading  the  volume  with- 
drawn. When  the  outflow  exceeds  a  rate  of  one  cubic  cen  - 
timeter  per  second,  the  opening  should  be  reduced  by 
carefully  heating  the  tip  in  the  flame  of  a  Bunsen  burner, 
while  constantly  rotating  it. 

When  not  in  use,  burettes  should  be  kept  filled  with  some 
fluid  which  will  keep  the  interior  chemically  clean,  such  as 
concentrated  sulf uric  acid  for  those  entirely  of  glass,  and  a 
weak  solution  of  chromic  acid  for  the  other  forms.  A  film 
of  grease  is  easiest  removed  by  rinsing  with  a  strong 
solution  of  caustic  potash  in  alcohol  or  allowing  to  stand  for 
some  time  filled  with  a  concentrated  solution  of  potassium  perman- 
ganate, then  rinsing  with  strong  hydrochloric  acid.  When  a  volu- 
metric solution  is  to  be  left  in  a  burette  for  any  length  of  time  the  top 
should  be  stopped  by  a  cork  or  capped  by  an  inverted  test-tube,  to  prevent 
evaporation  and  the  entrance  of  dust  and  fumes.  Absorption  of  acid  fumes 
from  the  air  by  caustic  alkali  solutions  is  guarded  against  by  a  cork  bearing  a 
tube  filled  with  soda-lime. 

Reading.  After  rinsing  with  the  solution  it  is  to  contain,  the  burette  is 
filled  to  above  the  zero  mark,  the  funnel  removed,  and  the  tap  opened  until  the 
bottom  of  the  meniscus  in  transparent  and  the  surface  in  opaque  solutions 
coincides  with  the  zero  mark.  With  opaque  solutions  the  exact  height  may  be 
read  without  difficulty,  but  with  the  lighter  colored  liquids  the  double  line  at 
the  meniscus,  most  apparent  when  the  light  falls  from  certain  directions,  is 
somewhat  confusing,  and  various  ways  have  been  devised  to  secure  a  sharper 
definition.* 

1.  A  card,  the  upper  half  white  and  the  lower  half  painted  a  lusterless  black, 
is  held  behind  the  burette  at  such  a  height  that  the  line  of   junction  of  the 
black  and  white  is  one  or  two  divisions  below  the  meniscus;    the  reflection 
of  the  black  surface  in  the  meniscus  shows  as  a  black  line  sharply  defined. 

2.  A  light  glass  buoy  (Erdmann's  or  Beutell's  float,  Fig.  100),  weighted  with 
mercury  so  that  it  remains  half  immersed,  and  circumscribed  with  a  line  coin- 
ciding with  the  division  to  be  read;  the  best  form  of  Erdmann's 
float  is  provided  with  six  blunt  protuberences  of  glass  in  order 
that  it  may  quickly  rise  through  the  liquid  when  submerged.    Or 
a  disk  of  hard  paraffin,  one  or  two  millimeters  thick  and  of  slightly 
less  diameter  than  the  interior  of  the  burette  may  float  on  the 
surface. 

3.  The  Shellbach  burette  has  inserted  at  the   back  a  stripe  of 
blue  glass  bordered  by  white,  the  combination  producing  at  the 
surface  of  the  solution  the  appearance  shown  in  Fig.  101;  the 
graduation  mark  directly  in  front  of  the  junction  of  the  apices  is 
read. 

4.  Carnegie  proposes  to  deposit  a  mirror  of  silver  on  the  back  of 
the  burette  by   means  of  a  silvering  solution.    The  graduation 
marks  are  made  to  reflect  more  distinctly  by  rubbing  into  them  a 

Fig  100.  2/3    paste  of  mercuric  iodide  in  turpentine. 


*  Chcm.  News,  1895—1—43;  Journ.  Amer.  Chem.  Socy.  1899—42. 


114 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


5.  The  simplest  method  and  one  quite  satisfactory,  is  to  rest  the  burette  - 
stand  on  a  window-sill  and  read  with  the  eye  aligned  with  the  bottom  of  the 
meniscus  and  a  horizontal  line  on  some  far  distant  object, 
such  as  the  window-cap  of  a  house  . 

Whatever  plan  be  adopted,  the  eye  should  always  be  at 
the  ievel  of  the  point  of  reading.  In  the  instruments  of  the 
German  Physical-technical  Institute  the  lines  of  the  macro- 
divisions  are  produced  entirely  round  the  burette,  the  sub- 
divisions half  round. 

The  narrower  the  burette,  the  more  easy 
to  read  by  reason  of  the  greater  space  be- 
tween the  lines  marking  the  fractions  of  the  Fig.  101. 

cubic  centimeter,  yet  for  technical  work  a  burette  longer  than 
about  30  inches  stands  too  high  above  the  table  for  convenience 
in  filling  and  drawing  off  the  titrand  to  the  zero  mark.  If  it  is 
deemed  necessary  to  read  the  volumes  withdrawn  to  closer  than 
one  -tenth  of  a  cubic  centimeter  some  special  modification  of 
the  burette  may  be  adopted. 

Melnicke's  burette,  Fig.  102,  has  two  branches  connected  by  a  stop- 
cock ;  the  burette  proper,  A,  holds  60  Cc.  and  Is  divided  in  units  only, 
while  the  narrower  branch,  B,  holds  but  one  Cc.  and  is  divided  from  the 
bottom  upward  into  hnndredths.  The  titration  is  done  from  A,  the  liquid 
in  B  standing  at  zero.  If,  at  the  end  of  the  titration,  the  titrand  is  not 
exactly  at  some  one  division,  the  stopcock  C  is  opened  until  this  occurs, 
when  the  volume  delivered  is  the  difference  between  the  readings  of 
the  two  branches. 

Some  chemists  prefer  to  weigh  the  titrand 
rather  than  to  measure  it,  and  for  this  purpose 
variously  shaped  glass  vessels  have  been  de- 
signed, compact  in  construction  to  admit  of 
suspension  within  the  pan-wire  of  a  balance,  or 
to  stand  on  the  pan.  One  of  these  is  illustrated  in  Fig.  103.  It  is  a 
short  inverted  burette,  A,  fitted  at  B  with  a  tap,  fused  into  the 
mouth  of  a  flask,  C,  having  a  lateral  branch,  D.  The  flask  is  partly 
filled  with  the  titrand  and  the  burette  filled  from 
the  flask  by  air  pressure  at  D.  The  flask  is  then 
weighed  and  fixed  in  an  inverted  position  in 
the  clamp  of  a  retort-stand;  the  titration  is  per- 
formed as  usual  and  the  flask  is  reweighed. 
The  burette  is  graduated  for  convenience  in 
drawing  out  nearly  the  volume  of  titrand  re- 
quired when  this  is  approximately  known. 

The  burette  is  held  vertically  in  the  screw- 
clamp  of  a  stand,  Fig.  104,  usually  made  of 
hard  wood  throughout,  the  clamp  sliding 
on  the  post  and  fixed  at  a  suitable  height 
by  a  set-screw.  On  the  base  is  placed  a 
sheet  of  white  paper  or  a  glazed  white 
porcelain  plate  to  show  plainly  a  slight  change  in  color  of  the 
titrate.  Wooden  stands  can  be  given  a  permanent  white  coat- 
ing by  painting  with  acid-proof  enamel. 


102. 


T 


L 


FiS« 


Chaddock's  burette  stand  has  a  wooden  base  in  which  is  inlaid  a  disk 
of  opal  glass;  from  a  post  rising  at  the  rear  of  the  base  project  heavy 
brass  wires  that  have  been  bent  to  such  forms  as  will  spring  together 
with  sufficient  force  to  hold  the  burette  at  any  height  by  friction  alone. 
Fig.  104.          Advantages  claimed  are  the  ease  of  removal  and  replacement  of  the 

burettes,  simplicity  of  construction  and  cheapness. 
Where  several  burettes  are  in  occasional  use,  each  reserred  for  a  special  solution,  a 


3 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


115 


rotary  stand  has  the  advantage  of  taking  up  but  little  room  on  the  work-table.  From  the 
center  of  a  circular  porcelain  plate  rises  a  brass  rod  carrying  a  slide  provided  with  eight 
radial  arms.  Each  arm  has  a  clamp  at  the  extremity  for  holding  a  burette  or  pipette;  the 
slide  may  be  rotated  to  bring  any  burette  to  the  front. 


Pipettes,  Fig.  105,  are  calibrated  to  deliver  certain  volumes  of  a  liquid,  the 
usual  sizes  having  a  capacity  of  1,  2,  5, 10,  25,  50, 100  and  200  Cc.  The  construc- 
tion is  that  of  a  cylindrical  bulb  terminated  by  narrow 
open  tubes,  the  lower  one  contracted  to  a  small  orifice, 
the  upper  circumscribed  with  a  line  from  which  the 
specified  volume  is  delivered.  In  the  smaller  sizes  the 
lower  tube  may  be  left  off,  the  bulb  narrowed  to  the 
proper  sized  orifice.  Tor  volumes  not  exceeding  ten 
Cc.  they  are  also  made  in  a  tubular  form  and  graduated 
like  a  burette  to  deliver  fractions  of  a  cubic  centi- 
meter. 

The  measurements  from  a  pipette  are  somewhat 
more  accurate  than  from  a  burette  on  account  of  the 
smaller  diameter  of  the  tube  at  the  line  of  calibra- 
bration.  The  orifice  should  be  so  small  that  not  less 
than  about  30  seconds  is  required  for  the  outflow; 
if  too  large  it  should  be  contracted  by  heating  in 
a  Bunsen  flame,  but  the  pipette  must  afterward  be 
restandardized  since  the  amount  of  liquid  adhering 
to  the  interior  varies  with  the  rapidity  of  the  out- 
flow. 

The  pipette  is  held  between  the  thumb  and  second 
finger  and  the  solution  drawn  in  by  suction  to  an 
inch  or  more  above  the  mark ;  then  loosely  stopping 
the  top  wtfh  the  forefinger  as  shown  in  Fig.  106, 


V 


V 

Fig.  105.     i/4 
meniscus  is  on  a  level 


allowed  to  run  out  until  the  bottom  of  the 

with  the  mark.  The  finger  is  then  pressed  down  firmly  and  the  pipette 
held  vertically  over  the  receiving  vessel.  After  discharging  the  con- 
tents, one  minute  is  allowed  for  draining,  and  the  orifice  is  touched 
to  the  surface  of  the  liquid  to  disengage  the  remaining  drop  uniformly 
though  only  partially. 

Only  by  adopting  some  one  rule  in  dealing  with  the  afterflow  in  the 
graduation  and  in  practice,  will  the  proper  volume  be  delivered.  The 
one  just  stated  is  sanctioned  by  most  author- 
ities, though  others  prefer  either  to  remove  the 
pipette  as  soon  as  the  flow  has  ceased,  to  blow 
out  the  remaining  drop,  or  to  keep  the  orifice 
immersed  while  the  pipette  is  emptying.  For 
normal  pipettes  of  their  certification,  the 
Deutsche  Physicalisch-technische  Reichsanstalt 
directs  that  the  outlet  be  held  against  the  wall  FiS-  106« 

of  the  receiving  vessel,  and  when  the  flow  has  ceased  to  allow  fifteen 
>  seconds  for  draining. 

A  pipette  is  calibrated  for  the  delivery  of  the  specified  volume  of 
Fig.  107.  pure  Water,  a  certain  small  part  of  the  contents  remaining  on  the  walls. 
For  dilute  aqueous  solutions  of  inorganic  bodies  the  difference  be- 
tween the  volume  retained  and  that  of  pure  water  is  practically  inconsiderable. 
But  for  concentrated  solutions  and  some  dilute  solutions  of  organic  matter  in 


116 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


water,  and  for  liquids  like  alcohol  and  ether,  the  volume  adhering  is  greater  or 
less  than  of  water,  and  the  volume  delivered  must  be  corrected  where  accuracy 
is  important,  or  the  pipette  recalibrated  for  the  particular  liquid  or  solution 
measured.  Some  analyses  call  for  two  or  more  equal  volumes  of  an  alcoholic 
or  ethereal  solution  approximating  a  round  number  of  cubic  centimeters,  and 
here  a  uniform  delivery  is  had  by  permitting  a  certain  arbitrary  number  of 
drops  — from  two  to  ten  —  to  fall  after  the  flow  has  ceased. 

In  dealing  with  a  liquid  evolving  a  poisonous  or  offensive  gas  it  is  well  to 
interpose  between  the  pipette  and  mouth  a  tube  packed  with  cotton  that  has 
been  moistened  with  water  or  some  absorbing  chemical  solution.  But  if  the 
proportion  of  a  gas  in  the  solution  is  to  be  determined,  it  is  advisable  to  weigh 
the  solution  rather  than  measure  it  out  from  a  pipette,  as  with  the  latter  there 
is  a  slight  loss  in  drawing  in  the  liquid  from  the  reduction  of  the  air  pressure. 
Special  pipettes  are  for  sale  adapted  for  measuring  highly  corrosive  and 
iuming  liquids. 

The  100  cubic  centimeter  pipette  used  to  measure  the  sodium  chloride 
solution  in  the  volumetric  assay  of  silver  alloys  differs  somewhat  in  form 
from  the  ordinary  pipette,  in  order  to  secure  greater  convenience  and  accurate 
measuring.  As  illustrated  in  Fig.  107,  the  upper  tube  is  contracted  to  a  small 
orifice,  and  the  pipette  filled  by  gravity  from  an  elevated  reservoir  containing 
the  standard  salt  solution.  The  two  are  connected  by  a  rubber  tube,  D,  the 
flow  being  regulated  by  a  pinchcock.  As  soon  as  the  solution 
overflows  into  the  waste -cup  E  the  orifice  F  is  stopped  by  the 
finger,  the  pinchcock  closed,  and  the  rubber  tube  removed.  The 
exterior  is  wiped  dry,  and  on  removing  the  finger  the  entire  con- 
tents (100  Cc.)  flow  out. 

A  modification  of  the  above  is  shown  in  Fig.  108.  At 
the  bottom  is  a  three-way  stopcock;  turned  in  one  direc- 
tion it  allows  the  pipette  to  fill  to  overflowing,  and  in  the 
opposite  direction  delivers  the  specified  volume.* 

A  convenient  form  of  pipette  for  delivering  a  number 
of  equal  volumes  of  a  liquid  is  shown  in  Fig.  109. f 

In  technical  work  dealing  with  a  liquid  of  constant 
density  a  special  pipette  may  be  provided  which  is  cali- 
brated to  deliver  a  certain  weight  of  the  specific  liquid, 
usually  a  round  figure.  Should  the  specific  gravity  of 
different  samples  of  the  liquid  vary  somewhat  from  the 
normal  or  average,  the  pipette  may  be  graduated  so  as  to 
bear  a  mark  corresponding  to  each  degree  of  gravity  for 
some  distance  above  and  below  the  average.  To  save 
calculations,  pipettes  can  also  be  ordered  of  a  capacity  to 
deliver  a  certain  weight  of  one  particular  liquid  —  e.g., 
milk,  sap,  vinegar  —  of  fairly  constant  gravity,  an  amount 
suitable  for  the  analysis.  Though  less  accurate  than 
weighing,  the  approximation  is  near  enough  for  practical 
purposes. 

Or,  if  a  certain  weight  of  a  solid  or  liquid  is  to  be  dissolved  in  a  sol- 
vent,'then  make  up  to  a  fixed  volume  and  an  aliquot  part  withdrawn  Fig.  108. 
for  analysis,  for  any  one  constituent  may  be  provided  a  special  pipette  that  shall  deliver 
exactly  the  same  number  of  cubic  centimeters  as  the  percentage  of  the  element  to  be 
determined  is  contained  in  the  compound  weighed,  or  some  multiple  or  fraction  thereof. 
Thus,  in  dealing  with  a  mixture  containing  X  per  cent  of  a  constituent  c;  a  weight  a  of 
the  mixture  is  dissolved  and  the  solution  made  up  to  a  volume  V;  from  Fis  drawn  out  an 
aliquot  part  v,  and  the  constituent  c  precipitated  from  it  as  the  compound  cr  that  is  found 

to  weight  grams  and  con  tains  p  per  cent  of  c.    Then,  X=—- — •     Now   if  v  be  made   to 


Fig.   109. 


*  Analyst,  1898—279 ;  Chem.  News,  1892—1—68. 

t  Chem.  News,  1992—1-125;  Analyst,  1898-55  and  223. 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


117 


equal  p,  then  X=  —;  or  If  v  be  made  to  equal  ?-,  then   JT  =  Vd;    or    if  v   be   made   to 
a  u 

equal  J5?,  then  X  =  mxf;  for  m  selecting  any  suitable  number. 
am 

Measuring  flasks,  Fig.  110,  are  calibrated  to  hold  accurately  a  round  number 
of  cubic  centimeters  at  a  given  temperature,  to  the  line  A  encircling  the  neck. 
A  second  mark  is  sometimes  provided  from  which  the  specified 
volume  may  be  poured,  the  difference  between  the  two  being  a 
volume  equal  to  that  of  the  liquid  remaining  adherent  to  the 
interior  of  the  flask;  a  pipette  is  more  suitable  for  this  purpose, 
however.  Each  flask  should  have  a  narrow  neck  closed  with  a 
well  fitting  glass  stopper  or  a  sound  cork,  and  a  suflicient  space 
left  above  the  mark  to  permit  thorough  mixing  of  the  contents. 
The  sizes  grade  from  25  to  2,000  cubic  centimeters  capacity. 
The  glass  should  be  well  annealed  to  allow  the  boiling  of  a 
solution  if  desired. 

Giles*  liter  flask*  has  a  bulb  in  the  neck  above  the  mark  and  a  sec- 
ond mark  at  1100  Cc.    It  is  designed  for  the  making  up  of  standard  vol- 
umetric solutions,  allowing  100  Cc.  to  be  withdrawn  for  standardizing,      Fig,  HO.     1/s 
and  leaving  an  entire  liter  of  solution  for  practical  use. 

Measuring  jars,  Fig.  Ill,  open  at  the  top  with  a  lip  for  pouring,  or  closed 
with  a  glass  stopper,  are  of  various  capacities,  the  sizes 
running  from  10  to  1000  cubic  centimeters.  The  gradua- 
tion is  in  whole  cubic  centimeters  or  fractions  thereof, 
according  to  the  size  of  the  jar,  and  is  usually  marked  on 
one  side  taking  the  bottom  of  the  jar  as  zero,  and  on  the 
other  down  from  the  top  line  taken  as  zero.  Their  com- 
paratively large  diameter  unfits  them  for  close  measure- 
ments, yet  they  are  very  convenient  where  this  is  not 
essential.  It  is  directed  that  normal  measuring  jars  be 
emptied  by  reversing  and  allowing  them  to  drip  for  one 
minute;  then  the  adhering  last  drop  is  taken  off  with  a 
glass  rod. 

For  general  work  the  following  assortment  of  glass- 
ware will  be  found  suflicient:  a  burette  of  50  or  100  Cc., 
measuring  flasks  of  100,  250,  500,  and  1000,  pipettes  of 
10,  25,  50,  and  100,  and  jars  of  10,50,  100  and  1000  Cc. 
capacity. 


Fig.  111. 


The  end-point  of  a  titration  is  shown  by  some  visible  physical  alteration  in 
the  titrate  or  a  portion  of  it.  The  change  is  sharply  marked  and  easily  dis- 
cernible in  all  commendable  methods  and  may  be  manifested  in  several  ways. 

A.  By  the  cessation  of  precipitation.  When  a  solution  of  lead  chloride  is 
titrated  by  a  solution  of  potassium  chromate  there  falls  a  precipitate  of  lead 
chromate;  the  titrand  is  run  in  until  the  last  drop  produces  no  turbidity  in  the 
clear  liquid  above  the  lead  chromate  which  rapidly  subsides. 

Again,  in  the  titration  of  methylamine  by  oenanthol,  both  dissolved  in 
benzene,  one  of  the  products  of  the  reaction  is  water  —  CH3.NH2  (methyla- 
mine) -J-CeHisCOH  ( oenanthol)  =  C6Hi3.CH3NOH  (methylamine-oenantholylene) 
+  H20.  Previous  to  the  titration  there  is  mixed  with  the  titrate  some  fused 


.  *  Chem.  News,  1894—1-1 


118  QUANTITATIVE   CHEMICAL    ANALYSIS. 

calcium  chloride ;  as  the  titrand  is  run  in  a  cloud  appears  from  the  separation 
of  water  (immiscible  with  benzene).  But  the  water  is  quickly  absorbed  by  the 
calcium  chloride  on  stirring,  and  the  titrate  becomes  clear,  allowing  the  end- 
pbint  to  be  easily  observed. 

The  possibility  of  distinguishing  the  end-point  in  titrations  of  this  kind  is 
determined  by  the  physical  character  of  the  precipitate  and  its  gravity  as  com- 
pared with  the  liquid  in  which  it  is  formed.  Most  precipitates  settle  or  clot  so 
slowly  that  a  small  portion  of  the  titrate  must  be  filtered  off  after  each  addi- 
tion of  the  titrand,  and  the  filtrate  tested  by  the  titrand  or  another  reagent. 
Various  forms  of  miniature  filtering  apparatus  have  been  brought  forward  for 
the  purpose. 

One  of  these  Is  simply  a  short  glass  tube,  the  lower  end  tightly  plugged  with  cotton  and 
dipped  into  the  solution.  Suction  is  applied  to  the  open  end,  and  when  a  few  drops  of 
clear  fluid  have  entered,  the  tube  is  removed  and  inverted  over  a  test-tube .  Another  plan 
is  to  immerse  the  apex  of  a  small  paper  filter  beneath  the  surface  of  the  solution ;  a  little 
of  the  liquid  passes  into  the  interior  and  can  be  withdrawn  by  a  small  pipette  or  medicine 
dropper.* 

B.  Conversely,  by  the  complete  solution  of  an  already  formed  precipitate. 
When  (insoluble)  mercuric  ammonium  chloride  suspended  in  water  is  titrated 
by  potassium  cyanide  solution,  a  soluble  double  salt  is  formed  and  the  precip- 
itate is  gradually  taken  up  by  the  water  until  at  the  end-point  the  last  trace  of 
opalescence  disappears.    The  success  of  a  titration  of  this  kind  depends,  of 
course,  on  the  readiness  with  which  the   precipitate  is  transformed  to  the 
soluble  combination,  and  its  insolubility  in  the  products  of  the  reaction. 

C.  By  the  formation  of  a  precipitate  in  a  clear  solution  through  the  decom- 
position of  an  already  formed  soluble  complex.    Thus   potassium    cyanide 
titrated  by  silver  nitrate;  there  is  formed  soluble  silver  potassium  cyanide 
AgK(CN)2,  but  the  least  excess  of  silver  nitrate  reacts  with  this  to  produce 
insoluble  silver  cyanide  —  AgK(CN)2  +  AgNO3  =  2AgCN  -f  KNO3.  Similarly,  in 
the  titration  of  a  hot  solution  of  sodium  phosphate  by  ammonium  molybdate  in 
presence  of  ammonium  nitrate  and  gelatin,  a  cloudiness  marks  the  end-point. 

D.  An  alteration  in  the  color  or  tint  of  the  titrate  is  noted.     If  to  a  solution 
of  a  copper  salt  is  added  hydrobromic  acid  there  results  a  deep  violet  color, 
and  on  titrating  the  mixture  by  stannous  chloride  the  color  persists  to  the  end, 
then  suddenly  bleaches.    In  the  titration  by  iodine  of  antipyrine  in  alcoholic 
solution  or  of  diazo-  compounds  in  ethereal  solution,  a  faint  yellow  or  red  tint 
indicates  the  end-point.    The  purple  of  potassium  permanganate  is  visible 
even  in  a  highly  dilute  solution  or  one  slightly  tinted  by  other  compounds,  and 
since  the  decomposition  products  of  this  reagent  by  a  reducer  are  colorless, 
the  titrate  remains  uncolored  up  to  the  end-point,  after  which  the  least  excess 
of  permanganate  is  shown  by  a  faint  purple  tint. 

A  passive  tinctorial  body,  that  may  be  added  to  the  titrate  or  one  of  the  products  of  the 
volumetric  reaction,  may  be  held  in  solution  by  the  aid  of  the  compound,  to  be  titrated, 
but  being  insoluble  in  one  of  the  products  of  the  volumetric  reaction,  will  wholly  preclp  - 
itate  at  the  close,  leaving  the  liquid  colorless  or  but  slightly  tinted.  Barely  a  change  in 
the  color  of  the  precipitate  itself  shows  the  end-point. 

Conversely,  there  may  be  compounded  with  the  titrate  an  immiscible  liquid  or  a  solid 
in  fine  powder  capable  of  withdrawing  and  permanently  retaining  either  (a),  the  chromo- 
gen  of  the  colored  titrate,  or  (b),  a  colored  product  of  a  secondary  reaction  initiated  only 
after  the  completion  of  the  volumetric  reaction ;  the  color  of  the  immiscible  liquid  is  vis- 
ible when  the  layers  have  separated.  For  example,  in  the  titration  of  sodium  chromate 
by  sulf  uric  acid  to  sodium  bichromate  (2Na2CrO4  +  H2SO4  =  Na2Cr2O7  +  Na2SO4  +  H2O) ;  into 
the  titrate  is  stirred  ether  containing  hydrogen  peroxide.  When  all  the  sodium  chromate 
has  become  bichromate  the  least  excess  of  sulfurlo  acid  sets  free  chromic  acid  (Na2Cr2O7  + 


*  Journ.  Anal.  Chem.  4—427. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  119 

H2SO4  +  H2O=  Na2SO4  +  2H2CrO4),  which  passes  at  once  Into  the  ether  and  reacts  with  the 
hydrogen  peroxide  to  produce  a  soluble  blue-colored  compound  plainly  visible  when  the 
ether  is  allowed  to  form  a  layer  above  the  water. 

Another  example  is  that  of  phosphorus  suspended  in  alcohol,  titrated  by  a  solution  of 
bromine  in  carbon  disulfide,  phosphorus  pentabromide  being  formed.  Any  excess  of 
bromine  passes  into  the  alcoholic  liquid  and  colors  it  yellow. 

E.  By  an  indicator.*    Previous  to  the  titration  an  adjective  soluble  in  the 
titrate  is  mixed  with  it.    The  adjective  reacts  with  both  titrand  and  titrate  and 
hence  a  permanent  compound  is  formed  with  the  titrand  only  after  the  volu- 
metric reaction  is  ended.    This  secondary  reaction  produces  some  visible 
effect,  usually  a  change  in  color,  less  often  a  turbidity,  opalescence  or  opacity. 

For  example,  the  titrate  a  solution  of  potassium  hydrate,  the  titrand  hydro- 
chloric acid,  and  the  indicator  the  sodium  compound  of  a  certain  weak  organic 
radical.  On  running  in  a  quantity  of  the  hydrochloric  acid  there  is  formed  an 
indeterminate  mixture  of  potassium  chloride,  sodium  chloride  and  free  organic 
acid,  but  the  latter  immediately  combines  with  the  remaining  potassium 
hydrate  to  form  the  potassium  compound  of  the  organic  radical.  At  the  end 
of  the  titration  when  all  the  potassium  hydrate  has  become  potassium  chloride, 
the  least  excess  of  hydrochloric  acid  reacts  with  the  organic  salt  and  perma- 
nently sets  free  an  equivalent  of  organic  acid.  At  this  point  occurs  a  marked 
change  in  color  of  the  titrate,  since  an  aqueous  solution  of  the  sodium  com- 
pound of  the  organic  radical  is  colorless  or  nearly  so,  while  that  of  the  free 
organic  radical  is  intensely  colored. 

For  the  indicator  is  selected  where  possible  a  compound  that  will  yield  a 
product  of  a  high  color  by  reaction  with  the  titrand.  Sometimes  the  product 
is  colloidal  or  insoluble  but  so  diffused  through  the  liquid  as  to  color  it  deeply 
and  uniformly.  For  example,  in  titrating  a  solution  of  sulfurous  acid  by 
solution  of  iodine,  the  adjective  being  soluble  starch ;  as  soon  as  the  volumet- 
ric reaction  is  ended  (H2S03+  I2  +  H2O  =  H2S04  +  2HI),  the  intensely  blue 
iodide  of  starch  is  formed  with  the  least  excess  of  iodine.  Similarly  in  the 
titration  of  ammonium  molybdate  by  lead  acetate,  the  reaction  producing 
insoluble  lead  molybdate;  the  adjective  tannin  yields  a  yellow  to  red  color. 

F.  Among  other  indications. that  are  of  use  in  special  cases  may  be  men- 
tioned the  disappearance  of  a  pungent  or  characteristic  odor,  and  the  ces- 
sation or  incipience  of  foam  on  stirring  the  titrate.    It  is  said  that  tastef  is  a 
highly  delicate  test  in  alkalimetric  titrations ;  in  titrating  picric  acid  by  solution 
of  berberine,  both  intensely  bitter  compounds,  the  end-point  is  observed  by  the 
absence  of  a  bitter  taste  in  a  little  of  the  clear  liquid  filtered  off  from  the  pre- 
cipitate of  berberine  picrate. 

The  potential  difference  of  two  electrodes  immersed  in  halves  of  one  solution 
separated  by  a  porous  diaphragm,  rises  from  zero  as  the  solute  in  one-half  is 
precipitated  by  the  titrate,  and  at  the  end  of  precipitation  shows  a  sudden  abnor- 
mal increase.  The  potential  difference  is  registered  by  a  delicate  galvanome- 
ter, t 

Spot  indications.  When  the  titrate  or  a  product  of  the  volumetric  reaction  is 
dark  in  color,  the  change  in  tint  of  an  indicator  cannot  be  perceived  with  certainty, 
if  at  all.  Again  there  are  times  when  for  some  reason  an  indicator  cannot  be 
incorporated  with  the  titrate,  even  near  the  close  of  titration.  In  such  cases  a 
*  spot  test '  takes  the  place  of  the  usual  modes  of  noting  the  end -point.  A  drop  of 
the  titrate  is  withdrawn  from  time  to  time  during  the  titration  and  tested  by  mixing 
with  a  drop  of  the  indicator  or  another  reagent,  or  in  some  other  way  whereby 


*  Lunge,  Chemisch-technlsche  Untersuchungsmethoden,  1— 56. 
t  Chem.  News,  1898—1—91. 
J  Chem.  News,  1896—2-270. 


120  QUANTITATIVE    CHEMICAL    ANALYSIS. 

the  color  of  the  titrate  does  not  obscure  the  exhibition.  The  test  is  repeated, 
with  each  addition  of  the  titrand  until  the  volumetric  reaction  is  complete. 
The  presence  of  a  precipitate  or  other  solid  suspended  in  the  titrate  has  less 
effect  on  the  distinctness  of  a  spot  reaction  than  the  same  indication  in  the 
titrate. 

An  example  is  the  familiar  method  of  the  titration  of  ferrous  chloride  by 
potassium  bichromate,  in  accordance  with  the  reaction  — 

6FeCl2  -f  K2Cr2O7  -f  14HC1  =  3Fe2Cl6  -f  Cr2C)6  +  2KC1  -f  7H2O, 
the  ferrous  chloride  being  oxidized  to  ferric.  The  pronounced  dark  yellow  and 
green  colors  of  the  ferric  and  chromic  chlorides  mask  the  yellow  tint  given  by 
an  excess  of  the  bichromate,  so  recourse  is  had  to  a  spot  indication.  A  solution 
of  potassium  ferricyanide  gives  no  precipitate  with  a  ferric  salt  but  an  intense 
blue  precipitate  with  a  ferrous  salt.  After  running  a  small  volume  of  the 
bichromate  solution  into  the  titrate,  a  drop  of  the  latter  is  taken  out  with  a 
glass  rod  and  mixed  on  a  porcelain  plate  with  a  drop  of  a  weak  solution  of 
ferricyanide.  An  intense  blue  precipitate  is  observed.  After  repeating  the 
above  several  times  it  is  noticed  that  the  precipitate  has  become  much  less 
voluminous,  this  serving  as  a  warning  to  run  in  the  titrand  in  smaller 
volumes  —  finally  the  mixed  drops  show  even  no  blue  coloration,  evidencing 
the  entire  conversion  of  the  iron  to  ferricum,  or  more  exactly,  that  the  remain- 
ing trace  of  ferrosum  is  too  minute  to  visibly  react  with  ferricyanide. 

Another  example  is  the  determination  of  chlorine  in  presence  of  a  salt  of 
copper  by  titration  with  silver  nitrate.  Instead  of  waiting  until  the  precipitate 
has  settled,  as  in  the  usual  method,  it  has  been  advised  to  take  out  a  drop  of 
the  turbid  liquid  and  let  it  fall  on  a  polished  copper  plate.  Any  excess  of 
silver  nitrate  produces  on  the  plate  a  gray  film  of  silver  —  2AgNOs  +  Cu  = 
Cu(NO3)2  + Ag.  The  insoluble  chloride  of  silver  does  not  react  with  the 
copper.* 

In  acidimetry  and  alkalimetry,  a  deep -colored  liquid  can  be  titrated  by  spotting  on  a 
test-paper  — filter  paper  that  has  been  impregnated  with  a  solution  of  litmus,  lacmoid, 
turmeric,  phenol -phthalein  or  other  medium,  and  dried ;  a  paper  coated  with  ultramarine 
is  discolored  by  acids  even  in  highly  dilute  solutions.  Waldblott  proposes  that  a  little  of  the 
titrate  be  taken  out  by  a  narrow  glass  tube  and  the  end  of  the  tube  pressed  against  the 
test-paper;  the  capillarity  of  the  paper  has  the  effect  of  conducting  away  the  water  from 
the  dissolved  matter  thus  concentrating  the  latter  at  the  center  of  the  stain  and  intensify- 
ing the  indication. 

When  a  colored  liquid  containing  a  precipitate  is  dropped  on  thick  filter  paper,  the 
precipitate  remains  at  the  spot,  while  the  liquid  extends  and  its  color  shows  plainly 
around  the  precipitate  and  on  the  back  of  the  paper.  A  drop  of  the  titrand  or  other 
reagent  let  fall  near  the  margin  of  the  moist  circle  creeps  in  contact  with  it  and  the  color- 
reaction  is  apparent  before  the  precipitate  is  reached. 

Indicators.  For  titrations  where  the  reaction  is  that  of  the  neutralization  of 
acids  by  alkalies  and  earths  or  their  carbonates,  or  the  reverse,  an  indicator  is 
mixed  with  the  titrate.  The  color  of  the  solution  of  an  indicator  changes  in- 
stantaneously when  the  reaction  of  the  liquid  containing  it  turns  from  acid  to 
alkaline  or  the  reverse. 

A  great  number  of  artificial  and  natural  organic  dyes  have  been  proposed  as 
indicators,  and  of  them  the  following,  with  perhaps  a  few  others,  have  come 
into  common  use.  They  are  made  up  and  kept  for  use  in  aqueous  or  alcoholic 
solution,  rarely  in  other  menstrua. 


*  Journ.  Anal.  Chem.  2—202. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  121 

One  part  With  With 

dissolved  in  acids  alkalies 

Phenol-phthalein  100  alcohol  Colorless  Intense  red 

Litmus  Infusion  Red  Blue 

Methyl  orange  1000  water  Pink  Yellow 

Cochineal  Tincture  Yellow-red  Violet 

Coralline  750  alcohol  Straw  Violet-red 

Lacmoid  500  dil.  alcohol  Bed  Blue 

Phenacetolln  600  alcohol  Gold-yellow  Dark  pink 

Turmeric  Tincture  Yellow  Red 

Brazil  wood  Infusion  Yellow  Purple-red 

Haematoxylin  100  alcohol  Yellow  Orange 

The  colors  may  be  modified  considerably  from  those  stated  above  by  impuri- 
ties in  the  indicator  and  extraneous  matters  in  the  titrate,  the  proportion  of 
indicator  to  the  titrate,  and  the  color  of  the  light. 

Indicators  are  either  free  acids  or  bases  or  their  compounds,  all  compara- 
tively weak,  though  of  different  strengths.  The  change  in  color  at  a  reversal 
of  reaction  is  accredited  to  an  alteration  in  molecular  complexity  —  thus  phenol- 
phthalein  is  colorless  when  in  the  molecular  state,*  but  red  when  dissociated 
into  ions;  the  molecule  of  methyl  orange  red,  the  ions  yellow,  etc.  It  was  re- 
marked by  Allen  that  the  indicator  must  always  be  weaker  in  chemical  affinity 
than  the  acid  or  alkali  to  be  titrated,  and  as  all  the  indicators  in  common  use 
are  weaker  than  the  mineral  acids  and  the  fixed  alkalies,  any  one  is  appli- 
cable for  their  reciprocal  titration ;  but  many  indicators  are  not  as  strong  as  the 
weaker  acids  and  bases. 

Thompson  divides  indicators  into  three  groups.  The  first  comprises  methyl 
orange,  cochineal,  Congo-red,  lacmoid,  indeosin,  and  dimethylamidobenzene ; 
these  react  with  strong  acids  only,  but  are  sensitive  to  bases  of  feeble  affinities 
such  as  many  alkaloids.  In  the  second  group  are  rosolic  acid  and  phenacetolin, 
reacting  with  weaker  acids  than  the  first  group.  In  the  third  are  phenol  - 
phthalein  and  turmeric,  sensitive  to  the  weakest  acids  and  indifferent  to  a  great 
many  of  the  vegetable  alkaloids. 

Lescoenr  defines  neutrality  as  a  state  wherein  on  the  one  hand  helianthine  remains 
yellow,  and  on  the  other,  phenol-phthalein  remains  colorless  and  litmus  red.  An  aqueous 
extract  of  cocoa-nibs  is  said  to  be  alkaline  to  methyl  orange  and  acid  to  phenol-phthalein, 
the  amphichroism  indicating  a  soluble  salt  of  a  weak  organic  acid  accompanied  by  a 
small  proportion  of  a  free  organic  acid. 

Litmus  Is  weaker  than  carbonic  acid  and  therefore  cannot  be  used  when  the  acid  Is 
present  in  the  free  or  half -bound  (page  v.  Water)  state,  while  the  stronger  cochineal  is 
unaffected.  The  alkaloids  are  bases  of  so  indifferent  a  character  that  the  acid  radical  of 
quinine  sulfate,  for  example,  may  be  titrated  by  an  alkali  and  phenol  phthalein  as  though 
it  were  combined  with  hydrogen. 

The  difference  in  the  strength  of  indicators  is  at  times  an  advantage,  permit- 
ting the  successive  titration  of  two  of  the  constituents  of  a  substance  without 
their  separation.  An  example  is  the  titration  by  a  caustic  alkali  of  a  mixture 
of  a  mineral  acid  with  one  of  the  higher  fatty  acids,  based  on  the  indifference 
of  methyl  orange  to  the  latter.  After  titrating  with  this  indicator  until  the  red 
has  changed  to  light  yellow,  showing  that  all  the  mineral  acid  has  been  neu- 
tralized, phenol-phthalein  is  added,  and  the  titration  continued  for  the  fatty 
acid. 

Again,  in  a  mixture  of  sodium  hydrate  and  sodium  carbonate  (e.  g.  the  commercial 
caustic  alkali),  the  proportions  of  each  can  be  determined  by  titration  by  a  standard  acid, 
first  with  phenol-phthalein,  the  solution  remaining  red  until  all  the  sodium  hydrate  and 
one-half  of  the  sodium  carbonate  are  neutralized  (sodium  bicarbonate  is  neutral  to  this 
indicator) 

NaCH  +  HC1  =  NaCl  +  H2O,  and  Na2CO3  +  HC1  =  NaHCOs  +  NaCl. 


*  Journ.  Amer.  Chem.  Socy.  1902—591. 


122  QUANTITATIVE    CHEMICAL    ANALYSIS. 

this  requiring  M  cubic  centimeters  of  the  acid.    The  sodium  bicarbonate  remaining  is  now 
titrated  with  the  acid  and  methyl  orange, 

NaHCOs  +  HC1  =  NaCl  +  H2OO3. 

requiring  NCcB.    The  "  total  alkalinity  "  is  expressed  by  M  +  tf,  and  the  "  causticity  "  by 
M  —  NCCB.  of  the  acid. 

The  color  of  an  indicator  is  modified  to  a  greater  or  less  extent  when  a  titratlon  is  done 
toy  artificial  light  (except  electric).  With  litmus  the  transition  is  obscured  by  gaslight, 
though  with  a  sodium  (monochromatic)  flame  the  sharpness  of  the  end- reaction  is  in- 
creased, the  red  appearing  colorless,  and  the  blue,  black. 

When  the  titrate  is  colored  by  organic  matter  or  other  bodies,  the  end-point  is  more 
easily  seen  through  a  flat  glass  cell  containing  a  liquid  of  the  same  hue.  Lupp,  as  an  aid 
in  discerning  the  end-point  by  change  in  color  or  formation  of  a  precipitate,  rests  the 
beaker  containing  the  titrate  on  a  hollow  tripod,  beneath  which  is  a  concave  mirror  re- 
flecting the  sun's  rays  up  through  the  titrate.  The  device  is  also  of  service  in  cloudy 
weather  from  the  increased  Illumination.  Some  titrations,  however,  as  of  acetone  by  a 
hypochlorite,  must  be  performed  in  subdued  light  only. 

Should  one  of  the  soluble  products  of  a  volumetric  reaction  deepen  the  color 
of  the  titrate,  possibly  it  may  be  withdrawn  from  solution  by  some  immiscible 
organic  solvent  stirred  up  with  the  titrate,  e.  g.t  iodine  absorbed  into  carbon 
disulflde.*  But  in  some  determinations  the  accuracy  is  prejudiced  by  the  con- 
tact of  organic  liquids  with  the  titrate  or  titrand,  and  many  indicators  show 
abnormal  colors  under  these  circumstances. f 

The  amount  of  indicator  to  be  used  in  the  titrate  should  be  limited  to  that 
actually  required,  since  an  excess  may  lessen  the  accuracy  of  a  delicate  titra- 
tion,  for  in  many  cases  all  the  indicator  present  must  react  with  the  titrand 
before  a  positive  change  in  color  ensues. 

Of  the  great  number  of  indicators  that  have  been  proposed,  probably  litmus, 
phenol-phthalein,  methyl  orange  and  cochineal  are  most  in  use.  Litmus,  the 
pioneer,  is  still  favored  by  some  chemists  over  those  of  more  recent  introduc- 
tion, notwithstanding  the  rather  laborious  process  of  preparing  the  solution  of 
the  principle  azolitmine  free  from  other  coloring  matters  of  the  lichen. :£  It  may 
be  used  in  hot  or  cold  solutions  for  the  mineral,  thiosulfuric,  and  nitrous  acids, 
the  alkalies  and  earths,  and  fairly  well  for  the  common  organic  acids  except 
citric.  As  it  is  sensitive  to  carbonic  and  hydrosulfuric  acids,  these  must  be 
entirely  removed  from  the  titrate ;  easiest  by  boiling  before  the  titration  is 
begun,  if  they  exist  in  the  free  state,  or  after  each  addition  of  the  titrand  if  they 
are  combined  with  a  base. 

Phenol-phthalein  ||  is  admirable  for  the  fixed  alkalies  and  mineral  and  organic 
acids,  but,  like  litmus,  it  is  sensitive  to  carbonic  acid.  For  free  ammonia  or 
in  titrating  in  presence  of  ammonium  salts,  a  large  proportion  of  the  indica- 
tor must  be  in  the  titrate. §  It  may  also  be  used  in  alcoholic  and  ethereal 
solutions. 

Methyl  orange  or  helianthin  is  less  delicate  than  the  above  even  when  used 
for  concentrated  solutions,  and  as  the  pink  color  is  but  faint  at  best,  close 
attention  is  needed  to  catch  the  change ;  moreover  it  cannot  be  used  for  organic 
acids  nor  in  hot  solutions,  yet  in  spite  of  these  and  other  drawbacks,  it 
is  of  great  value  from  its  indifference  to  carbonic  and  other  weak  acids. 

Cochineal  is  but  slightly  affected  by  carbonic  acid,  but  cannot  be  used  for 
organic  acids  or  in  presence  of  iron  or  aluminum  salts.  Lacraoid  is  very  sensi- 
tive and  may  be  used  in  a  strong  alcoholic  solution  where  methyl  orange  is 
indistinct. 


*  Zelts.  anal.  1896—305. 

t  Journ.  Phys.  Chem.  1898—171. 

J  Chem.  News,  1889-2-306  and  1894—2—225. 

||  Journ.  Anal.  Appl.  Chem.  1893—204. 

§  Chem.  News,  1899-1—214. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  123 

Other  indicators  have  special  characteristics  and  are  chosen  in  preference 
to  the  foregoing  for  some  special  determinations  where  these  peculiarities 
are  of  advantage.  But  for  general  work,  an  indicator  as  delicate  and  brilliant 
asphenol-phthalein,  and  at  the  same  time  unaffected  by  carbonic  and  hydro- 
sulfuric  acids,  is  yet  unknown. 

Whenever  the  titre  of  a  standard  acid  or  alkali  or  similar  compound  is 
stated  the  indicator  used  in  the  standardization  should  be  named. 


A  '  standard '  or  *  set '  solution  is  one  in  which  the  weight  of  reagent  in  the 
unit  of  volume  (in  the  metric  system,  grams  in  one  cubic  centimeter)  or  the 
chemical  change  that  a  unit  of  volume  will  effect,  is  exactly  known.  It  may  be 
stated  as  (a),  the  weight  of  reagent  in  the  unit  of  volume;  or  (6),  the  active 
constituent  of  the  reagent  in  the  unit  of  volume;  or  (c),  the  weight  of  a 
certain  body  with  which  the  unit  of  volume  reacts. 

Thus,  a  cubic  centimeter  of  u  chameleon  "  solution  may  be  said  to  contain 
a  grams  of  potassium  permanganate  or  b  grams  of  available  oxygen,  or  to 
be  equivalent  to  c  grams  of  oxalic  acid.  It  is  immaterial  which  is  chosen,  as 
one  may  be  calculated  from  another,  except  in  empirical  processes  where  the 
reaction  is  more  or  less  vague,  incomplete,  or  modified  by  the  experimental 
variants  of  temperature,  dilution,  etc.,  and  one  result  comparable  with  another 
only  when  all  the  conditions  are  alike — here  only  the  third  form  of  expression 
is  admissible. 

The  concentration  of  a  standard  solution  may  vary,  usually  between  wide 
limits.  Exceptions  are  where  the  solubility  of  the  reagent  or  its  volatility  in 
aqueous  solution  limits  the  strength ;  where  the  volumetric  reaction  takes 
place  normally  only  in  a  very  concentrated  or  dilute  solution,  with  a  reversed 
or  some  secondary  reaction  at  a  different  concentration ;  or  where  the  indicator 
is  such  that  a  prompt  or  decisive  change  is  shown  only  at  certain  concentrations. 
But  there  is  never  any  advantage  in  making  a  standard  solution  weaker  than 
when  one  drop  will  distinctly  bring  out  the  end  point  of  a  titration. 

The  solvent  is  usually  water,  or  for  reagents  insoluble  or  but  sparingly 
soluble,  an  aqueous  solution  of  some  compound  that  will  not  interfere  with  the 
volumetric  reactions.  For  some  reagents  alcohol  more  or  less  diluted  with 
water  is  necessary;  bromine  and  iodine  and  some  organic  compounds  are  dis- 
solved in  chloroform,  ether,  benzol,  or  carbon  tetrachloride,  the  titrate  being 
a  solution  in  the  same  liquid;  but  on  account  of  the  volatility  of  these  solvents, 
great  care  must  be  taken  to  prevent  evaporation  during  titration,  and  standard- 
izing be  done  just  before  a  titration, 

A  few  solutions  are  best  made  up  by  dissolving  two  or  more  bodies  that 
mutually  react  to  set  free  the  active  constituent;  this  plan  is  adopted  in  cases 
where  it  is  difficult  to  purify  or  weigh  the  reagent.  For  example,  six  atoms  of 
bromine  are  set  free  on  dissolving  five  molecules  of  potassium  bromide  and 
one  molecule  of  potassium  bromate  in  dilute  hydrochloric  acid.  Frequently 
the  reaction  is  so  definite  and  complete  that  the  standardization  of  the  solution 
can  be  omitted. 

Standardizing.  Having  decided  as  to  the  quantity  of  solution  to  be  prepared, 
the  calculated  weight  of  reagent  is  to  be  dissolved  in  water  and  made  up  to  the 
proper  volume.  For  ammonia  and  the  mineral  acids  and  some  other  liquids 
the  original  specific  gravity  affords  a  ready  means  of  preparing  the  volumetric 
solutions  by  simple  dilution,  and  the  gravity  of  the  solution  is  a  rough  measure 
of  the  concentration.  Tables  of  concentrations  corresponding  to  the  various 


1LM  fjUJANTITATIVK    <   INIMICAI,     ANALYSIS. 

gravities  will  bo  found  In  tnoHt  works  on  analysis,  imi.  an  allowance  intiHt   In- 

Ml... !••    f(»r   lll<-   rolll.l.'l.   Moll    ill    volllliif   Oil   ililill.lon. 

The  rxarl,    Illic     (concentration)     of     the    Holiltion    is    now    to    l)«:     established . 

Several  method  a  are  available,* 

A.  A  Hiiiull  iii--:i  UK  -I  (or  weighed)  volume  of  MM;  solution  Is  evaporated  to 
dryneHS  In  u  tared  <ti:ili  and  tin-  n-sldnc  wei^ln-d.  This  process  assumes  Mint, 
iiui.li  tin-  iv.-cviil.  :ind  solvent  an:  perfectly  pure,  and  that  the  rca^«:iil.  is  not 
oxldlxod  or  otherwise  decomposed  or  volatile  at  the  temperature  of  boiling 
water. 

If  thu  reagent  Is  an  acid,  a  measured  volume  may  be  HUporHaturatod  by  ammo- 
nia :ind  evaporated  l.o  dryness  at  a  low  temperature.  A  pure  ainmonluni 
.salt  IM  left  and  the  proportion  of  acid  can  be  calculated  from  UN  weight.  Or  the 
acid  may  be  poured  on  a  weighed  amount  of  a  fixed  metallic  oxide  or  carbonate 
In  lino  powder  that  readily  combines  with  the  acid.  The  liquid  Is  evaporated, 
dried  and  weighed,  and  from  the  Increase  ID  weight,  due  to  the  combined  acid 
radical,  10  calculated  the  tltre,  Thus,  lead  protoxide  with  sulfurlc  acid. 

D.  The  reagent  or  eome  constituent  thereof  is  precipitated  from  a  measured 

volume  of  the  solution  ami  t.lie  precipitate  weighed;  as  hydrochloric,  uc.ld  prc- 
clpltated  by  silver  nitrate,  the  silver  chloride  weighed,  and  the  corresponding 
acid  calculated.  Hero  enter  all  the  errors  of  precipitation,  nitration,  Ignition 
and  wrii'jiinf,,  ami  the  process  IH  necessarily  slow,  so  that  it  is  leas  used  than 
the  following. 

0.  Thu  most  common  method  Is  that  of  ascertaining  what  volume  Is  required 
to  neutralize  or  otherwise  react  with  a  weighed  quantity  of  some  solid  with 
which  it  combines.  For  thin  purpose  wo  select  a  substance  of  a  definite  chem- 

ic:tl  composition  and  prepare  a  (piunl  ity  with  such  preejtu!  Ions  us  to  be  positive 

of  ii s  purity,  dissolve  a  suitable  weight  in  water  or  other  solvent,  and  titrate 
the  solution  directly. 

The  presumption  would  bo,  from  the  abundance  of  chemical  compounds  easy 
to  prepare  in  a  pure  crystallized  or  anhydrous  form)  that  no  difficulty  should 
l»e  experienced  In  Hndlng  one,  if  not  several,  milted  to  any  given  volumetric 
solution,  thus  providing  a  ready  and  exact  means  of  standardization.  Yet  for 
quite  a  number  of  solutions  even  one  compound,  satisfactory  in  all  respects,  is 
wanting. 

(  i  ystalllno  double  Halts,  whose  bases  are  respectively  the  active  and  an  In- 
different principle,  have  the  advantage  over  simple  salts  of  bearing  a  higher 
molecular  weight,  thus  counteracting  to  some  extent  the  errors  of  weighing, 
etc.  But,  on  the  other  hand,  the  composition  of  a  double  salt  is  apt  to  be  un- 
certain, the  ratio  of  the  bases  varying  according  to  the  conditions  of  crystal - 
II  eat  Ion. 

Some  elements  or  compounds  that  are  volatile,  readily  oxldlzable,  or  other- 
wine  unsuited  for  direct  weighing,  may  bo  liberated  in  situ  from  suitable  com* 
pounds  by  cortaln  reagents;  0.  g.%  Iodine  is  liberated  from  potassium  iodide  In 
aqueous  solution  by  potassium  permanganate  (the  solid  or  In  solution)  accord- 
log  to  the  equation  — 

10KI  4-  KaO.2MnO.Ofl  +  UHg804 -  Bla  +  2Mn804  +  12KH804  +  8HaO. 

Our    iiiHhiHl    «>l   iilnmhinll/liiK  I"   hv  ntln-liiK  lulu    u   mi>amir«il  volume  of  Mir  itoliition  a 

weighed  Amount  (an  OXOOHM)  of  loma  compound  in  fine  powder  that  is  Insoluble  In  water 

l.ul  n-ii.-:.-i  l.i  Moh.M,-  pi-M.lurlH  will.  Ihr  i ,-,.,/., •„(  of  (In-  Holul  I.MI  .  I  lu-u  <l.-lrriiilnliiK  wl.nl 
proportion  of  th«  pow.l.T  rt-inaliiM  iiiiillNHolvrd.  anil  hy  illllrronr.ii  wluil  IIIIH  rrnrltMl.  Kor 

example,  Imrliun  oarbonnto  mod  to  determine  the  strength  of  hydroohlorlo  aoid.  The 
prooen  U  aabjeot  to  At  lenit  two  form*  of  error ;  one,  that  the  point  where  the  aold  becomes 
by  noutrnlUnUon  so  dilate  Unit  it  hits  no  action  on  the  bAse,  is  Indefinite,  varying  with 


•  Jonr,  Amer.0hem.8ooy.  1901-797. 


(..I-AMITMIN  I)    riMvMICAl,    ANALYSIS.  I2.r» 


the  i'-iii|iiTJiltir«,  frciqiM-ricy  <»f  MllrrniK,  <'oiirrni  ml  Ion,  ;i  ml  (In-  pi.  •  .-'n  jil  •  ••unlit  .ion  >,t  \\\<- 
,"oll't  ,  I  In-  «.i  ln-i  ,  I  ti.il  I  he  Hnhilloii  nl  it  niMilral  nail,  may  have  a  Kiealer  nolv<'iil,  powi  i  l<» 
Um  povvdor  than  hurt  puro  waier.  \\  <•  may  th-iilon-  have  I  n  Holiitlon  froo  add  or  froo 
bum)  with  Hi"  in-iil.ial  >ulL  Mini-  (In-  two  eiiont  have  a  n  o|.|>  »-.-\  I  <  <ll<il  i.n  I  In-  i  .-HII  II 
tlioy  art)  to  MOUIII  <«xt«!Mt  uiiiliiully  corn-dlvr. 

I  or    l!i<-  lili:ill'.n  «l  :i  r.oiu-l  1  1  IK  n  I  o|  a  <-.,IM  p  !<•  \  IMI  l.ul  aii<  •«•  where  UK    volume  of    l.llrand  IM 

iiien-a:,ed  <>i  dimim-hed  Homrwiiai  i.',  n«-  1  1  1  n  u  •  i,  <  <  <  .  r  aMooUUed  bodies,  tb«  volution  lit 
Mtandardtzod  on  (i  lynlhoUc  proof  M  oo*r  to  tba  mixture  In  oorapoiltlon  ft>  poNilbio,  or 

on     .•inolln-r     .••linllar      inlxliin-      \vlici<ln      UK-     n-acl.itiK     >  on-  I  I  I.IK  n  I      IIIIM      |.  r  i-  v  ion:-  1  •       I..-I-M 

del  c  rin  i  n  i-d  uravliiMitrlcally. 

I).  A  few  Hpcclal  methods  not  Included  In  the  foregoing.    For  example, 

Ui<-  Hlandanli/Jrip;  of  :in  ji(|in-«)iiM  Holnt.ion  ol  liydco-M-n  pf|-o\  nl«-  hy  I  ihi-ratlnj^ 
HIM!  Mi<-:iMiinM,",  1,li<'  (<loill)l<i«l  )  voluirur  of  UK-  ;iv;ii  liihlc  oxyvi'"  (»1  l,h<-  perox- 

ide —  In  the  reaction  with  potaHHlum  ix^rmanganate  In  an  acid  nolutlon,  an 
atom  of  oxygen  from  the  peroxide  unlt<:n  witti  an  atom  from  the  permanganate 
to  form  a  molecule  — 

511,0.0  +  KaO.2MnO.Oft  +  8H|804  -  60s  +  8H|0  +  K|804  +  2MnS04, 
one   molecule  of  potaselum  permanganate  reacting  with  five  molecules  of 
hydrogen  peroxide. 

Of  two  mutually  reacting  HolutlonM,  If  one  be  itandardlzed,  the  strength  of 
the  other  may  be  found  from  their  volumetric  relation  and  the  combining 
weights  of  their  reagents. 


Normal  solutions.  A  normal  solution  Is  a  standard  solution  made  up  of  such 
a  strength  that  In  a  unit  of  volume  (one  liter)  Is  dissolved  a  weight  of  reagent 
equivalent  In  chemUm  to  a  unit  of  weight  (one  gram)  of  hydrogen ;  for  example, 

1.  Cl  (atomic  weight  85.45)  +  II  —  HC1  (molecular  weight  86.458) ;  a  normal 
solution  of  chlorine  contains  85,45  gramsof  Cl;  and  one  of  hydrochloric  acid, 
36.458  grams  of  HC1  per  liter. 

2.  AgNOs  (molec.  weight  109.06)  +  HC1  —  AgCl  +  UNO.,. 
normal  sliver  nitrate  contains  169,96  grams  AgNOs  per  liter. 

8.  KCNB  (molec.  weight  97.22)  +  AgNOs  —  AgCNS  +  KNO«. 
M  .im:ii  potartslura  sulfocyanlde  contains  07.22  grams  KCNB  per  liter. 

4.  iiraii,o,  OJ0.082)  +  KOH  —KCsHsOs-f  HsO. 
Hs8O<      (98.086)  -f  2KOH  =  Kj804  +  2HfO. 
n  i  -o,      (98.044)  -f  8KOH  =  K8PO4  +  8H|O. 

normal  acetic,  sulf uric  and  phosphoric  acids  contain  respectively  60.082,  one> 
knifot  98.086,  and  one-third  of  98.044  grams  per  liter. 

6.  KsO.CrgOa.Os    (294.42)  +  6H  (nascent)  —2KOH  +  CrsOs  +  iflM>. 
normal  potassium  bichromate  contains  one-sixth  of  294.42  grams  of  KfOftOr 
per  liter. 

As  Indicated  in  the  above,  In  a  reaction  between  any  two  normal  solutions 
exactly  equal  volumes  of  each  take  part.  The  molecular  weights  are  often  the 
<  omhlning  ones  and  equal  the  number  of  grams  to  be  dlsitoived  In  a  liter  of 
water,  but  there  are  numerous  exceptions. 

From  the  dissimilar  nature  of  the  reactions,  two  solutions  that  each  react 
volume  for  volume  with  a  third  solution  may  not  agree  with  each  other.  If 

nifiirouH  acid  and  potassium  bichromate  be  made  normal  toward  potassium 
iiyirate  they  will  not  mutually  react  In  equal  volumes;  for  although 

11*80,  j  2KOH  _  Ks80s  -fSHsO. 
and     KtCrftOr  -f  2KOH  =  2KsCr04  +  HsO. 
yet      KzCrsOr  -f-  3H«80s  -f  Hs804  -  Cr*  (804)s  +  KfS04  -f  4HsO. 


126  QUANTITATIVE    CHEMICAL    ANALYSIS. 

With  some  writer?,  particularly  those  of  Great  Britain,  the  term  '  normal'  has  not  the 
significance  generally  adopted,  but  is  held  to  represent  the  solution  of  a  molecular  weight 
of  a  bivalent  reagent  in  the  unit  of  volume. 

When  an  exactly  normal  solution  is  to  be  made,  the  calculated  weight  (or  as 
near  it  as  practicable) ,  is  dissolved  in  water  and  the  solution  made  up  to  the 
corresponding  volume;  the  concentration  is  then  ascertained  by  a  suitable 
method.  From  this  datum  it  is  easy  to  compute  (page  180)  how  much  of  either 
the  reagent  is  to  be  added  to  bring  up  the  solution  to  normal,  or  of  water  to 
reduce  it  thereto,  but  from  several  causes,  this  process  often  fails  to  yield  an 
exactly  normal  titre,  and  the  testing  and  adjusting  may  have  to  be  repeated 
one  or  more  times  before  success  is  attained. 

Strictly  normal  and  subnormal  solutions  of  the  mineral  acids  can  be  pre- 
pared by  the  electrolysis  of  a  metallic  sulf ate  giving  free  sulf uric  acid ;  for 
hydrochloric  and  nitric  acids  the  corresponding  barium  salts  are  decomposed 
by  the  sulf  uric  acid.* 

But  considering  the  liability  of  solutions  to  gain  or  lose  in  strength  by  varia- 
tions in  temperature,  evaporation  or  decomposition,  a  simpler  and  much  less 
laborious  plan  is  to  forego  attempts  to  prepare  and  maintain  the  solutions  of  a 
strictly  normal  concentration,  and  make  them  up  only  approximately  normal, 
correcting  for  the  variations  in  calculating  the  results  of  the  titrations. 

Sub -normal  and  super-normal  solutions.  Instead  of  the  various  devices  for 
delivering  and  reading  fractions  of  a  cubic  centimeter  smaller  than  tenths,  the 
same  accuracy  can  be  attained  by  the  use  of  solutions  weaker  than  normal  — 
dilution  with  an  equal  volume  of  water  giving  semi-normal;  with  nine  parts, 
decinormal,  etc.  By  reason  of  the  volatile  nature  of  some  compounds  and  the 
limited  solubility  of  some  salts,  it  is  practicable  to  make  up  their  solutions 
only  semi-normal  or  weaker.  As  to  the  question  of  what  strength  it  is  best  to 
make  up  a  standard  solution  —  whether  normal,  decinormal  or  centinormal  — 
it  is  to  be  considered  that  although  the  errors  in  measuring  the  volumes  and 
other  factors  are  less  in  proportion  as  the  solution  is  more  dilute,  yet  the 
volume  needed  to  show  the  end-point  distinctly  is  greater.  In  general,  the 
concentration  of  the  titrand  should  be  adjusted  to  the  weight  of  the  body 
to  be  titrated  or  vice  versa  as  circumstances  admit. 

Solutions  stronger  than  normal  are  used  when  it  is  an  object  to  keep  the 
total  volume  of  the  titrate  small,  or  when  the  reaction  or  end -point  indication 
proceeds  best  in  a  highly  concentrated  solution.  It  is  seldom  that  one  stronger 
than  binormal  will  be  needed. 

A  list  of  volumetric  solutions  commonly  used  in  inorganic  analysis  follows. 
The  weights  are  grams  of  the  reagent  per  liter. 

Hydrochloric  acid Normal  36.456 

Nitric  acid "  63.048 

Sulfuric  acid "  49.043 

Oxalic  acid,  anhydrous "  45.008 

Oxalic  acid,  crystallized "  63 .024 

Potassium  hydroxide,  anhydrous "  56.118 

Sodium  hydrate,  anhydrous "  40058 

Barium  hydrate,  anhydrous Decinormal  8.572 

Ammonia  (NHs) "  1706 

Sodium  carbonate,  anhydrous  Normal  53.050 

Potassium  carbonate,  anhydrous "  69.110 

Arsenions  oxide. . : Decinormal  4  950 

Iodine "  12.685 

Sodium  thiosulfate,  cryst "  24832 

Potassium  bichromate...                                                                                         "  4.908 


Journ.  Amer.  Chem.  8ocy.  1901—12  and  343. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  127 

Potassium  permanganate ..Decinormal     3.162 

Iron  (as  ferrous  salt) "  5.602 

Ferrous  sulfate,  crystallized "  27.820 

Ferrous  ammonium  sulf ate,  cryst "  39.240 

Stannous  chloride,  anhydrous "  9.493 

Tin  (as  stannous  chloride) "  5.953 

Silver  (as  silver  nitrate) "  10.792 

Silver  nitrate "  16.996 

Sodium  chloride "  5.850 

Potassium  sulf ocyanide "  9.722 

Empirical  Solutions.  In  routine  technical  work  where  a  number  of  samples 
of  a  given  raw  material  or  product  are  periodically  tested,  the  usual  calcula- 
tions are  dispensed  with  by  so  adjusting  the  standard  solution  that  one  cubic 
centimeter  corresponds  to  one  per  cent,  one- half  per  cent,  one-tenth  per  cent, 
etc.,  of  the  reacting  constituent  of  the  substance  analyzed,  of  which  a  certain 
fixed  weight  is  taken  for  the  titration.  A  "  decimal  solution  "  is  one  of  which 
ten  cubic  centimeters  reacts  with  one  gram  of  a  given  solid  or  one  cubic  centi- 
meter of  a  given  liquid. 

The  concentration  of  an  empirical  solution  designed  for  a  special  material 
should  be  such  that  the  smallest  volume  that  can  be  easily  read  on  the  burette 
used  is  approximately  equivalent  to  say  one-fifth  of  the  probable  error  of  the 
determination,  and  the  total  volume  used  for  a  titration  of  any  sample  not 
abnormal  in  composition  be  within  the  capacity  of  the  burette. 

Common  reagents  for  titration.  For  alkalimetry,  sulfuric  acid  is  easily 
obtained  in  the  market  almost  perfectly  pure,  and  can  be  diluted  to  a  perfectly 
stable  solution.  Precipitates  are  formed  with  hydrates  of  the  barium  group 
and  certain  lead  compounds,  yet  the  end-point  can  nevertheless  be  seen  with 
sufficient  distinctness.  Hydrochloric  acid  forms  soluble  chlorides  with  nearly 
all  the  bases  and  its  titre  is  readily  found  by  means  of  silver  nitrate,  but  a 
serious  objection  is  its  volatility  and  although  it  has  repeatedly  been  claimed 
that  a  weak  solution  can  be  heated  and  even  boiled  without  loss  by  volatiliza- 
tion, yet  a  conservative  operator  will  generally  adopt  the  certainly  non- 
volatile sulfuric.  Of  the  other  acids  nitric  offers  no  advantages  except  possibly 
where  secondary  reactions  might  take  place  with  the  above ;  oxalic  and  tar- 
taric  and  their  acid  salts  are  not  easily  procured  of  so  definite  a  composition  as 
to  be  weighed  directly,  they  form  insoluble  compounds  with  many  bases,  and 
are  liable  to  decompose  when  in  solution ;  chromic  acid  (potassium  bichromate) 
has  been  proposed,  but  its  deep  yellow  color  obscures  the  tint  of  the  indicator, 
especially  in  a  strong  solution. 

Many  schemes  for  standardizing  the  acids  have  been  brought  forward, 
though  none  are  entirely  satisfactory  as  combining  accuracy  and  convenience. 
Potassium  and  sodium  hydrates  are  not  admissible  as  they  always  contain  more 
or  less  water  and  carbonates,  and  are  extremely  hygroscopic.  Sodium  carbonate, 
however,  can  readily  be  prepared  pure  and  anhydrous,  and  not  being  hygro- 
scopic can  be  accurately  weighed.  The  carbonic  acid  liberated  during  the 
neutralization  must  be  expelled  by  boiling  after  the  end -point  is  shown,  unless 
an  indicator  can  be  used  that  is  not  affected  by  it.  The  alkaline  reaction  is 
restored  by  the  removal  of  the  carbon  dioxide,  and  alternate  additions  of  acid 
and  ebullition  are  necessary  until  no  change  in  color  is  produced  by  the  latter. 
A  less  tedious  way  is  to  supersaturate  at  once  with  an  observed  volume  of  acidr 
and  after  boiling  determine  the  excess  of  acid  with  standard  alkali.  Richmond 
calls  attention  to  the  error  that  may  arise  from  the  absorption  of  carbon  diox- 
ide by  standard  acids,  the  acidity  increased  proportionately. 

Other  methods,  more  or  less  used,  are  the  precipitation  of  sulfuric  acid  by 
barium  chloride,  the  precipitated  barium  sulphate  weighed  and  the  concentration 


128  QUANTITATIVE    CHEMICAL    ANALYSIS. 

of  the  acid  calculated,  this  process  assuming  that  the  solution  contains  no 
sulf  ate  of  a  base ;  similarly,  hydrochloric  acid  is  precipitated  by  silver  nitrate, 
oxalic  acid  by  calcium  chloride,  etc.  A  solution  of  pure  sodium  hydrate  can 
be  prepared  by  weighing  a  freshly-cut  lump  of  sodium  under  gasoline,  after- 
ward dissolving  in  alcohol  of  90  per  cent,  and  diluting  with  water.  The  base 
of  a  metallic  compound  with  a  weak  acid  radical,  such  as  boracic,  can  be 
titrated  directly  by  a  standard  mineral  acid,  with  a  suitable  indicator.  Sodium 
oxalate,  a  salt  that  can  be  prepared  pure  by  repeated  crystallization,  leaves  on 
ignition  pure  sodium  carbonate  that  may  be  titrated  directly.  Sulfuric  acid 
diluted  with  hydrogen  peroxide  solution  reacts  with  standard  potassium  per- 
mangate  in  a  definite  ratio  (page  125).  And  it  has  been  advised  to  make  up 
normal  sulf  uric  acid  by  dilution  of  a  weighed  quantity  of  concentrated  acid 
with  strict  attention  to  temperature.* 

Of  the  corresponding  bases,  potassium  and  sodium  hydrates  are  most  in  use, 
though  against  them  may  be  charged  the  difficulty  of  obtaining  them  free  from 
carbonic  acid  and  protecting  their  solutions  from  its  absorption,  and  the  cor- 
rosion of  the  glass  of  bottles  used  for  their  storage.  The  solutions  may  be 
purified  by  the  introduction  of  barium  hydrate,  avoiding  an  excess,  and  Mil- 
lon's  base  has  been  advocated  for  the  purpose,  itself  practically  insoluble. 
But  if  practicable,  it  is  the  better  plan  to  arrange  the  titration  so  that  the 
carbonic  acid  shall  not  interfere. 

Potassium  tetroxalate,  sodium  biborate,  and  other  acid  salts  and  certain  crys- 
tallized acids  are  used  for  standardization  by  titration  to  the  normal  salts; 
thus,  KHC^Oe  (potassium  hydrotartrate)  -f-  KOH  =  I&C^Oe  (potassium 
tartrate)  -f-  H2O,  but  only  after  a  tedious  purification  and  testing  has  insured  a 
product  of  exactly  the  assumed  formula  unmixed  with  either  the  normal  salt  or 
the  free  acid.  Titration  of  the  sulf  uric  acid  liberated  by  the  electrolysis  of  a 
weighed  amount  of  pure  cupric  sulfate  in  aqueous  solution  is  perhaps  the  most 
accurate  method  that  has  been  proposed. 

However,  the  usual  method  for  standardization  is  by  titration  of  a  standard 
acid  and  calculating  from  their  combining  weights. 

Pure  ammonia  is  easy  to  purchase,  acts  but  slightly  on  glass,  and  unlike  the 
fixed  alkalies,  does  not  introduce  a  fixed  base  into  the  titrate,  which  is  some- 
times an  advantage.  Like  hydrochloric  acid,  however,  its  volatility  is  a  seri- 
ous drawback  for  general  use.  Lime  water  and  calcium  saccharate  are  highly 
recommended  for  the  titration  of  free  carbonic  acid  and  such  organic  acids 
as  are  found  in  wine-must:  strontia  water  has  some  advantages  over  lime  water, 
as  in  the  titration  of  free  acid  in  fermented  milk.  Baryta  water  is  better  suited 
for  the  titration  of  alkaloidal  compounds  than  the  caustic  alkalies,  and  any 
carbon  dioxide  absorbed  from  the  air  immediately  combines  with  barium  and 
precipitates,  leaving  the  liquid  perfectly  caustic.  Potassium  bichromate  may 
be  used  for  standardizing,  with  phenol-phthalein  as  indicator. 

Sodium  carbonate  can  be  obtained  quite  pure  and  weighed  without  difficulty. 
Carbon  dioxide  is  liberated  on  titrating  an  acid,  and  the  titrate  must  be 
intermittently  boiled  or  a  strong  indicator  employed.  It  has  been  proposed 
to  determine  the  end-point  by  adding  a  little  barium  chloride  to  the  titrate 
and  note  the  instant  when  a  persistent  cloudiness  (BaCOs)  appears.  In 
presence  of  certain  organic  matters  a  troublesome  frothing  ensues,  but  can 
be  dissipated  by  a  thin  layer  of  paraffin  or  paraffin  oil  on  the  titrate. 

Alkali  solutions  of  various  bases,  as  zinc  oxide  in  potassium  hydrate,  copper 
oxide  in  ammonia,  etc.,  have  the  advantage  of  being  available  for  dark  colored 
clear  liquids,  since  the  end-point  is  shown  by  a  turbidity  due  to  the  separation 


*  Chem.  News,  1895-2-5. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  129 

of  the  base  of  the  titrand.  They  are  not  applicable  for  acids  forming  insoluble 
compounds  with  the  base  of  the  titrand  nor  in  presence  of  their  metallic 
salts. 

2.  Titration  by  iodine  (held  in  aqueous  solution  by  potassium  iodide)  may 
be  applied  for  the  determination  of  a  number  of  bodies,  either  directly,  as  for 
hydrogen  sulflde,  sulfurous  acid,  etc.,  which  react  with  the  decomposition  of 
water,  or  by  the  intervention  of  an  iodide  as  in  the  case  of  chlorine  (Cl%  -f  2KI 


Arsenious  acid  reacts  with  iodine  becoming  oxidized  to  arsenic  acid.  The 
solution  is  made  by  combining  the  but  slightly  soluble  acid  with  sodium  carbon- 
ate. Sodium  thiosulfate  with  iodine  forms  sodium  tetrathionate  (2Na2S2Os  -{- 
2I  =  Na2S406  +  2NaI).  The  end-point  of  these  titrations  is  shown  by  the 
bleaching  of  the  yellow  color  of  the  iodine  solution  (iodine  in  potassium  iodide) 
or  more  decisively  by  the  incipience  or  disappearance  of  the  blue  color  of 
iodide  of  starch  when  starch  paste  has  been  previously  introduced  in  the 
solution. 

3.  Potassium     bichromate    (K2O.Cr203.O3)     easily    parts    with    three,    and 
potassium  permanganate  (K2O.2MnO.05)  with  five  atoms  of  oxygen  to  reduc- 
ing agents.    The  former  is  in  common  use  in  acid  solution  for  the  titration  of 
ferrous  salts  and  a  few  organic  bodies,  while  the  latter  is  more  sensitive  and 
decomposes  many  organic  bodies.    Ferrous   salts  are  commonly   used   for 
standardization,  sometimes  a  hot  solution  of  oxalic  acid  for  permanganate. 
Bichromate  may  be  used  for  titrates  containing  free  hydrochloric   acid,  but 
with  permanganate  a  secondary  reaction  occurs  interfering  with  the  titration.* 

Of  all  volumetric  solutions,  that  of  potassium  permanganate  perhaps 
approaches  nearer  the  ideal  solution  than  any  other.  It  has  a  wide  applica- 
tion, oxidizing  most  of  the  lower  inorganic  and  many  organic  compounds,  is 
fairly  stable  and  not  decomposed  by  light,  air  or  carbon  dioxide,  is  easily  made 
up  and  standardized,  and  from  its  intense  color  needs  no  extrinsic  indicator. 

The  purple  tint  that  marks  the  end-point  fades  rapidly,  due  principally  to 
the  spontaneous  decomposition  of  the  free  permanganic  acid  to  water,  man- 
ganic oxide  and  oxygen.  The  decomposition  is  hastened  by  the  presence  in 
the  titrate  of  a  manganous  salt  or  organic  matter. 

Permanganate  is  used  chiefly  for  the  titration  of  acidified  aqueous  solutions; 
the  most  suitable  acid  is  sulf  uric,  though  dilute  nitric  is  equally  good  for  some 
compounds.  It  is  asserted  that  potassium  manganate  has  some  advantage  over 
permanganate  as  an  oxidizer.f 

Potassium  (or  sodium)  bichromate  dissolves  to  a  stable  yellow  solution  well 
adapted  for  the  titration  of  strong  reducers,  though  from  the  deep  green 
color  of  the  chromium  salts  formed  in  its  reduction,  a  spot  indication  in  neces- 
sary. Bichromate  is  also  a  medium  for  the  titration  of  barium  and  lead  com- 
pounds, their  chromates  falling. 

Stannous  chloride  is  sometimes  used  for  the  titration  of  ferric  chloride,  re- 
ducing it  to  ferrous  chloride;  the  indicator  is  a  sulfocyanide,  the  red  ferric 
sulfocyanide  being  reduced  to  the  corresponding  colorless  compound.  The 
solution  is  readily  oxidized  by  the  air  and  must  be  restandardized  before  each 
set  of  titrations. 

4.  Silver  and  chlorine  unite  to  form  the  highly  insoluble  silver  chloride.    For 
the  titration  of  solutions  containing  silver  salts  is  used  standard  sodium  chlo- 
ride, and  for  chlorides,  standard  silver  nitrate.    The  end-point  is  the  cessation 
of  precipitation,  easy  to  be  seen  as  the  precipitate  quickly  clots,  more  readily 


*  Amer.  Chem.  Journ.  1899—461. 
f  Chem.  News,  1889—1-301. 


130  QUANTITATIVE    CHEMICAL    ANALYSIS. 

•when  silver  is  in  excess.  In  a  neutral  titrate  of  a  chloride  there  may  be  con- 
tained potassium  chromate  as  an  indicator,  any  excess  of  silver  precipitating 
red  silver  chromate. 

A  sulfocyanide  precipitates  silver  as  sulf ocyanide ;  the  indicator  is  a  ferric 
salt,  developing  the  familiar  intense  red  coloration  of  ferric  sulfocyanide  with 
the  least  excess  of  the  titrand. 


Residual  titration  —  also  known  as 're  verse  titration'  or  '  titrating  back  '- 
This  modification  is  practiced  in  titrations  where  the  end-point  is  not  easy  to 
discern  or  the  titration  tedious  or  difficult  for  other  reasons.  To  the  titrate  is 
added  of  the  titrand  a  measured  volume  in  quantity  more  than  sufficient 
to  complete  the  reaction;  the  excess  is  then  determined  by  titration  by 
another  volumetric  solution  reacting  with  the  first  and  whose  volumetric  rela- 
tion to  it  has  been  ascertained.  It  is  premised,  of  course,  that  the  products 
of  the  first  reaction  are  indifferent  to  the  second  solution.  For  the  first  solu- 
tion it  is  often  more  convenient  to  substitute  a  weighed  amount  of  the  solid 
reagent  that  forms  its  basis. 

For  example,  formaldehyd  reacts  with  ammonia  to  produce  (neutral)  hexamethylene- 
tetramine  — 6CHOH+4NH4OH==(CH2)6N4  +  10H2O  — but  instead  of  titrating  the  formal- 
dehyd directly  by  a  standard  solution  of  ammonia,  it  is  more  satisfactory  to  add  an  excess 
of  standard  ammonia  at  once,  then  determine  the  excess  by  titration  with  standard 
hydrochloric  acid  and  a  suitable  indicator.* 

The  principle  of  reversed  titration  has  several  applications  that  are  the  bases 
of  technical  methods,  as  the  following. 

Of  a  mixture  of  solids,  the  proportion  that  is  soluble  in  a  reagent  can  be 
found  by  treating  the  mixture  with  an  excess  of  a  solution  of  the  reagent  of 
known  concentration,  then  titrating  back  with  a  solution  that  reacts  with  the 
solvent  but  not  with  the  dissolved  matter.  The  end- point  is  observed  by  an 
indicator  or  the  turbidity  coming  from  the  precipitation  of  one  of  the  dissolved 
constituents. 

An  element  or  compound  to  be  determined  is  precipitated  from  solution  by 
a  measured  volume  of  one  volumetric  solution;  after  filtering  or  decanting,  an 
aliquot  part  of  the  filtrate  is  titrated  back  by  a  second  volumetric  solution.  A 
modification  useful  in  some  cases  is  to  dispense  with  filtration  by  removing 
the  excess  of  the  first  volumetric  solution  by  boiling  or  other  means;  the  sus- 
pended precipitate  Is  dissolved  in  an  excess  of  the  second  solution,  and  the 
excess  titrated  back  by  the  first. 

The  determination  of  the  saponiflcation  value  as  applied  to  an  oil  or  fat  is 
described  under  Oils  and  Fats;  the  process  is  also  resorted  to  for  various 
other  compounds. 

In  a  direct  titration  when  the  end-point  is  inadvertently  overstepped,  a  small 
measured  volume  of  the  second  solution  may  be  introduced  and  the  titration 
resumed  more  cautiously,  not  forgetting  to  deduct  in  the  calculation  for  a 
volume  of  the  titrand  equivalent  to  that  of  the  second  solution  added. 

An  elaboration  of  the  principle  of  residual  titratton  is  shown  in  the  following  examples. 
The  direct  titration  of  sulfuric  acid  or  a  soluble  sulfate  by  barium  chloride  is  open  to  the 
objection  that  the  precipitated  barium  sulfate  is  finely  divided  and  slow  to  settle,  so  that 
the  end-point  cannot  be  observed  by  noting  when  the  formation  ceases;  and  filtering  a 
little  of  the  turbid  fluid  after  each  addition  of  the  titrand  and  testing  the  filtrate  is  at  best 
tedious.  Indirect  methods  avoid  the  filtration  and  testing,  though  at  the  expense  of  sim- 
plicity ;  a  few  only  are  trustworthy.  That  of  Edmunds  f  requires  four  standard  solutions, 
and  is  as  follows. 


*  Chem.  News  *1893-2— 2,  and  Journ.  Anal.  Chem.  3—459. 
t  Chem.  News,  1896-2—194  and  246. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  131 

1.  The  neutral  solution  of  a  sulfate  is  precipitated  by  a  measured  volume  (an  excess)  of 
standard  barium  nitrate ; 

NasS04  +  Ba(NOs)2  =  BaS04  +  2NaNOs 

2.  What  remains  of  the  barium  nitrate  is  precipitated  by  a  measured  volume  (an  excess) 
of  standard  potassium  chromate ; 

Ba(NOs)2  +  K2CrO4  =  BaCrO4  +  2KNO3. 

3.  What  remains  of  the  potassium  chromate  is  precipitated  by  a  measured  volume  (an 
excess)  of  standard  silver  nitrate; 

K2CrO4  +  2AgNO3  =  Ag2CrO4  +  2KNOs. 

4.  The  precipitate,  a  mixture  of  barium  sulfate,  barium  chromate  and  silver  chromate, 
is  filtered  off  and  in  the  filtrate  the  excess  of  silver  nitrate  is  determined  by  titration  with 
standard  sodium  chloride; 

AgNOs  +  NaOl  •=  AgCl  +  NaNOs. 

Another  method  for  the  same  determination  is  due  to  Stolle.  The  principles  are  these : 
(1)  When  barium  chromate  is  dissolved  in  hydrochloric  acid,  there  are  formed  barium 
chloride  and  chromic  acid  both  passing  into  solution  — 

BaCrO4  +  2HC1  =  CrOs  +  BaCl2  +  H2O. 

(2).  If  an  excess  of  ammonia  be  added  to  this  solution  the  reverse  reaction  takes  place 
and  all  the  chromic  acid  falls  as  barium  chromate  — 

CrOa  +  BaCl2  +  2HN4  OH  =  BaCrO4  +  2NH4C1  +  H2O. 
But  if  previous  to  (2),  all  the  barium  is  removed  by  precipitation  as  barium  sulfate  — 

CrOs  +  BaCla  +  H2SO4=  CrO3  +  BaSO4  +  2HC1— 

then  all  the  chromic  acid  combines  with  ammonia  on  afterward  supersaturing  the  solution 
with  ammonia,  and  remains  in  solution  — 

CrO3  +  BaSO4  +  2BC1  +  4NH4OH  =  (NH4)2CrO4  +  BaSO4  +  2NH4C1  +  3H2O. 
and  if  less  than  the  total  barium  be  precipitated  by  sulf  uric  acid  a  proportionally  less 
amount  than  the  total  chromic  acid  will  remain  in  solution,  the  remainder  falling  as 
barium  chromate. 

Of  a  freshly*  made  solution  of  a  known  weight  of  barium  chromate  in  a  certain  volume 
of  dilute  hydrochloric  acid,  an  excess  is  added  to  the  sulfuric  solution  to  be  assayed. 
After  stirring,  an  excess  of  ammonium  hydrate  is  run  in,  and  the  precipitated  barium  sul- 
fate and  barium  chromate  filtered.  The  filtrate  is  acidified,  and  the  chromic  acid  titrated 
by  reducing  to  chromic  oxide  by  a  standard  reducing  solution,  then  the  excess  of  the 
latter  titrated  back  by  an  oxidizer  like  permanganate  or  bichromate. 


The  basis  of  a  few  methods  is  the  titration  by  a  volumetric  solution  that  acts 
as  a  solvent  of  solid  matter  suspended  in  water  or  in  some  solution;  even 
titrations  wherein  the  solid  is  transformed  to  another  insoluble  compound  have 
been  proposed.  The  success  of  a  titration  of  this  nature  depends  on  several 
conditions,  such  as  the  volume  and  temperature  of  the  titrate,  the  rapidity  of 
the  reaction,  and  the  subdivision  of  the  suspended  matter  —  thus,  a  freshly 
formed  voluminous  precipitate  will  dissolve  much  more  promptly  than  after 
standing  for  a  time  or  becoming  crystalline  or  compacted.  But  as  a  rule  better 
results  are  obtained  and  more  quickly  by  a  reverse  titration. 

The  titration  of  a  liquid  in  which  is  suspended  a  precipitate  or  other  solid 
matter  that  also  reacts  with  titrand  is  not  uncommon,  it  being  assumed  that  the 
reaction  will  take  place  preferentially  with  the  solution  and  the  end-point  show 
momentarily  before  the  reaction  with  the  solid  begins.  Of  this  class  are  meth- 
ods where  the  base  of  a  neutral  metallic  salt  is  precipitated  by  an  excess  of 
standard  alkali,  and,  without  filtering,  the  excess  titrated  by  standard  acid; 
since  in  a  cold  dilute  solution  the  indicator  turns  before  any  of  the  precipitate 
is  acted  on  by  the  acid.  Occasionally  the  nature  of  the  titrand  and  of  the 
bodies  suspended  and  in  solution  is  such  that  should  any  of  the  solid  be  acted 
on  by  the  titrand,  the  product  of  the  reaction  will  itself  at  once  react  with 
the  uncombined  soluble  matter  as  if  it  were  the  titrand ;  here  the  presence  of 
the  suspended  matter  may  be  ignored,  although  the  customary  indication  of 


*  Chem.  News.  1892—2—168. 


132  QUANTITATIVE    CHEMICAL    ANALYSIS. 

the  end -point  may  be  so  transitory  as  to  require  a  special  provision  for  the 
purpose.  In  all  other  cases  the  success  of  a  titration  of  this  kind  depends 
mainly  on  a  high  degree  of  insolubility  of  the  suspended  matter  and  its  resist- 
ance to  combination  with  the  titrand.  It  need  hardly  be  mentioned  that 
the  titrand  should  be  run  in  slowly  —  even  by  drops  —  and  that  the  liquid  be  not 
too  concentrated. 

In  a  few  instances  the  compound  to  be  titrated  is  a  comparatively  insoluble 
gaseous  radical  that  must  be  liberated  from  the  compound  during  titration  by 
acidification  of  the  solution,  e.  g.t  a  nitrite  titrated  by  potassium  permanganate, 
liberating  the  nitrous  acid  and  oxidizing  it  by  alternate  additions  of  sulf  uric  acid 
and  permanganate  to  the  cold  dilute  solution. 

A  few  titrations  are  made  fractionally,  changing  the  reaction  of  the  titrate, 
raising  the  temperature,  or  otherwise  altering  the  conditions  before  the  end- 
point  is  reached.  In  these  cases  two  or  more  reactions  are  involved,  each  pro- 
ceeding only  under  certain  suitable  conditions.  Only  the  final  product  may 
cause  the  exhibition  of  the  end-point,  or  the  end  of  the  first  reaction  may  also 
be  manifest. 

An  example*  is  a  method  for  the  titration  of  cuprous  sulfocyanide  by  potassium  per- 
manganate. By  the  action  of  permanganate  the  cuprous  compound  is  broken  up  to  cupric 
sulfate  and  hydrocyanic  acid  according  to  the  equation  — 

lOCuCNS  +  7K2  MD2O8  +  21H2SO4  =  10CuSO4  +  10HCN  +  7K2SO4  +  14MnSO4  +  16H2O (1). 

In  the  first  installment  of  the  titration  the  cuprous  sulfocyanide  is  decomposed  by 
sodium  hydrate,  then  the  cuprous  hydrate  oxidized  to  cupric  hydrate  by  titration,  a  slight 
excess  of  permanganate  being  run  in  to  complete  oxidation.  In  the  second  instalment 
the  titrate  is  acidified  by  sulfuric  acid,  and  the  sulfocyanic  acid  oxidized  to  hydrocyanic 
acid  by  titration.  The  weight  of  the  original  cuprous  sulfocyanide  is  calculated  from  the 
above  equation. 

Several  reactions  are  involved  in  the  process.  On  treating  the  cuprous  sulfocyanide 
with  sodium  hydrate  it  is  decomposed  to  cuprous  hydrate  and  sodium  sulfocyanide  — 

CuCNS  +  NaOH  =  CuOH  +  NaCNS (2). 

On  titrating  this  solution  by  permanganate  there  are  produced  cupric  and  manganic 
hydrates  — 

SCuOH  +  K2Mn2O8  +  8H2O  =  8Cu(OH)a  +  MnsO3.3H2O  +  2KOH (3). 

The  slight  excess  of  permanganate  added  acts  on  the  sodium  snlfocyanlde  — 
NaCNS  +  4K2Mn2O8  +  lONaOH  =  4K2MnO4  +  4Na2MnO4  +  NaCNO  +  N82SO4  + 

5H2O (4). 

On  acidifying  by  sulfuric  acid,  the  manganic  hydrate  in  equation  (3)  and  the  potassium 
and  sodium  manganates  and  the  sodium  cyanate  in  equation  (4)  react  with  the  sodium 
sulfocyanlde  of  (2)  to  form  hydrocyanic  acid — 

6Mn203.3H20  +  2NaCNS  +  11H2SO4  =  12MnSO4  +  2HCN  +  28H2O+  Na2SO4 (5) . 

3K2MnO4  +  3Na2MnO4  +  4NaCN8  +  10H2SO4  +  4H2O  =  3K2SO4  +  5Na2S04  + 

6MnSO4  +  4HCN  +  12B2O (6). 

3  NaCNO  +  NaCNS  +  H2SO4  +  H2O=4HCN  +2Xa2SO4 (7). 

And  on  titrating  by  permanganate  — 

5HCNS  +  3K2Mn2O8  +  4H2SO4  =  3K2SO4  +  6MnSO4  +  5HCN  +  4H2O. . .   (8) . 

That  the  process  as  conducted  answers  to  the  equation  (1)  is  proved  by  the  net  con- 
sumption of  oxygen  from  permanganate  for  equations  (3)  and  (8)  being  35  atoms  to  10 
molecules  of  cuprous  sulfocyanide;  and  the  net  consumption  in  equation  (4)  agreeing 
with  that  In  equation  (8). 

In  theory  it  is  immaterial  as  regards  the  ratio  of  their  combining  volumes, 
which  of  two  inter- reacting  solutions  is  made  the  titrand,  but  in  practice  there 
will  be  found  a  slight,  sometimes  a  marked,  difference  due  to  the  excess  of  the 
titrand  needed  to  show  the  end- point,  or  for  other  reasons.  It  is  sometimes 
the  better  plan  to  dissolve  a  material  to  be  assayed  in  water,  make  up  the 
solution  to  a  definite  volume,  and  with  it  titrate  a  measured  volume  of  a  react- 
ing solution  than  to  proceed  in  the  usual  way ;  here  the  standardizing  follows 
the  same  routine  as  the  titration. 


*  Journ.  Amer.  Chem.  Socy.  1900—685  and  1902—580. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  133 

For  example,  picric  acid  forms  an  insoluble  plcrate  (C25H31N3.C6H2(NO2)3.OH)  with 
crystal  violet  (an  alinln  dye,  the  hydrochloride  of  hexamethyl-rosanilin).  For  the  de- 
termination of  picric  acid  in  a  commercial  sample,  one  gram  of  the  sample  is  dissolved 
in  a  liter  of  water  and' added  from  a  burette  to  a  measured  volume  of  a  solution  of  crystal 
violet  until  the  supernatant  liquid  assumes  a  faint  yellow  tint  announcing  an  excess  of 
picric  acid.  An  equal  volume  of  the  violet  solution  is  then  standardized  by  titrating  it  by 
a  solution  containing  a  known  weight  of  chemically  pure  picric  acid. 

A  complex  substance  containing  two  analogous  elements  or  compounds  in 
the  free  state  or  each  combined  with  the  same  or  with  different  radicals,  may 
often  be  volumetrically  determined  without  separation. 

1 .  By  means  of  one  volumetric  solution.    One  of  the  bodies  may  react  pre- 
cedently  to  the  other  and  the  intermission  allow  a  transient  exhibition  of  the 
end-point,  or  the  incipiency  of  one  of  the  products  of  the  reaction  with  the 
second  member  be  utilized  to  show  the  dividing  line.*    Thus,  sulfindigotic  acid 
is  completely  oxidized  by  a  permanganate  before  sulflndirubic  acid  is  acted  on; 
lactic  acid  is  neutralized  by  an  alkali  before  lactic  anhydride  is  saponified. 

In  a  few  cases  also,  the  inequality  in  strength  of  two  indicators  may  be 
applied.  For  the  determination  of  the  free  acid  and  the  base  in  an  acid  solu- 
tion of  a  metallic  salt,  it  has  been  proposed  that  the  indicator  be  a  mixture  of 
phenol-phthalein  and  methyl  orange.f  At  the  beginning  the  solution  is  pink 
from  the  methyl  orange,  phenol-phthalein  being  colorless  in  an  acid  solution. 
On  titrating  by  an  alkali  the  solution  turns  to  yellow  at  the  point  where  the 
free  acid  is  neutralized,  and  the  titrate  remains  yellow  until  all  the  base  is 
precipitated,  when  the  least  excess  of  alkali  develops  the  red  of  the  phenol- 
phthalein,  its  intensity  overpowering  the  faint  yellow  of  the  methyl  orange. 

The  members  of  some  mixtures  may  be  titrated  successively  under  different 
conditions,  one  member  reacting  in  the  cold,  the  other  only  at  the  boiling 
point;  one  reacts  in  an  acid,  the  other  only  in  a  neutral  or  alkaline  solution; 
etc. 

A  mixture  of  two  bodies  A  and  B,  that  react  simultaneously  but  in  dissimilar 
ratios  may  be  determined  in  one  titration.  If  x  represents  the  percentage  of 
A  in  the  mixture;  y,  that  of  B;  and  100  of  the  two;  a,  the  volume  of  titrand 
reacting  with  one  gram  of  A,  and  b  and  d  the  corresponding  volumes  for  B, 

d  —  b 
and  the  mixture ;  then  x  =  100  ^j~^  and  y  =100  —  x. 

In  technical  work  are  found  certain  materials  that  contain  two  or  more  analogous- 
constituents  possessing  an  equal  or  nearly  equal  value  for  some  practical  purpose,,and 
can  be  determined  together  by  one  titration.  If  there  be  no  reason  to  the  contrary,  the 
result  can  be  reported  as  the  joint  content  of  the  constituents,  or  as  the  leading  member 
if  one  greatly  preponderates,  or  the  result  calculated  to  units  of  value  for  the  use  in- 
tended. 

2.  The  mixture  may  be  successively  titrated  by  two  different  volumetric 
solutions.    Between  the  titrations  some  alteration  is  made  in  the  titrate,  such 
as  changing  the  reaction  from  acid  to  alkaline,  filtering  off  an  insoluble  product 
of  the  first  operation,  boiling  off  a  volatile  product,  oxidation,  etc.    Thus,  a 
solution  of  two  inorganic  salts  containing  free  acid;  first  the  free  acid  is  neu- 
tralized by  standard  alkali,  then  one  base  changed  in  valence  by  a  standard 
oxidizing  or  reducing  solution,  and  finally  the  other  base  precipitated  by  a 
suitable  standard  precipitant.    However,  it  is  more  usual  to  effect  a  partial  or 
complete  separation  before  proceeding  with  the  volumetric  determinations. 

One  element  or  compound  existing  in  a  solution  in  two  different  combina- 
tions may  be  successively  titrated  by  two  solutions  each  reacting  with  only  one. 


Allen,  Coml.  Org.  Anal.  3--1-123. 
Zeits.  anal.  1883—397. 


134  QUANTITATIVE    CHEMICAL    ANALYSIS. 

form  of  combination.    An  example  is  a  mixture  of  sodium  chloride  and  sodium 
hypochlorite  titrated  by  silver  nitrate  and  arsenious  acid.* 

3.  The  titrate  may  be  divided  and  one  element  or  compound  determined  in 
each  half  by  different  solutions;  for  example,  in  one-half  a  constituent  is  ox- 
idized or  reduced,  and  in  the  other  half  another  constituent  is  precipitated. 


Preservation.  Volumetric  solutions  should  be  stored  in  a  cool  dark  place  in 
tightly  stoppered  bottles  of  chemical  glass.  The  absorption  of  carbonic  and 
other  acids  from  the  air  by  solutions  of  the  caustic  alkalies  can  be  prevented 
by  withdrawing  the  liquid  through  a  syphon,  admitting  air  to  the  bottle  through 
a  guard-tube  containing  soda-lime  or  solid  caustic  potash.  A  thin  layer  of 
paraffin  oil  or  kerosene  floating  on  the  solution  will  tend  to  prevent  contact  of 
air,  though  the  protective  power  against  oxidation  has  been  overestimated. 
Where  the  reagent  is  an  easily  fermentable  or  putrescible  organic  body,  a  trace 
of  a  preservative,  as  a  mercuric  salt,  phenol,  or  salicylic  acid,  can  usually  be 
introduced  without  its  interfering  with  titrations.  The  fungoid  growth  some- 
times observed  in  fifth -normal  sulfuric  acid  is  prevented  by  the  addition  of  a 
few  drops  of  chloroform. 

Too  much  reliance  must  not  be  placed  on  the  unalterability  of  any  volumet- 
ric solution.  With  many  of  these  the  efficiency  of  the  reagent  lies  in  the  facil- 
ity and  rapidity  with  which  it  is  decomposed  by  other  bodies,  and  the  solution 
is  correspondingly  prone  to  spontaneous  decomposition  or  sensitive  to  light, 
heat,  dust,  etc.  Stannous  chloride  in  hydrochloric  acid,  hydrogen  peroxide  in 
water,  bromine  in  carbon  disulfide,  and  the  like,  change  perceptibly  from  day 
to  day.  Other  reagents  are  more  stable  and  can  be  preserved  for  a  consider- 
able time  without  alteration.  Yet,  all  things  considered,  it  is  always  advisable 
to  verify  every  solution  before  each  set  of  determinations.  This  precaution 
al«o  eliminates  a  possible  source  of  error  due  to  difference  in  temperature  at 
the  times  of  standardization  and  analysis. 

A  volumetric  solution  compounded  of  two  others,  one  containing  the  essen- 
tial principle  of  the  reaction,  the  other  an  adjective,  may  become  less  active  on 
standing,  though  apparently  the  composition  remains  unchanged.  The  solu- 
tions are  best  kept  separate  up  to  the  time  of  use,  then  a  sufficient  quantity 
mixed  in  the  proper  proportions.  And  where  a  solution  is  made  up  in  consid- 
erable quantity  for  regular  use,  it  is  well  to  divide  it  among  several  small  sealed 
bottles  opened  only  as  required,  or  to  preserve  it  in  a  large  stock  bottle  kept 
sealed  and  in  a  dark  cool  closet,  transferring  to  a  smaller  one  as  needed. 


Titration.  After  filling  the  burette  and  drawing  off  the  solution  to  the  zero 
mark,  the  tap  is  opened  and  the  solution  run  into  the  titrate  until  the  end-point 
is  reached.  Allowing  a  few  minutes  for  the  liquid  to  collect  from  above  the 
surface,  the  volume  withdrawn  is  read  off.  In  most  titrations,  especially  when 
the  end  is  shown  by  a  decided  change  in  color,  one  can  follow  the  progress  of 
the  reaction  closely  enough  to  slacken  the  stream  when  near  the  close  and 
finish  by  delivering  a  drop  at  a  time,  not  forgetting  to  stir  continuously 
throughout. 

The  most  suitable  vessel  to  contain  the  titrate  is  a  casserole  or  a  wide  beaker 
set  on  a  porcelain  plate,  as  the  white  surface  allows  a  slight  change  in  color  to 
be  more  plainly  seen. 


Ohem.  News,  1892-2-114. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  135 

A  floccalent,  dark  colored  precipitate  obscures  the  change  In  color  of  an  indicator,  but 
If  the  titrate  is  held  In  a  porcelain  dish  or  a  casserole,  a  margin  of  clear  liquid  In  which  the 
color  can  be  plainly  seen  is  exposed  by  allowing  the  precipitate  to  settle  for  a  moment  and 
slightly  inclining  the  dish. 

In  the  titratlon  of  a  chloride  by  silver  nitrate  or  the  reverse,  the  precipitate  of  silver 
chloride  quickly  balls  together  on  violent  agitation  of  the  titrate,  and  here  a  flask  with  a 
glass  stopper  is  better.  If  by  long  continued  shaking  the  interior  of  the  flask  becomes 
clouded  with  a  film  of  silver  chloride,  a  portion  of  the  clear  liquid  is  transferred  by  a 
pipette  to  another  flask  for  testing. 

In  titrating  hot  solutions  sufficient  heat  may  be  radiated  to  expand  the  con- 
tents of  the  burette,  and  a  shield  should  be  interposed,  such  as  a  perforated 
card  fixed  above  the  tap;  or  the  tap  may  enter  a  long  rubber  tube  terminated 
by  a  glass  tip,  allowing  the  beaker  containing  the  titrate  to  stand  some  distance 
aside  from  the  burette,  and  over  a  burner  if  it  is  desired  to  maintain  a  boiling 
heat. 

Should  the  basis  of  the  colored  titrate  be  a  fixed  alkali,  there  may  be  added 
an  excess  of  ammonium  sulfate  and  the  liquid  distilled ;  ammonia  is  liberated 
equivalent  in  alkalinity  to  the  fixed  alkali  and  passes  into  the  colorless  or 
slightly  colored  distillate  and  is  titrated  therein  by  standard  acid.  On  the  other 
hand,  titrates  containing  free  acid  are  distilled  with  ammonium  sulfate  and  a 
measured  quantity  of  standard  potassium  hydrate ;  the  ammonia  in  the  dis- 
tillate is  equivalent  to  that  liberated  from  the  volume  of  potassium  hydrate 
added,  less  the  amount  neutralized  by  the  acid  of  the  titrate. 

But  in  most  cases  it  is  the  better  plan  wherever  possible,  to  remove  coloring 
matter  by  oxidation,  evaporation  or  precipitation;  finely  divided  suspended 
matter  may  be  entangled  by  a  voluminous  precipitate  added  to  or  formed  in  the 
liquid,  and  filtered  off.  Or  the  basis  of  the  titrate  can  oftentimes  be  precipi- 
tated or  salted  out  and  filtered  off,  then  dissolved. 

In  a  few  titrations,  as  an  acid  solution  of  auric  chloride  titrated  by  oxalic  acid,  the 
titrate  must  be  protected  from  oxidation  by  the  air.  A  convenient  receptacle  is  a  three- 
necked  Woolf 's  bottle,  the  burette  tip  passing  through  a  cork  In  the  center  neck,  and  a 
current  of  carbon  dioxide  or  coal-gas  passed  in  and  out  of  the  other  two. 

When  the  end-point  is  not  to  be  perceived  by  a  visible  change  in  the  titrate, 
and  the  approximate  volume  of  titrand  required  is  unknown,  the  titration  is 
likely  to  be  very  tedious,  and  time  will  be  saved  by  duplication,  in  the  first 
^ssay  roughly  finding  the  volume  to  within  a  few  cubic  centimeters,  and  in  the 
second  to  run  in  at  once  as  much  as  the  first  shows  can  be  done  with  safety. 
Or  the  titrate  may  be  divided  into  three  unequal  parts,  the  largest  roughly 
titrated,  the  second  part  added  and  the  titration  continued,  then  the  smallest 
added  and  the  titration  concluded. 

Gutzow  *  describes  an  apparatus  wherein  the  titrate,  made  up  to  a  definite 
volume,  is  put  in  a  bottle  through  the  cork  of  which  passes  the  stem  of  a 
thistle-tube  reaching  to  the  bottom  of  the  bottle.  By  compressing  the  air  in 
the  bottle  an  aliquot  part  of  the  titrate  is  forced  up  into  the  thistle  and  is  there 
titrated,  then  allowed  to  recede  into  the  bottle  and  mix  with  the  remainder  of 
the  titrate.  The  volume  of  titrand  required  for  the  entire  titrate  is  found  ap- 
proximately by  a  simple  calculation,  and  the  titration  concluded  in  the  usual 
way;  or  if  a  closer  approximation  is  desired,  the  above  may  be  repeated.  All 
methods  on  this  principle  assume  that  it  is  immaterial  whether  the  titrate 
receives  the  titrand  or  conversely. 

When  the  end -point  is  observed  by  the  change  in  color  of  an  indicator  and 
the  titrate  is  turbid  originally  or  from  the  separation  of  a  slowly  subsiding 
precipitate,  the  color  can  be  made  more  evident  by  preparing  for  comparison  a 


*  Chem.  News,  1888-2— 190. 


13(3  QUANTITATIVE    CHEMICAL    ANALYSIS  . 

solution  identical  with  the  titrate  and  containing  the  same  proportion  of  the 
indicator  and  of  the  titrand  in  quantity  just  short  of  the  end -point. 

A  device  frequently  employed  where  the  change  in  color  at  the  end- point 
occurs  only  after  some  time  or  when  it  passes  through  a  transition  tint,  is  that 
of  preparing  a  number  of  test-tubes  containing  equal  volumes  of  the  titrate  and 
running  In  a  progressive  series  of  volumes  of  the  titrand.  After  standing  in 
the  cold  or  at  a  boiling  heat  for  the  proper  time,  the  color  or  clearness  of  one 
of  the  tubes  in  relation  to  the  one  adjoining  shows  it  to  contain  equivalent  vol- 
umes of  the  reacting  solutions. 

For  example,*  glycerol  gives  a  colorless  solution  In  dilate  sulfuric  acid,  potassium 
bichromate  a  yellow,  and  chromic  sulfate  a  green.  On  boiling  a  weak  solution  of  glycerol 
in  dilute  snlfuric  acid  with  an  equivalent  amount  or  less  of  bichromate  the  reaction  be- 
tween the  two  converts  the  latter  entirely  to  chromic  sulfate  and  the  solution  becomes 
pure  green.  Any  excess  of  bichromate  would  give  a  yellowish  tint  to  the  green.  A  deter- 
mination of  glycerol  in  weak  aqueous  solution  can  be  made  by  placing  5  cc.  in  each  of  four 
test-tubes,  acidifying  by  sulfuric  acid,  and  running  In  .5  cc.,  1.0  cc.,  1.5  cc.,  and  2.0  cc.  of 
standard  bichromate.  After  boiling,  two  adjacent  tubes  show  the  dividing  line,  one  being 
a  pure  green,  the  other  a  yellowish-green.  The  latter  is  made  a  type  for  a  more  exact 
determination;  a  second  series  is  made  up  as  before  but  with  volumes  of  the  titrand  here 
running  tenths  of  one  cc.  below  that  used  in  the  type,  and  on  boiling,  the  last  tube  show- 
ing pure  green  is  considered  as  containing  equivalent  proportions  of  glycerol  and 
bichromate. 


Confirming  the  graduation.  The  accuracy  of  all  volumetric  ware  should  be 
verified  by  the  user  —  the  flasks,  by  weighing  when  empty  and  dry,  then  filled 
with  water  to  the  mark;  the  pipettes,  by  weighing  the  amounts  of  water  de- 
livered ;  and  the  burettes,  by  weighing  every  five  or  ten  cubic  centimeters  drawn 
consecutively  into  a  light  flask;  the  variation  in  volume  of  water  at  different 
temperatures  being  allowed  for  by  the  table  given,  q.  v.  All  articles  for 
measuring  can  be  purchased  accompanied  by  certificates  issued  by  an  institu- 
tion of  high  repute  and  responsibility,  attesting  their  accuracy  or  itemizing  the 
extent  of  the  departure  therefrom. 

Few  burettes  have  an  exactly  equal  internal  area  throughout  the  graduated 
part,  and  to  learn  whether  the  maker  has  spaced  the  divisions  with  regard  to 
the  variations  —  i.  e.,  how  nearly  they  represent  true  metric  volumes — the 
weights  of  water  drawn  from  aliquot  parts  are  compared  with  the  weight  of  the 
same  volume  of  water  at  the  temperature  of  the  experiment.!  If  errors  of  con- 
sequence are  found,  corrections  are  applied  in  practice  to  every  reading  affected 
by  them. 

Unless  the  interior  of  the  burette  has  been  thoroughly  cleansed,  water  will 
not  flow  out  completely  but  leave  drops  adhering  here  and  there.  An  effective 
abstersive  is  a  strong  alcoholic  solution  of  potassium  hydrate,  afterward  rins- 
ing with  water,  dilute  hydrochloric  acid  and  water.  If  a  film  of  grease  resists 
this  treatment,  it  can  be  removed  by  allowing  the  burette  to  stand  for  a  time 
filled  with  a  solution  of  potassium  bichromate  in  dilute  sulfuric  acid. 

A  light  flask  of  a  capacity  of  50  to  100  cc.  is  cleaned  and  dried  by  warming  and  aspirat- 
ing the  air  from  the  interior  by  suction  through  a  glass  tube;  it  is  then  cooled  and 
weighed. 

The  burette  is  fixed  vertically  in  Its  stand,  in  front  of  a  window  if  convenient,  and  after 
rinsing,  filled  from  a  flask  of  distilled  water  that  has  been  standing  in  the  balance  room 
for  some  hours;  most  of  the  water  is  then  run  out  rapidly  to  carry  away  any  air- bubbles 
imprisoned  in  the  tap.  After  refilling,  the  water  is  drawn  off  until  the  bottom  of  the  men 
iscus  is  just  behind  the  zero  mark  read  with  the  eye  at  its  level;  the  outside  of  the  tap  is 
then  wiped  dry. 


*  Analyst,  1898-8. 

t  Journ.  Amer.  Chem.  Socy.,  1898—912. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  137 

Ten  cubic  centimeters  of  water  is  drawn  into  the  flask  and  weighed;  then  an  additional 
ten  cc.,  and  so  on  up  to  the  capacity  of  the  burette.  It  can  safely  be  assumed  that  the 
area  of  the  interior  doe§  not  vary  to  any  appreciable  extent  within  a  space  of  ten  cc.  The 
temperature  of  the  water  is  noted,  and  the  weights  compared  with  those  for  the  corre- 
sponding temperature  in  the  table. 

Example.  A  50  cc.  burette  was  tested  as  directed  above,  the  temperature  of  the  water 
registering  26®  Cent. 

Weight  of  Weight  of  True  weight       Actual  volume       Error 

Flask.       Grams.  water.  at26o.  in  ccs.  in  cos. 

18.589 

28.527  9.938  9.962  9.976  —.024 

38.450  9.923  "  9.961  —.039 

48.428  9.978  "  10.016  +  .016 

58.418  9.990  "  10.028  +  .028 

68.400  9.982  "  10.020  +  .020 


Total  49.811  49.810  50.001  +  .001 

From  the  above  we  see  that  the  first  ten  cubic  centimeters  drawn  out  has  actually  a 
volume  of  only  10  x  9.938  -i-  9.962  =  9.976  cc. ;  therefore  the  true  volume  of  a  reading  of  say 

7  cc.  is  7  X  .9976  =  6.983  cc. 

13  CC.  is  9.976  +  3  X  .9961  =  12.964  CC. 

29  CC.  Is  9.976  +  9.961  +  9  X  1.0016  =  28.951  CC.,  etc. 

In  this  way  a  table  may  be  drawn  up  showing  the  true  value  of  every  cubic  centimeter 
of  the  burette.  For  all  ordinary  work,  however,  no  correction  need  be  made  for  such  small 
inaccuracies  as  the  above. 

A  method  of  testing  the  relative  accuracy  of  the  graduations  is  due  to  Carnegie.*  The 
burette  is  carefully  cleaned,  and  the  tap  joined  by  a  long  rubber  tube  to  a  funnel  contain- 
ing water  and  elevated  above  the  burette.  Opening  the  stopcock,  water  is  allowed  to 
enter  and  rise  to  the  lowest  mark.  By  means  of  a  long-stemmed  funnel  about  five  cubic 
centimeters  of  some  light  organic  fluid  immiscible  with  water  is  run  in,  and  the  space  it 
occupies  Is  read.  More  water  is  allowed  to  enter  until  the  organic  fluid  occupies  the 
space  next  above ;  the  volume  occupied  Is  read,  and  this  process  continued  throughout  the 
entire  graduation. 

PIPETTES. 

The  volume  of  water  delivered  by  each  pipette  is  weighed  in  a  light  tared 
flask.  The  pipette  is  mounted  in  a  burette  stand,  and  the  end  of  a  long  rubber 
tube  drawn  over  the  top.  The  tube  is  closed  by  a  pinchcock,  this  plan  allow- 
ing the  adjustment  of  the  meniscus  to  the  graduation  line  to  be  made  more 
closely  than  when  the  pipette  is  stopped  by  the  finger.  Water  at  the  temper- 
ature of  the  room  is  drawn  in  and  run  out  until  the  bottom  of  the  meniscus  is 
aligned  with  the  mark,  and  the  drop  hanging  to  the  orifice  taken  off  by  a  glass 
rod.  The  water  is  then  run  into  a  tared  flask,  and  one  minute  after  the  flow 
has  ceased  the  hanging  drop  is  removed  by  touching  it  to  the  inside  of  the 
flask.  The  flask  is  weighed,  the  temperature  of  the  water  observed,  and  the 
volume  delivered  calculated  as  for  the  burette.  It  is  the  better  plan  to  cor- 
rect for  any  material  inaccuracy  in  the  analyses  than  to  attempt  to  establish  a 
new  mark  should  the  original  one  be  wrongly  located.  • 

MEASURING   FLASKS. 

These  are  weighed,  first  empty  and  dry,  then  filled  to  the  mark  with  water  at 
the  temperature  stated  on  the  flask.  If  a  balance  of  the  required  capacity  is 
not  at  hand,  they  may  be  calibrated  with  fair  accuracy  by  filling  from  a  tested 
pipette.  If  found  inaccurate,  a  second  mark  is  made  by  a  writing-diamond  or 
a  keen  triangular  file,  or  a  record  is  made  of  the  true  volume  contained  and  a 
correction  applied  in  practice. 

Should  the  water  be  at  a  higher  temperature  than  marked  on  the  flask,  a  cor- 
rection is  applied  to  the  weighings  by  the  formula 


*  Chem.  News,  1891—2—42. 


138 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


V=  W—W 


where    F  is  the  volume  of  the  flask  in  liters  at 


1  -f  .000025  d 

w' 

T°  ;  W,  the  weight  of  the  flask  and  water  at  t° ,  the  temperature  of  weighing; 
w,  the  weight  of  the  empty  flask;  w',  the  weight  of  one  liter  of  water  at  t°  by 
brass  weights  in  air;  d,  the  difference  between  T°  and  t°  ;  and  .000025,  the 
cubic  expansion  of  glass  for  one  degree  Cent. 

MEASURING  JARS. 

As  a  rule,  the  jars  provided  with  a  glass  stopper  are  calibrated  to  contain, 
and  those  with  a  pouring  lip  to  deliver  the  volumes  designated,  but  some  mak- 
ers graduate  all  jars  to  contain  the  specified  volumes.  If  used  for  any  accu- 
rate work,  therefore,  the  calibration  should  be  carefully  examined  by  means  of 
a  tested  pipette  of  say  one-fifth  or  one-tenth  of  the  capacity  of  the  jar. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  139 


CHAPTER  6. 

GASOMETRY. 

The  practice  of  gasometry  or  aerometry  may  be  divided  into  two  classes, 
scientific  and  technical  gas  analysis.  In  the  former  the  highest  accuracy  is 
aimed  at,  while  in  the  latter,  strict  accuracy  is  not  so  much  an  object  as  that 
the  results  of  an  analysis  be  obtained  quickly.  The  difference  between  the 
two  lies  mainly  in  the  apparatus  and  appurtenances  used,  those  for  technical 
analysis  being  necessarily  more  compact  and  portable. 

Under  a  pressure  not  exceeding  1500  millimeters  of  mercury  and  at  a  tem- 
perature above  zero  Cent.,  it  may  be  assumed  that  the  volume  of  a  gas 
varies  directly  with  the  temperature  and  inversely  with  the  pressure  upon  it, 
and  that  the  volume  of  a  mixture  of  gases  equals  the  sum  of  the  volumes  of 
the  constituents. 


A.  In  accurate  analyses,  two  graduated  glass  tubes  are  used  — the  measuring- 
tube  in  which  the  sample  of  gas  is  gauged  and  its  constituents  absorbed, 
and  the  eudiometer,  in  which  is  brought  about  a  combination  of  hydrogen  or 
hydrocarbons  with  oxygen.  The  former  is  simply  a  straight  tube  of  thick 
glass,  say  25  cm.  long  and  20  mm.  in  diameter,  one  end  sealed  and  the  other 
provided  with  a  lip  for  transferring  without  loss  an  inclosed  gas  to  another 
container.  The  eudiometer  differs  in  being  about  double  the  length  and  hav- 
ing two  platinum  wires  sealed  in  near  the  closed  end,  their  inner  extremities 
almost  touching,  while  the  outer  projections  are  either  bent  into  loops  or  cut 
off  close  to  the  glass.  On  connecting  the  projections  to  a  source  of  electricity 
of  high  tension  a  spark  leaps  across  the  space  between  their  inner  ends,  kind- 
ling the  mixture  of  gases  within,  which  is  either  combustible  of  itself  or  has 
been  made  so  by  the  addition  of  oxygen. 

Both  tubes  are  graduated  in  millimeters,  or  In  cubic  centimeters  and  tenths, 
from  the  closed  end  as  zero.  Before  putting  a  tube  into  use  it  must  be  ascer- 
tained whether  the  graduation  is  correct  throughout,  and  if  found  inaccurate  at 
any  point,  the  proper  correction  must  be  applied  to  the  reading  of  every  gas 
volume  comprehending  the  defective  part.  The  simplest  way  by  which  any 
variation  can  be  detected  is  to  support  the  tube  vertically  and  pour  in  small 
equal  volumes  of  a  liquid  and  note  if  the  height  of  the  column  as  read  on  the  scale 
is  each  time  increased  to  the  same  extent.  Mercury  is  the  most  suitable  fluid 
and  is  measured  into  the  tube  from  a  cup  made  of  heavy  glass  tubing.  The 
cup  has  a  flat  edge  and  holds  a  volume  of  mercury  approximating  one -tenth  or 
less  of  the  capacity  of  the  tube.  After  filling  the  cup  and  removing  any  air 
bubbles  adhering  to  the  glass,  the  excess  of  mercury  due  to  the  meniscus  is 
expelled  by  pressing  down  to  the  edge  a  flat  glass  plate.  The  cup  and  plate 
have  each  a  long  handle  of  twisted  wire,  that  the  heat  of  the  hand  may  not 
expand  the  mercury. 

The  mercury  in  the  cup  is  poured  into  the  measuring  tube,  with  care  to  avoid 
air  bubbles.  As  mercury  does  not  adhere  to  glass,  the  surface  is  convex,  and 


140 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


L 


i. 


the  highest  point  at  the  center  is  taken  for  the  reading.  Obvi- 
ously, the  empty  space  surrounding  the  meniscus,  of  the  form  of 
a  plano-concave  disk,  is  included  in  the  reading,  and  to  obtain  a 
true  reading  it  is  necessary  to  flatten  the  surface  which  can  be 
done  by  covering  it  with  a  layer  of  a  solution  of  mercuric  chlo- 
ride, the  height  being  thus  lowered  to  the  line  D  E,  Fig.  112. 
Since  the  tube  is  practically  of  equal  bore  throughout,  all  the 
subsequent  readings  may  be  taken  at  C  and  the  difference  between 
Fig.  112.  c  and  D  E  deducted. 

The  volume  of  mercury  held  by  the  cup  is  found  by  multiplying  its  weight 
by  .07355  (1  -|-  .0001814  «),  the  volume  of  one  gram  of  mercury  at  the  tempera- 
ture t°  of  the  experiment.  From  this  datum 
the  capacity  corresponding  to  each  space  of  the 
tube  is  calculated,  and  a  table  is  drawn  up 
showing  the  true  volume  of  any  reading  in  prac- 
tice. Where  the  tube  is  marked  off  in  milli- 
meters, the  capacity  per  millimeter  is  found  In 
the  same  way. 

Analysis.  A  drop  of  water  is  introduced  into 
the  measuring  tube,  and  through  a  funnel  whose 
stem  is  long  enough  to  reach  to  the  bottom,  it 
is  filled  with  mercury.  The  open  end  is  firmly 
closed  by  the  thumb,  and  the  tube  inverted  into  a 
mercury  trough  and  held  in  a  vertical  or  inclined 
position  by  the  clamp  of  a  heavy  retort  stand. 
The  trough  is  made  of  wood  or  iron  with  the 
front  and  back  of  plate  glass,  or  better  is  made 
entirely  of  heavy  glass,  Fig.  113. 

The  gas  to  be  analyzed  is  now  passed  up,  care  being  taken  to  avoid  loss  or 
introduction  of  air.  If  contained  in  another  tube  standing  over  mercury  the 
transfer  is  made  by  upward  displacement;  if  in  a  bulb  terminated  by  narrow 
sealed  tubes,  the  bulb  is  held  vertically  beneath  the  measuring  tube  and  the 
ends  of  the  prolongs  broken  off  by  nippers;  and  if  in  an  aspirator  bottle,  the  end 
of  the  rubber  tube  is  held  under  the  mercury  in  the  trough  and  gas  forced  out 
until  it  is  judged  that  all  the  air  has  been  expelled;  then  the  tube  is  moved 
under  the  measuring  tube  and  the  proper  volume  of  gas  is  allowed  to  pass  up. 

A  pipette  for  transferring  a  gas  from  a  gas  tube 
or  gasometer  to  another  tube  is  shown  in  Fig.  114. 
The  glass  bulbs  a  and  b  are  of  about  50  cc.  capac- 
ity and  connected  by  the  capillary  tube  c.  Over 
the  prolong  of  a  is  drawn  a  rubber  tube  e.  To  fill 
the  pipette,  b,  c,  and  d  are  filled  with  mercury  by 
suction  at  e,  the  orifice  of  d  is  passed  up  into  the 
gas  of  a  gas  tube  standing  in  a  mercury  trough, 
and  suction  applied  to  e  to  draw  the  mercury  from 
b  to  a.  When  b  has  filled  with  the  gas,  sufficient 
mercury  remains  in  the  bend  of  d  to  trap  it.  To 
discharge  the  gas,  d  is  immersed  in  a  mercury 
trough,  when  mercury  displaces  the  air  beyond  the 
bend ;  then  the  orifice  of  d  is  inserted  in  the  mouth 
of  a  gas  tube  filled  with  mercury  and  standing  in 
the  same  trough,  and  air  blown  into  e. 


Fig.  113. 


Fig.  114. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


141 


In  reading  the  volume  of  a  gas  it  must  be  observed  that 

1.  The  closed  end  of  the  tube,  below  during  the  calibration,  is  now  above, 
so  that  the  error  caused  by  reading  the  top  of  the  meniscus  is  doubled  and  the 
correction  must  be  doubled  accordingly,  Fig.  115. 

2.  The  volume  of  the  gas  varies  directly  with  the  temperature  and  inversely 
with  the  pressure  upon  it,  and  the  observed  volume  is  to  be  reduced  to  the 
standard    temperature   (zero    Centigrade)    and  pressure   (760  millimeters  of 
mercury). 

3.  The  drop  of  water  introduced  in  the  tube  before 
filling  with  mercury  has  saturated  the  gas  with  aqueous 
vapor  whose  tension  or  pressure  increases  that  of  the 


gas. 


i 

X 


4.  The  pressure  of  the  external  air  on  the  gas  is  dimin- 
ished by  the  difference  in  vertical  height,  if  any,  between 
the  surfaces  of  the  mercury  in  the  measuring  tube  and 
the  trough. 

5.  For  the  most  accurate  analyses  must  be  taken  into 
account  the  temperature  of  the  mercury  of  the  barome- 
ter (altering  its  density)  and  of  the  barometer  scale,  the  Fig.  115. 
height  above  sea   level,  and  the  capillary  attraction  between  mercury  and. 
^lass. 

The  room  is  left  unoccupied  for  an  hour  or  more  that  the  gas  and  mercury 
may  come  to  its  temperature.  On  returning,  the  operator  reads  the  heights  of 
the  mercury  in  the  tube  and  trough,  a  thermometer  hung  beside  the  tube,  and 
the  barometer.  These  observations  are  made  through  a  telescope  or  opera 
glass  from  a  distance  of  several  feet  that  the  gas  may  not  be  expanded  by  the 
heat  of  the  body.  The  normal  volume  of  gas  is  then  calculated  by  the  rules  on 
page  183. 

The  proximate  constituents  of  the  gas  are  now  to  be  absorbed  seriatim  by 
suitable  reagents.  Those  commonly  used  are 

Water.  Absorbs  bromine,  chlorine,  ammonia  and  the  gaseous  acids;  carbon 
dioxide  slowly. 

Absolute  alcohol.  Hydrocarbons  of  the  series  CnH2n  -j-  1  and  CnH2n  -{-  2. 

Sodium  phosphate.    The  halogen  acids. 

Moist  potassium  hydrate.  All  acid  gases,  carbon  dioxide,  hydrogen  sulfide  and 
water. 

Dilute  sulfuric  acid.  Ammonia. 

Concentrated  sulfuric  acid.  Water,  alcohol,  ether,  methyl  oxide,  propylene. 

Fuming  sulfuric  acid,  or  bromine  water.  As  in  (4)  and  also  methane  and  its 
liomologues;*  and  by  the  acid,  benzene. 

Phosphorus.  Oxygen. 

Palladium.  Hydrogen. 
Reagents  used  in  aqueous  solution  are 

Alkaline  pyrogallate  or  ferrous  tartrate.  Oxygen. 

Cuprous  chloride  in  acid  or  ammoniacal  solution.  Carbon  monoxide,  acety- 
lene, oxygen. 

Ferrous  sulfate.  Nitrogen  protoxide  (N20). 

Chromic  acid  or  potassium  permanganate.  Hydrogen  sulfide  and  sulfurous  acid. 

Palladious  chloride.  Oxygen. 

Potassium  hydrate  is  introduced  into  the  measuring  tube  in  the  form  of  a 
solid  ball  on  the  end  of  a  long  platinum  wire,  made  by  casting  the  melted  com- 
mercial hydroxide  in  a  bullet  mold  having  a  groove  filed  in  one  of  the  jaws  to 


*  Journ.  Amer.  Chem.  Socy.  1899—245. 


1-12  QUANTITATIVE    rilK.MH    \L     ANALYSIS. 

admit,  the  wire.  Tin:  ball  IH  moistened,  Immersed  In  the  mercury  in  DM;  trough, 
wiped  free  of  uir  bubbles  with  the  lingers,  and  passed  up  In  the  tube  (which  IM 
Inclined  i.o  about  45°)  Into  the  gas.  It  IH  allowed  to  n-nia.ni  then;  for  an  hour 
or  more;  after  withdrawing  and  reading  the  volume  of  the  remaining  gases 

ball   IM   passed   up  to  le.Ht  the  completeness  of  the;  absorption ;    if  com 
the  diminution  of  tho    original  volume  IH  the  volume  of  MM-  ;;us  or  gases 
abHorbed   by  the  alkali. 

Liquids  are  Introduced  by  means  of  a  pipette  with  upturned  orifice,  and 
withdrawn  by  the  sumo  instrument  or  by  punning  up  u  tuft  of  moistened  absorb- 
ent cotton.  Or  a  porous  ball  of  papier-mache  or  of  carbon  (mado  by  Igniting  a 
mixture  of  powdered  coke  and  bituminous  coal)  may  be  fixed  to  the  end  of  a 
long  wire  and  saturated  with  the  liquid. 

Small  amounts  of  certain  gases  can  be  more  accurately  determined  by  with- 
drawing them  from  tho  mixture  Into  solid  or  liquid  reagents,  then  dissolving 
the  reagent  In  some  solvent  that  will  retain  tho  gas,  and  determine  It  by  a 
gravimetric  or  volumetric  process.  Thus,  hydrogen  sulflde  can  be  absorbed  by 
potassium  hydrate,  afterward  dissolved  In  very  dilute  hydrochloric  acid  and  the 
gas  titrated  by  iodine. 

After  the  absorbable  gases  and  the  moisture  have  boon  eliminated,  there  may 
remain  a  mixture  of  hydrogen,  various  hydrocarbons  and  nitrogen.*  This 
residue,  or  a  portion  of  It,  is  transferred  to  the  eudiometer,  and  pure  oxygen,  In 
excess  of  what  Is  judged  will  bo  needed  to  form  water  and  carbon  dioxide 
when  combined  with  the  hydrogen  and  carbon  of  the  gases,  Is  Introduced. 
After  observing  the  total  volume,  the  lower  end  of  the  eudiometer  Is  clamped 
down  firmly  upon  a  sheet  of  rubber  cemented  to  the  bottom  of  the  trough,  or 
If  the  eudiometer  Is  provided  with  a  stopcock,  It  Is  closed.  The  exterior  pro- 
jecting ends  of  tho  platinum  wires  are  connected  with  an  Induction  coil  excited 
by  a  battery.  A  Mpark  loapn  from  the  Inner  extremity  of  one  wire  to  the  other,, 
the  complete  combination  of  the  gases  being  evidenced  by  a  sharp  flash  of  light 
throughout  tho  mixture. 

To  Increase  tho  certainty  of  a  complete  combustion,  a  measured  volume  of 
oxy-hydrogon  gas  may  be  passed  up  Into  the  tube  before  tho  explosion.  Pure 
oxygen  and  hydrogen,  mixed  in  the  exact  ratio  to  recomblne  to  water,  are  fur- 
nished by  the  decomposition  of  dilute  sulfurlc  acid  by  the  electric  current 
(H8804  -f-  HaO  =-  Ha  f  SOiHa  -f  0) , 

llnnHon'ftnpparnluB  for  tho  oloctrolyHla  IH  Hliown  in  Klg. 
110.  At  tho  HurfacoM  of  tho  two  platinum  platoH  arc  given 
olF  hydrogon  and  oxygon  rospocllvoly,  IhoHo  mixing  In  tln- 
roMorvoIr  and  displacing  tho  dlluto  add.  AH  tho  milubll- 
Itloti  of  tho  gUBOi  In  tho  dlluto  aold  vary  ununlformly  with 
tho  tmnporaturo,  It  is  Important  that  the  temperature  of 
tho  liquid  romiilns  tho  HUIUO  during  tho  generation,  and 
this  IH  provided  for  by  Burroundmg  tho  Jar  with  wtttor. 
The  ;-.:i'i  evolved  at  the  beginning  of  (in-  dcci  i<>l\  M|M  \-. 
allowed  to  oncapo,  tho  composition  being  doubtful.  Ho- 
lore  piiHKlng  to  the  <-mll<>mrin  ih.-  gat*  IH  dried  by  panning 
through  a  Lube  containing  concentrated  milfurlo  aold. 

To  guard  against  the  danger  of  the  eudiometer 
breaking  from  too  great  expansion  of  the  gases  at 
the  Instant  of  combination,  the  pressure  on  the  gas 
Is  reduced  to  below  atmospheric  by  raising  tho 

eudiometer  so   hii^h    lh:it    there    is  a    eonsid<T:ibl<- 
difference   between   the   levels    of   mercury    in    Mie 
Pig.  11C,  tube  ainl  the  trough.     This  also  lessens  tin-  danger 


Journ.  Amor.  Ohom.  Sony.  180!)— 308. 


<%M    AN  I  I  I  A  I  IN  I'!     (III.MK'AL     ANALYSIS.  M  ,'{ 

of  converting  a  part  of  any  nitrogen  in  tin-  mixture  to  an  o\i<l<-  «>i  mi  rogcn  from 
a  high  temperature  indneed  by  the  exploHion. 

After  tlie  explosion  there,  remains  water  (in  vapor  or  condensed),  carbon 
dioxide,  and  nitrogen.  Tho  mixture  in  transferred  to  a  measuring  tubo  and 
the  carbon  dioxide  determined  by  absorption  by  potassium  hydrate.  If  the 
original  gases  were  perfectly  dry,  the  water  formed  may  be  measured  by  rais- 
ing the  temperature  to  near  I  no*  and  reducing  the  pressure  to  one-half  JID 
atmosphere,  when  the  water  becomes  .steam. 

Krotn  the  data  of  the  volumes  of  the  combustible  gases,  the  oxygen  added, 
and  carbon  dioxide  and  nitrogen,  may  bo  calculated  the  proportions  of  the 
hydrogen,  hydrocarbons,  and  nitrogen  In  the  original  mixture.* 

A  number  of  modifications  of  the  original  apparatus,  more  or  less  compli- 
cated, have  been  designed  to  secure  greater  convenience  and  safety,  and  shorten 
and  simplify  the  operations.  Full  descriptions  of  these  will  be  found  In  works 
on  gas  analysis  and  elsewhere. f 

Simple  devices  for  evading  the  calculations  of  gas  volumes  from  observed 
conditions  to  the  normal  (dry  gas  at  zero  and  760  mm.  of  mercury)  have  been 
described  by  Qlbbs  and  others.  In  that  of  Qlbbs  moist  air  Is  Inclosed  In  a 
measuring  tube,  measured,  and  the  volume  calculated  to  normal  conditions* 
This  "  companion  tube  "  hangs  In  a  largo  trough  of  mercury,  and  whenever  a 
volume  of  gas  Is  to  be  measured  the  tube  containing  It  Is  brought  to  the 
side  of  the  companion  tube  and  raised  or  lowered  until  the  surfaces  of 
mercury  In  the  tubes  are  at  the  same  height.  Since  both  the  air  and  gas  are  at 
the  same  temperature  and  under  the  same  pressure,  then  — 

Observed  volume  of  gas  i  Its  true  volume  :  :  observed  volume  of  air  t  Its  true- 
volume. 

Kelaor'H  apparatus)  dispenses  entirely  with  graduated  tube*.    Tho  principle  Is,  that  HH 
tho-gttH  to  be  measured  enters  a  bulb  Ailed  with  mercury,  an  equal  volume  of  mercury  U 
forced  out  and  can  bo  collected  and  weighed  and  ltd 
volume  calculated.    Omitting  some  details  of  con- 
struction, It  may  be  described  as  follows:  — 

In  Fig.  117,  A  and  It  are  two  cylindrical  glass 
bulbs,  ouch  of  IfiO  cc.  capacity,  terminated  below  by 
tubci)  uniting  In  the  three-way  stopcock  0  which  Is  so 
< -oMHi.rueted  that  a  pannage  may  bo  opened  between 
A  and  15  or  tin;  ronU-ntM  of  H  drawn  out  through  the 
tap  I),  IIH  doBirod.  Tho  upper  end  of  A  is  closed  by 
the  three-way  cock  ft  which  may  be  turned  to  con- 
nect either  with  F  or  the  U-tube  G,  both  these 
having  a  capillary  bore.  The  bulb  Bis  closed  by  a  F'K> 

rubber  stopper  holding  a  tubo  J  attached  to  a  rubber  bulb  K  with  reversible  valves,  whlcl* 
on  compression  and  relaxation,  either  forces  air  Into  It  or  withdraws  It,  according  to  the 
position  of  the  valves. 

As  an  Illustration  of  the  manipulation  of  tho  apparatus,  let  It  bo  required  to  determine 
the  carbon  dioxide  In  a  chimney-gas.  Mercury  Is  poured  Into  II  and  by  working  tho 
putnp  K,  A  Is  completely  filled  with  mercury  to  E;  B  Is  then  emptied  through  0.  Tho 
tube  T,  containing  a  drop  of  water,  is  connected  with  tho  chimney,  and  0  Is  turned  to 
opm  A  mi.,  u;  tiutrcury  Hows  Into  B  until  the  surfaces  are  at  the  same  level,  drawing  in 
Kas  from  tho  chimney  (about  75  co.).  0  If  turned  to  the  left,  and  tho  mercury  In  It  run* 
into  a  tared  beaker  and  Is  weighed.  Dividing  tho  weight  by  the  specific  gravity  of  mer- 
cury at  the  obHorvod  temperature  gives  tho  volume  of  gas  indrawn;  this  volume  may  he 
reduced  to  the  normal  by  tho  usual  calculation  (page  188).  If  desired,  any  greater  volume 
of  gas  up  to  the  capacity  of  A  may  be  drawn  In  by  means  of  the  pump  K. 

The  absorption  pipette  Is  shown  In  Fig.  117.    It  consists  of  two  glass  globes  L  and  M 

*  SuUon,  Volum.-lrlr  Anal.  fid.'. 

I  Watt's  Diet,  of  Ohem.  1-242;  Ghom.  News,  1890-2— 11W;  Journ.  Amor.  Ohom.  Socy. 

r.MHi    :H:J. 

1  (;hom.  News,  1887— 2— :JO. 


144 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


connected  at  the  bottom.  At  the  top  of  L  is  a  stopcock  N  connecting  it  with  the  capillary 
tube  O.  L  and  M  are  half  filled  with  the  absorbing  reagent,  here  a  concentrated  solution 
of  potassium  hydrate.  To  transfer  the  gas  from  A  to  L,  the  reagent  is  forced  up  In  L  to 
the  stopcock  N,  and  the  tubes  F  and  O  are  joined  by  rubber  tubing;  by  compressing  the 
bulb  K  the  gas  Is  made  to  pass  into  L.  When  the  absorption  is  complete  the  residual  gas 
is  returned  to  A,  its  volume  ascertained  as  described  above.  To  be  assured  that  the  gas  in 
A  Is  neither  above  nor  below  atmospheric  pressure,  the  stopcock  E  is  turned  so  that  A 
communicates  with  the  manometer  tube  G;  this  is  half  filled  with  water,  and  the  levels 
in  the  arms  will  be  unchanged  if  the  pressure  of  the  gas  in  the  right  limb  equals  that 
of  the  atmosphere  in  the  left. 

When,  as  is  usual,  the.  temperature  and  pressure  of  the  atmosphere  do  not  materially 
change  during  an  analysis,  the  weights  of  mercury  corresponding  to  the  different  con- 
stituents of  the  gas  are  directly  proportional  to  their  volumes,  and  the  calculation  is 
very  simple. 

The  "  nitrometer  "  is  an  apparatus  designed  particularly  for  the  generation 
and  measurement  of  nitrogen  or  nitrogen  dioxide,  though  often  employed  with 

advantage  for  other  gases.  One  of  the  many 
forms  is  shown  in  Fig.  118.  The  burette  A  is 
closed  at  the  top  with  a  stopcock  B  opening  in 
the  funnel  C.  The  burette  is  graduated  down- 
ward, from  the  stopcock  taken  as  zero,  to  50  Cc. 
The  lower  end  is  contracted  and  joined  by  a 
long  rubber  tube  to  an  open  level-tube  D  of 
the  same  diameter  as  A. 

The  burette  is  filled  with  mercury  by  pouring 
in  at  D  until  A  and  D  are  more  than  half  full; 
then  elevating  D  until  the  mercury  in  A  rises  to 
the  stopcock,  and  closing  B,  when  the  appara- 
tus is  in  readiness  for  a  determination. 

Given  a  sample  of  an  impure  nitre  to  deter- 
mine the  KNOs  contained.  A  solution  of  a  ni- 
trate mixed  with  mercury  and  an  excess  of  sul- 
f uric  acid  reacts  to  form  nitrogen  dioxide  — 

Ol       I      2KN03  +  5H2S04  +  3Hg  =  3HgSO4  +  2KHSO4 
I  +  N202  +  4H20, 

one  cubic  centimeter  of  nitrogen  dioxide  meas- 
ured at  zero  and  760  mm.  being  evolved  from 
.004524  gram  of  KNO3. 

About  .2  gram  of  the  sample  is  weighed,  dis- 
solved in  a  little  water,  and  transferred  to  the 
funnel  C;  D  is  lowered  and  the  stopcock  opened  and  when  the  solution  is  drawn 
into  the  burette  it  is  followed  by  a  little  rinsing  water.  Double  the  volume  of  con- 
centrated sulfuric  acid  is  then  drawn  in,  cautiously,  that  no  air  may  enter. 
The  burette  is  shaken,  B  being  closed,  until  the  reaction  is  at  an  end,  and  after 
cooling,  the  tube  D  is  raised  to  such  a  height  that  the  pressure  on  the  nitrogen 
dioxide  equals  that  of  the  atmosphere.  The  volume  is  read,  reduced  to  stand- 
ard conditions,  and  the  percentage  of  KNOs  in  the  sample  calculated  from  the 
equation  above. 

Since  an  aqueous  solution  floats  on  the  mercury  in  A  and  the  reading  is  taken 
at  its  surface,  the  level  of  the  mercury  in  D  must  be  somewhat  above  that  in  A 

hg 
that  the  pressure  be  atmospheric.    The  difference  in  height  is  --„   h  being  the 

height  of  the  column  of  aqueous  solution,  g  its  specific  gravity,  and  g'  the  spe- 
cific gravity  of  mercury.  In  technical  analyses  of  a  given  material  where  the 
volumes  of  the  solutions  and  their  specific  gravities  are  practically  constant  in 


Fig.  118. 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


145 


all  analyses,  the  tube  D  may  be  so  graduated  as  to  show  the  proper  level  for 
any  volume  of  gas  to  be  measured. 

To  save  the  trouble  of  calculating  the  gas  volume  from  the  observed  tem- 
perature and  pressure  to  the  standard,  in  Lunge's  **  gas-volumeter  ",  *  Fig.  118, 
a  •  reduction  tube '  is  introduced.  The  rubber  tube  joining  A  to  D  has  a  T- 
tube  inserted  at  E,  the  third  branch  being  joined  to  the  reduction  tube  F;  this 
is  calibrated  to  hold  exactly  100  cc.  from  the  orifice  G  to  the  mark  H,  and  be- 
yond II  graduated  down  to  140  cc.  It  is  prepared  once  for  all  in  the  following 
way:  The  temperature  and  barometric  pressure  of  the  air  of  the  laboratory 
are  noted  and  the  volume  saturated  with  moisture  corresponding  to  100  cc.  of 
dry  air  at  zero  and  760  mm.  of  mercury  is  calculated.  A  drop  of  water  is 
put  into  G  and  mercury  poured  into  D  until  it  has  risen  to  the  division 
on  F  corresponding  to  the  calculated  volume.  The  orifice  G  is  then  sealed 
with  the  blowpipe  and  the  apparatus  is  in  readiness  for  use. 

Having  the  gas  to  be  measured  (saturated  with  water)  in  A,  the  heights  of 
D  and  F  are  so  adjusted  that  the  mercury  in  F  stands  at  the  mark  H,  and  in 
A  at  the  same  lev«l.  Then,  since  both  the  gas  and  the  air  in  the  reduction 
tube  are  saturated  with  moisture,  and  at  the  same  temperature  and  under  the 
same  atmospheric  pressure,  the  volume  of  gas  in  A  as  read  is  the  volume  of 
dry  gas  at  zero  and  760  mm. 

Moore  f  proposes  to  determine  the  weight  of  certain  metals  In  solution  by  the  weight  of 
oxygen  combined  in  the  course  of  a  reaction  that  converts  a  proto- to  a  per-salt,  e.  g.t 
cobaltous  to  cobaltic  sulfate.  — 

2CoS04.(NH4)23O4  +  O  +  H2O  =  Co2(SO4)3.(NH4)SSO4  +  2NH4OH. 

the  conversion  to  persulfate  takes  place  only  in  an  alkaline,  not  in  an  acid,  solution. 
The  weight  of  the  reacting  oxygen  is  determined  from  Its  volume,  this  by  the  diminution 
in  volume  of  a  measured  volume  of  air. 

The  apparatus  Fig.  119  A  consists  of  a  tube  A  graduated  in  cubic  centimeters  and  deci- 
mals upward  from  zero,  and  terminated  at  each  extremity  by  a  bulb  B  and  C,  each  pro- 
vided with  a  stopcock,  B'  and  G'.  The  acidified  solution  of  cobalt  sulfate 
is  run  into  B  and  made  up  to  the  zero  mark  with  water.  The  stopcock  C'  is 
closed  and  strong  ammonia  water  forced  into  the  apparatus  through  B'  (by 
connecting  the  prolong  of  C'  by  a  long  rubber  tube  to  an  elevated  aspirator- 
bottle  filled  with  ammonia  water).  B'  is  closed,  and  the  apparatus  shaken 
vigorously  to  mix  the  air  of  C  and  A  with  the  liquid.  Then  the  solution  in 
B  is  replaced  by  water  by  admitting  water  through  C',  at  the  same  time 
running  out  the  solution  by  B'.  Finally  the  temperature  of  the  remaining 
air  plus  nitrogen  is  brought  to  that  at  the  beginning  of  the  experiment, 
the  pressure  made  atmospheric  by  means  of  a  level-tube,  and  the 
height  of  the  liquid  read  on  the  scale.  The  diminution  in  volume  is  the 
oxygen  that  has  taken  part  in  the  reaction,  one  atom  corresponding  to  two 
atoms  of  cobalt.  The  absorption  of  oxygen  due  to  sulfates  of  metals  other 
than  cobalt  associated  with  it  may  be  neglected  provided  they  have  been 
previously  brought  to  the  per-state;  an  exception  is  manganous  sulfate, 
whose  interference,  however,  can  be  minimized  by  the  addition  of  citric 
acid. 

The  gas  balance  of  LuxJ  is  a  glass  globe  mounted  at  one  end 
of  a  balance  beam  whose  opposite  end  is  a  pointer  traversing  a 
scale.  The  gas  enters  at  the  fulcrum  passing  by  a  narrow  tube 
Fig.  119A.  into  the  globe  and  out  again  at  the  fulcrum.  By  means  of  a 
counterpoise  on  the  pointer-arm  the  pointer  is  made  to  register 
100  when  the  globe  is  filled  with  air;  the  scale  is  divided  into  100  equal  parts 
so  spaced  that  the  pointer  shall  mark  7  when  the  globe  is  filled  with  hydrogen 
(specific  gravity  .07,  air  at  1).  The  sensibility  of  the  beam  can  be  altered  by 
adjusting  screws  at  the  fulcrum. 

*  Chrm.  News,  1892—1—45. 
t  Chem.  News,  1892—1—76. 
{  Chem.  News,  1888—2—4  and  Journ.  Franklin  Inst.  1898—206. 

10 


3* 


146  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Lux  (loc.  cit.)  has  devised  a  method  for  the  analysis  of  a  mixture  of  gases  by 
a  series  of  the  gas-balances  connected  by  tubing  and  having  between  each 
adjoining  two  a  bulb  holding  a  reagent  to  absorb  a  specific  gas.  When  a 
mixture  is  run  through  the  series,  from  the  differences  in  the  readings  of  the 
balances  may  be  calculated  the  proportions  of  the  constituent  gases  by  the 

d  —  b 
formula  X=  100  a_^   where  d  is  the  specific  gravity  of  the  original  gas;  a, 

the  gravity  of  the  gas  absorbed,  and  X  its  percentage  in  the  original  mixture  j 
and  ft,  the  gravity  of  the  residual  mixture. 

Siegurt  and  Dnerr  manufacture  aii  apparatus  on  a  somewhat  similar  plan  for  continu- 
ously showing  the  relative  proportions  of  carbon  dioxide  and  carbon  monoxide  in  a  pro- 
ducer gas  or  a  chimney  gas.  Down  to  a  certain  economical  minimum,  the  smaller  the 
proportion  of  carbon  monoxide  In  a  chimney  gas,  the  more  perfect  the  combustion  of  the 
fuel;  on  the  other  hand  the  calorific  value  of  a  producer  gas  is  in  proportion  to  the  con- 
tent of  carbon  monoxide,  neglecting  the  hydrogen  and  hydrocarbons  contained.  The 
apparatus  Is  a  glass  globe  sealed  up  and  fixed  to  one  end  of  a  balance  beam  and  counter- 
poised by  a  weight  at  the  other  end;  a  pointer  traverses  a  scale  and  registers  the  rise  and 
fail  of  the  globe.  Through  an  air  tight  box  Inclosing  the  globe  and  beam  there  flows  con- 
tinuously a  slow  current  of  the  gas.  As  the  ratio  by  volume  of  the  carbon  dioxide 
(specific  gravity  1 .5290)  to  carbon  monoxide  (specific  gravity  .9674)  increases,  the  globe  is 
bnoyea  up  in  proportion  and  the  pointer  rises,  and  conversely. 


The  determination  of  a  gas  by  absorption  and  weighing,  though  otherwise  un- 
exceptionable, requires  a  much  larger  volume  than  when  measured,  by  rea- 
son of  the  low  specific  gravity  of  the  gas.  The  mixed  gases  are  slowly  forced 
through  a  train  of  U-tubes  and  bulb-tubes  filled  with  granular  solids  and 
liquids,  each  tube  or  bulb  absorbing  a  single  gas  or  the  members  of  a  group. 
The  general  arrangement  of  such  a  train  is  determined  by  the  nature  of  the 
gases  to  be  absorbed  and  their  proportion  in  the  mixture. 


Fig.  119. 

One  is  shown  in  Fig.  119;  the  measured  quantity  of  gas  enters  the  U-tube  A 
containing  dry  calcium  chloride  to  retain  aqueous  vapor,  then  passes  through  B 
filled  with  a  strong  solution  of  potassium  hydrate  to  absorb  carbon  dioxide  and 
other  acid  gases ;  then  to  C  containing  a  solution  of  pyrogallol  in  potash  lye 
for  the  absorption  of  oxygen ;  then  through  a  porcelain  tube  D  filled  with 
granular  copper  oxide  kept  at  a  red  heat  by  burners  below,  the  hydrogen, 
carbon  monoxide  and  hydrocarbons  being  burned  to  water  and  carbon  diox- 
ide. Beyond  D  is  a  calcium  chloride  tube  F  and  the  potash  bulb  E  whose  ob- 
jects are  the  same  as  A  and  B  (in  the  cut  F  should  precede  E).  The  tubes  and 
bulbs  are  weighed  before  and  after  the  passage  of  the  gases,  the  increase 
representing  the  weight  of  gas  retained.  Guard  tubes  of  calcium  chloride 
may  be  attached  to  the  absorption  bulbs  with  advantage,  for  the  purpose  of 
retaining  any  moisture  carried  off  from  the  absorbents  by  the  gas  current. 

A  process  that  has  many  applications  is  that  of  liberating  a  gaseous  radical, 
alone  or  in  combination,  by  heating,  treatment  with  an  acid  or  alkali,  or  other- 
wise. The  evolved  gas  may  be  measured,  but  more  usually  is  passed  into 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


147 


some  solution  with  which  it  reacts.  If  the  absorbent  is  a  measured  volume  of 
a  volumetric  solution,  the  excess  may  be  titrated  back;  otherwise  the  weight 
of  the  gas  is  found  by  a  gravimetric  determination. 


B.  Technical  analysis.  While  the  results  obtained  by  the  foregoing  methods 
are  unexceptionable  as  regards  exactness,  the  unhandi- 
ness of  the  apparatus  and  the  length  of  time  consumed 
in  an  analysis  have  led  Hempel,  Winkler,  Elliott,  and 
others  to  devise  portable  apparatus  for  technical  work  by 
which  results  accurate  enough  for  the  purpose  can  be 
quickly  obtained.  The  principal  differences  are  the  sub- 
stitution of  water  (rarely  saturated  brine  or  petroleum) 
for  mercury  as  a  trapping  liquid,  and  the  provision  of  a 
convenient  means  for  bringing  the  gases  in  contact  with 
the  absorbents.  Of  the  many  types,  two  will  be  de- 
scribed —  those  of  Bunte  and  Orsat. 

Bunte's  burette,  Fig.  120  holds  100  Cc.  from  the  stop- 
cock B  to  C,  graduated  into  cubic  centimeters  and 
tenths.  The  stopcock  Bis  a 'three -way*  and  accord- 
ing to  the  position  of  the  plug  opens  a  passage  from  A  to 
B  or  from  A  to  D  or  closes  all  communication.  An  ordi- 
nary stopcock  F  terminates  the  burette  at  the  bottom. 
An  aspirator  bottle  G  filled  with  water  is  connected  to  F 
by  a  long  rubber  tube  H. 

Given  a  mixture  of  say  carbon  monoxide,  carbon 
dioxide,  oxygen  and  hydrogen  to  be  analyzed.  The 
stopcocks  F  and  B  are  opened  and  G  raised  until  the 
burette  is  filled  with  water;  D  is  then  connected  with 
the  gas  reservoir  and  G  lowered  until  somewhat  more 
than  100  Cc.  of  the  gas  is  drawn  into  the  burette.  B  is  then  opened  to  E, 
and  G  raised  until  the  surface  of  water  in  the  burette  stands  at  zero;  F  and 
B  are  closed.  There  is  now  inclosed  in  the  burette  exactly  100  Cc.  of  gas  at 
the  temperature  and  pressure  of  the  air  of  the  laboratory,  and  saturated  with 
aqueous  vapor. 

The  funnel  E  is  now  filled  with  a  strong  solution  of  caustic  potash ;  after 
removing  the  rubber  tube  H  from  F,  a  beaker  is  placed  beneath  the  burette. 
B  is  then  opened  slightly  so  that  a  slow  stream  of  the  solution  may  flow  down 
the  interior  of  the  burette  and  absorb  the  carbon  dioxide.  When  no  more  will 
enter,  F  is  opened  and  the  remainder  of  the  potash  run  through,  followed  by 
enough  water  through  B  to  wash  it  entirely  into  the  beaker.  These  opera- 
tions can  be  easily  performed  without  any  loss  of  gas  or  ingress  of  air.  Bis 
closed,  the  tube  H  slipped  over  F,  and  G  raised  until  the  surfaces  of  water 
coincide.  The  volume  of  the  remaining  gases  is  deducted  from  100,  the 
difference  being  the  volume  of  carbon  dioxide  absorbed. 

The  oxygen  is  taken  up  in  the  same  manner  by  an  alkaline  solution  of 
pyrogallin,  and  the  carbon  monoxide  by  an  acid  solution  of  cuprous  chloride, 
the  nitrogen  remaining. 

To  provide  against  changes  in  the  temperature  of  the  laboratory  during  the 
analysis,  the  burette  is  inclosed  in  a  water  jacket.  This  is  a  large  cylindrical 


148 


QUANTITATIVE   CHEMICAL   ANALYSIS. 


Fig.   121. 


glass  tube,  the  ends  closed  by  rubber  stop- 
pers through  which  pass  the  terminal  tubes 
of  the  burette.  The  space  bet  ween  the  bu- 
rette and  jacket  is  nearly  filled  with  water 
whose  change  in  temperature  is  so  slow  that 
the  temperature  of  the  gas  in  the  burette  is 
maintained  practically  constant  during  an 
analysis  despite  a  moderate  rise  or  fall  in  the 
temperature  of  the  surrounding  air. 

The  original  apparatus  of  Orsat  has  been 
modified  by  Fischer,  Muencke,  Lunge,  Sal- 
leron,  and  others.*  All  the  parts  are  of 
glass  and  are  inclosed  in  a  portable  wooden 
case  A,  Fig.  121,  the  front  and  back  being 
removable.  The  gas  burette  B  is  graduated 
in  cubic  centimeters  from  the  zero  near  the 
bottom,  the  100  mark  being  located  just  at 
the  stopcock  J.  A  water  jacket  surrounds 
the  burette.  A  long  rubber  tube  E  joins  the 
bottom  of  the  burette  to  the  tnbulus  of  a 
water  bottle  F,  and  the  top  is  joined  to  the 


capillary  tube  C  having  three  branches  each  joined  to  a  large  U-tube  G,  H,  and 
I,  each  containing  a  solution  of  a  reagent.  I  is  half  filled  with  strong  caustic 
potash  solution  for  the  absorption  of  carbon  dioxide,  H  with  an  alkaline  solu- 
tion of  pyrogallol  to  retain  oxygen ;  and  G  (here  turned  through  90°  from  its 
normal  position)  with  a  hydrochloric  solution  of  cuprous  chloride  to  absorb 
carbon  monoxide.  The  outer  end  of  the  tube  C  is  connected  to  the  gas  reser- 
voir, interposing  a  tube  filled  with  cotton  to  retain  soot  and  tarry  matter  of  the 
gas. 

The  performance  of  the  analysis  is  simple  and  rapid.  The  burette  being 
completely  filled  with  water,  the  front  limb  of  each  U-tube  with  its  reagent  and 
their  stopcocks  closed,  100  cc.  of  the  gas  is  drawn  into  the  burette  by  lowering 

F.  J  is  then  closed  and  the  bottle  raised  higher  than  C.    On  opening  the  stop- 
cock of  I  water  rises  in  the  burette  and  forces  the  gas  into  the  front  limb  of 
I,  the  reagent  receding  into  the  rear  limb.    After  running  the  gas  back  and 
forth  a  few  times   to  promote  its  intimate  contact  with  the  potash,  it  is 
brought  entirely  into  the  burette,  the  stopcock  of  I  closed,  the  surfaces  of 
water  in  B  and  F  brought  to  a  level,  and  the  diminution  of  the  gas  volume 
read. 

The  oxygen  is  absorbed  in  a  similar  way  in  H,  and  the  carbon  monoxide  in 

G.  To  burn  the  hydrogen  and  hydrocarbons  remaining,  a  measured  excess  of 
air  is  drawn  in  through  J,  and  the  mixture  run  into  a  receiver  and  back  through 
a  heated  capillary  tube  containing  finely  divided  palladium;  this  metal  has 
the   power  to  induce  the  gases  to   burn  in   transitu    to  water  and  carbon 
dioxide,  marsh  gas  excepted. 

The  lower  half  of  the  burette  is  of  tubing  considerably  smaller  than  the 
upper  half.  The  reason  for  the  inequality  is  that  the  apparatus  is  designed 
mainly  for  producer  and  flue  gases  of  whose  constitution  by  volume  nitrogen 
forms  more  than  one-half,  and  as  each  of  the  other  gases  is  absorbed,  the 
volume  of  the  remaining  gases  is  read  on  the  graduation  of  a  tube  so  narrow 
as  to  admit  of  a  comparatively  accurate  observation.! 


*  Journ.  Amer.  Chem.  Socy.  1899—1108  and  1898—343. 
t  Idem,  1897—869. 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


149 


Fig.  122. 


A  short  account  of  some  practical  applications  of  gasometry  may  be  of  Interest. 

1.  Determination   of  carbon  In  steel   and  Iron.    Lunge's  improvement*  of  Wiborg's 
apparatus  Is  shown  In  -Fig.  122.    It  Is  made  entirely  of  glass,  for 

the  reason  that  the  chemicals  used  in  the  flask  might  act  on  cork  or 
rubber  fittings  and  carbon  dioxide  be  generated.  The  flask  A  has 
a  funnel-tube  t  sealed  in,  and  into  the  mouth  is  ground  the  lower 
end  of  a  condenser  d.  The  upper  end  of  the  condenser  connects 
by  a  capillary  tube  f  to  the  three-way  stopcock  h  of  the  Lunge 
burette  B.  The  burette  is  fitted  with  a  level-tube  D  and  a  reduc- 
tion tube  C  . 

For  an  analysis  a  weighed  amount  of  steel  drillings  is  placed 
in  A  and  covered  with  a  saturated  neutral  solution  of  cupric  sul- 
fate.  In  the  ensuing  reaction  (page  345),  the  carbon  of  the  steel 
separates  as  a  black  powder  in  combination  with  hydrogen  and 
oxygen.  The  apparatus  is  connected  as  shown  and  the  air  removed 
from  A  by  turning  h  to  connect  with  A  and  lowering  D.  The 
mercury  sinks  in  B,  rarifying  the  air  in  A.  Then  h  is  turned  to 
open  B  to  i,  and  D  is  raised  expelling  the  air  from  B.  These  manipulations  are  repeated 
until  a  fairly  high  vacuum  is  produced  in  A,  finally  leaving  B  filled  with  mercury. 

Through  the  funnel  t  is  introduced  into  A  a  solution  of  chromic  acid  in  sulf uric  acid. - 
When  heated,  chromic  acid  reacts  with  carbon  to  form  carbon  dioxide  (30  -f-  4CrOs  ==  3CO2+ 
2Cr2Os) .  D  is  lowered,  h  is  opened  to  f,  and  the  solution  boiled  for  an  hour.  The  carbon 
dioxide  with  some  oxygen  and  the  remaining  air  is  cooled  in  passing  through  the  con- 
denser, and  the  excess  of  water  vapor  condensed  and  returned  to  A.  Part  of  the  carbon 
dioxide  remains  in  the  liquid  and  space  in  A,  and  to  transfer  it  to  the  burette  a  few  cubic 
centimeters  of  solution  of  hydrogen  peroxide  in  water  is  run  in  through  t,  and  mixing 
with  the  excess  of  chromic  acid  reacts  with  the  generation  of  oxygen  (2CrOs  -f  3II2O2  =• 
CraOa  +  3H2O  +  3O2),  which  displaces  the  carbon  dioxide.  Hot  water  is  then  drawn  in  at  t 
(by  lowering  D)  until  it  reaches  h  which  is  then  closed. 

There  is  now  in  B  all  the  carbon  dioxide  mixed  with  oxygen  and  the  residual  air.  D  is 
raised  until  the  mercury  in  C  stands  at  100;  then  D  and  C  raised  or  lowered  until  the  levels 
of  mercury  in  B  and  C  are  the  same.  The  volume  of  the  mixed  gases  is  then  read . 

An  Orsat's  tube  E  F  (page  148)  containing  caustic  alkali  solution  is  attached  to  i  and  the 
mixed  gases  forced  into  it ;  the  carbon  dioxide  is  absorbed.  The  oxygen  and  nitrogen  are 
brought  back  into  B  and  their  united  volume  read  as  before.  The  difference  in  the  read- 
ings is  the  volume  of  carbon  dioxide  under  normal  conditions,  from  which  the  weight  of 
the  original  carbon  may  be  calculated. 

2.  An  apparatus  constructed  by  Schiebler  was  formerly  in  extensive  use  in  sugar  re- 
fineries for  the  periodical  testing  of  bone-black  for  calcium  caibonate,  its  proportion 
diminishing  as    the   bleaching  power   of  the    bone-black 

weakens  by  use.  The  apparatus  is  arranged  for  a  rapid  and 
fairly  accurate  estimation  on  the  principle  of  measuring  the 
volume  of  carbon  dioxide  evolved  on  treating  the  bone- 
black  with  an  acid.  A  modification  due  to  Bernard  is  shown 
in  Fig.  123. 

The  apparatus  is  in  three  parts,  a  flask  for  the  generation 
of  the  gas,  a  burette  for  measuring  it,  and  a  level-tube .  The 
funnel  A  is  supported  by  a  shelf  above,  and  the  bottom  con- 
nected by  a  long  rubber  tube  to  the  bottom  of  the  burette  B. 
The  burette  has  a  capacity  of  100  Cc.  and  is  graduated  down 
from  zero  near  the  top  in  cubic  centimeters  and  tenths.  The 
top  is  closed  by  a  rubber  stopper  holding  a  downward-bent 
glass  tube,  this  joined  by  a  rubber  tube  to  the  flask  O.  Through 
the  stopper  of  the  flask  passes  a  thermometer  T,  and  inside 
is  a  test  tube  D  of  such  a  length  that  It  stands  in  an  inclined 
position  as  shown. 

Water  is  poured  into  A  until  it  has  risen  to  a  short  dis- 
tance above  the  zero  mark  on  the  burette.  A  weighed 
quantity  of  the  bone-black  is  placed  in  C,  and  the  test-tube  D 
is  three-quarters  filled  with  dilute  hydrochloric  acid  and 
carefully  lowered  to  the  position  shown.  The  stopper  of  the 
flask  is  pushed  in  tightly,  this  compressing  the  air  within  to 
such  a  degree  as  to  depress  the  water  in  the  burette  a  few 
divisions  below  the  zero  mark.  A  is  then  lowered  until  the 
levels  of  water  coincide,  and  the  burette  is  read. 


L 


Fig.  123. 


*  Engineering  and  Mining  Journ.  1891—68. 


150 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


The  flask  is  tilted  so  far  that  the  acid  runs  out  of  the  test  tube  and  saturates  the  bone- 
black  ;  the  carbon  dioxide  evolved  depresses  the  water  level  in  the  burette  and  corre- 
spondingly raises  it  in  A.  But  at  the  same  time  A  is  lowered  at  a  rate  that  will  keep  the 
levels  about  the  same  —this  to  guard  against  leakage  through  imperfect  connections  by 
A  pressure  above  atmospheric  of  the  air  and  gas  within. 

When  no  more  gas  is  evolved,  A  is  raised  or  lowered  until  the  levels  are  the  same,  and 
the  burette  is  read.  This  reading,  minus  the  previous  one,  is  the  volume  of  carbon  di- 
oxide evolved  at  the  temperature  shown  by  the  thermometer,  and  from  it  is  calculated  the 
weight  of  the  calcium  carbonate.  The  usual  precautions  are  taken  against  a  rise  in  tem- 
perature of  the  air  and  gas.  A  source  of  error  is  the  retention  of  gas  dissolved  in  the  acid 
and  is  recognized  in  the  tables  furnished  with  the  Schiebler  apparatus  showing  the  per- 
centage of  calcium  carbonate  corresponding  to  any  volume  of  gas.  The  absorption  of 
carbon  dioxide  that  has  diffused  Into  the  burette  and  come  in  contact  with  the  water  is 
practically  inconsiderable;  in  Schiebler's  apparatus  the  air  and  gas  are  prevented  from 
mixing  by  a  slack  partition  of  thin  sheet  rubber.* 

8.  Reichenbergt  has  devised  a  modification  of  Coquilion's  apparatus  for  the  examina- 
tion of  air  containing  fire-damp.  As  the  apparatus  is  to  be  used  in  mines  it  is  made  as 
compact  and  portable  as  possible.  In  Fig.  124,  A  is  a  pipette  whose  outlet  tube  Is  grad- 
uated and  enters  a  rubber  bulb  C  filled  with  mercury.  By 
turning  a  thumbscrew  fixed  in  the  bottom  of  the  jar  of 
water  B,  the  bulb  is  compressed  and  mercury  forced  up 
to  fill  the  pipette.  The  top  of  the  pipette  is  connected 
with  a  capillary  T-tube,  the  right-hand  limb  having  a 
bulb  D  containing  a  platinum  spiral  that  may  be  heated 
to  redness  by  an  electric  current,  and  terminates  In  the 
gasometer  E  containing  caustic  soda  solution. 

The  pipette  Is  filled  with  mercury,  and  the  air  to  be 
tested  is  drawn  in  through  F.  The  air  is  forced  into  E  to 
absorb  any  carbon  dioxide  contained,  then  drawn  back 
into  A  and  measured.  The  spiral  in  D  is  heated  to  red- 
ness and  the  air  drawn  back  and  forth  over  it  to  burn  the 
methane  to  carbon  dioxide  and  water  by  combination 
with  the  oxygen  of  the  air,  the  former  being  absorbed  by 
the  soda.  Finally  the  residue  is  drawn  Into  A  and  meas- 
ured. The  diminution  in  volume  divided  by  three  (one  of 
O  methane  to  two  of  oxygen)  is  the  volume  of  methane  in 

E  the  air. 

4.  ClowesJ  has  proposed  to  determine  the  carbon  mo- 
noxide in  air  by  drawing  it  through  a  box  with  a  glass 
front  in  which  is  a  burner  fed  with  hydrogen.  The 
almost  invisible  flame  is  regulated  to  be  .4  inch  high 
When  the  air  passing  through  the  box  contains  .25  per 


Fig.  124. 


cent  of  carbon  monoxide  or  other  combustible  gas  the  flame-cap  (ghost)  is  Increased  to  .5 
inch.  A  scale  is  fixed  behind  the  flame,  and  a  pointer  runs  through  the  top  of  the  box 
which  may  be  raised  or  lowered  so  that  the  point  at  the  bottom  touches  the  flame.  The 
box  is  covered  with  a  focusing  cloth,  the  pointer  depressed  until  the  flame  Is  touched, 
the  cloth  is  removed  and  the  height  of  the  pointer  read  on  the  scale. 


In  solids,  gases  may  be  occluded  in  cavities  (blowholes,  the  pipe)  or  in 
pores,  and  in  viscous  liquids  as  bubbles  minute  and  slow  to  segregate.  The 
identification  and  determination  of  these  gases  is  often  a  matter  of  scientific 
and  practical  interest  as  denoting  the  origin  of  a  material  or  the  circumstances 
of  its  production  or  subsequent  treatment. 

Where  the  cell-walls  are  thin  and  easily  ruptured  by  internal  pressure,  the 
inclosed  gases  can  be  withdrawn  in  large  measure  by  strongly  heating  in 
vacuo,  thereby  producing  a  considerable  pressure  on  the  cell-walls  and  at  the 
same  time  lessening  their  tenacity.  A  practically  complete  evolution  follows 


*  Crookes'  Select  Methods,  586  and  594. 
t  Chem   News,  1896-1  -158. 
J  Journ.  Chem.  Socy.  1896—742. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  151 

when  the  substance  can  be  melted  at  a  moderate  heat  or  at  least  becomes  a 
semi  fluid.  An  apparatus  for  the  purpose  is  shown  in  Fig.  168;  the  substance 
is  contained  in  the  combustion  tube,  which  is  exhausted  by  the  mercury  pump, 
the  evolved  gases  passing  to  the  gas-measuring  tube. 

An  infusible  material  is  reduced  to  powder,  the  fineness  depending  on  the 
size  and  shape  of  the  gas-containing  cavities  and  their  proportion  in  the  solid, 
as  if  comparatively  large  and  abundant,  a  considerable  part  of  the  gas  would 
be  lost  on  trituration ;  on  the  other  hand,  if  small  and  scattered,  the  rupture 
and  penetration  of  the  cell-walls  by  the  inclosed  gases  is  less  difficult  the  more 
the  stability  and  resisting  strength  is  reduced  by  subdivision. 

Should  the  solid  be  soluble  in  a  simple  solvent  or  a  solution  of  some  reagent 
not  acting  on  the  gases,  they  may  be  liberated  by  dissolving  therein  in  vacuo 
and  pumped  into  a  gas-measuring  tube.  A  fair  proportion  of  the  gas  contained 
in  bodies  of  a  fibrous  structure  and  with  elongated  pores  (e.  g.y  charcoal)  is  ex- 
pelled when  the  solid  is  suspended  in  water  which  is  then  alternately  boiled  and 
cooled  while  maintaining  a  vacuum. 

A  molten  metal  may  absorb  or  occlude  many  times  its  volume  of  gases,  lib  - 
erating  them  in  part  before  or  at  the  moment  of  solidification.  The  phenomenon 
of  '  sprouting*  noticed  in  a  large  silver  button  as  it  chills  in  a  cupel,  and  the 
boiling  up  of  a  steel  ingot  in  a  mold,  are  examples. 

Metallurgists  agree  that  all  varieties  of  iron  and  steel  contain  more  or  less  of 
the  gases  carbon  monoxide,  nitrogen,  hydrogen,  and  oxygen,  and  perhaps  carbon 
dioxide  and  hydrocarbons,  but  differ  as  to  their  proportion  and  influence  on  the 
physical  qualities  of  the  metals.  Pig  iron  is  believed  to  contain  the  least,  wrought 
iron  more,  and  steel  —  exposed  as  it  is  to  the  intimate  contact  of  air  while  in  a 
molten  condition,  inducing  extensive  gas -gene  rating  molecular  reactions,  and  by 
reason  of  the  rapid  passage  from  a  liquid  to  a  solid  — the  greatest  amount.  Ac- 
cording to  Howe,*  nitrogen,  hydrogen,  and  carbon  monoxide  may  exist  in  iron  or 
steel  in  three  if  not  four  fairly  distinct  conditions :  A,  non-gaseous  or  condensed, 
(1)  in  chemical  combination,  exemplified  by  particles  of  cuprous  oxide  diffused 
through  melted  copper;  (2),  in  solution,  corresponding  to  the  solution  of  a 
solid  in  a  liquid;  (3),  in  adhesion,  where  physical  forces  predominate,  as  in  the 
absorption  of  ammonia  in  the  cells  of  charcoal :  and  B,  gaseous,  (4) ,  mechan- 
ically retained  in  the  pores  or  cavities,  as  bubbles  of  air  in  ice.  A  purely 
mechanical  treatment  will  set  free  all  the  gases  in  (4),  perhaps  also  in  (2)  and 
(3),  but  not  in  (1), 

It  will  readily  be  seen  that  a  determination  under  such  conditions  presents 
many  difficulties,  and  while  many  methods  have  been  proposed,  all  are  lacking 
in  practicability  or  certainty.  The  main  reasons  are,  that  weight  for  weight, 
the  proportion  of  gas  to  metal  is  excessively  minute,  and  that  the  analytical 
difficulties,  such  as  the  hazard  of  introducing  a  trace  of  air  or  the  products  of 
combustion  of  the  heating  arrangement  into  the  complicated  apparatus  for  the 
determination,  are  almost  insurmountable.  That  the  results  on  the  same 
material  do  not  agree  is  further  explained  by  (1),  the  metal  being  heterogene- 
ous as  regards  the  gases,  from  the  unequal  size  and  ununiform  distribution  of 
the  cavities  due  to  conditions  of  manufacture  or  coming  from  cold- working, 
local  expulsion  of  gas  by  heating  or  distortion  of  the  metal,  or  possibly  by 
osmosis  or  migration;  (2),  heterogeneous  as  regards  the  impurities  of  the 
metal,  some  (perhaps  all)  of  these  influencing  the  metal  to  retain  the  gas,  or 
by  inter -reactions  of  the  gases  with  the  metal  or  its  impurities  on  heating  or 
distortion  of  the  metal  during  or  after  manufacture. 

The  <rases  mechanically  retained  in  the  pores  of  the  metal  may  be  liberated 


Howe,  Metallurgy  of  Steel  105  et.  seq. 


152  QUANTITATIVE    CHEMICAL    ANALYSIS. 

by  simple  comminution.  Mueller's  apparatus  for  the  purpose  is  an  ordinary 
dull  flat  steel  drill  fixed  vertically  in  a  tank  of  water,  oil,  or  mercury,  with  the 
cutting  edge  upward.  Beneath  the  surface  of  the  liquid  and  resting  on  the 
point  of  the  drill  is  the  lower  end  of  a  bar  of  metal  which  is  slowly  rotated  by 
a  simple  mechanism.  As  the  drill  penetrates  the  bar  the  metal  is  ground  off, 
rather  than  cut,  by  the  dull  drill,  and  the  gases  are  set  free  and  collect  in  the 
conical  depression,  from  whence  they  may  be  removed  from  time  to  time  by  a 
gas  pipette  and  transferred  to  a  eudiometer  for  analysis. 

On  heating  in  vacuo  a  metal  in  the  form  of  powder  or  foil,  the  gases,  except 
those  chemically  combined,  are  set  free,  and  may  be  pumped  into  a  gas  meas- 
uring-tube. The  completeness  of  the  evolution  depends  on  the  attenuation 
of  the  metal,  the  degree  of  vacuity  and  the  temperature. 

Hydrogen  may  be  determined  by  heating  the  finely  divided  metal  alone  or 
intimately  mixed  with  powdered  cupric  oxide,  in  a  current  of  oxygen.  The 
water  formed  is  caught  in  a  tube  containing  phosphoric  anhydride,  and  weighed. 
Conversely,  oxygen  by  heating  in  a  current  of  hydrogen,  but  here  as  the  metal 
itself  does  not  enter  into  chemical  combination  with  the  transmitted  gas,  the 
evolution  is  less  complete  than  in  the  case  of  heating  in  oxygen  whereby  the 
metal  is  nearly  or  entirely  converted  into  oxide.  For  the  current  of  hydrogen 
may  be  substituted  nitrogen.  It  has  been  proposed  to  determine  the  oxygen 
in  commercial  copper  by  melting  with  tin  in  an  electric  furnace  in  a  current 
of  carbon  monoxide,  weighing  the  carbon  dioxide  produced. 

Nitrogen  can  be  determined  in  steel  and  iron  by  the  Will  and  Varrentrapp 
process  as  used  for  the  nitrogen  of  an  organic  body.  The  metallic  powder 
is  mixed  with  soda-lime  and  heated  in  a  combustion  tube  and  the  ammonia 
formed  by  combination  of  the  nitrogen  with  hydrogen  collected  in  a  dilute 
acid  and  determined.* 

A  more  accurate  scheme  is  to  dissolve  the  steel  in  hydrochloric  acid,  the 
nitrogen  combining  with  the  nascent  hydrogen  evolved.  The  solution,  now 
containing  ammonium  chloride,  is  gradually  added  to  a  boiling  solution  of 
sodium  hydrate,  the  alkali  being  in  excess  of  what  is  required  for  neutrali- 
zation of  the  free  acid  and  precipitation  of  the  iron  as  ferrous  hydrate.  The 
alkali  is  contained  in  a  closed  flask  connected  to  a  bulb-tube  containing  water 
or  a  dilute  acid  into  which  the  liberated  ammonia  distills. 

In  either  case  the  ammonia  is  determined  by  Nessler's  solution  (page—). 
The  result  includes  any  ammonia  present  as  such  in  the  steel  —  several  observers 
have  reported  the  escape  of  ammonia  gas  from  fresh  fractures  of  cold  steel. 

Carbon  monoxide  burns  to  carbon  dioxide  when  the  metal  is  ignited  in  a  cur- 
rent of  oxygen.  After  drying  the  gases  by  passing  over  anhydrous  phosphoric 
acid,  the  carbon  dioxide  is  caught  in  a  potash  bulb  and  weighed.  The  result 
includes  any  carbon  dioxide  occluded  in  the  steel. 

Carbon  dioxide  is  expelled  more  or  less  completely  when  the  finely  divided 
metal  is  heated  in  nitrogen  or  hydrogen ;  it  is  caught  and  weighed  in  the  usual 
way.  

ILLUMINATING  GAS. 

In  ordinary  coal-gas  are  contained  various  heavy  hydrocarbons,  mainly 
ethylene,  but  also  propylene,  acetylene,  butylene,  various  members  of  the 
paraffin  series,  etc. ;  hydrogen  and  carbon  monoxide ;  also  smaller  amounts  of 
carbon  dioxide,  oxygen,  nitrogen,  hydrogen  sulfide,  carbon  disulfide,  and 
allied  bodies. 


*  Blair,  Chem.  Anal,  of  Iron  and  Steel,  195. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  153 

A  scheme  of  analysis*  follows,  conducted  in  the  ordinary  forms  of  ap- 
paratus for  gasometry. 

1.  The  carbon  dioxide,  usually  very  small  in  amount,  is  determined  by  pass- 
ing the  gas  through  a  potash  bulb  and  noting  the  increase  in  weight. 

2.  The  oxygen,  by  absorption  in  a  freshly  made  up  solution  of  pyrogallol  in 
potassium  hydrate. 

3.  Ethylene  is  absorbed  by  bromine  water,  limiting  the  time  of  contact.    The 
fumes  of    bromine  remaining  in  the  residual  gas  are  removed  by  sodium 
hydrate  solution. 

4.  Benzene  is  absorbed  by  Nordhausen  sulf uric  acid,  afterward  removing  the 
fumes  by  sodium  hydrate. 

The  separation  of  ethylene  and  benzene  is  only  approximate,  as  bromine 
water  slowly  absorbs  benzene. 

6.  Carbon  monoxide  is  taken  up  by  a  freshly  made  up  solution  of  cuprous 
chloride  in  ammonia.  The  ammonia  fumes  are  removed  by  dilute  sulfuric 
acid.f 

6.  Ethane,  propane,  butane  and  most  of  the  methane  is  absorbed  by  paraffin 
oil,  which  should  contain  no  members  volatile  below  100  ° . 

7.  The  residue  is  mixed  in  a  eudiometer  with  oxygen  and  exploded.     The 
carbon  dioxide  formed  comes  chiefly  from  the  methane  unabsorbed  in  (6) ;  it 
is  determined  by  absorption  in  potassium  hydrate. 

8.  A  volume  of  the  original  gas  is  mixed  with  oxygen  and  exploded.    The 
carbon  dioxide  formed  is  absorbed  by  potassium  hydrate,  and  the  excess  of 
oxygen  by  alkaline  pyrogallol;  the  residue  isnitrogen.    Hydrogen  is  determined 
by  difference. 

9.  Ammonia  is  determined  by  passing  a  volume  of  ten  cubic  feet  or  more  very 
slowly  through  a  tube  filled  with  glass  beads  saturated  with  weak  standard 
sulfuric  acid.    The  excess  of  acid  unneutralized  by  ammonia  is  titrated  by 
standard  ammonia  and  haematoxylin. 

10.  Hydrogen  sulflde.  From  40  to  50  liters  of  the  gas  is  passed  through  an 
ammoniacal  solution  of  lead  acetate.    The  precipitated  leadsulflde,  mixed  with 
tarry  matter,  is  filtered  and  oxidized  by  fuming  nitric  acid,  converting  the  sul- 
flde  to  sulfate  and  destroying  the  tarry  matter.    The  lead  sulfate  is  determined 
by  the  usual  gravimetric  method. 

In  the  method  of  Wright,  the  gas  is  first  passed  through  dilute  sulfuric  acid 
to  absorb  ammonia,  and  calcium  chloride  to  dry  it,  then  through  a  tared  tube 
filled  with  cupric  phosphate  which  retains  the  hydrogen  sulflde. 

11.  For  carbon   disulfide   and    allied  bodies  have  been    proposed    several 
methods. 

The  carefully  dried  gas  is  passed  through  a  saturated  solution  of  potassium 
hydrate  in  alcohol.  The  carbon  disulfide  reacts  to  form  potassium  xan- 
thate  —  C2H5OH  -f  KOH  -f  CS2  =  KC2H6COS2  -f-  H2O.  The  solution  is  diluted 
with  water  and  neutralized  by  acetic  acid ;  the  xanthic  acid  is  titrated  by  stand- 
ard cupric  sulfate  which  precipitates  cuprous  xanthate,  the  end-point  shown 
by  potassium  ferricyanide.  Or  the  titration  may  be  done  by  standard  iodine  — 
HC2H6COS2  +  I  =  C2H5COS2  +  HI. 

In  another  method,  the  gas,  mixed  with  five  volumes  of  pure  air,  is  passed 
first  through  lead  acetate  solution  to  remove  hydrogen  sulflde,  then  over  heated 
platinized  asbestos  or  spongy  platinum  which  burns  the  carbon  disulfide  to 
sulfuric  acid;  then  into  a  solution  of  potassium  carbonate,  The  potassium 
sulfate  produced  is  precipitated  by  barium  chloride  as  usual. 

*  Chem.  News,  1891—1—15. 

f  Journ.  Amer.  Chem.  Socy.  1898—343  and  1900—16. 


154  QUANTITATIVE    CHEMICAL    ANALYSIS. 

12.  Total  sulfur.  The  gas  is  passed  through  a  scrubber  containing  dilute 
sulfuric  acid  to  retain  ammonia.  It  then  passes  to  a  small  burner  which  is 
fixed  beneath  a  cone-shaped  glass  shade.  To  the  burner  is  supplied  pure  air 
for  combustion,  and  a  constant  supply  of  ammonia  gas  is  passed  over  the  gas  jet. 
The  ammonium  sulfate  formed  by  the  combustion  of  the  sulfur  under  these 
conditions  deposits  on  the  sides  of  the  shade  and  may  be  dissolved  off,  pre- 
cipitated as  barium  sulfate,  and  weighed. 

Another  method  directs  to  burn  the  gas  in  pure  air  and  collect  the  sulfuric 
acid  formed  in  absorption  bottles  containing  a  dilute  solution  of  potassium 
carbonate.  The  gas  passes  through  a  small  gas  meter,  thence  to  a  micro - 
burner  surrounded  by  a  metal  cylinder.  Above  the  cylinder  is  a  glass  cone, 
the  two  joined  by  a  gutter  filled  with  mercury.  The  products  of  combustion 
pass  from  the  cone  to  three  absorption  bottles  whose  exit  tube  is  connected  to 
a  vacuum  air  pump.  Air  is  supplied  to  the  burner  through  a  pipe  leading  from 
a  jar  filled  with  pumice  saturated  with  potassium  hydrate  solution.* 
Below  is  an  analysis  of  South  Metropolitan  gas  made  by  the  above  method. 

Hydrogen 47.9 

Ethylene  series,  approximate 3.5 

Benzene  series,  "  9 

Methane   series   {by  explosion 33.3 

I  by  paraffin  oil 7.9 

Carbon  monoxide 6.0 

Carbon  dioxide • 

Oxygen 5 

Nitrogen    

100.0 
Total  illuminants  (ethylene,  benzene,  methane) 45.6 


Illuminants 


*  Journ.  Amer.  Chem.  Socy.  1898—702. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  155 


CHAPTER  7. 

ATTRIBUTIVE  METHODS. 

In  the  preceding  pages  has  been  described  the  direct  determination  of  the 
mass  of  a  body  by  means  of  the  properties  of  weight  and  volume,  or  the  meas- 
ure of  a  chemical  reaction.  We  will  now  consider  certain  methods  based  on 
other  properties,  common  or  special,  that  stand  in  a  definite  relation  to  mass. 
These  properties  may  be  either  intrinsic  or  extrinsic,  physical  or  chemical. 

In  discussing  these  methods  let  us  first  regard  a  simple  mixture  of  the  body 
to  be  determined  A  with  another  B.  Two  cases  are  presented. 

1.  Where  A  has  a  certain  measurable  property  not  exhibited  by  B  or  prac  - 
tically  inconsiderable,  and  its  value  for  A  is  (a),  beyond  a  normal  reduction, 
not  modified  by  admixture  with  B;  or  (b),  is  altered  more  or  less  by  their  jux- 
taposition or  an  inter-reaction. 

(a)  Here  only  the  determinations  of  a  constant  of  A  in  the  pure  state  and 
of  the  mixture  are  required.    The  percentage  of  A  is  found  from  the  simple 
proportion  — 

The  constant  of  the  mixture  :  the  constant  of  A  : :  the  percentage  of  A  in 
the  mixture  :  100  per  cent. 

For  example,  a  red  pigment  in  flne  powder  has  a  color  expressed  by  42  units.  The  pig- 
ment Is  made  up  of  a  compound  whose  color  value  Is  78  units  and  a  pure  white  powder; 
hence  the  percentage  of  the  compound  In  the  mixture  Is  53.8. 

(b)  In  most  cases  the  physical  constants  of  the  mixture  are  not  in  direct 
ratios  to  the  proportion  of  A  but  are  modified  by  the  influence  of  B  on  A. 
Sometimes  the  influence  of  B  is  so  small  as  to  be  practically  negligible,  bnt 
more  often  seriously  affects  the  resultant  value.    The  aberration  may  pro- 
ceed regularly  as  the  proportion  of  A  in  the  mixture  increases,  when  the  cal- 
culation under  (a)  applies  followed  by  a  predetermined  correction. 

But  if  proceeding  irregularly,  or  when  the  correction  cannot  be  expressed  by 
other  than  an  approximate  formula  the  only  recourse  is  to  determine  the  con- 
stants corresponding  to  a  progressive  series  of  mixtures,  and  compare  there- 
with the  constant  of  the  mixture  in  hand.  The  magnitude  of  the  intervals 
between  successive  members  of  such  a  table,  intermediate  numbers  being 
obtained  by  interpolation,  is  governed  by  the  aberration  from  the  normal 
resulting  from  the  presence  of  B.  Obviously,  the  labor  of  compiling  a  table 
of  this  kind  is  only  repaid  where  a  large  number  of  determinations  are  to 
be  made,  although  for  technical  work  it  need  only  comprise  such  proportions 
of  A  to  B  as  are  commonly  found  in  the  article  examined,  the  limits  being 
usually  far  less  than  a  range  from  none  to  100  per  cent  of  each  body. 

2.  Where 'both  A  and  B  possess  a  common  property  but  to  an  unequal  extent, 
the  two  mixing  (a)  without,  or  (b)  with  modification  of  the  composite  value  of 
the  property. 

(a)  The  equation  representing  the  relation  of  a  constant  of  two  bodies  to 
the  resultant  constant  of  their  mixture  is  aX-f  bT=  100<Z,  where  X-f-  Y=  100 
per  cent.  Hence  the  formulae 

X=100  ~-    and  F=100  =  100  — X. 


156  QUANTITATIVE   CHEMICAL   ANALYSIS. 

X  being  the  percentage  of  A  in  the  mixture,  and  Fthat  of  B;  a,  the  value  of  a 
certain  constant  of  A;  6,  that  of  B;  and  d,  that  of  the  mixture. 

Graphically,  If  the  ordinates  are  percentages  from  eero  to  100,  and  abscissae  the  con- 
stants from  a  to  &,  the  value  of  X  is  read  at  the  intersection  of  the  abscissa  of  the  constant 
of  the  mixture  with  a  right  line  from  a  to  6. 

The  above  formulae  hold  good  where  one  of  the  constants  is  negative  as 
related  to  the  other.  They  also  apply  not  only  where  Xand  Y  are  elements  or 
definite  compounds,  but  also  where  either  or  both  are  mixtures  of  two  or  more 
bodies  having  an  identical  —  or  at  least  an  approximately  equal  —  constant. 

In  applying  the  above  formulae,  it  must  be  observed  that  the  constants  are 
expressed  as  related  to  a  uniform  weight  of  the  bodies  they  represent,  and  not 
as  a  fixed  constant  to  different  weights  of  the  bodies.  For  example,  the  Koetts- 
torfer  Number  of  an  oil  is  the  number  of  milligrams  of  potassium  hydrate 
required  to  saponify  one  gram  of  the  oil,  and  here  the  above  formulae 
apply  :  but  the  saponification  equivalent  of  an  oil  is  the  number  of  grams  of 
oil  saponified  by  one  liter  of  normal  alkali,  and  here  for  the  values  of  a,  6,  and 
d  in  the  equations  must  be  substituted  their  reciprocals. 

The  above  equations  may  be  combined  to  the  forms  — 
100  (d  —  fr)  +  fr  X  __  WOd  —  bY 


X  100—  Y 

WOd—aX  =  IGOd  —  a  (100—  Y) 
=    100—  X    ''  ~Y~ 

d  =  «-3T  +  ft  OOP  —  -3Q    __  a  (100—  T)  -f  bY 

100  ~~  100 

for  the  purpose  of  determining  a  constant  of  either  body  when  the  constant  of 
the  other  and  the  proportions  of  the  constituents  are  known,  or  to  find  the  theo- 
retical value  of  d.  .The  former  is  of  use  when  one  body  may  be  any  member 
of  a  group  which  it  is  desired  to  identify,  the  latter  in  determining  experi- 
mentally whether  the  two  constituents  are  miscible  without  alteration  of  the 
resultant  constant. 

In  the  technical  analysis  of  a  mixture  one  may  be  able  to  easily  determine 
the  nature  and  proportion  of  one  body,  while  the  identity  of  one  or  two  others 
cannot  readily  be  made  out.  But  if  several  constants  of  the  known  body  and 
the  mixture  be  ascertained,  the  corresponding  constants  of  the  other  may  be 
calculated  from  the  above  equations,  and  by  comparison  with  tables  of  con- 
stants the  natu«re  of  the  unknown  body  be  inferred.  And  sometimes  with  two 
or  even  three  unknown  bodies,  the  figures  corresponding  to  the  mean  of  their 
constants  may  at  least  authorize  a  conjecture  as  to  their  identity. 

For  example,  a  lubricating  grease,  after  the  elimination  of  some  mineral  matter,  was 
separated  by  saponification  into  a  certain  percentage  of  paraffin  and  a  fatty  acid  from  a 
fat.  The  specific  gravity,  ether  value,  and  heat  of  sulfuric  decomposition  was  determined 
for  the  paraffin  and  the  mixture,  and  these  constants  calculated  for  the  saponifiable  fat  by 
the  above  formula.  By  comparison  with  a  table  for  the  animal  and  vegetable  fats  it  was 
found  that  those  for  tallow  best  agreed.  Other  considerations  tended  to  confirm  the 
deduction. 

A  physical  property  exhibited  by  solutions  of  some  bodies  may  diminish 
regularly  as  the  solutions  are  diluted,  while  of  others  the  diminution  proceeds 
abnormally,  and  in  a  mixture  of  a  body  of  each  class,  observations  taken  at 
various  dilutions  may  allow  a  deduction  of  the  proportions  of  each. 

If  the  two  bodies  A  and  B  do  not  make  up  the  whole  of  a  mixture,  but  there 
is  also  Z  per  cent,  deter  minable  directly  or  by  difference,  of  a  third  constituent 
C  having  a  constant-value  of  zero,  the  formulae  become 

^^  100  (d—  5)+6Z     ftnd  r_100(d  —  a) 
a—  b  b—  a 


QUANTITATIVE   CHEMICAL   ANALYSIS.  157 

In  some  mixtures  the  constant  of  one  or  both  constituents  may  be  so  great  that 
experimental  errors  become  excessive,  and  to  reduce  it  to  a  reasonable  figure, 
both  the  pure  constituent  and  the  mixture  are  diluted  with  a  known  proportion 
of  another  body  whose  corresponding  constant  is  practically  nil,  or  at  most  is 
a  definite  small  number.  On  the  other  hand,  where  a  mixture  has  such  a  com- 
position that  the  resultant  constant  is  not  exhibited  to  a  degree  that  is  easily 
measured,  a  weighed  quantity  of  one  of  the  constituents  in  a  pure  state 
is  added  to  the  mixture  and  corrected  for  in  the  calculation  of  the  results. 

But  if,  as  is  more  usual,  the  third  constituent  has  also  the  property  common 
to  the  other  two,  then  the  values  of  a  second  common  constant  must  be 
determined  for  each  constituent  and  the  mixture,  the  equations  standing  — 


in  which  a,  6,  c,  and  d  are  respectively  constants  of  the  three  constituents  X, 
J,  Z,  and  their  mixture,  and  a',  &',  c',  and  d'  are  the  corresponding  values  of 
the  other  constant. 

(b)  The  resultant  value  of  the  physical  constant  is  modified  by  the  admix- 
ture of  the  constituents.  If,  as  in  1  (b),  the  divergence  proceeds  symmetri- 
cally with  the  increase  of  A  in  the  mixture,  a  correction  may  be  applied  by 
means  of  an  algebraic  or  logarithmic  expression.  But  in  most  cases  a  correc- 
tion of  this  kind  is  applicable  only  for  a  limited  range  of  the  constitution  of 
the  mixture. 

For  occasional  determinations  of  mixtures  of  two  bodies  by  this  method,  an  approxi- 
mate correction  for  the  inter  -reaction  may  be  calculated  as  follows.  Having  determined 
d  and  calculated  X  and  Fby  the  usual  formula  (page  155),  a  synthetic  proof,  X'  +  I>,  is 
made  up  with  the  same  percentages.  On  the  proof  is  determined  d';  should  there  have 
been  no  inter-reaction,  then  d  would  equal  d'.  If  there  is  a  difference  beyond  experi- 
mental ^errors,  it  is  due  to  the  inter-  reaction  of  the  constituents.  X"  is  calculated,  using 
d'  in  the  formula,  and  the  proportion  X"  :  X1  :  :  X'  :  m  solved,  m  being  the  true  percent- 
age of  X  in  the  mixture.  This  is  on  the  presumption  that  the  variation  of  d  from  the 
normal  is  practically  the  same  throughout  a  small  range  In  the  composition  of  the  mix- 
ture, and  is  manifestly  less  true  as  the  aberration  is  greater.  It  is  often  necessary,  and 
generally  advisable  to  compound  a  second  proof  in  conformity  with  the  corrected  per- 
centages, then  repeat  the  process  as  above. 

Of  the  various  physical  characteristics,  those  of  density,  color,  rotation  of 
polarized  light,  boiling,  melting  and  congealing  points,  and  refraction  of  light 
are  in  frequent  use  ;  there  may  also  be  applied  viscosity,  thermal  and  electrical 
conductivity,  specific  heat,  vapor  pressure,  capillary  ascent,  voltaic  energy,  and 
penetrability. 

SPECIFIC  GRAVITY. 

We  may  define  the  mass  of  a  body  as  the  total  amount  of  matter  irrespective 
of  external  conditions  ;  the  density  as  the  volume  of  the  body  under  specific 
conditions  of  temperature,  and  pressure  ;  the  weight  as  an  expression  desig- 
nating the  force  of  gravity  upon  the  body  at  any  given  locality;  and  the 
specific  gravity  as  the  ratio  existing  between  the  weight  and  that  of  an  equal 
volume  of  a  given  standard  under  like  conditions  of  temperature  and  pressure. 

In  determining  the  specific  gravity  of  a  solid  or  liquid,  pure  water  is  made 
the  standard  of  comparison.  Since  the  density  of  water  and  other  liquids  and 
solutions  varies  considerably  with  the  temperature,  it  is  important  that  the 


158 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


temperature  of  the  water  be  stated,  as  also  that  of  a  liquid  compared  with  it. 
The  usual  temperatures  chosen  for  the  purpose  are  zero  or  15  o  Cent.,  some- 
times 17.5°,  20°,  or  100°. 

In  taking  the  specific  gravity  of  a  liquid  it  is  all-important 
that  the  volumes  be  measured  with  great  exactness.  The  or- 
dinary specific  gravity  bottle  is  a  light  glass  flask,  Fig.  125; 
the  glass  stopper  is  hollow  and  ground  to  fit  the  flask,  the 
upper  part  of  the  bore  being  a  capillary  tube.  They  may  be 
purchased  of  a  capacity  of  exactly  25,  50,  or  100  Cc.  at  a  fixed 
temperature  —  usually  15  o  Cent.,  or  holding  approximately 
one  of  these  volumes,  the  exact  capacity  to  be  determined  by 
Fig.  125.  the  operator. 
The  size  of  the  flask  best  suited  for  a  given  determination  depends  on  the 
degree  of  accuracy  required  in  the  result,  equal  errors  in  weighing  and  else- 
where having  less  effect  the  greater  the  volume  used  in  the  test.  For  liquids 
that  differ  but  slightly  in  gravity  from  pure  water,  one -half  or  one  liter  is  a 
suitable  volume. 

The  liquid  to  be  tested  is  brought  to  the  temperature  specified  and  the  bottle 
filled  to  the  brim;  the  stopper  is  inserted  and  fills,  the  excess  overflowing 
through  the  tube.  After  wiping  the  exterior,  the  whole  is  weighed  and  the 
gravity  calculated  from  the  difference  in  weight  from  pure  water  at  that  tem- 
perature. 

A  serious  objection  to  this  apparatus  is  the  difficulty  of  retaining  exactly  the 
same  temperature  during  the  weighing,  a  slight  increase  causing  an  overflow 
through  expansion.  Improved  forms  have  a  long  narrow  tube  extending  from 
the  stopper  marked  at  a  point  near  the  bottom,  so  that  the  liquid  may  expand 
into  it ;  in  some  forms  the  tube  is  so  graduated  from  zero  near  the  bottom  as  to 
show  the  height  to  which  it  is  to  be  filled  with  a  liquid  at  any  given  higher 
temperature  to  equal  the  capacity  of  the  flask  at  the  normal  temperature.* 

Since  glass  vessels  expand  on  heating,  the  capacity  of  the  flask  or  tube  will 
increase  with  the  temperature  of  the  liquid  inclosed.  Landolt's  formula, 

p 
neglecting  a  correction  for  vacuum  weights,  is  V=  -r,  V  being  the  capacity  of 

the  flask;  dy  the  density  of  water  at  the  temperature  t;  and  P  the  weight.    For 

p 

any  higher  temperature  t',  the  volume  V  is  V  =  -T  [1  +  .000025  (tf  —  £)]>  tak- 
ing .000025  to  be  the  cubic  expansion  coefficient  of  chemical  glass  for  1  o  Cent. 
The  employment  of  temperatures  higher  than  ordinary  is  a  necessity  with 
bodies  melting  at  a  moderate  heat. 

A  very  convenient  and  simple  apparatus  is  the  Sprengel  tube,  Fig.  126.  A 
light  glass  U-tube  is  terminated  by  two  capillary  branches 
C  and  D  respectively  .25  and  .50  Mm.  in  bore,  with  a  mark 
E  around  the  middle  of  D;  during  the  weighing  the  orifices 
are  closed  by  small  glass  caps  not  shown.  C  is  inserted  in 
the  liquid,  which  is  then  drawn  in  by  suction  at  D  until  the 
tube  is  filled  to  beyond  E.  By  touching  a  strip  of  filter  paper 
to  C  the  column  of  liquid  in  D  is  made  to  recede  until  the 
meniscus  reaches  E,  the  tube  C  remaining  filled  by  reason  of 
its  smaller  bore.  If  inadvertently  the  liquid  is  drawn  be- 
yond E,  a  drop  of  the  liquid  hanging  to  a  glass  rod  will  be 
taken  in  if  touched  to  D. 


\J 

Fig.  126. 


For  determinations  at  temperatures  above  the  normal,  the  tube  is 
filled  and  hung  in  a  beaker  of  water  or  oil  BO  that  the  body  of  the 


Ephemcrls,  1897. 


QUANTITATIVE    CHEMICAL   ANALYSIS, 


159 


tube  is  completely  immersed.  The  bath  is  heated  to  the  required  temperature,  and  as  the 
liquid  expands  the  excess  escapes  through  D.  The  meniscus  is  then  brought  to  E,  the 
tube  removed  from  the  bath  and  cooled,  and  weighed  after  closing  by  the  caps. 

That  a  solid  immersed  in  a  liquid  loses  in  weight  to  the  extent  of  the  weight 
of  an  equal  volume  of  liquid  is  applied  in  the  Westphal  and  similar  balances. 
The  one  shown  in  Fig.  127  is  simply  a  light  horizontal  balance  beam  A  sup- 
ported near  one  end  on  a  knife-edge.  On  the  left-hand  end  is  a  pointer  show- 
ing that  the  beam  is  horizontal  by  its  alignment  with  a  stud  B  fixed  to  the  sup- 
porting frame;  and  to  the  opposite  end  is  hung  by  a  fine  platinum  wire  a  plum- 
met C  whose  volume  is  exactly  five  or  tenCc.  — At  is  usually  a  thermometer. 
The  left  arm  of  the  beam  is  made  so  heavy  that  the  beam  will  float  when 
the  plummet  is  attached,  and  also  when  the  plum- 
met is  immersed  in  water  at  a  temperature  of  1 5  o 
and  a  weight  of  five  or  ten  grams  (in  the  form  of 
a  hook  D)  is  hung  on  the  arm  at  the  division 
marked  1.  The  space  between  the  knife-edge 
and  1  is  divided  into  ten  equal  parts,  and  if  the 
plummet  be  immersed  in  say  an  oil  of  .900 
specific  gravity,  the  beam  will  be  poised  when 
the  weight  is  at  the  division  marked  .9,  etc. 
Weights  of  .500,  .050  and  .005  gram  are  also  pro- 
vided for  determining  the  gravity  to  the  third  Fig.  127. 
decimal.  For  liquids  of  greater  specific  gravity  than  water,  a  heavier  plummet 
is  provided. 

Taylor*  proposes  as  a  ready  means  of  determining  the  gravity  of  a  liquid  to 
weigh  therein  a  plummet  whose  specific  gravity  equals  its  weight  in  air  in 
grams.  The  difference  between  the  weight  of  the  plummet  in  air  and  in  the 
liquid  is  the  specific  gravity  of  the  liquid.  If  more  convenient  the  weight  of 
the  plummet  may  be  a  decimal  multiple  or  divisor  of  its  specific  gravity,  with  a 
corresponding  correction  of  the  result. 

The  hydrometer,  Fig.  128,  is  a  light  glass  tube  weighted  at  the  bottom  with 
mercury  or  shot  so  that  the  hydrometer  will  float  in  a  liquid  in  a  vertical 
position  with  the  surface  of  the  liquid  meeting  some  point  on  the 
upper  narrow  stem;  in  some  instruments  the  mercury  forms  also 
the  bulb  of  a  thermometer  inclosed  within  the  tube.  Within  the 
stem  is  a  scale,  the  division  which  coincides  with  the  surface  of 
the  liquid  being  taken  as  the  reading  point.  In  the  instrument  for 
heavy  liquids  the  scale  usually  ranges  from  1.000  near  the  top  to 
1.800  near  the  bottom,  and  in  that  for  light  liquids  the  1.000  mark 
is  near  the  bottom  decreasing  upwards,  usually  to  .700.  A  hydro- 
meter graduated  to  compass  so  great  a  variance  cannot  be  divided 
closer  than  the  hundredths  place  of  decimals  and  be  of  convenient 
length,  therefore  this  range  is  better  distributed  among  four  hy- 
drometers graduated  in  thousandths,  say  1.000  to  1.200,  1.200  to 
1.400,  etc. 

Warringtonf  claims  that  an  accuracy  as  great  as  one  part  in  one  million 
may  be  obtained  by  a  form  of  hydrometer  that  can  be  wholly  immersed  in 
the  liquid.  Small  ring-shaped  platinum  weights  are  slipped  over  theun- 
gradnated  neck  of  a  glass  hydrometer  until  the  latter  has  nearly  attained 
the  specific  gravity  of  the  liquid  to  be  tested.  The  temperature  of  the  liquid 
is  then  slowly  altered  until  the  hydrometer  and  liquid  have  exactly  the 
Fig.  128.  Vs  same  density. 


*  Chem.  News,  1888-1-138. 
t  Chem.  News,  1898-2-H4. 


1(50  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Instead  of  the  graduation  in  units  and  decimals  of  specific  gravity,  certain 
hydrometers  are  marked  with  empirical  scales.  The  most  common  of  these 
scales  are  the  Beaume",  for  liquids  lighter  or  heavier  than  water,  and  the 
Twaddle,  for  heavy  liquids  only.  They  are  exclusively  used  in  some  lines  of 
manufacturing,  for  acids,  alkali  solutions,  and  other  liquids,  though  having  no 
real  advantage  over  the  rational  scale.  The  hydrometer  of  Brix  is  so  grad- 
uated that  the  degrees  express  directly  the  percentage  of  a  given  solid  in  the 
solution  tested,  and  consequently  any  one  instrument  can  only  be  applied  to 
the  specific  solution  for  which  it  has  been  adjusted.  Tables  for  the  conversion 
of  the  various  empirical  scales  to  specific  gravity  will  be  found  in  compilations 
of  chemical  tables.* 

Bohnf  proposes  to  determine  the  relative  specific  gravities  of  liquids  by  measuring  the 
heights  of  columns  that  exert  an  equal  pressure. 

Tbe  densities  of  most  mixtures  of  liquids  do  not  follow  the  ratio  of  the  pro- 
portions of  the  constituents,  the  volume  of  the  mixture  being  somewhat  less 
than  the  sum  of  the  constituents.  The  same  is  true  of  solutions  of  solids  in 
liquids,  except  in  quite  dilute  solutions,  where  up  to  a  certain  specific  concen- 
tration ranging  from  one  to  five  per  cent  of  the  solid,  the  excess  of  specific 
gravity  over  pure  water  is  directly  proportional  to  the  weight  of  salt  in  solu- 
tion—  the  equation  Dt  =  dt-\-  kP  holding,  where  D  is  the  density  of  the 
solution ;  d,  the  density  of  water  at  t  °  ;  P,  the  percentage  of  anhydrous  salt, 
and  k,  a  factor  constant  for  the  given  salt. 

The  specific  gravity  of  solids  is  less  frequently  called  for  in  analysis.  If  the 
substance  is  sensibly  porous,  a  distinction  is  to  be  made  between  the  real  and 
the  apparent  density  —  the  former  being  that  of  the  finely  powdered  substance, 
and  the  latter  that  of  a  fragment  with  whatever  air  is  inclosed  within  its  sensible 
pores. 

If  a  lump  of  moderate  size  is  obtainable,  the  most  accurate  method  is  to  sus- 
pend it  by  a  hair  or  fine  wire  from  the  hook  of  the  stirrup  of  an  analytical 
balance  and  weigh,  and  by  the  aid  of  the  wooden  bridge  over  the  pan,  again 
weigh  when  submerged  in  a  beaker  of  water;  the  difference  is  the  weight  of 
an  equal  volume  of  water  at  the  temperature  of  the  experiment.  Should  the 
-substance  be  soluble  in  water  another  liquid  of  known  gravity  is  substituted. 

For  powders,  Hogarth's  flask,  Fig.  129,  a  modification  of  the  Sprengel 
tuhe,  will  be  found  convenient.  Its  capacity  is  ascertained,  and  the  dry 
flask  weighed  before  and  after  introduction  of  the  powder;  then  partly 
filled  with  water.  After  boiling  to  remove  any  air  adhering  to  the 
powder,  the  flask  is  filled  and  adjusted  to  the  mark  A,  and  again  weighed. 
The  specific  gravity  bottle  for  liquids  may  be  employed,  though  not  so 
conveniently. 

The  volume  of  a  heavy  solid  in  the  form  of  fragments  or 
nodules  maybe  found  by  filling  the  body  of  a  small  long-necked 
flask  and  weighing,  then  pouring  in  mercury  to  the  mark  and 

w 
again  weighing.    The  volume  of  the  solid  is    V—  —  and    its 

W.a 
specific  gravity   v    _w    where  V is  the  capacity  of  the  flask 

to  the  mark ;  gr,  the  specific  gravity  of  mercury  at  the  tempera- 
Fig.  129.   Vs    ture  Of  the  experiment;  w,  the  weight  of  mercury  poured  into 
the  flask;  and  W,  the  weight  of  the  solid. J 


*  Journ.  Amer.  Chem.  Socy.  1899—199. 

t  Chem.  News,  1888—1—83. 

\  Journ.  Franklin  Inst.  1899— 200. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


161 


Another  device  is  shown  in  Fig.  130.  A  ground  stopper  elongated  to  an  open 
capillary  tube,  fits  the  glass  bulb  A  whose  bottom  is  drawn  out  to  ananow  tube 
and  connected  by  a  rubber  tube  C  to  a  narrow  10  Cc.  bu- 
rStte  D.  The  bulb  is  filled  with  water  and  raised  or 
lowered  until  the  surface  reaches  a  mark  B  on  the  tube  of  the 
stopper,  when  the  level  in  the  burette  is  read.  The  bulb  is 
now  raised  until  the  water  recedes  somewhat,  the  stopper  is 
withdrawn,  and  the  weighed  fragment  or  fragments  of  the 
(insoluble)  solid  introduced.  The  stopper  is  replaced  and 
the  bulb  lowered  until  the  level  of  the  water  rises  to  B,  and 
the  burette  again  read.  The  difference  between  the  two 
readings  is  the  volume  of  the  solid,  and  having  its  weight, 
the  gravity  is  easily  calculated. 

The  volumenometer  of  Regnault-Dnpre  is  constructed  on  the  prin- 
ciple that  when  into  a  given  volume  of  air  v  at  normal  pressure  is 
forced  another  volume  of  air  V  also  at  normal  pressure,  the  tension 
of  V  is  increased  in  proportion  to  the  ratio  between  v  and  V.  Also  if 
the  volume  V  be  reduced,  as  by  the  introduction  of  a  solid  into  the 
vessel  containing  it  to  that  extent  will  the  tension  be  further  Increased. 

A  simple  means  of  determining  the  gravity  of  a  light  solid 
In  fragments  is  to  prepare  a  liquid  in  which  it  is  insoluble, 
of  such  a  density  that  the  solid  will  remain  suspended,  neither 
floating  or  sinking  to  the  bottom  of  the  vessel ;  then  observing 
the  density  of  the  liquid. 

Thus,  raw  coffee-beans  (sp.  gr.  1.041  io  1.368)  maybe  thrown  into 
a  strong  solution  of  calcium  chloride,  this  lightened  with  water  until 
the  berries  just  sink  below  the  surface.  For  roasted  coffee  (sp.  gr. 
.500  to  .635)  the  lightest  grade  of  gasoline  is  adjusted  with  kerosene. 
Schultze  recommend  that  a  fat  or  wax  be  melted  and  the  fluid  dropped  into  cold  alcohol ; 
after  twenty-four  hours  several  of  the  globules  are  transferred  to  dilute  alcohol  followed 
by  the  addition  of  water  or  strong  alcohol  as  needed.  The  liquid  is  filtered  and  its 
density  determined  by  the  alcoholometer.  Drops  containing  air-bubbles-behave  differently 
from  the  others  and  are  rejected.  Fibrous  bodies  are  tested  under  reduced  atmospheric 
pressure  to  eliminate  the  air  contained  in  the  open  pores. 

Blyth  has  proposed  to  approximately  determine  quantities  of  the  alkaloids  or  their  salts 
too  minute  for  weighing  (such  as  are  obtained  in  toxicological  examinations)  by  crystal- 
lizing and  measuring  the  crystals  under  the  microscope  with  the  aid  of  a  micrometer. 
Knowing  the  crystalline  form  and  specific  gravity  it  is  a  simple  mathematical  problem  to 
compute  the  volume  and  weight. 

A  fair  approach  to  the  proportion  of  a  solid  or  liquid  in  an  aqueous  solution 
can  often  be  reached  by  a  calculation  from  the  decrease  or  increase  in  density 
of  the  solvent  due  to  the  elimination  of  the  solute  by  precipitation  or  evapo- 
ration, or  by  a  change  in  chemical  combination.  The  presence  of  other  bodies 
remaining  in  the  solution  does  not  interfere.  This  principle  has  many  analyti- 
cal applications. 

Examples  are  these.  For  the  determination  of  tannin  in  an  aqueous  extract  of  oak- 
bark,  the  specific  gravity  of  the  extract  is  observed,  and  the  tannin  precipitated  from  a 
measured  volume  by  cnpric  oxide;  after  filtering,  the  gravity  is  again  observed.  The  dif- 
ference between  the  two  is  compared  with  a  table  drawn  up  from  the  results  of  direct  ex- 
periments on  the  purest  tannin  obtainable.  Similarly,  the  density  of  a  mixture  of  alcohol 
and  water  increases  in  proportion  as  the  alcohol  is  dissipated  by  boiling,  and  the  per- 
centage of  alcohol  in  the  mixture  can  be  calculated  from  a  second  density  determination 
made  after  replacing  the  liquid  evaporated  by  an  equal  volume  of  water.  Urine  (average 
specific  gravity  1.020)  containing  albumen  (specific  gravity  1.314)  is  of  a  lower  gravity  after 
the  albumen  has  been  removed  by  coagulation  and  filtering.  A  dilute  acid,  such  as  vine- 
gar, on  shaking  with  (insoluble)  calcium  carbonate,  takes  into  solution  an  equivalent  of 
calcium  and  the  density  is  Increased  proportionally. 

11 


130. 


Fresenius  and 


162  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Instead  of  the  usual  procedure  of  drying  and  weighing  a  precipitate,  it  may,, 
after  thorough  washing,  be  rinsed  with  water  into  a  specific  gravity  flask, 
water  added  to  the  mark,  and  the  flask  weighed.  Obviously  the  difference 
between  this  weight  and  that  of  the  flask  filled  with  pure  water  at  the  same 
temperature  is  that  of  the  precipitate  less  the  weight  of  an  equal  volume 
of  water.  Or  if  the  precipitate  is  of  such  a  nature  that  it  will  remain 
for  a  time  uniformly  diffused  in  the  liquid  in  which  it  has  been  formed, 
from  the  difference  in  specific  gravity  of  the  liquid  before  and  after  clarifica- 
tion may  be  calculated  the  weight  of  the  precipitate,  assuming  that  its  specific 
gravity  has  been  previously  determined.  The  formula  for  densimetric  methods 

is  W=  8  ^~g  where  W  is  the  weight  of  the  precipitate;    S,  the   specific 

gravity  of  the  precipitate ;  s,  the  specific  gravity  of  the  solution  (or  water)  in 
which  the  precipitate  is  suspended;  G,  the  total  weight  of  the  picnometer,  solu- 
tion and  precipitate ;  and  g,  the  weight  of  the  picnometer  filled  with  the  clear 
solution. 

The  purity  of  a  given  sample  of  a  commercial  article  can  sometimes  be 
judged  by  determining  the  gravity  at  several  different  temperatures,  consider- 
able variations  being  shown  by  adulterated  samples  as  the  temperature  is 
raised,  though  at  ordinary  temperatures  there  may  be  no  marked  difference 
between  the  pure  and  adulterated.  And  where  a  certain  minimum  or  maximum 
density  characterizes  an  article  of  standard  quality,  a  gravity  determination 
alone  will  indicate  the  grade  more  or  less  accurately.  Similarly,  the  propor- 
tion of  impurities  in  a  fairly  pure  commercial  compound,  both  being  freely 
soluble  in  some  liquid,  can  be  determined  by  making  a  saturated  solution  at 
a  given  temperature  and  noting  the  variation  in  gravity  from  that  of  a  satu- 
rated solution  of  the  pure  compound  at  the  same  temperature.  But  in  many 
cases,  changes  in  constitution,  such  as  through  fermentation  and  concentra- 
tion by  evaporation,  that  may  follow  age  or  exposure,  may  considerably  alter 
the  normal  gravity. 

The  specific  gravity  of  a  mixture  of  gases  is  determined  by 
weighing  the  gas  in  a  glass  globe  (page  38),  and  comparing  the 
weight  with  that  of  pure  air  or  hydrogen  under  similar  con- 
ditions. 

A  method  sometimes  applied  to  illuminating  gases,  is  based 
on  the  law  that  gases  transfuse  through  a  small  orifice  at  rates 
proportional  to  the  square  roots  of  their  densities.  A  simple 
apparatus  for  the  purpose  is  shown  in  Fig.  131.  A  glass  tube 
A  B  is  closed  air-tight  at  the  top  by  a  thin  platinum  plate  per- 
forated at  the  center  by  a  hole  .1  Mm.  in  diameter.  The  lower 
end  of  the  tube  is  open.  A  glass  float  C  slides  in  the  tube  and 
has  two  marks  at  the  extremities  a  and  b.  The  tube  is  lowered 
into  a  jar  of  mercury  and  filled  with  the  gas  to  be  tested;  the 
stopcock  D  is  opened  until  the  float  rises  so  far  that  the  mark 
a  coincides  with  the  surface  of  mercury  outside  the  tube.  The 
gas  is  now  under  a  pressure  p.  During  a  time  £,  so  much  gas 
has  effused  that  the  mark  b  coincides  with  the  surface  of  mer- 
cury, the  gas  being  now  at  a  pressure  p'.  From  another  experi-- 
ment  with  hydrogen  or  air,  the  density  of  the  gas  is  easily  calculated.* 


Electrical  Engr.  25—311. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  163 


MELTING  AND  CONGEALING  POINTS. 

Determinations  by  the  temperatures  of  liquefaction  and  solidification  after 
fusion  are  practically  restricted  to  bodies  that  melt  and  congeal  at  compara- 
tively low  temperatures,  such  as  the  fats,  fatty  acids,  waxes,  etc. 

Where  a  reasonably  large  quantity  of  the  substance  to  be  tested  is  at  hand, 
twenty  grams  or  more  is  placed  in  a  beaker  provided  with  an  arrangement  for 
continuous  stirring  and  an  accurate  thermometer.  The  beaker  is  set  in  another 
containing  water  or  oil  heated  a  little  above  the  melting  point  of  the  substance. 
As  soon  as  partial  fusion  takes  place  the  stirrer  is  rapidly  turned;  so  long  as 
some  of  the  substance  remains  unfused  the  thermometer  will  remain  stationary 
at  the  melting  point. 

For  paraffin  and  like  bodies,  Kissllng  *  recommends  to  heat  a  quantity  in  a  covered 
beaker  to  ten  degrees  above  the  melting  point,  then  surround  the  beaker  with  an  air- 
jacket  in  the  form  of  an  open  glass  cylinder.  The  paraffin  is  allowed  to  cool  slowly  while 
stirring  with  a  thermometer,  until  a  thin  skin  forms  on  the  surface  and  bottom  of  the 
liquid  and  it  becomes  cloudy.  This  temperature  is  taken  as  the  melting  point. 

Where  the  quantity  of  substance  available  for  the  test  is  limited,  or  when  it 
is  desirable  for  other  reasons  to  use  only  a  small  amount,  the  above  process 
will  not  answer,  and  several  plans  suited  to  the  reduced  quantity  have  been 
proposed.  Most  of  these  assume  the  melting  point  to  be  the  temperature  at 
which  some  physical  alteration  denoting  incipient  or  entire  fusion  becomes 
apparent;  for  example,  when  a  disk  of  the  solid  becomes  spherical,  a  globule 
transparent,  a  layer  softens  so  far  as  to  yield  to  a  given  continuous  uniform 
pressure,  or  the  edges  of  a  cube  become  rounded.  Since  there  is  always  a 
small,  sometimes  a  great  difference  in  the  melting  point  obtained  by  methods 
of  this  class,  whenever  a  melting  point  is  stated  there  should  always  be 
appended  a  note  of  the  method  used  for  the  determination. 

1.  A  short  glass  tube  of  narrow  bore  is  filled  with  the  melted  substance  by 
suction;  after  cooling  and  remaining  solid  fora  sufficient  period,  the  tube  is 
tied  to  a  delicate  thermometer  and  the  two  lowered  into  a  beaker  of  water, 
which  is  then  heated  slowly  on  the  water  bath.    The  temperature  is  observed 
when  the  contents  of  the  tube  become  transparent.    For  powders,  the  tube  is 
closed  at  the  bottom  and  the  point  of  liquefaction  observed.   Somewhat  different 
results  are  obtained  with  tubes  of  different  diameters  of  bore.f 

2.  A  drop  of  the  melted  fat  may  be  placed  on  a  globule  of  mercury  contained 
in  a  porcelain  crucible.    When  the  fat  has  solidified,  the  crucible  is  floated  in  a 
dish  of  water  and  a  delicate  thermometer  inserted  in  the  mercury.    The  water 
is  slowly  warmed  until  the  fat  spreads  over  the  surface  of  the  mercury;  the 
reading  of  the  thermometer  at  this  moment  is  taken  as  the  melting  point. 

3.  Wiley  recommends  to  drop  a  melted  fat  on  a  cake  of  ice,  the  drops  con- 
gealing to  form  disks  of  a  uniform  size,     A  test-tube  is  half  filled  with  water, 
then  nearly  filled  with  alcohol,  retaining  their  integrity  as  far  as  may  be.  Near  the 
junction  is  a  point  where  the  specific  gravity  of  the  somewhat  diffused  liquids 
is  the  same  as  that  of  the  fat,  and  consequently  the  disks  will  remain  suspended 
at  that  point.    One  of  the  disks  is  dropped  into  the  tube  and  a  delicate  ther- 
mometer suspended  close  to  it,  and  the  tube  heated  in  a  water  bath  until  the 
disk,  after  shriveling,  becomes  spheroidal.    Precautions  must  be  taken  against 
air  bubbles  attaching  to  the  disk,  and  to  prevent  the  disk  touching  the  side  of  the 
tube. 


*  Journ.  Socy.  Chem,  Ind.  1898—380. 
t  Allen,  Coml.  Org.  Anal.  3—2—520. 


164  QUANTITATIVE    CHEMICAL   ANALYSIS. 

With  oils  that  remain  liquid  below  zero,  disks  are  made  by  dropping  the  oil 
into  a  glass  spoon  that  has  been  chilled  by  solid  carbon  dioxide.  A  disk  is  then 
dropped  into  a  test-tube  containing  a  lower  layer  of  a  mixture  of  concentrated 
sulfuric  acid  and  abolute  alcohol,  and  an  upper  layer  of  absolute  alcohol  float- 
ing on  the  lower.  The  disk  remains  suspended  at  some  point  in  the  lower 
layer.  The  tube  is  immersed  in  a  beaker  of  alcohol  cooled  by  solid 
carbon  dioxide  in  a  surrounding  beaker,  this  jacketed  by  a  third  beaker  con- 
taining a  little  concentrated  sulfuric  acid  to  prevent  clouding  by  deposition  of 
frost.  The  carbon  dioxide  is  removed  when  the  test-tube  has  become  of  the 
temperature  of  the  surrounding  alcohol,  and  the  latter  allowed  to  rise  in  tem- 
perature while  being  constantly  stirred. 

4 .  Damien,  by  means  of  a  special  apparatus,  heats  a  layer  of  a  fat  to  slightly  above  the 
melting  point,  then  cools  one-half  the  layer  until  it  just  solidifies.    The  change  is  shown 
by  contrast  of  the  melted  and  congealed  halves,  and  the  mean  of  two  thermometers,  one 
in  each  part,  is  taken  as  the  melting  point. 

5.  In  several  apparatus  the  melting  point  is  indicated  by  the  closing  of  an 
electric  circuit.    In  one,  the  bend  of  a  small  U-tube  is  filled  with  the  melted 
fat  which  is  then  allowed  to  solidify.    Two  platinum  wires  whose  inner  ends 
are  nearly  in  contact  are  sealed  in  one  limb  near  the  bottom,  and  each  wire 
connected  to  the  pole  of  a  galvanic  battery  in  circuit  with  an  electric  bell. 
Into  the  other  limb  of  the  U-tube  is  poured  a  quantity  of  mercury.     The  U-tube 
is  slowly  heated  in  a  water  bath,  and  when  the  fat  melts  the  mercury  flows 
into  the  other  limb  and  makes  an  electrical  contact  between  the  wires,  com- 
pleting the  circuit  and  causing  the  bell  to  ring. 

6.  The    temperature  at  which  a  disk  of  a  fat  liquefies  sufficiently  to  stain 
paper  on  which  it  rests  has  been  proposed  at  the  melting  point.    In  the  appar- 
atus designed,  the  stain  is  observed  in  an  inclined  mirror  stationed  below  the 
paper,  the  disk  heated  by  a  water-bath.* 


In  general  a  simple  chemical  compound  passes  sharply  from  the  solid  to  the 
liquid  state  and  the  reverse,  while  mixtures  exhibit  a  more  or  less  prolonged 
transition  period.  The  interval  of  semi-fluidity  is  well  marked  with  certain 
complex  fats  and  preparations  from  them.  In  the  case  of  alloys  and  amalgams 
it  is  probable  that  their  bases  are  certain  eutechtic  compounds  of  two  metals 
admixed  with  an  excess  of  one  of  the  metals,  and  the  difference  in  melting 
points  between  them  gives  rise  to  anomalous  results. 

Whatever  method  of  determining  the  melting  point  be  adopted  it  is  to  be 
remembered  that  the  melting  point  of  a  fat  or  fatty  acid  may  be  altered  as 
much  as  several  degrees  by  a  previous  melting,  and  the  original  figure  is 
regained  only  after  standing  several  hours  after  solidification. 

As  to  whether  the  melting  point  of  a  given  mixture  is  the  mean  of  the 
melting  points  of  the  several  constituents  can  only  be  determined  by  direct 
experiment.  Thus,  mixtures  of  butter-fat  with  oleo-oil  or  neutrallard  or  both, 
show  melting  points  agreeing  with  the  calculated  values ;  while  mixtures  of 
solid  fatty  acids,  as  stearic  and  palmitic,  have,  in  certain  relative  proportions, 
a  lower  melting  point  than  either  constituent. 

The  point  of  solidification  after  melting  generally  differs  somewhat  from  the 
fusing  point,  as  from  a  disengagement  of  the  latent  heat  of  fusion,  a  recal- 
escence  takes  place,  the  thermometer  remaining  stationary  for  a  few  moments, 
or  even  risiug  a  degree  or  more. 


*  Analyst,  1899-84. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


165 


The  determination  has  been  applied  to  various  mixtures  of  analogous  compounds. 
Thus,  for  the  determination  of  paratoluidin  In  commercial  toluids,  Raabe  adds  a  weighed 
quantity  of  pure  paratoluidin  In  such  an  amount  as  will  give  a  mixture  whose  congealing 
point  can  be  readily  determined.  The  thermometer  is  graduated  in  twentieths  of  a  de- 
gree from  30°  to  60°,  each  twentieth  corresponding  to  .2  of  one  per  cent  of  paratoluidin. 
Standards  are  made  up  conforming  to  the  usual  composition  of  commercial  samples,  and 
their  congealing  points  observed. 

According  to  Pickering,*  out  of  many  thousands  of  compounds  that  have 
been  investigated,  there  are  but  one  or  two  in  which  the  addition  of  another 
body  has  been  found  to  raise  the  congealing  point  of  a  liquid;  in  all  other 
cases  it  is  lowered.  Finkener  f  states  that  on  warming  a  mixture  of  chem- 
ically pure  substances  of  different  melting  points,  which  have  no  chemical 
action  on  each  other  except  solvent  action,  the  temperature  remains  almost 
constant  about  the  melting  point  of  the  lower  melting  constituent  until  this 
has  ceased  to  dissolve  the  more  solid  constituent,  and  then  rapidly  rises. 


POLARIZED  LIGHT. 

The  polariscope  has  a  limited  use  in  pathological  examinations,  determina- 
tion of  the  alkaloids,  camphors,  oils,  etc. ;  while  apparatus  of  special  design 
are  extensively  employed  for  the  determination  of  the  sugars. 

The  simplest  form  of  polariscope  is  that  devised  by  Mitscherlich,  Fig.  132. 
A  is  a  source  of  monochromatic  light, 
a  Bunsen  burner  whose  flame  impinges 
on  a  bead  of  sodium  chloride.  The 
rays  pass  through  a  calcite  prism  B  by 
which  the  light  is  polarized,  so  con- 
structed that  the  extraordinary  ray  is 
extinguished  while  the  ordinary  ray 
proceeds  through  a  lens  into  the 
analyzer  C.  This  is  another  prism  so 
mounted  that  it  may  be  rotated  on  its 
long  axis  by  moving  the  arm  D.  The 
angle  of  rotation  is  shown  on  the 
vernier  of  the  scale  F  divided  into 
degrees.  When  the  vernier  points  to 
0°  or  1800  on  the  scale,  the  light 
transmitted  to  the  eye  at  G  is  at  a 

minimum;  on  rotating  the  analyzer  C  the  light  gradually  increases  until  at 
90  o  or  270  °  it  is  at  a  maximum. 

If  now  a  tube  H  of  standard  length  whose  ends  are  closed  by  glass  plates  be 
filled  'with  an  optically  active  solution,  as  of  a  sugar,  and  interposed  between 
the  polarizer  and  analyzer,  the  plane  of  the  light  coming  from  the  polarizer  is 
turned  to  an  extent  determined  by  the  concentration  of  the  solution,  and  the 
point  of  minimum  brightness  is  no  longer  at  0°  or  180°,  but  at  some  inter- 
mediate division.  Knowing  by  calculation  or  previous  experiments  to  what 
extent  the  ray  is  deviated  by  a  sugar  solution  of  given  strength,  it  is  easy  to 
calculate  the  weight  of  sugar  in  the  tube. 

Several  greatly  improved  forms  of  the  polariscope  have  been  invented,  prin- 
cipally for  the  determination  of  sugar  and  known  as  saccharimeters.  In  these 


Fig.  132. 


*Chem.  News,  1892—1-50;  Idem,  1892—2—109. 
t  Analyst,  1899—269. 


166  QUANTITATINE    CHEMICAL   ANALYSIS. 

the  reading  point  is  shown  by  the  identity  in  tint  of  two  luminous  adjacent 
semi-circles,  the  appearance  or  disappearance  of  black  bands  on  an  illumi- 
nated field,  etc.  A  full  description  of  these  will  be  found  in  works  on  the 
polariscope  and  sugar  analysis. 

The  specific  rotary  power  of  a  body  is  the  angular  deviation  produced  when 
the  ray  passes  through  an  optically  active  substance  in  a  solution  of  a  concen- 
tration of  one  gram  per  cubic  centimeter,  and  of  a  length  of  one  decimeter. 
The  amount  of  rotation  depends  on  the  nature  of  the  substance  and  the  solvent, 
the  length  of  the  column  of  liquid,  the  temperature,  and  the  kind  of  mono- 
chromatic light  employed.  If  (a) &  represents  the  specific  rotary  power  of  a 
solution  with  the  D  or  sodium  light;  a,  the  angular  displacement  of  the  ray;  1, 
the  length  of  the  column  of  liquid  expressed  in  decimeters;  d,  the  specific 
gravity  of  the  solution ;  and  p,  the  percentage  of  the  solid  in  the  solution  by 
weight,  then 

100  a 


SPECTRUM  ANALYSIS* 

Although  originated  many  years  ago  and  revived  and  improved  from  time  to 
time,  and  apparently  capable  of  practical  application  in  many  examinations, 
lor  several  reasons  none  of  the  methods  have  come  into  practical  use.  Most 
of  the  processes  have  been  devised  for  the  alkalies,  some  for  the  analysis  of 
the  coal -tar  colors,  the  valuation  of  commercial  indigo,  f  the  determination  of 
the  haemoglobin  of  bloodj  etc. 

The  method  of  Vierordt  depends  on  the  principle  that  if  the  slit  of  a  spec- 
troscope be  divided  transversely  in  halves,  each  independently  adjustable,  the 
intensity  of  the  two  spectra  formed  from  one  source  of  light  is  proportional  to 
the  widths  of  the  slits;  and  if  separate  lights  enter  the  slits  and  the  widths  of 
these  be  so  adjusted  that  the  intensities  of  the  spectra  are  equal,  then  the  in- 
tensities of  the  lights  are  proportional  to  the  widths  of  the  slits. 

The  apparatus  used  is  the  universal  spectroscope  of  Kruess.  The  halves  of 
the  divided  slit  are  each  provided  with  an  accurate  measuring  apparatus.  In 
front  of  the  slits  is  placed  a  flat  glass  cell  with  parallel  sides  containing  the 
liquid  to  be  examined.  The  upper  half  of  the  cell  is  eleven  millimeters  between 
the  sides,  the  lower  half  (reduced  by  the  insertion  of  a  block  of  glass  ten  milli- 
meters thick)  only  one  millimeter  between  the  sides.  On  adjusting  the  open- 
ings of  the  slits  until  the  spectra  are  of  the  same  brightness,  the  ratio 
between  the  widths  of  the  slits  is  the  intensity  of  the  light  emerging 
from  a  layer  of  the  liquid  ten  millimeters  in  thickness,  the  intensity  of 
the  original  light  expressed  as  unity.  From  the  ratio  is  calculated  the  '  ex- 
tinction coefficient'  which  is  defined  as  the  " reciprocal  value  of  the*thick- 
ness  which  a  substance  must  have  in  order  to  decrease  the  intensity  of  the  light 
which  passes  through  it  to  one-tenth  of  the  original  intensity."  In  the  case 
of  solutions  the  extinction  coefficient  depends  on  the  concentration  c  of  the 
solution. 

Truchott's  method  differs  from  the  above  in  that  the  brightness  and  duration 
of  the  spectra  are  compared  when  definite  quantities  of  a  solution  of  an  alkali 
salt  and  one  of  a  standard  solution  of  the  pure  salt  are  brought  into  a  Bunsen 
fiame.  The  comparison  may  be  made  by  diluting  each  solution  to  the  extinction 
of  the  brightest  lines.  Another  plan  is  to  add  from  a  burette  to  a  measured 


Thorp,  Diet.  Applied  Chem.  3—345. 


QUANTITATIVE   CHEMICAL   ANALYSIS.  167 

volume  of  pure  water  smaM  quantities  of  a  weak  standard  solution  of  the  metal- 
lic compound  to  be  determined,  until  a  characteristic  line  of  the  spectrum  just 
appears;  then  repeat  with  the  solution  to  be  examined.  The  standard  solution 
should  also  contain  approximately  the  same  amount  of  associated  metals  as  are 
in  the  sample  to  be  tested. 


REFRACTIVE  INDEX  OF  LIGHT. 

For  measuring  the  specific  refraction  of  light  by  liquids,  various  instruments 
known  as  f  ref  ractometers  '  have  been  invented.    Some  of  these  are  adapted  to 
any  refracting   body,    others    are  constructed  with 
special   reference  to    the   examination  of  a  certain 
liquid,  such  as  the  « butyro-ref ractometer »  for  butter- 
fat. 

The  refracto- 
meter  of  Abbe, 
Fig.  !33,  has  a 
double  prism  E 

E          SS*  W     /->\\\      cut     obliquely, 

between  whose 
inclined  faces  is 
held  a  layer  of 
Fig-  133.  the  liquid  to  be 

tested.  The  prism  moves  in  an  arc  with  the  vernier  D.  A  telescope  A  is 
attached  to  the  alhidade  B  and  moves  with  it  in  the  same  arc  as  the  vernier. 
Monocromatic  .light  is  reflected  from  a  mirror  through  the  prism  into  the  tele- 
scope. If  a  liquid  of  a  smaller  refractive  index  than  the  glass  of  the  prism 
be  inclosed  therein,  then  for  a  certain  position  of  E  as  regards  A,  one-half  the 
field  appears  dark,  the  other  half  light.  The  refractive  index  is  read  directly 
on  the  scale  of  the  alhidade.  If  white  instead  of  monochromatic  light  is  the 
illuminant,  the  dividing  line  of  the  field  is  colored  (due  to  dispersion),  and 
must  be  made  to  appear  sharp  and  colorless  by  the  adjustment  of  a  compensat- 
ing apparatus. 

The  refractive  index  of  a  body  is  expressed  by  the  quotient  of  the  sine  of  the 
angle  of  the  ray  incident  to  the  body  divided  by  the  sine  of  the  refracted  ray. 

According  to  Gladstone  molecular  refraction  and  dispersion  may  be  safely 
deduced  from  the  substance  in  solution  where  the  solvent  is  chemically  inac- 
tive, but  that  in  the  case  of  water  (refractive  index  at  20  o  for  sodium  light 
1.3329)  there  is  some  profound  change  effected  upon  the  constitution  of  hy- 
dracids,  haloid  salts,  and  probably  some  other  compounds  by  the  action  of 
solution.  Although  the  rule  holds  that  a  solid  when  dissolved  retains  its  for- 
mer refractive  power,  there  are  some  exceptions. 

Marked  differences  are  found  in  the  refractions  of  different  varieties  of  the  fixed  and 
essential  oils,  but  the  data  are  somewhat  conflicting  since  the  refractive  index  is  modified 
by  the  age  of  the  oil,  process  of  refining,  the  presence  of  free  fatty  acids  and  oxidation 
products,  etc. 


168 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


B 


CAPILLARY  ASCENT  OF  LIQUIDS. 

The  rise  of  a  liquid  in  a  capillary  tube  is  observed  in  an  instrument  known 
as  the  capillarimeter.  As  shown  in  Fig.  134,  it  is  a  capillary  tube 
BB,  150  to  200  Mm.  in  length,  fixed  to  a  scale  A  graduated  in  half- 
millimeters.  The  radius  of  the  bore  of  the  tube  is  determined  by 
introducing  threads  of  mercury,  measuring  their  lengths,  and 
taking  their  weights;  the  radius  should  be  uniform  throughout. 

The  scale  is  fixed  in  a  vertical  position  over  a  dish  of  the  liquid 
to  be  tested  and  lowered  until  the  points  a  and  a'  (the  zero  of  the 
scale)  just  touch  the  surface  of  the  liquid.  By  means  of  a  rubber 
tube  slipped  over  the  upper  end  of  the  capillary  tube  the  liquid  is 
drawn  up  a  few  centimeters  above  the  final  position  of  the  meniscus 
of  the  liquid,  then  allowed  to  recede  till  stationary,  and  the  height 
read  on  the  scale. 

The  capillarimeter  has  been  applied  to  the  determination  of 
alcohol  in  spirits,  and  to  mixtures  of  the  alcohols.  According  to 
Traube,  the  minor  constituents  of  fusel  oil  and  various  aldehyds 
reduce  the  ascent  to  a  greater  degree  than  ethyl  alcohol  but  less 
than  amyl  alcohol. 

It  is  said  that  of  solutions  of  equal  concentration  of  some 
homologous  series  of  organic  bodies,  the  height  of  the  rise 
is  inversely  proportional  to  the  molecular  weight  of  the 
Fig.  134.  members. 
Paterson  utilizes  the  difference  in  capillary  adhesion  for  the  detection  of  the  various 
coloring  matters  of  a  mixture.  When  a  dilute  solution  of  the  dye  is  dropped  on  filter 
paper  a  simple  coloring  matter  forms  a  homogeneous  blot,  but  if  complex  there  are 
formed  concentric  rings.  Taking  for  a  basis  water  as  100,  the  speed  of  diffusion  of  solu- 
tion of  acid  magenta  is  100;  of  uranin  is  78.5;  of  rhodamin  is  42.8;  of  methyl  violet  is 
14.2,  etc. 


VISCOSITY. 

The  viscosity  or  internal  friction  of  a  liquid  is  generally  measured  by  the 
time  required  for  a  standard  volume  at  a  standard  temperature  to  flow  through 
an  orifice  of  standard  area.  Usually  the  viscosity  is  referred  to  that  of  pure 
water  under  the  same  conditions,  determined  in  the  same  instrument. 

Applications  are  mainly  to  the  fixed  and  essential  oils,  though  usually  in  a 
qualitative  way  only  or  as  a  criterion  of  the  lubricating  quality  of  the  former; 
and  to  the  potash  soaps  made  up  from  standard  weights  of  saponifiable  oils 
acted  on  by  a  standard  volume  of  lye  of  a  given  concentration. 

Of  gum  arable  and  gum  tragacanth,*  ten  per  cent  solutions  are  compared  with  similar 
solutions  freshly  made  up  of  the  best  quality  of  these  gams  as  standards.  Lungef  has 
devised  a  form  of  viscosimeter  for  mucilage  of  tragacanth  and  gum  thickenings;  it 
resembles  a  hydrometer,  and  the  viscosity  is  determined  by  the  number  of  minutes  re- 
quired for  the  instrument  to  sink  to  a  mark  on  the  stem,  it  having  previously  been  dipped 
in  the  solution,  but  it  is  doubtful  whether  reliance  can  be  placed  on  an  estimation  of  the 
proportion  of  an  impurity,  or  that  the  process  is  capable  of  giving  more  than  an  idea  as 
to  the  relative  quality  of  two  samples. 

Prollius  states  that  the  viscosity  of  solutions  of  isinglass  of  a  concentration  of  one  part 


*  Allen,  Coml.  Org.  Anal.  1—428  and  4—483. 
t  Jonrn.  Socy.  Dyers  &  Col.  1896—12. 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


169 


In  ninety  parts  of  water  ranges  from  360  to  507  seconds  according  to  the  quality  of  the 
samples  tested,  when  measured  by  the  same  Instrument. 

The  viscosity  of  mllfc  is  a  fairly  constant  value,  diminished  by  watering  and  ferment- 
ation. 


VAPOR  TEMPERATURE. 

The  temperature  of  the  vapor  of  a  mixture  of  two  liquids  lies  between 
those  of  the  constituents.  For  the  determination  of  alcohol  in  beverages 

an  instrument  known  as  the  ebulliometer  has 
come  into  considerable  use  in  some  European 
countries,  based  on  the  principle  that  steam 
at  atmospheric  pressure  has  normally  a  tem- 
perature of  100®,  while  the  vapor  of  alcohol 
at  the  same  pressure  is  only  at  78.4°  ,  and  that 
of  a  spirit  is  in  proportion  to  the  alcoholic 
strength. 

A  recent  form  of  apparatus*  is  shown  in  Fig. 
135.  The  flask  F  holds  the  spirit  to  be  tested ; 
through  the  cork  passes  a  delicate  thermometer 
B  graduated  from  95  o  near  the  bulb  to  100°  at 
the  top,  and  also  a  tube  entering  the  condenser 
D.  To  the  lower  end  of  D  is  joined  a  tube  E 
entering  the  flask,  the  end  dipping  below  the 
surface  of  the  liquid.  To  prevent  cooling  of 
the  flask  by  draughts  of  air  it  is  surrounded  by 
a  glass  cylinder  covered  with  a  rubber  plate 
carrying  a  thermometer. 

The  apparatus  being  connected  as  shown,  the 
spirit   in   F  is  heated   to  boiling,  the  vapor  of 
alcohol   and  water  emitted   passing  through   C 
Fig.  135.  into  D  where  it  is  condensed  and  returns  through 

E  to  F.  When  the  thermometer  has  reached  90  ° ,  B  is  read  and  the  tempera- 
ture corrected  for  any  variation  of  barometric  pressure  from  the  normal 
at  the  time  of  reading. 

The  temperature  of  the  vapor  of  a  spirit  is  depressed  from  that  of  pure 
water  by  .8  o  Cent,  for  each  per  cent  by  volume  of  alcohol  contained  in  the 
spirit,  this  ratio  holding  good  up  to  five  per  cent  of  alcohol.  Wines  or  liquors 
above  this  strength  are  best  diluted  before  testing.  If  acetic  acid  in  any  great 
proportion  is  contained  a  previous  distillation  with  caustic  potash  is  ad  vised. 


PENETRABILITY. 

The  resistance  of  a  solid  or  semi-liquid  body  to  penetration  is  occasionally  applied  in 
analysis,  principally  to  the  fats  and  fatty  acids.  The  *  oleogrammeter '  of  Brull6 1  is  a  ver- 
tical metal  rod,  surmounted  by  a  scale-pan,  which  slides  freely  in  a  guide-ring.  The  lower 
flat  end  of  the  rod  presses  on  the  fat  to  be  tested  brought  to  a  temperature  of  21  o .  The 
scale-pan  is  weighted  with  increasing  loads  until  the  rod  penetrates  the  fat. 

Under  a  rod  of  standard  diameter,  pure  butter  fat  yields  to  a  weight  of  250  grams  while 
margarine  requires  5,000  grams,  and  intermediate  figures  obtained  with  mixtures  of  the 


*  Journ.  Amer.  Chem.  Socy.  1896—1063. 
t  Chem.  News,  1893-2—2. 


170  QUANTITATIVE   CHEMICAL  ANALYSIS. 

two  are  said  to  allow  an  approximate  calculation,  except  for  mixtures  containing  seed  oils, 
which  greatly  modify  the  resistance.  Olive  oil  is  tested  for  adulteration  with  cottonseed 
oil  by  first  freezing  the  sample  thoroughly;  genuine  olive  oil  yields  under  1,700  grams  and 
cottonseed  oil  under  1,000  grams. 

The  relative  hardness  of  alloys  of  certain  metals  Is  observed  by  an  apparatus  which  is 
essentially  a  diamond  fixed  to  the  beam  of  a  balance,  ruling  lines  to  be  measured  In  the 
microscope  by  a  micrometer.  Alloys  of  copper  and  tin  show  a  hardness  of  364  units 
when  composed  of  17  per  cent  of  copper  with  83  per  cent  of  tin,  rising  to  1100  units  at  75 
per  cent  of  copper  with  25  per  cent  of  tin,  thence  decreasing  to  675  units  at  96  per  cent  of 
copper  with  4  per  cent  of  tin. 

The  temperature  to  which  a  fat  must  be  heated  so  that  a  glass  bulb  of  specified 
weight  and  dimensions  will  sink  therein  has  been  termed  the  '  sinking  point.'  For 
butter-fat  the  bulb  Is  pear-shaped,  has  a  volume  of  one  cubic  centimeter  and  a  specific 
gravity  of  2.4.  Hehner  and  Angell  found  the  average  sinking  point  of  butter-fat  to  be 
35.50  ;  of  lard,  430  ;  and  of  beef -tallow,  50.6  o,  mixtures  following  the  usual  formulae. 

Hassell*  calls  the  'rising  point*  the  temperature  at  which  a  bulb  of  .5  cubic  centi- 
meters volume  and  .18  gram  in  weight  will  rise  In  a  previously  solidified  fat  contained 
In  a  test-tube  one-third  inch  in  diameter  by  four  Inches  in  height  heated  In  a  water- 
bath. 


VOLTAIC  ENERGY. 

For  a  determination  of  a  salt  in  aqueous  solution,  Gore  f  proposes  to  measure  the 
energy  excited  In  a  galvanic  couple  of  platinum  and  zinc,  referred  to  a  standard  of  that 
excited  In  pure  water,  both  at  atmospheric  temperatures.  The  plates  Immersed  in  the 
liquid  to  be  tested  are  connected  with  a  delicate  galvanometer,  then  the  solution  diluted 
until  the  voltaic  current  generated  is  sufficient  only  to  visibly  move  the  needle,  the  vol- 
ume of  the  diluted  solution  being  directly  as  the  weight  of  salt  contained.  The  results  on 
mineral  acids,  ammonia,  sodium  chloride  and  sodium  carbonate  up  to  ten  per  cent  solu- 
tions agreed  fairly  well  with  the  determinations  by  specific  gravity  and  chemical  analysis, 
and  the  method  is  claimed  to  be  quick  and  easy,  and  to  require  less  substance  than  other 
analytical  methods. 


ADHESION. 

Certain  pure  essential  oils  when  treated  with  concentrated  sulfuric  acid  pass  to  a  viscid 
liquid,  later  to  a  viscous  solid  of  a  definite  adhesiveness.  The  common  adulterants  of 
these  oils  lessen  the  adhesiveness  directly,  since  they  do  not  solidify.  To  determine  the 
;  .proximate  proportion  of  an  adulterant  it  has  been  suggested):  to  place  .020  to  .030 
gram  of  the  sample  on  a  ground -glass  plate  and  mix  with  one  drop  of  concentrated  sulfuric 
acid.  A  glass  rod  whose  lower  end  has  been  ground  flat,  is  hung  from  a  balance-beam  and 
counterpoised,  and  the  end  brought  Into  contact  with  the  mixture  on  the  plate.  The 
minimum  weight  In  the  opposite  pan  that  will  lift  the  rod  is  a  measure  of  the  adhesion 
and  consequently  of  the  purity  of  the  oil. 


FLASH  POINT. 

The  temperature  at  which  a  mixture  of  the  vapor  of  a  volatile  liquid  and  air  will  Ignite 
when  brought  In  contact  with  a  flamelet  Is  called  the  flash  point.  Mixtures  of  two  liquids, 
the  vapor  of  .one  Inflammable,  the  other  not,  show  a  flash  point  varying  inversely  with  the 
proportion  of  the  former  in  the  mixture,  but  only  approximate  results  can  usually  be 
obtained  by  this  process.  For  example,  the  flashing  temperature  in  degrees  Cent,  of  a 
mixture  of  alcohol  and  water,  §  A  being  the  percentage  of  alcohol,  and  t  o  the  flash  point. 
A.  100  90  80  70  60  50  40  30  20  10  5  A 

to.  12.         16.5       19.        21.         22.3       24          26.3       29.5       36.8       49  62.       t®. 


*  Prescott  Coml.  Org.  Anal. ! 
t  Chem.  News,  1889—1—243. 
\  Odorographla,  374. 
§  Analyst,  1899-132. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  171 

Other  physical  constants  that  have  been  proposed  are  electrical  conductivity 
and  resistance,  vapor  density,  conduction  of  he'at,  thermal  expansion, 
cryoscopy,  etc. 


CHEMICAL  METHODS. 

The  proportions  of  two  bodies  in  a  homogeneous  mixture  can  be  determined 
without  separation  by  finding  the  united  weights  of  a  common  constituent 
and  calculating  from  the  usual  formula  — 

X=100^ZI_6and  F=100d~"a  =  100  —  X. 
a  —  b  b  —  a 

where  JTand  Fare  respectively  the  weights  in  grams  in  100  grams  of  the  mix- 
ture of  A  and  B;  a  and  6,  the  proportions  of  a  common  constituent  of 
A  and  B  in  one  gram;  and  d  the  proportion  of  the  common  constituent  in  one 
gram  of  the  mixture.  Several  cases  are  presented. 

1.  When  each  member  of  the  mixture  contains  a  known  definite  proportion 
of  a  common  constituent,  the  proportions  being  unequal.    The  above  formulae 
apply  for  the  calculation,  and  the  greater  the  divergence  in  the  ratios  of  the 
constituents  to  the  bodies  themselves,  the  more  nearly  will  the  result  of  a 
determination  approach  the  truth,  other  conditions  being  the  same. 

The  same  principle  covers  cases  where  a  certain  extrinsic  associate  accompanies  each 
body  in  a  reasonably  constant  proportion  —  such  as  are  sometimes  found  in  commercial 
articles  and  natural  products,  either  originally  present,  acquired  by  age  or  exposure,  or 
developed  during  refining  or  other  treatment  in  their  manufacture.  Of  course,  some 
doubt  always  attaches  to  a  determination  made  on  this  basis. 

If  with  a  complex  body  M,  bearing  a  known  proportion  of  a  constituent  m, 
be  admixed  an  adulterant  N  containing  a  known  proportion  of  a  cons  tituent  n 
which  is  identical  with  m  or  analytically  equivalent  to  it,  the  extent  of  the  adul- 
teration may  be  calculated  from  the  usual  formula  F=100  a  where  T  is 

the  percentage  of  Nin  the  mixture;  a,  the  percentage  of  m  in  M;  6,  the  per- 
centage of  n  in  N;  and  dt  the  percentage  of  m  -\-n  in  the  mixture.  For  exam- 
ple, an  organic  substance  leaving  when  pure  17  per  cent  of  ash,  and  an  adulter- 
ant leaving  88  per  cent  of  ash;  if  the  ash  in  a  given  mixture  is  70  per  cent,  the 
percentage  of  N  is  74.6.  Where  N  is  anhydrous  mineral  matter,  n  =  100. 

Where  a  mixture  has  such  a  composition  that  a  chemical  constant  is  not  ex- 
hibited to  a  degree  sufficiently  great  to  be  easily  measured,  a  weighed  quantity 
of  one  of  the  constituents  in  the  pure  state  may  be  added  to  the  mixture  and 
its  effect  allowed  for  in  the  calculation. 

2.  When  both  members  are  brought  to  a  form  that  contains  a  common  con- 
stituent.   The  usual  formula  apply  to  all  the  following  cases. 

A.  When  the  proportion  of  the  constituent  in  one  body  becomes  a  determin- 
able  quantity,  in  the  other  remains  practically  at  zero.    The  addition  of  an  ele- 
ment to  one  of  the  bodies  may  be  done  by  a  simple  operation,  e.  g.9  the  conver- 
sion of  an  alloy  of  platinum  and  zinc  to  a  mixture  of  platinum  and  zinc  oxide, 
or  both  bodies  may  be  brought  to  a  different  combination,  then  reduced  to  the 
above  condition.    The  reverse  of  this  may  be  availed  at  times  —  the  withdrawal 
of  a  common  constituent  from  one  of  the  bodies,  leaving  the  constituent  in  the 
other  body  in  the  original  amount;  or,  when  only  one  of  the  bodies  contains  a 
constituent  that  can  be  wholly  expelled. 

B.  When  the  constituent  is  introduced  into  both  bodies  to  a  measurable 
though  differing  proportion,  the  general  formula  applies.    Frequently  the  in- 


172  QUANTITATIVE    CHEMICAL    ANALYSIS. 

crease  in  weight  or  volume  alone  will  furnish  an  easy  means  of  determination : 
the  absorption  of  a  halogen  or  oxygen  by  certain  oils  is  an  example.  Other- 
wise, as  when  an  acid  radical  is  replaced  by  another  (e.  g.,  a  metal  with 
an  organic  radical  changed  to  a  carbonate),  a  simple  calculation  is  required. 
As  in  A,  the  complete  removal  of  a  common  constituent  from  both  bodies 
may  be  made  the  basis  for  a  determination. 

Basic  radicals,  difficult  of  direct  separation,  may  be  combined  with  one  acid 
radical,  and  acid  radicals  with  one  base;  thus,  valeric  and  acetic  radicals  with 
barium,  barium  valerate  containing  40.41  per  cent  of  barium,  and  barium  ace- 
tate, 53.72  per  cent. 

3.  By  the  action  of  a  reagent  there  is  produced  with  or  from  both  members 
a  third  body  that  can  be  separated  and  determined.    This  is  usually  a  precipi- 
tate, but  may  be  a  gas,  as  where  two  carbonates  are  treated  by  hydrochloric  acid 
and  the  evolved  carbon  dioxide  caught  and  weighed. 

4.  With  a  mixture  of  two  bodies  having  an  element  or  radical  in  common, 
the  proportion  may  be  calculated  from  the  increase  or  decrease  in  weight  when 
one  body  is  transformed  to  the  composition  of  the  other.    Thus  ferrous  oxide 
with  ferric  oxide,  lead  sesquioxide  with  lead  protoxide,  tungsten  with  tungstic 
acid,  cupric  oxide  with  metallic  copper  —  in  each  case  one  being  oxidized  or 
reduced  to  the  composition  of  the  other. 

The  usual  formula  applies,  a  being  the  theoretical  weight  were  the  percent- 
age of  the  transformable  constituent  100;  6,  the  original  weight  of  the  mixture; 
and  rf,  the  weight  after  transformation. 

Even  the  combination  of  both  bodies  may  be  changed  to  other  dissimilar 
forms,  as  an  alloy  of  silver  and  copper  to  silver  nitrate  mixed  with  cupric 
oxide.  Owing,  however,  to  the  uncertainty  that  each  body  is  brought  entirely 
to  the  presumed  condition,  the  method  is  but  seldom  used. 

5.  Where  the  two  bodies  react  with  a  third  but  in  unlike  ratios:  e.  #.,  one 
part  of  methyl  alcohol  reduces  9.22  parts  of  potassium  bichromate;  one  of 
ethyl  alcohol,  4.28  parts;  and  one  of  propyl  alcohol,  3.28  parts. 

The  two  mixed  bodies  may  each  react  with  a  reagent  but  a  certain  deter- 
minable  product  be  formed  from  only  one.  For  example,  dinitro- phenol  and 
picric  acid  when  acted  on  by  bromine  are  transformed  according  to  the  equa- 
tions — 

C6H3(NO2)2OH  +  Br2  =  C6H2Br(N02)2OB  +  HBr. 
C6H2(N02)2OH  +  Br2+  H2O  =  C6H2Br(NO2)2OH  +  HBr  +  HNO3. 
both  reacting  to  form  bromo-dinitrophenol  and  hydrobromic  acid,  but  picric 
acid  yielding  also  nitric  acid.    The  determination  of  the  nitric  acid  is  the  basis 
of  a  method  for  the  organic  mixture. 

In  volumetric  analysis,  a  mixture  of  two  bodies  reacting  in  unlike  ratios  to 
the  titrand  may  be  determined  in  one  titration.  In  the  formula  a  is  the  volume 
in  cubic  centimeters  reacting  with  one  gram  of  X,  and  b  and  d  the  correspond- 
ing volumes  for  Y  and  the  mixture.  An  example  is  the  titration  of  a  mixture 
of  potassium  and  sodium  hydrates  by  a  standard  acid. 

The  combining  weights  of  organic  acids  and  bases  are  often  made  the  basis 
of  their  determinations.  Thus  the  saponiflcation  equivalent  (page  240)  of  oils; 
usually  saponiflable  fats  and  oils  have  nearly  the  same  equivalent,  but  the 
mineral  and  rosin  oils  have  comparatively  low  figures,  and  mixtures  of  one  of 
these  with  a  fat  oil  may  be  quite  accurately  determined.  The  proportions  of 
one  of  the  elementary  constituents  of  organic  bodies  may  sometimes  be  used, 
but  as  a  rule  in  such  analogous  bodies  as  are  likely  to  be  found  together  there 
is  so  little  variance  in  the  proportions  of  any  one  element  that  the  results  are 
but  approximations. 


QUANTITATIVE   CHEMICAL   ANALYSIS.  173 

Solubility.  The  proportions  of  a  mixture  can  be  calculated  from  the  coeffi- 
cients of  solubility  at  a  given  temperature  in  a  simple  solvent  or  a  solution  of 
a  given  concentration.  Obviously,  this  method  is  more  practicable  with 
solvents  that  exhibit  an  apparent  chemical  reaction  with  the  bodies  than  if  the 
solution  is  but  a  simple  one,  since  with  the  latter  a  prolonged  digestion  is 
needed.  In  any  case  the  process  is  empirical,  as  the  solvent  power  of  a  liquid 
for  one  body  is  altered  by  the  presence  of  the  other. 

By  the  rate  of  solubility  of  a  third  body  In  a  mixture  of  two  liquids  simple  or  complex. 
As  examples,  the  difference  In  solubility  of  dry  clnchonine  in  alcohol  and  in  chloroform 
Is  a  basis  for  the  determination  of  small  amounts  of  the  former  liquid  in  the  latter;  zein 
(a  proteid  of  maize)  is  practically  insoluble  in  water  and  in  alcohol,  but  dissolves  in  a  mix- 
ture of  the  two  according  to  their  relative  proportions.  Isoterebenthene  absorbs  (at  24  o 
and  724  Mm.  of  mercury)  34  per  cent  of  gaseous  hydrochloric  acid,  while  metaterebenthene 
absorbs  only  17.7  per  cent  under  the  same  conditions;  similarly,  one  volume  of  oil  of  tur- 
pentine absorbs  7.5  volumes  of  ammonia  at  16 o  Cent.,  while  one  volume  of  oil  of  lavender 
absorbs  49  volumes.  Glycerol  mixes  with  water  to  a  clear  liquid  in  all  proportions,  but  if 
to  commercial  glycerin  containing  water  is  added  a  certain  weight  of  anhydrous  phenol, 
the  mixture  becomes  turbid  on  the  addition  of  water  beyond  a  certain  proportion,  the 
amount  varying  inversely  with  the  water-content  of  the  original  glycerin.  A  mixture  of 
acetic  and  formic  acids  boiled  withwater  and  yellow  mercuric  oxide  takes  up  mercury 
equivalent  to  the  acetic  acid  only. 

Two  liquids  immiscible  or  nearly  so  at  ordinary  temperatures  and  pressures  coalesce 
to  a  homogeneous  mixture  at  a  given  higher  temperature  and  pressure  called  by  Cris- 
mer  "  the  critical  temperature  of  dissolution."  He  has  applied  the  principle  to  the  de- 
termination of  mixtures  of  certain  fats. 

6.  Where  a  chemical  reaction  originates  a  measurable  physical  attribute  in 
one  or  both  members.  The  most  common  of  these  are  the  exothermic  reactions 
of  various  organic  bodies  when  treated  with  certain  reagents,  such  as  concen- 
trated sulfuric  acid,  sulfur  chloride,  or  bromine.  Where  the  specific  rise  in 
temperature  is  great  enough  to  be  measured  by  an  ordinary  thermometer 
the  process  furnishes  a  simple  practical  test,  though  the  results  are  but 
approximate  at  best.  An  example  is  the  determination  of  monomethylamin  in 
commercial  dimethylanilin;  on  treatment  with  an  equal  volume  of  acetic 
anhydride  there  is  a  rise  of  about  .82  o  Cent,  for  each  unit  of  monomethylamin 
in  the  mixture;  but,  as  in  most  other  mixtures,  the  constant  applies  only 
through  a  limited  range  in  the  proportions  of  the  mixtures  and  in  the  absence 
of  certain  common  associates. 

The  heat  of  combustion  of  the  animal  and  vegetable  oils  ranges  from  8835  to  10797 
calories  as  determined  in  the  Atwater-Blakeslee  calorimeter,  the  sperm,  rosin  and  mineral 
oils  showing  considerably  higher  values  than  the  fatty  oils.* 

When  for  any  reason  the  chemical  reaction  is  slow  or  incomplete  with  the  proportions 
of  the  constituents  of  a  given  mixture,  the  observation  can  often  be  facilitated  by  incor- 
porating a  weighed  amount  of  one  constituent  or  a  passive  diluent.  The  calculation  is 
then  as  follows:  given  the  weight  of  the  original  mixture  W;  the  weight  of  the  diluent 
added.ro;  and  the  constant  of  the  compounded  mixture  d';  then  Y',  the  percentage  of  B 

fir  a 

in  the  latter  is  found  from  the  equation  F'  =  100  ~b_a;  and  F,  the  percentage  of  B  in  the 

original  mixture,  is  F=  F'  w+w. 
W 

*  Journ.  Amer.  Chem.  Socy.  1901—170. 


174  QUANTITATIVE   CHEMICAL   ANALYSIS. 


CHAPTER  8. 

THE  CALCULATION  OF  ANALYSES. 

The  results  of  analyses  are  expressed  decimally  for  uniformity  and  ease  of 
comparison  with  others  of  the  same  kind.  A  few  exceptions  are  met  with,  as 
in  reporting  an  assay  of  gold  ore,  where  the  value  in  dollars  per  ton  of  ore  is 
given,  or  a  natural  water  in  grains  per  gallon  or  parts  per  million. 

Percentages  always  refer  to  100  parts  by  weight  of  a  solid,  and  also  of  a 
liquid  unless  volumes  are  specified.  Should  the  constituents  of  a  liquid  be 
stated  as  grams  per  liter  or  grains  per  gallon,  the  specific  gravity  of  the 
liquid  should  also  be  recorded  to  allow  a  recalculation  to  weight,  if  desired 
for  comparison  with  other  analyses  so  expressed.  Many  commercial  solutions, 
however,  have  practically  the  same  gravity  as  water,  and  others  vary  but  little 
from  a  well-known  specific  standard. 

Gas  analyses  are  stated  either  as  volumes  per  hundred  or  in  percentages  by 
weight  or  both.  Either  one  can  easily  be  calculated  from  the  other. 

In  reporting  a  result  it  is  the  custom  to  follow  the  general  rule  of  investiga- 
tors that  only  the  extreme  right-hand  digit  may  be  inaccurate.  This  in  most 
analyses  is  considered  to  be  the  second  beyond  the  decimal  point,  though  often 
the  tenth  or  even  the  unit  figures  may  not  be  above  suspicion.  On  the  other 
hand,  if  the  proportion  of  a  constituent  be  very  small  and  the  method  for  its 
determination  exceptionally  accurate,  the  result  may  with  propriety  be  extended 
to  as  many  as  four  or  five  places  of  decimals;  e.  <?.,  in  the  following  analysis  of 
bar  lead. 

Arsenic trace  Iron .00496 

Antimony ^ 00347  Zinc 00517 

Silver 00105  Nickel 00125 

Copper 00946  Lead  (by  difference)  99.97464 

But  in  all  ordinary  analyses  of  substances  whose  constituents  are  contained 
in  considerable  proportions,  where  results  are  reported  to  the  fourth  or  fifth 
place  of  decimals,  the  absurdity  warrants  a  suspicion  of  the  competence  of 
the  chemist  if  not  of  charlatanry. 

In  many  analyses  one  is  at  liberty  to  tabulate  the  results  either  as  elements, 
radicals,  molecules  or  compounds,  and  as  these  are  calculated  one  from 
another  by  stoichoimetrical  rules  there  can  be  no  preference  on  the  score  of 
numerical  accuracy.  Some  authorities  would  adopt  the  first  form  exclusively  — 
that  in  every  analysis  there  be  reported  the  elements  uncombined,  as  in  this 
way  the  chemist  inclines  to  no  particular  theory  as  to  the  manner  in  which 
they  are  associated.  As  carrying  out  this  principle  literally  would  mean  the 
exclusion  of  most  proximate  analyses,  we  must  presume  it  intended  to  apply 
only  to  a  single  class  of  analyses  whose  purpose  is  to  fix  the  elementary  con- 
stitution of  pure  chemical  compounds,  and  as  a  protest  against  grouping 
elements  in  a  manner  not  sanctioned  by  modern  theories. 

In  technical  analysis,  where  the  subject  is  a  raw  material  or  its  products  or 
a  commercial  article,  the  adoption  of  such  a  course  would  greatly  lessen  the 
intelligibility  of  the  larger  number  of  analyses,  more  especially  as  they  are 


QUANTITATIVE    CHEMICAL    ANALYSIS.  175 

usually  intended  for  the  information  of  those  seldom  so  versed  in  chemistry 
as  to  be  able  to  infer  the  molecular  structure  from  a  report  of  the  uncombined 
elements.  For  to  the  extent  that  an  analysis  is  to  be  an  arbiter  of  the  fitness 
or  value  for  a  particular  use  or  to  describe  the  general  character  of  a  sub- 
stance, that  form  of  a  report  conveying  this  information  in  the  fullest  and 
clearest  manner  is  certainly  to  be  preferred,  irrespective  of  its  accordance  with 
the  views  held  at  present  regarding  molecular  structure  and  dissociation. 
With  minerals,  ores,  and  inorganic  bodies  generally,  the  metals  and  metalloids 
have  heretofore  been  combined  with  oxygen,  the  halogens,  etc.,  and  this  prac- 
tice is  likely  to  be  retained  for  some  time  to  come,  since  as  one  or  another 
of  these  known  or  assumed  compounds  predominates,  it  will  often  impress  its 
individual  characteristics  and  valuable  or  detrimental  properties  on  the 
substance  of  which  it  forms  a  part. 

All  things  considered,  as  a  rule  it  seems  best,  at  least  in  technical  analyses, 
to  report  the  elements  or  radicals  combined  into  such  molecules  as  exist  in  the 
substance  under  examination,  inferring  this  from  its  origin  or  general  charac- 
ter or  by  qualitative  tests;  lacking  this  information  they  may  be  set  down  as 
ions  or  elements  as  seems  best  suited  for  the  purpose  for  which  the  analysis  is 
to  be  employed. 

If  the  results  of  two  or  more  determinations  of  a  constituent  of  a  material 
agree  fairly  well,  the  average  of  the  two  is  reported.  When  the  two  results 
are  both  greater  or  both  less  than  the  true  content,  the  average  will  be  inter- 
mediate in  accuracy  between  the  two,  while  if  there  is  a  minus  error  on  one  and  a 
plus  error  on  the  other,  the  average  will  usually  be  more  accurate  than  either. 
If,  however,  one  is  known  to  be  slightly  inferior  from  faulty  manipulation  or 
other  causes,  or  where  there  is  a  considerable  difference  between  the  two  and 
in  one  there  is  known  to  have  been  incurred  a  considerable  loss  or  gain,  the 
result  obtained  on  the  other  may  be  reported ;  but  it  is  always  more  prudent  to 
repeat  the  analysis. 

In  averaging  the  results  of  duplicate  determinations  made  on  unequal  weights 
of  material,  the  calculated  percentages  are  added  together  and  the  sum  divided 
by  the  number  of  determinations. 

An  indeterminate  mixture  of  certain  analogous  bodies  may  sometimes  be  obtained  in 
technical  analysis,  and  if  for  any  reason  a  separation  is  not  practicable  or  not  deemed 
of  enough  importance  to  repay  the  time  and  labor  required,  the  mixture  may  be  re- 
ported as  composed  only  of  the  typical  constituent  or  the  one  present  In  the  greatest 
proportion.  Clearly  this  is  permissible  only  with  comparatively  unimportant  constit- 
uents and  where  the  custom  generally  obtains. 


A.  The  computations  to  be  made  in  most  gravimetric  analyses  are  two  in 
number:  first  to  eliminate  from  the  formula  of  the  precipitate  as  weighed  all 
the  elements  except  those  comprised  in  the  compound  sought,  and  second,  to 
reduce  this  remainder  to  parts  per  hundred  of  the  substance  analyzed.  The 
following  examples  show  this  to  be  a  very  simple  matter. 

Having  weighed  1.203  grams  of  a  speiss  and  by  the  routine  of  analysis  ob- 
tained all  the  arsenic  in  a  precipitate  which  after  Ignition  has  the  formula 
AS2P2O7  and  weighs  .5040  grams,  to  calculate  the  weight  and  percentage  of 
arsenic  in  the  speiss. 

From  the  formula  we  find  the  molecular  weight  by  multiplying  the  number  of 
atoms  of  each  element  by  its  atomic  weight: 

Arsenic 2  atoms  X  75  =  15° 

Phosphorus 2  atoms  X  31=  62 

Oxygen 7  atomsX16=112 

Molecular  weight , 324 


176  QUANTITATIVE    CHEMICAL    ANALYSIS. 

That  is,  in  324  parts  of  the  precipitate  there  are  150  parts  of  arsenic,  62  parts 

of  phosphorus,  and  112  parts  of  oxygen.     And  from  the  proportions, 

324  :  150  :  :  .504  grams  As2P207  :  X.     X=  .2333  grams  As, 

"    :    62  :  :  "  :  I".     Y=  .0965       "       P. 

"    :  112  :  :  "  :  Z.     Z  =  .1742       "       O. 


.5040       « 
To  reduce  the  above  to  percentage  : 

1.203  grams  speiss  :  .2333  grams  As  :  :  100  per  cent  :  F. 
F  =  19.39  per  cent  of  arsenic  in  the  speiss. 

In  general,  if  a  is  the  weight  in  grams  of  the  substance  taken  for  analysis;  6, 
the  weight  of  the  precipitate  ;  c,  the  molecular  weight  of  b  ;  d,  the  molecular 
weight  in  b  of  the  compound  sought;  and  X,  the  percentage  in  a  of  the  com- 

100.6.  d 
pound  sought;  then  JT=  - 

The  calculation  may  be  somewhat  abbreviated  by  the  use  of  the   decimal 

equivalent  for  —  ,  of  which  lists  for  the  commoner  elements  are  to  be  found 

c  . 

in  books  of  chemical  tables.*    Or  when  several  analyses  of  one  kind  are  in 

hand,  by  making  a  =  -,  X  will  equal  100  b.    As  oftentimes    this    weight    of 
c 

sample  ia  smaller  than  desirable  it  may  be  doubled  or  tripled  and  b  divided  by 
two  or  three  accordingly.  Logarithms  miy  be  used  for  the  calculations,  though 
here  their  advantage  is  not  very  evident  —  (log  d  -f-  log  6)  —  (log  c  -f-  log  a)  = 

log  X;  or,  log  ~  +  log  b  —  log  a  =  log  X. 

c 

The  element  or  compound  to  be  determined  may  not  be  comprehended   in 
the  precipitate  weighed,  as  in  the  example  given  on  page  13,  where  the  follow- 
ing reactions  are  involved  : 

2(C6H5OH)        +2H2S04  =  2(C6H5HS04)    +  2H2O  ...........  1. 

2(C6H5HSO4)    +  BaCO3  =Ba(C6H6SO4)2  +  H2OO3  ..........  2. 

Ba(C6H5SO4)2  +  Na2C03  =  Na2(C6H5SO4)2  +  BaCO3  ..........  3. 

The  calculation  is 

A.  BaCOs,  molec.  wt.  197.4  contains  Ba,  molec.  wt.  137.4. 

B.  Ba,  molec.  wt.  137.4  combines  with  (C6H5SO4)2  giving  Ba(C6H5SO4)2  molec. 

wt.  483.62. 

C.  Ba(C6H5SO4)2  molec.  wt.  483.62  is  formed  from  2(C6H5HSO4),  molec.  wt. 

348.236. 

D.  2(C6H5H304)  molec.  wt.  348.236  is    formed  from  2(C6H6OH),  molec.   wt. 

188.096. 
Hence, 

E.  197.4       :      137.4  :  :  weight  of  BaC03  :  weight  of  Ba  =  x. 

F.  137.4       :       483.62  :  :  x  :  weight  of  barium  phenolsulfonate  =  y. 

G.  483.62     :       348.236  :  :  y   :  weight  of  phenol-sulfonic  acid  =z. 
H.     348.236   :      188.096  :  :  0  :  weight  of  phenol. 

But  in  each  of  the  three  equations  we  see  that  two  CeHs  groups  are  con- 
cerned, hence  for  the  proportions  E,  F,  G,  and  H  may  be  substituted  the 
simple  one  — 

Molecular  wt.  of  BaCO3  (197.4)  :  twice  the  molec.  wt.  of  phenol  (188.096} 
:  :  weight  of  BaC03  found  :  weight  of  phenol  sought. 

Assaying.  If  an  assay-ton  (page  41)  of  ore  is  taken  for  the  assay  each  mil- 
ligram of  metal  obtained  represents  one  ounce  per  ton  of  2,000  pounds,  or  if  a 


*  Journ.  Amer.  Clietn.  Socy.  1896—903. 


QUANTITATIVE    CHLMICAL   ANALYSIS.  177 

grams  are  taken,  then  29.167  times  the  weight  of  gold  or  silver  divided  by  a 
equals  the  ounces  per  ton.  In  assaying  separately  the  malleable  scales  that 
refuse  to  pass  the  finer  selves  and  the  powder  passing  through,  the  calcula- 
tions should  be  made  as  for  two  separate  ores  and  the  sum  of  the  results 
figured  to  the  sum  of  the  weights  of  scales  and  sittings. 

In  taking  a  given  volume  of  a  liquid  for  analysis  it  is  often  more  convenient 
to  measure  than  to  weigh,  but  here  the  specific  gravity  and  temperature  must 
be  taken  into  consideration.  The  weight  in  grams  is  the  pro  duct  of  the  volume 
in  cubic  centimeters  multiplied  by  the  specific  gravity  of  the  liquid,  both  at  the 
temperature  of  the  experiment. 

At  times,  in  deciding  the  value  of  a  commercial  article  by  comparison  of  an  analysis 
with  others  of  the  same  material,  some  exogenous  or  variable  factor  must  be  eliminated  or 
introduced,  such  as  moisture  acquired  from  the  air  or  rain.  The  following  analyses,  A 
and  B,  of  two  shipments  of  malt,  are  not  directly  comparable  since  one  refers  to  the  mate- 
rial in  a  moist  condition  as  received,  the  other  after  drying;  hence  either  A  is  changed 
to  the  form  of  B  (as  in  a)  by  proportionally  increasing  the  percentage  of  each  element 
by  dividing  by  the  difference  between  the  total  and  the  percentage  of  the  moieture  to  be 
eliminated;  or  B  to  the  form  of  A,  as  in  b,  through  multiplication  by  the  same  remainder. 

A.  B.  a.  b. 

Sugar  and  carbohydrates  18.03  24.48  19.44  22.06 

Starch 48.22  48.58  52.00  43.77 

Cellulose,  etc 9.21  9.32  9.95  8.40 

Proteids 11.92  11.00  12.86  9.91 

Fat 1.81  1.29  1.95  1.16 

Ash 2.97  4.24  3.20  3.82 

Moisture 7.28  9.90 

Undetermined  and  loss 56  1.09  .60  .98 


100.00       100.00       100.00       100.00 
Moisture 9.90          7.28 

B.  The  percentage  composition  of  a  substance  is  derived  from  its  formula 
by  dividing  the  product  of  the  number  of  atoms  of  each  element  times  its 
atomic  weight  by  the  molecular  weight  of  the  compound  and  multiplying  the 
quotients  t  by    100.    For    example,    the  percentage  composition  of    camphor 
(QsoHieO)  is  deduced  — 

Carbon,       10  X  12.000  =  120.000  X 100  -5-  152.128  =  78.88  per  cent 
Hydrogen,  16  X    1.008=    16.128  X  "  -*•       "       =10.60        " 
Oxygen,         1 X  16.000=  16. 000  X  "  -*-       "       =10.52        « 

Molecular  weight,  162.128  100.00        «« 

C.  The  empirical  formula  of  a  chemical  compound  Is  deduced  from  an 
analysis  by  dividing  the  percentage  of  each  element  by  its  atomic  weight,  and 
the  quotients  by  their  greatest  common  divisor;  thus: 

Carbon,        78. 88-*- 12.       =  6. 573 -5- .657  =  10  atoms. 
Hydrogen,   10.60-*-   1.008=10.516-5-    "    =16      tf 
Oxygen,        10.52 -,-16.       =     .657-=-    "    =   1      •< 
Hence  the  formula  CioHieO. 

Where  isomorphous  elements  replace  those  constituting  the  compound,  they 
are  first  eliminated,  as  in  the  following  example  of  a  native  silicate. 

Silica 62.38      Magnesia 10.37 

Titanic  acid 24      Lime * 12 

Alumina 26.29      Combined  water 50 

Ferric  oxide 15  

Total 100.05 

12 


178  QUANTITATIVE    CHEMICAL   ANALYSIS. 

It  might  be  inferred  from  the  analysis  that  the  mineral  is  a  hydrated  silicate 
and  titanate  of  alumina,  lime,  magnesia  and  ferric  oxide,  but  in  view  of  the 
relatively  minute  proportions  of  water  and  the  oxides  of  iron,  calcium  and 
titanium,  we  are  justified  in  regarding  them  as  incidental  only,  and  the  mineral 
as  an  anhydrous  compound  of  silica,  alumina,  and  magnesia,  the  ferric  oxide 
replacing  alumina,  the  lime,  magnesia,  and  the  titanic  acid,  silica. 

With  this  view  we  first  eliminate  the  water  by  using  as  a  divisor 
100.05—  .50 


100 


=  .9955. 


Silica 62.38  -^-  .9955  =  62.66 

Titanic  acid 24-=-     "    =     .24 

Alumina 26.29-j-     "     =26.41 

Ferricoxide 15-s-     "    =      .15 

Magnesia 10.37--     "    =10.42 

Lime 12-n      «    =      .12 


99.65  100.00 

Next  we  find  how  much  alumina  the  .15  per  cent  of  ferric  oxide  replaces  and 
substitute  it  therefor : 

Fe2O3   (mol.  wt.  160)    :  A12O8  (mol.   wt.    102.2)    :  :  .15   :  X.     X  =  .09,   an 
26.41 +  .09  =  26.50. 

Similarly  we  find  the  silica  increased  to  62.85,  and  the  magnesia  to  10.50 
The  analysis  now  stands 

Silica,  62.85 
Alumina,  26.50 
Magnesia,  10.50 


99.85 
Dividing  the  above  by  the  molecular  weights  : 

Silica  62.85  -*-  60.40  =  1.041 
Alumina  26.50  --  102.20  =  .259 
Magnesia  10.50  -=-  40.30  =  .261 

The  empirical   formula   is  therefore  (Al2O3).259CMgO).26i(SiO2).i.04i;   or  di- 
viding by  .260  and  allowing  for  the  errors  of  analysis,  AlsO3.MgO.4SiO2,  or 


A  rational  formula  can  only  be  deduced  after  certain  additional  data  have 
been  obtained.* 

D.  An  indirect  analysis  is  applicable  for  material  wherein  two  elements  whose 
atomic  weights  differ  considerably  each  combines  with  a  third  element  or  com- 
pound. The  calculation  may  be  illustrated  as  follows  :  — 

If  we  weigh  one  gram  of  metallic  lead  and  (by  solution  in  nitric  acid,  evapo- 
ration and  ignition)  convert  it  to  lead  protoxide,  we  will  find  the  weight  of  the 

latter  to  be  1  X  '       =  1  .0773  grams  ;  similarly,  one  gram  of  magnesium 

206.92 

forms  1  X  2^  3  =  1  fifiSA  grams  of   magnesium  oxide.    An  alloy  of  one 

24.3 

gram  of  each  metal  would  give  2.7357  grams  of  the  mixed  oxides,  and  with 
other  proportions  of  the  metals  we  would  find  — 


*  lloecoe  &  Schorlemmer's  Chemistry,  3—1—84. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  179 

Metals.  Oxides. 

Grams  Pb  -f  Grams  Mg  =  Total.  Grams  PbO  -f-  Grams  MgO  =  Total. 

2.000  None  2.000  2.1546  None  2.1546 

1.999  .001  .      "  2.1536  .0017  2.1553 

1.998  .002  "  2.1525  .0033  2.1558 

1.997  .003  "  2.15U  .0050  2.1564 

1.996  .004  "  2.1503  .0066  2.1569 

and  so  on  to 
None  2.000  "  None  •  3.3168  3.3168 

Notice  that  the  weight  of  the  mixed  oxides  increases  directly  and  uniformly 
with  the  weight  of  the  magnesium  in  the  alloy  and  inversely  with  that  of  the 
lead. 

The  above  table  might  be  extrapolated  to  show  the  exact  proportion  of  each 
metal  for  every  weight  of  the  oxides,  since  a  given  weight  of  oxide  can  be 
produced  from  only  one  combination  of  metals,  but  this  is  superfluous  since  a 
simple  calculation  will  answer  the  same  purpose  —  for  example,  given  the 
weight  of  the  alloy  2  grams,  and  that  of  the  oxides  2.3289  grams;  the 
difference,  .3289,  is  oxygen,  and  since  2  grams  of  lead  combine  with  only 
.1546  of  oxygen,  the  excess  of  .1743  gram  is  due  to  the  presence  of  a  certain 
weight  of  magnesium  oxide.  Now,  since  1  gram  of  lead  combines  with  .0773 
of  oxygen  and  1  gram  of  magnesium  with  .6584  of  oxygen,  the  difference, 
.5811,  is  to  1  gram  of  magnesium  as  .1743  is  to  the  weight  of  magnesium  in  the 
alloy,  that  is  to  .300  gram. 

In  general,  if  Xis  the  percentage  of  one  metal  A  in  the  alloy,  and  T  the 
percentage  of  the  other,  B;  a,  the  weight  of  the  oxygen  combining  with  100 
grams  of  the  metal  A;  b,  the  oxygen  combining  with  100  grams  of  the  metal  B; 
and  dt  the  found  weight  of  oxygen  combined  with  100  grams  of  the  alloy; 
then  — 

X=100  d~b  and  F=100  d~~a   =  100  —  X. 
a  —  b  b  —  a 

It  will  be  seen  that  the  greater  the  difference  between  the  atomic  weights  of 
the  metals  concerned,  the  less  is  the  correctness  of  the  result  affected  by 
errors  of  analysis. 

The  same  principle  Is  applied  to  a  mixture  of  potassium  and  sodium  chlorides  by  deter- 
mining the  chlorine,  to  barium  and  calcium  carbonate,  etc.  Landls  *  gives  the  following 
general  formulae. 

Let  fFbe  the  weight  of  a  mixture  of  two  salts;  w  that  of  the  constituent  common  to 
both  ;  x  the  weight  of  salt  containing  the  greatest  amount  of  common  constituent,  and  v 
that  containing  the  least;  a  the  weight  of  common  constituent  in  one  gram  of  a;,  and  6  that 
in  one  gram  of  y: 

r      W-x 


Or  if  c  is  the  molecular  weight  of  x,  and  d  that  of  y  ;  and  V  the  calculated  weight  were 
all  the  common  constltutent  combined  with  the  base  of  Y: 

V—  W 
Then  #=  c  d  _  c  ,  andy  =  W—  x. 

A  variation  in  practice  is  that  of  determining  the  weights  of  the  common  constituent 
added  and  of  the  product.  Thus,  a  solution  of  potassium  chloride  and  potassium  bromide 
is  titrated  by  silver  nitrate,  requiring  of  the  titrand  a  volume  containing  a  grams  of 
metallic  silver;  the  precipitated  silver  chloride  and  bromide  is  weighed,  and  the  calcula- 

tion made  from  these  data.    The  formula  X=  100  —  ^-  applies.  X  being  the  percentage 

a  —  o 

of  potassium  chloride  in  the  mixture  ;  a,  the  calculated  weight  of  chlorine  in  the  substance 
were  it  all  potassium  chloride  ;  b,  the  calculated  weight  of  bromine  in  the  substance  were 


Journ.  Amer.  Chem.  Socy.  1896—182. 


180  QUANTITATIVE    CHEMICAL    ANALYSIS. 

It  all  potassium  bromide;  'and  d,  the  difference  between  the  weight  of  the  precipitate  and 
the  weight  of  silver  used  for  precipitation. 

The  principle  may  be  extended  to  three  elements  combining  with  a  fourth,  as  chlorine, 
bromine  and  iodine.    A  solution  containing  the  elements  Is  divided  Into  three  equal  parts 
and  each  precipitated  by  silver  nitrate.    One  precipitate,  made  up  of  AgCl  +  AgBr  +  Agl 
is  weighed;  the  second  weighed  after  digestion  with  sodium  bromide  by  which  the  chlo- 
rine is  replaced  by  bromine  while  the  Agl  is  unaffected;  and  the  third  after  digestion  with 
sodium  iodide  which  leaves  the  precipitate  entirely  composed  of  silver  iodide. 
Letarbe  the  weight  of  AgCl,  and  a  Its  molecular  weight. 
V  .  "  AgBr,    "    6 

z  Agl,       "    c 

iv  "  Agl  +  AgBr  +  AgCl  =  x  +  y  +  z. 

W  Agl  +  AgBr  +  AgBr. 

w"  "  Agl  +  Agl  +  Agl, 

Then 

a(w'  —  w)  b(w"  —  w'}         b(w'  —  w}  b(w"—w') 

*=—  &_0      J  y=—c—b  -  -  ~~b-a      '•  and*=«;'-        c_6 

And  generally,  when  two  bodies  or  constituents  thereof  have  a   common 
physical  attribute  varying  with  the  conditions  of  temperature,  etc.,  the  measure 
of  the  attribute  in  a  mixture  will  serve  as  a  basis  for  calculating  the  propor- 
tions of  the  bodies.    If  X  is  the  percentage  of  one  body  A;  Y,  that  of  the 
other  B;    a',  a",  a'"  ........  ,  the  values  of  a  constant  of  A  under   different 

conditions;  &',    b"t  b'"  ........  ,  the  corresponding  values  of  B;  a,nd  d',  d", 

d'"   ..,....,  the  corresponding  values  of  the  mixture;  then  X  :   Y  :  :  b'  — 

d'  :  d'--a'  :  :  b"  —  d"  :  d"  —  a"  :  :  etc. 

A  mixture  containing  among  other  constituents  two  having  a  common 
element  or  compound;  the  proportions  of  the  two  are  calculated  from  the 
formulae  — 

100  (d  —  b)  +  bZ,  100  (d  —  d) 

X=  -  --  andF  =  -- 


where  Xand  Yare  the  percentages  of  the  two  constituents  and  Z  the  percent- 
age of  the  remainder  of  the  mixture  ;  a  and  6,  the  proportions  of  the  common 
element  in  the  two  constituents,  and  d  the  proportion  in  the  mixture  as 
found  by  analysis.  For  example,  a  mixture  of  potassium  chromate,  potassium 
bichromate,  and  17  per  cent  of  other  alkali  salts,  analyzed  12.19  per  cent  of 
available  oxygen;  since  potassium  chromate  contains  12.35  per  cent,  and 
potassium  bichromate  16.30  per  cent  of  available  oxygen,  the  percentage  of  the 
former  compound  was  33.80,  and  of  the  latter  49.10  per  cent. 

E.  Volumetric  analysis  —Normal  solutions.  To  prepare  a  normal  solution 
of  sodium  chloride  (58.50  grams  per  liter),  about  60  grams  of  common  salt  was 
dissolved  in  a  liter  of  water,  and  20  Cc.  of  the  solution  precipitated  by  silver 
nitrate  — 

NaCl  (58.50)  +  AgNOs  (169.96)  =  AgCl  (143.37)  +NaNO3. 

giving  2.940  grams  of  AgCl  ;  at  this  rate  1000  Cc.  would  give    1(^00  X  2-940  =  1*7 

grams,  and  the  proportion  of  NaCl  in  solution  is  therefore  59.980  grams  since 
143.37  :  58.50  :  :  147  :  59.980. 

A  normal  solution  should  contain  58.500  grams  NaCl. 

This  solution  contains  59.981      "         " 

There  is  in  excess  therefore  1.481      u         " 

We  may  consider  that  we  have  a  liter  of  strictly  normal  solution  containing 
an  additional  1.481  grams  of  NaCl,  and  the  problem  is  to  find  in  how  much 
water  this  excess  is  to  be  dissolved  to  make  it  of  normal  strength  —  in  other 
words,  how  much  water  is  to  be  added  to  the  liter  to  compensate  for  this  extra 
amount  of  NaCl,  It  is  25.3  Cc.,  since  58.50  :  1000  :  :  1.481  :  25.3. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  181 

But  a  portion  of  the  original  solution  was  used  in  making  the  test  by  silver 
nitrate,  so  from  the  remainder  of  the  liter  is  measured  say  950  Cc.,  and  24  Cc. 
of  water  added  to  it,  since  1000  :  950  :  :  25.3  :  24. 

Should  the  original  solution  prove  weaker  than  normal  instead  of  stronger, 
the  proper  additional  weight  of  salt  to  be  dissolved  may  be  ascertained  by  a 
similar  calculation. 

It  is  often  convenient  to  make  up  two  solutions,  one  slightly  stronger,  the 
other  slightly  weaker  than  normal  and  mix  them  in  the  proper  proportions; 
this  avoids  in  great  measure  the  concentration  that  would  follow  the  dilution 
of  a  strong  solutiou  with  water  and  the  correction  therefor.  The  formula 

X  =  1000 =•  applies,  where  X  is  the  volume  of  the  stronger  solution  and 

1000  —  X  that  of  the  weaker  to  be  united ;  a,  the  litre  of  the  stronger  solution 
and  b,  that  of  the  weaker;  and  d,  the  concentration  of  a  strictly  normal  solution. 
A  strong  solution  may  be  diluted  with  water  according  to  this  formula,  6  being 
of  course  zero. 

In  most  cases  it  will  answer  all  purposes  to  make  volumetric  solutions  only 
approximately  normal  and  allow  for  the  variation  in  the  calculation  of  results. 
The  corresponding  volume  of  a  strictly  normal  solution  is  found  by  multiplying 
the  volume  used  in  a  titration  by  a  factor  obtained  by  dividing  the  actual  con- 
tent by  the  normal  content. 

If  m  represents  the  weight  of  a  pure  reagent  to  be  dissolved  in  a  liter  of  water  to  form 
a  strictly  normal  solution  S,  then  ~  grams  will  exactly  combine  with  100  Cc.  of  any  other 

normal  solution  S'  reacting  with  it;  but  should  the  reagent  contain  impurities  (of  such  a 
nature  as  not  to  react  with  S')»  then  the  number  of  cubic  centimeters  of  S'  required  ex- 
presses directly  the  percentage  of  the  pure  reagent.  And  in  practice-^  grams  of  a  com- 
mercial substance  may  be  dissolved  and  titrated ;  e.  g.,  when  10.792  grams  of  a  silver-copper 
alloy  is  dissolved  in  nitric  acid  and  titrated  by  normal  sodium  bromide,  the  number  of 
cubic  centimeters  gives  the  percentage  of  silver  without  further  calculation ;  though  for 
practical  reasons  it  would  be  better  to  dissolve  1.0792  grams  and  titrate  by  a  decinormal 
solution. 

For  standardizing  some  solutions  it  may  be  an  advantage  to  liberate  the  reacting  body 
in  situ  just  before  the  titration  by  means  of  a  second  standard  solution.  Thus,  for  stand- 
ardizing sodium  thiosulfate  solution  by  iodine  (page  129),  instead  of  weighing  the  pure 
iodine,  to  a  solution  of  an  excess  of  potassium  iodide  is  added  a  measured  volume  of 
standard  potassium  bichromate  whose  strength  in  terms  of  iron  is  known.  The  weight  of 
the  reacting  body  liberated  is  calculated  as  follows:  if  one  molecule  of  the  second  solu- 
tion liberates  a  molecular  weight  a  of  the  reacting  body,  and  also  reacts  with  a  mole- 
cular weight  6  of  the  standard;  and  if  one  Cc.  of  the  second  solution  is  equivalent  to  c 
grams  of  the  standard;  and  the  volume  of  the  second  solution  used  is  V;  then  the  weight 

of  the  reacting  body  liberated  is  VC-^.  Thus,  one  molecule  of  potassium  bichromate  lib- 
erates six  atoms  of  iodine  from  potassium  iodide,  and  also  converts  six  atoms  of  iron  from 
the  ferrous  to  the  ferric  state;  if  one  Cc.  of  standard  bichromate  is  equivalent  to  .0071 

gram  iron,  then  20  Cc.  would  liberate  20  x  .-^X  76  1.1  =  3217 gram  lodlne> 

•86.13 

If  there  be  ascertained  the  relation  between  a  volumetric  solution  and  a  given 
chemical  compound,  from  the  ratio  of  their  combining  weights  may  be  cal- 
culated not  only  the  corresponding  relation  between  the  solution  and  any  radi- 
cal of  the  compound,  but  also  that  of  any  other  combination  into  which  the 
radical  may  enter.  For  example,  if  one  cubic  centimeter  of  a  potassium  per- 
manganate solution  reacts  with  a  grams  of  crystallized  oxalic  acid  (H2C2O4. 

2H20,  126.048),  it  is  also  equivalent  to  a  X  12d'Q68  grams  of  th«  anhydrous  acid 

88 
(H2C204,  90.016)  and  to  a  X  126  Q48  grams  of  the  C2O4  radical.     And  should  a 


182  QUANTITATIVE    CHEMICAL   ANALYSIS. 

precipitate  of  zinc  oxalate  be  decomposed  by  sulfuric  acid  and  the  acid  radical 
titrated  by  permanganate,  each  cubic  centimeter  of  the  permanganate  reduced 

65.4  81  4 

will  represent  a  X  i^Q48  gram  of  zinc  (65A^  or  «  X  grams    of    zinc 


oxide  (81.4). 

Similarly,  if  two  or  more  chemical  compounds  A1  A',  A", react  with  a 

volumetric  solution  S}  and  the  relation  between  S  and  A  is  known,  the  relation 

to  J.',  A" maybe  calculated.    Let  a  be  the  molecular  weight  of  A;  b, 

the  coefficient  of  A  in  the  equation  expressing  the  reaction ;  c,  the  coefficient 
of  the  molecule  of  the  reagent  of  S  in  the  equation;  and  w,  the  weight  of  A 
equivalent  to  one  cubic  centimeter  of  S:  and  let  a',  b',  c',  and  w'  be  the  cor- 
responding values  of  A';  then 

w.a'.b'.c 
a.b.c' 

For  example,  In  the  titrations  of  potassium  nitrite  and  molybdenum  suboxide  by  potas- 
sium permanganate,  the  equations  are 

2K2Mn2O8  +  10KNO2  +  11H28O4  =  lOHNOs  +  7K2SO4  +  4MnSO4  +  6H2O. 
17K2Mn2O8  +  5M012O19  +  61H2SO4  =  60MoOs  +  34MnSO4  +  17K2SO4  +  51H2O. 

If  one  cubic  centimeter  of  permanganate  oxidizes  say  .007  gram  of  KNO2,  it  is  also 
equivalent  to  .00704  gram  of  Moi2Oi9,  since 

.007X1456X5X2 
1     85.15  X  10  X  17      =  •°0704' 

Empirical  standard  solutions.  To  escape  the  tedium  of  lengthy  calculations, 
these  are  made  up  to  express  directly  the  percentages  of  the  body  titrated  by 
the  number  of  cubic  centimeters  of  the  solution.  For  example,  in  the  determi- 
nation of  calcium  bicarbonate  in  the  analysis  of  a  natural  water,  to  prepare  a 
solution  of  such  a  strength  that  each  cubic  centimeter  shall  represent  one  grain 
of  calcium  carbonate  per  U.  S.  gallon  of  water  when  500  Cc.  is  titrated  by  the 
acid  and  lacmoid  indicator. 

One  grain  =  .0648  gram,  and  one  gallon  =  3785  Cc. ;  hence  .0648  :  3785  :  :  X  : 
500.  X=  .00856  gram,  the  corresponding  weight  of  CaCOs  in  500  Cc. 

Also,  CaCO3  (100.1)  :  H2S04  (98.086)  :  :  .00856  :  y.  y  =  .00839,  the  weight  in 
grams  of  H2SO4  in  one  Cc.  of  the  proposed  standard  solution. 

One  liter  of  the  standard  solution  contains  1000  X  .00839  =8.390  grams. 
Hence  8.390  :  49.043  :  :  z  :  1000.  0  =  171  Cc.,  the  volume  of  normal  sulfuric 
acid  containing  8.390  grams  of  H2SO4.  Therefore,  if  171  Cc.  of  normal  sulfuric 
acid  be  diluted  to  one  liter  each  cubic  centimeter  will  represent  one  grain  of 
calcium  carbonate  as  above. 

Residual  titration.  A  weight  of  1.121  grams  of  impure  zinc  oxide  was  treated 
with  50  Cc.  of  hydrochloric  acid,  one  Cc.  containing  .0337  grams  of  HC1,  and 
after  solution,  the  excess  of  acid  was  titrated  by  potassium  hydrate,  requiring 
23.4  Cc.,  one  Cc.  containing  .0498  grams  of  KOH.  What  is  the  percentage  of 
zinc  oxide  in  the  sample? 

50.0  Cc.  of  the  acid  contains  50. OX  .0337  =  1.6850  grams  of  HC1. 
23.4  Cc.  of  the  alkali  contains  23.4  X  .0498  =  1.1653  grams  of  KOH. 

Consider  the  1.6850  grams  of  acid  divided  into  two  parts,  X,  the  weight  neu- 
tralized by  the  zinc  oxide,  and  Y,  the  weight  neutralized  by  the  alkali ;  then, 
since  one  molecule  of  KOH  combines  with  one  molecule  of  HC1, 

56.118  (KOH)  :  36.458  (HC1)  :  :  1.1653  :  Y.     F=.7571  grams,  and      . 
X=  1.6850—  .7571  =  ,9279  grams. 

Two  molecules  of  hydrochloric  acid  combine  with  one  molecule  of  zinc  oxide, 
hence, 


QUANTITATIVE    CHEMICAL    ANALYSIS.  183 

72.916  (2HC1)  :  81.4  (ZnO)  :  :  .9279  :  Z,  the  weight  of  the  zinc  oxide. 
Z X  ICO X  1-121  =  92.42,  the  percentage  of  zinc  oxide  in  the  sample;  it  is 
assumed  that  there  was  no  other  compound  in  the  mixture  capable  of  neutral- 
izing the  acid. 

F.  Colorimetry.  When  there  are  dissolved  the  weights  Wot  a  standard  and 
W  of  a  sample,  containing  respectively  a  and  Xper  cent  of  a  chromogen,  and 

the  solutions  diluted  to  an  equal  depth  of  color,  then  measuring  respectively  V 
and  V  cubic  centimeters,  the  percentage  of  the  chromogen  of  the  sample  is 

a.  V.  W 
found  from  the  formula  JT=   y  ™., 

G.  To  calculate  the  weight  of  a  reagent  required  for  the  solution  or  precipi- 
tation of  a  given  body.    The  equation  of  the  reaction  is  written,  and  from  the 
combining  weights  the  proportion  is  furnished.    If,  in  the  equation,  a  repre- 
sents the  combining  weight  of  the  compound  to  be  dissolved  or  precipitated ; 
b,  the  combining  weight  of  the  reagent ;  c,  the  weight  of  the  sample  taken  for 

analysis;  and  x,  the  necessary  weight  of  the  reagent;  then  x=  a'c 

b 

For  example,  having  weighed  one  gram  of  impure  potassium  chromate  for 
analysis,  and  desiring  to  know  how  much  crystallized  lead  acetate  to  use  for 
precipitating  the  chromic  acid;  from  the  equation  — 

K2Cr04  (194.32)  -f  Pb  (C2H302)2.3aq  (379.016)  =PbCrO4  +  2KC2H302  +  H2O, 
we  see  that  379.016  grams  of  lead  acetate  precipitate  194.32  grams  of  potassium 
chromate.    The  potassium  chromate  in  the  sample  cannot  exceed  one  gram, 

379.016  V  1 
so  that  the  reagent  need  not  be  over  — .    .  .... —  =  1.950    grams  or  about  20 

cubic  centimeters  of  a  ten  per  cent  solution. 

Some  idea  can  generally  be  formed  as  to  the  proportion  of  the  constituent  to 
be  precipitated  and  the  precipitant  reduced  accordingly,  always  allowing  a 
slight  excess. 

H.  Gasometry.  The  volume  of  a  gas  or  mixture  of  gases  is  a  function  of 
its  temperature  and  the  pressure  upon  it,  and  usually  is  measured  at  the  tem- 
perature and  pressure  of  the  surrounding  air  at  the  time  of  reading,  and 
saturated  with  the  vapor  of  water.  It  has  been  agreed,  however,  that  all 
results  of  gas  analysis  shall  be  expressed  in  volumes  of  dry  gas  at  a  temper- 
ature of  zero  Cent,  under  a  pressure  of  760  Mm.  of  mercury.  The  conversion 
is  made  as  follows : 

1.  Aqueous  vapor  exerts  a   pressure  or  tension  varying  directly,  though 
not  uniformly,  with  the  temperature.    At  zero  it  equals  that  of  4.5  Mm.  of 
mercury,  and  at  40  ° ,  54.9  Mm.    A  table  for  each  intermediate  degree  is  given 
under  Tables,  post. 

2.  The  volume  of  a  gas  is  in  inverse  ratio  to  the  pressure  upon  it — that 
is,  the  volume  times  the  barometric  pressure  divided  by  760  gives  the  volume 
at  760  Mm.  of  mercury. 

3.  A  gas  expands   from  zero   l-273rd    or   .00367  of  its  volume    for    each 
degree  Cent,  of  a  rise  in  temperature,  so  that  the  volume  as  read  multiplied 
by  273  and  the  product  divided  by  273  -f- 1°  (the  observed  temperature)  gives 
the  volume  at  zero. 

Hence  the  rules  — 

1.  From  the  observed  barometric  pressure,  less  the  difference  in  height, 
if  any,  between  the  mercury  in  the  trough  and  measuring  tube,  deduct  the 
tension  of  aqueous  vapor  at  the  observed  temperature. 

2.  Multiply  the  remainder  by  the  observed  volume  of  the  gas  and  divide 
the  product  by  760.     The  quotient  is  the  volume  of  dry  gas  at  a  pressure  of 
760  Mm.  of  mercury  at  t  °  . 


184  QUANTITATIVE    CHEMICAL    ANALYSIS. 

3.  Multiply  the  quotient  by  273,  and  divide  the  product  by  273  -+-  t.  The  quo- 
tient is  the  volume  of  dry  gas  at  760  Mm.  and  zero. 

Shortly,  if  F  Is  the  volume  and  d  the  density  of  a  gas  at  t  °  and  b  Mm. 
pressure,  then  the  volume  V  and  the  density  d'  at  zero  and  760  Mm.  — 


>  and  *  =d   ' 

Tables  for  1.00367  t  when  £  is  between  2  °  to  40°  Cent.,  and  for  —  when    b 

760 

is  between  10  and  840  Mm.  may  be  found  in  works  on  gas  analysis.* 
Having  the  analysis  of  a  mixed  gas  expressed  in  volumes  per  hundred,  to 

change  to  percentage  by  weight. 
Multiply  the  percentage  of  each  constituent  by  the  weight  of  one  cubic  cen- 

timeter of  the  gas  under  standard  conditions  of  temperature  and  pressure. 

Divide  each  product  by  their  sum,  and  multiply  the  quotient  by  100. 
Thus, 

Percentage  Percentage 

by  volume.  by  weight. 

Carbon  monoxide  ............  52.23  X  -001251  =  .06534X  100  -s-  .13767  =  47-46 

Carbon  dioxide  ...............  17.10X  .001977=  .  03381  X100-r-  .13767  =  24.56 

Nitrogen  .....................  30.67X  -001256=  .03852  X  100  -r-  .13767  =  27.98 

The  change  from  weight  to  volume  is  made  by  a  similar  calculation. 

I.  The  specific  gravity  of  a  mixture  of  two  bodies  which  coalesce  without 
inter-reaction  varies  directly  with  their  proportions  and  serves  to  determine 
the  percentage  of  each,  the  more  accurately  the  greater  the  difference  between 
their  respective  densities.  If  it  be  known  that  any  two  mix  without  change  in 
volume,  in  other  words,  that  the  curve  whose  ordinates  represent  units  or 
decimals  of  specific  gravity,  and  abscissae  the  relative  percentages  in  the  mix- 
ture, is  a  straight  line,  the  gravity  of  the  mixture  is  expressed  by  the  equation 

VD'  4-  V"D" 
D=  -     '     yn  —    where  V  and  V"  are  the  respective  volumes  of  the  com- 

ponents ;  D  '  and  D  ",  their  respective  densities  ;  and  Z>,  the  density  of  the 
mixture.  Conversely,  when  the  gravity  of  the  mixture  and  the  densities  of  the 
components  are  known,  the  volumes  composing  the  mixture  are 


And  the  weight  of  one  component  is  found  by  the  formula 


W  =  D'  AD  "  r,,  ~~      •»  -4  being  the  weight  of  the  mixture  in  air,  and 

By  the  weight  in  water  (applied,  e.g.,  to  mixtures  of  minerals,  as  for  gold  in 
quartz,  galena  incalcite,  etc.). 

For  aqueous  solutions  of  metallic  salts  a  specific  formula  may  be  deduced 
from  the  general  one  D  =  1  -{-  ap  -f  bp2  —  cp3:  for  example  one  for  potas- 
sium chloride  D  =  1  +  .0062170  -f  .00003574p2_Q0000018p3,  in  which  p  is  the 
percentage  of  the  salt  in  solution,  and  a,  6,  and  c  are  empirical  coefficients 
determined  by  experiment. 

Formulae  for  the  calculation  of  specific  gravity  by  different  methods  are 
given  below.  The  principles  will  be  readily  understood  by  bearing  in  mind 
that  specific  gravity  is  the  ratio  of  the  weight  of  a  body  to  that  of  an  equal 
volume  of  water  or  other  standard  taken  as  unity,  and  that  a  solid  immersed 
in  a  liquid  loses  the  weight  of  an  equal  volume  of  the  liquid. 


*  Biedermann'H  Chemiker  Kalander,  15. 


QUANTITATIVE    CHEMICAL   ANALYSIS.  185 


SOLIDS. 

A  solid  Insoluble  In  ,water  and  of  greater  gravity.    Specific  gravity  =-  G. 

Weight  of  the  solid  In  air  =  A.  A      = 

Weight  of  the  solid  in  water  =  B.  A  —  B 

By  Nicholson's  aerometer. 

Weight  to  be  added  to  upper  pan  to  sink  hydrometer  to  the  mark  =  A. 

Substance  In  the  upper  pan,  the  weight  In  upper  pan  =  B. 

Substance  in  lower  pan,  the  weight  in  upper  pan  =  C. 

By  the  specific  gravity  bottle  or  Hogarth's  flask,  etc. 

Weight  of  substance  In  air  =  A.  A 

Weight  of  flask  and  water  =  B.  A  ^  £_c  =  O. 

Weight  of  flask,  substance  and  water  =  C. 

A  solid  Insoluble  in  water  but  lighter. 

Weigh  a  heavier  piece  of  metal  in  water,  attach  to  the  solid  and  weigh  both  In  water. 

Weight  of  the  substance  In  air  =  A.  A 

Weight  of  the  substance  and  metal  In  water  =  B.          ~A  —  B  +  C  =  G' 

Weight  of  the  metal  In  water  =  C. 

A  solid  heavier  or  lighter  than  water,  but  soluble  therein.  Select  another  fluid  of 
known  specific  gravity,  lighter  than  the  solid  and  having  no  solvent  actionuponitje.gr. 
an  oil. 

Weight  of  the  solid  In  air  =  A. 

Weight  of  the  solid  In  oil  =  B.  AS    _ 

Specific  gravity  of  water  =  1.  A  —  B 

Specific  gravity  of  oil        =  S. 

LIQUIDS. 

By  the  specific  gravity  flask. 
.    Weight  of  flask  alone  =  A.  C—A 

Weight  of  flask  and  water          =  B.  B—  A  =  G' 

Weight  of  flask  and  the  liquid  =  C. 
By  weighing  a  solid  In  water  and  the  liquid 
Weight  of  the  solid  in  the  air      =  A.          C  —  A 
Weight  of  the  solid  in  water        •=  B.          B^A  =  Gn 
Weight  of  the  solid  In  the  liquid  =  C. 

GASES.    (AIR  =  1.) 

Weight  of  globe  filled  with  dry  air  =  A. 
Weight  of  vacuous  globe  =  B. 

Weight  of  globe  filled  with  dry  gas  =  C. 

^?  =  density  of  dry  gas  at  t°  Cent,  and  6  Mm.  barometer. 

-4  —  x> 

§^l  X  Vx  ^0012934*  x  IT  =  density  at  zero  and  76°  Mm-  barometer. 

To  dilute  a  volume  V  of  a  solution  of  a  specific  gravity  S  to  a  specific  gravity  S'  ;  the 
volume  of  water  to  be  added  to  Fis  r,  and 

s-s- 


To  convert  the  specific  gravity  S  of  a  substance  at  Ta  Cent,  against  water  at  T'Q,  to 
gravity  S'  at  t°  against  water  at  t'9.  Let  C  be  the  coefficient  of  expansion  of  the  substance 
for  one  degree;  V,  the  volume  of  one  gram  of  water  at  T'°\  and  V,  the  volume  at  t'°; 
V  S. 


then  S' 


V  +  V(t  —  T)  C 


In  an  aqueous  solution  of  a  volatile  liquid  and  non-volatile  matter  (as  a  solu- 
tion of  alcohol  and  extractive),  the  volume  of  the  former  may  be  determined 
from  the  difference  in  specific  gravity  before  and  after  its  removal  by  evapora- 
tion. A  measured  volume  F"of  the  solution  of  the  specific  gravity  G  is  boiled 
until  the  volatile  body  is  expelled,  the  solution  cooled  and  made  up  to  the 
original  volume  F"with  water,  and  the  specific  gravity  G'  again  observed.  If 


186  QUANTITATIVE    CHEMICAL,    ANALYSIS. 

the  gravity  of  the  volatile  body  is  G  ",  then  its  proportion  by  volume  v  in  the 

TT  fQ.  Q.'\ 

original  mixture  is  v  =  — ^77- ~ .    This  formula  is  applicable  only  where 

no  contraction  occurs  on  mixing  the  volatile  liquid  with  the  solution,  other- 
wise a  correction  must  be  applied  since  the  volume  of  water  added  after  boiling 
will  be  less  than  the  volume  of  the  volatile  liquid. 

Densimetric  methods.  In  these  a  precipitate  is  weighed  while  in  suspension 
in  water  or  a  solution.  Obviously  when  a  precipitate  is  introduced  into  a  given 
fixed  volume  of  water  or  a  solution,  the  weight  of  the  latter  is  increased  by  the 
weight  of  the  precipitate  and  diminished  by  the  weight  of  a  volume  of  water 
or  the  solution  equal  to  the  volume  of  the  precipitate.  Let  x  be  the  weight  of 
the  precipitate;  a,  the  specific  gravity  of  the  dry  precipitate;  6,  the  specific 
gravity  of  the  solution  at  t  ~  ,  the  temperature  of  the  experiment;  c,  the  volume 
held  by  the  picnometer  at  t  °  ;  d,  the  weight  of  the  volume  c  of  the  solution 
with  the  precipitate  in  suspension ;  then 

x  ^  a  (d  —  6c) 

d  =  b  (c  —  -  )  +  x,  whence  x  =      ^_b 

Where  a  precipitate  is  formed  in  a  liquid  and  a  portion  of  the  latter  with- 
drawn for  further  analysis,  the  ratio  of  the  part  to  the  total  liquid  can  be  found 
as  follows.  Let  the  total  weight  of  the  liquid,  precipitate  and  flask  containing 
them  be  A;  after  drawing  off  a  part  of  the  clear  liquid,  the  total  weight  of  the 
remaining  liquid,  precipitate  and  flask  be  B:  the  remaining  liquid  is  filtered,  the 
precipitate  washed,  ignited  and  weighed,  its  weight  being  C;  and  the  weight 
of  the  flask  alone  is  D.  Then  the  weight  of  the  whole  of  the  clear  liquid  is 
A — (C -f  D),  and  the  weight  of  the  portion  withdrawn  is  A  —  B.  Hence 

^  JCJ 

the  ratio  of  the  liquid  withdrawn  to  the  whole  liquid  is  - .     This 

«  —  w  ~r  JJ) 

formula  is  only  correct  where  nothing  is  dissolved  from  the  precipitate  on 
washing  with  water. 

J.  In  technical  analysis,  especially  organic,  there  are  many  mixtures  that 
cannot  be  separated  by  any  of  the  known  methods,  or  bat  imperfectly,  yet  by 
computations  from  data  obtained  by  various  transmutations  and  the  determi- 
nation of  constants  under  different  conditions,  the  proportions  of  the  constitu- 
ents may  be  arrived  at  with  a  greater  or  less  degree  of  accuracy.  It  would 
occupy  too  much  space  to  detail  any  number  of  examples,  but  the  following 
will  indicate  the  general  trend  of  schemes  of  this  kind. 


The  first  is  the  method  of  Mebus  for  mixtures  of  sodium  (or  potassium)  carbonate  and 
bicarbonate. 

Two  equal  weights,  A  and  B,  of  the  sample  are  dissolved  in  cold  water.  A  Is  titrated  by 
normal  acid  and  methyl  orange  for  the  total  alkali,  requiring  a  Cc.  of  acid.  To  B  is  added 
normal  sodium  hydrate  (perfectly  free  from  carbon  dioxide  and  baryta)  in  quantity  6  Ccs. 
exactly  equivalent  to  a  Cc.  of  the  acid,  and  also  an  excess  of  neutral  barium  chloride. 
The  precipitated  barium  carbonate  is  filtered  oil,  and  the  filtrate  titrated  by  normal  acid, 
requiring  c  Cc.  From  these  volumes  are  calculated  the  relative  proportions  of  monocar- 
bonate  and  bicarbonate  in  the  mixture. 

To  more  readily  understand  the  reactions  and  calculation  let  us  consider  what  c  would 
amount  to 

1.  Were  the  sample  entirely  sodium  monocarbonate.  Here,  since  2NaOH  Is  equivalent 
to2HCl,  the  reactions  would  be 

A.  Na2COs  +  2HC1  (a)  =  2NaCl  +  H2COs;  and 

B.  Na2C'O3  +2NaOH  (6)  +  BaCl2  =  2NaCl  +  BaCOs  +  2NaOH  (6),  and  c  would  be  exactly 

equal  to  a. 


QUANTITATIVE    CHEMICAL,    ANALYSIS.  187 

2.  Were  the  sample  entirely  sodium  bicarbonate.    Here  the  reactions  would  be 

A.  Na2CO3.H2C03  +  2HC1  (a)  =  2NaCl  +  2H2CO3,  and 

B.  The  caustic  alkali  abstracts  one -half  of  the  COs  from  the  bicarbonate  — 

Na2CO3.H2CO3  +  2NaOH  =  2Xa2CO3  +  2H2O ;  and  with  barium  chloride, 

2N82CO3  +  2BaCl2  =  4NaCl  +  2BaCOs. 

The  filtrate  is  exactly  neutral  and  c  is  zero. 

3.  Hence  100  per  cent  of  monocarbonate  is  shown  when  c  equals  a,  and  100  per  cent  of 
bicarbonate  when  c  equals  zero.    The  difference  between  a  and  c  in  a  determination  Is  a 
volume  of  normal  acid  equal  to  the  volume  of  normal  sodium  hydrate  reacting  with  the 
bicarbonate.    This  volume  in  cubic  centimeters  times  .040058  (the  weight  of  sodium  hydrate 
in  one  cubic  centimeter  of  the  normal  solution)  is  the  weight  d  of  sodium  hydrate  react- 
ing; and  since  NaoCO3.K2CO3  (168.116)  +  2NaOH  (80.116)  =  2Na2CO3  +  2H2O,  from  the  pro- 
portion 168.116  :  80.116  :  :  X  :  d  may  be  calculated  X,  the  weight  of  sodium  bicarbonate  In 
the  sample. 


For  the  analysis  of  crude  soda-  lyes  that  contain  sodium  carbonate,  hydrate,  sulflde, 
sullite  and  thiosulfate,  a  volumetric  method  is  due  to  Kalmann  and  Spueller.*  The 
method  is  based  on  the  insolubility  of  barium  sulfite  and  solubility  of  the  thiosulfate,  and 
precipitation  of  a  sulfide  by  zinc  in  alkaline  solutions. 

Five  equal  volumes  of  the  liquid  are  measured  and  titrated  as  follows  by  normal  acid 
and  decinormal  iodine  solutions. 

A.  By  acid  and  methyl  orange,  neutralizing  the  carbonate,  sulfide,  hydrate  and  one-half  of 

the  sulfite  (sodium  bisulfite  is  neutral  to  methyl  orange). 

B.  By  iodine,  reacting  with  the  sulfide,  sulfite,  and  thiosulfate. 

C.  The  sulfide  is  precipitated  by  a  zinc  salt,  the  zinc  sulfide  filtered  off,  and  the  filtrate 

acidified  and  titrated  by  iodine  ;  there  are  oxidized  the  sulfite  and  thiosulfate. 

D.  The  sulfite  is  precipitated  by  barium  chloride,  and  the  filtrate  titrated  by  acid  ;  there 

are  neutralized  the  hydrate  and  sulfide. 

E.  The  sulfite  is  precipitated  by  barium  chloride,  the  solution  filtered,  and  the  filtrate 

acidified  and  titrated  by  iodine  ;  there  are  oxidized  the  sulfide  and  thiosulfate. 
Combining  the  above  data  we  obtain  the  volumes  of    the  acid  and   iodine   solutions 
corresponding  to  the  following:  — 

B.  Na2S  +  NaaSOs  +  Na2S2O3  |  p  j  The  difference  is  iodine  solution 

E.  Na2S  +  Na£S2O3  \"  '  (     equivalent  to  the  Na2SOs. 

B.  Na2S  +  Na2SO3  +  Na2S2Os  )  G  (  The  difference  is  iodine  solution 

C.  Na25Os  +  Na2S2Os  *"  "I     equivalent  to  the  Na2S. 

E.  NaaS  +Na2S2O3  )  ff  (  The  difference  is  iodine  solution 

G.  Na2S  '  (     equivalent  to  the  Na2S2O3. 

D.  Na2S  +  NaOH  \  ^     The  difference  is  acld  solutlon 
Jo-  Na2§                 )    '  '  I     equivalent  to  the  NaOH. 


A.   Na2COs  +  Na2S  +  NaOH  +  _    N 

2  I  _  |  The   difference  is  acid  solution 

D  +  £.          Na2S  +  NaOH  +  !  Na2SOs  [  '     equivalent  to  the  Na2CO3. 

In  computing  /only  one-tenth  of  G  is  subtracted  from  D  since  the  iodine  solution  is 
decinormal  while  the  acid  solution  is  normal;  similarly  in  computing  J,  one  -tenth  of  one- 
half  of  Flu  subtracted. 


Given  a  mixture  containing  the  three  sugars,  sucrose,  dextrose,  and  levnlose;  an 
aqueous  solution  is  clarified  and  diluted  to  a  definite  volume,  and  three  aliquot  parts,  A, 
B,  and  C  are  withdrawn.  Each  part  is  boiled  with  Fehling's  solution  (vide  Sugar), 
producing  a  precipitate  of  cuprous  oxide,  as  follows  — 

A.  Directly.    The  precipitate  is  a  grams  of  cuprous  oxide  from  the  reaction  with  the 
dextrose  and  levulose,  sucrose  having  no  action  on  Fehling's  solution. 

B.  After  conversion  of  the  sucrose  into  equal  parts  of  dextrose  and  levulose  by  inver- 
sion by  a  dilute  acid ;  the  precipitate  is  b  grams  of  cuprous  oxide  coming  from  the  reaction 
with  the  original  dextrose  and  levulose,  and  the  dextrose  and  levnlose  from  the  sucrose. 

C.  After  destruction  of  the  levulose  by  heating  with  concentrated  hydrochloric  acid ;  - 


*  Dingl.  Polyt.  Journ.  264—456. 


188  QUANTITATIVE    CHEMICAL    ANALYSIS. 

this  also  inverts  the  sucrose  and  destroys  the  levulose  formed.  There  is  precipitated  c 
grams  of  cuprous  oxide  from  the  reaction  with  the  original  dextrose  and  the  dextrose 
from  the  sucrose. 

Calling  the  weight  of  the  original  dextrose  D  and  levulose  L,  and  the  dextrose  from  the 
sucrose  Z>'  and  the  levulose  L' ;  then  the  weight  of  the  precipitate 
From  D  +  L  =  a 

"      D   !  L  +  Z>'  H  L'  =  b 
"      D  +  D'  =  c 

"       D'+L'  =  2D'        =6  —  a 
"      L  +  L'  =  6  —  c 


"      D 

"      L  =  a—  [c  —    i  (&  —  a)] 

Now,  one  gram  of  cuprous  oxide  results  from  the  reaction  with  either  .260  gram  dex- 
trose, .253  gram  levnlose,  or  .262  gram  inverted  sucrose;  and  one  gram  of  inverted  sucrose 
results  from  the  inversion  of  .950  gram  sucrose ;  hence 

.95 
(6  —  o)  x  ~^r>  =  weight  of  sucrose  in  the  mixture. 

[c  —  -g  (6  —  a)  X  -Tjt-Q  =  weight  of  dextrose  in  the  mixture. 
(a  — [c— (6  — a)])  X   253  =  weight  of  levulose  in  the  mixture. 


A  number  of  organic  and  inorganic  compounds  are  quantitatively  oxidized  by  potas- 
sium permanganate  in  an  alkaline  solution,  the  permanganate  breaking  up  in  this  way  — 
K2O+2MnO2+3O,  the  binoxide  of  manganese  precipitating  as  a  fine  hydrated  powder, 
and  the  three  atoms  of  oxygen  acting  to  oxidize  the  organic  compound. 

A  determination  by  this  reaction  may  be  done  by  (1)  filtering  off  the  manganese  bin- 
oxide  and  determining  its  weight  by  a  gravimetric  or  volumetric  process;  or  (2),  having 
used  a  known  volume  of  standard  permanganate  solution  for  the  operation,  to  filter 
through  asbestos  and  determine  the  unreduced  permanganate  in  the  filtrate  by  a  volu- 
metric tltration .  In  either  case  the  weight  of  the  organic  compound  is  learned  by  a  simple 
calculation. 

A  more  expeditious  plan  is  (3)  to  employ  a  known  volume,  a  moderate  excess,  of 
standard  permanganate  for  the  determination,  then,  without  filtering,  to  run  in  a  known 
volume,  an  excess,  of  a  standard  reducing  solution,  this  reducing  both  the  excess  of  per- 
manganate and  the  precipitated  manganese  binoxide ;  finally  the  excess  of  the  reducing 
solution  is  determined  by  titration  by  standard  permanganate. 

The  calculation  for  the  last  named  process  can  be  made  in  several  ways  of  which  two 
are  given  below.  Taking  sodium  thiosulf  ate  as  an  example,  the  reaction  between  it  and 
potassium  permanganate  is  expressed  by  the  equation  — 

(1).  6NTa2S2O3.5aq  +  8K2O.(MnO)2.O5  +  2HaO  =  IGMnOa  +  6Na2SO4  +  6K2SO4  +  4KOH  +  5aq. 
Of  the  40  atoms  of  available  oxygen  of  the  permanganate,  16  go  to  form  MnO2  and  24  to 
oxidize  the  thlosulfate. 

On  now  adding  sulfuric  acid  and  an  excess  of  a  standard  solution  of  ferrous  sulfate  to 
the  turbid  liquid,  first  the  excess  of  the  permanganate  is  reduced  — 
(2).  10FeSO4+  K2O.(MnO)2.O5  +  8H2SO4  =  5Fe2(SO4)3  +  K2SO4  +  2MnSO4  +  8H2O. 
then  the  manganese  binoxide  is  dissolved  — 
(3).  2FeSO4  +  Mn02  +  2H2SO4  =>  Fe2(SO4)3  +  MnSO4  +  2H2O. 

Finally  the  excess  of  ferrous  sulfate  is  titrated  by  standard  permanganate  of  the  same 
strength  as  originally  used,  as  per  equation  (2)  above. 

A.  Calling  the  original  volume  of  permanganate  solution  c,  and  the  volume  used  in 
the  final  titration  d,  then  the  total  volume  used  is  c  +  d  cubic  centimeters. 

Now  this  volume  c  +  d  contains  a  certain  weight  of  potassium  permanganate  and  this 
a  certain  proportion  of  available  oxygen.  The  available  oxygen  we  may  consider  as 
divided  into  two  parts;  one  part  goes  to  oxidize  the  ferrous  snlfateto  ferric  sulfate,  the 
other  to  oxidize  the  thiosulf  ate  as  in  equation  (1)  above.  This  will  be  evident  when  the 
rationale  of  the  process  is  considered.  On  mixing  the  permanganate  and  thiosulfate 
solutions,  the  oxygen  of  the  original  volume  of  permanganate  divides in'o  three  parts; 
the  first  part  (A)  oxidizes  the  thiosulf  ace  to  sulfate;  the  second  part  (B)  unites  with 


QUANTITATIVE    CHEMICAL    ANALYSIS.  189 

MnO  to  form  MnCte;  the  third  (C),  that  of  the  excess  of  permanganate,  remains  unsepa- 
rated.  Now,  on  adding  the  ferrous  solution,  the  oxygen  of  (B)  plus  (C)  plus  that  of  the 
permanganate  used  in  the  titration  react  with  the  iron  to  exactly  convert  it  to  ferric 
sulfate. 

We  have  therefore  to  calculate  the  available  oxygen  in  the  total  permanganate  and  sub- 
tract from  it  the  part  required  to  oxidize  the  ferrous  sulfate;  the  remainder  is  the 
oxygen  taken  up  by  the  thiosulfate,  from  which  the  weight  of  the  latter  can  be  calculated. 

1.  From  equation   (2)  we  see  that  ten  atoms  of  iron  react  with  five  atoms  of  oxygen  ; 

that  is,  in  the  ratio  of  10  x  56  to  5  x  16,  or  as  7  to  1.    Hence  —  of   the   weight  a  of  iron 

oxidized  by  one  cubic  centimeter  of  the  permanganate  solution  gives  the  weight  of  avail- 
able oxygen  in  one  cubic  centimeter  of  the  permanganate  solution.  And  the  total  volume 

of  permanganate  solution  used  contains.-  .a  .(c  +  d)  grams  of  available  oxygen. 

2.  Now,  if  in  the  volume  of  ferrous  solution  added  there  are  b  grams  of  iron,  then  -^  .6 
will  be  the  weight  of  available  oxygen  of  the  permanganate  required  to  oxidize  the  iron. 

3.  Therefore  —  .a  .(c  +  d)  --  -  .&  =  the  weight  of  oxygen  reacting  with  the  thiosulfate. 

4.  From  equation  (1)  we  learn  that  six  molecules  of  thiosulfate   (6  x  248.32  =  1489.92) 
react  with  24  atoms  of  oxygen  of  the  permanganate  (24  X  16  =  384)  .    Hence  one  gram  of 

1489.92 
oxygen  corresponds  to     ^     grams  of  thiosulfate. 

(1  IN          1489  9^ 

-i  .o.(c  +  e&)  —  ^.b)  x    -^-  =>  X  =  the   weight   of  crystallized   sodium 

thiosulfate  in  the  material  analyzed.    This  may  be  reduced  to  X  =  .5543  (o.c  +  a.d  —  &). 

B.  A  somewhat  similar  calculation  is  as  follows.  It  is  based  on  the  principles  that  cer- 
tain proportional  weights  of  oxygen  are  required  to  oxidize  respectively  one  gram  of 
thiosulfate  to  sodium  sulfate  (in  an  acid  solution),  and  one  gram  of  iron  from  ferrous  sul- 
fate to  ferric  sulfate,  the  proportions  shown  by  the  equations  — 

4K2MU2O8  +  5Na2S2O3.5aq  +  7H2SO4  =  5Na2SC>4  +  4K2SO4  +  8MnSO4  +  7H2O;  and 
4K2Mn2O8  +  40FeSO4  +  32H2SO4  =  20Fe2(SO4)3  +  4KaSO4  +  8MnSO4  +  32H2O. 

From  these  equations  it  is  seen  that  four  molecules  of  permanganate  oxidize  respect- 
ively five  molecules  of  thiosulfate  and  40  atoms  of  iron.  Hence  one  cubic  centimeter  of 
permanganate  solution  will  oxidize  iron  and  thiosulfate  in  the  ratio  of  40  X  56  to  5  X 
248.32;  or,  if  one  cubic  centimeter  of  permanganate  solution  oxidizes  a  grams  of  iron,  it 

5  X  248.32 
will  oxidize  a  X  '  40  x  56  S1*11118  of  thiosulfate. 

If  e  cubic  centimeters  of  permanganate  solution  oxidizes  the  volume  of  ferrous  solution 
added,  and  c  +  d  is  the  total  volume  of  permanganate  solution  used,  then  c  +  d  —  e  is  the 
volume  of  permanganate  solution  oxidizing  the  thiosulfate. 


Hence  a  X  ie   X  (c  +  d  —  e)  =  X  =  the  weight  of  crystallized  thiosulfate.    More 

conveniently  expressed,  X  =  .5643  .a.(c  +  d—e). 


190  QUANTITATIVE    CHEMICAL    ANALYSIS. 


CHAPTEE  9. 

ERRORS  AND  PRECAUTIONS. 

On  finishing  the  calculation  of  an  analysis  there  may  arise  two  questions: 
how  nearly  should  the  results  agree  with  those  deduced  from  the  formula,  or 
otherwise  ascertained,  to  be  considered  satisfactory?  and  to  what  cause 
should  a  failure,  manifest  or  inferential,  be  attributed? 

It  is  plain  that,  as  a  general  proposition,  the  first  question  admits  of  no 
answer;  for  although  depending  primarily  on  the  intrinsic  accuracy  of  the 
method  employed,  yet  the  correctness  of  every  determination  is  contingent  to  a 
great  extent  on  the  skill  of  the  analyst,  the  care  and  attention  given  at  the  vari- 
ous stages  of  the  analysis,  and  other  conditions.  So  that  no  general  rule  can  be 
laid  down,  even  approximately.  As  to  the  methods  themselves,  the  most  accur- 
ate are  capable,  when  prosecuted  with  the  utmost  care  in  every  respect,  of 
affording  results  withina  variation  of  not  over  one-tenth  of  one  per  cent,  grading 
down  to  those  where  an  inaccuracy  as  high  as  five  or  ten  per  cent  is  not 
uncommon. 

Perhaps  the  best  criterion  is  to  be  found  in  the  test-analyses  appended  to 
the  original  description  of  the  methods  (which  should  always  be  consulted 
when  possible),  however  to  be  accepted  at  times  with  some  allowance  for  the 
natural  inclination  of  the  deviser  to  excuse  and  suppress  the  more  unfavorable 
results. 

And  to  the  second  query  only  a  like  indefinite  reply  can  be  made.  Assum- 
ing that  the  method  of  analysis  followed  is  unimpeachable,  one  should  first 
repeat  the  calculations;  then  examine  each  precipitate  (always  to  be  preserved 
until  the  entire  analysis  is  finished)  to  learn  whether  it  is  of  the  assumed  com- 
position and  for  impurities  from  foreign  sources  or  in  the  form  of  other  con- 
stituents of  the  sample  that  have  been  imperfectly  separated;  then  the  strength 
and  purity  of  the  reagents  used;  and  lastly  the  substance  analyzed  in  respect  to 
purity  and  freedom  from  moisture.  If  none  of  the  above  is  found  defective, 
the  analyst  is  forced  to  the  conclusion  that  faulty  manipulation  at  some  stage 
has  been  responsible,  and  his  only  recourse  is  a  repetition  of  the  entire 
analysis. 

Outside  of  defects  in  the  method  itself,  the  errors  most  likely  to  be  incurred 
in  the  course  of  an  analysis  may  be  recounted  as  follows : 

1.  A  defect  in  the  sample.  Although  directions  for  sampling  and  the  prepa- 
ration of  the  sample  are  not  commonly  included  in  the  brief  of  a  method,  yet 
the  importance  of  properly  conducting  these  operations  cannot  be  too  much 
emphasized.  Discrepancies  in  analyses  of  one  material  that  are  due  solely 
to  an  imperfect  sample  are  frequently  charged  to  the  remissness  of  the  chemist 
or  a  faulty  method.  Within  the  observation  of  the  author,  the  great  majority 
of  the  variances  among  chemists  have  arisen  from  ignorance  or  carelessness 
in  this  respect. 

In  general,  the  irregular  distribution  of  the  various  constituents  in  animal 
and  vegetable  matter  and  heterogeneous  materials  both  natural  and  artificial, 
and  the  effects  of  imperfect  mixing,  liquation  and  segregation  in  manufac- 
tured products  demand  the  greatest  care  in  the  selection  of  what  is  to  be  a 


QUANTITATIVE    CHKMICAL    ANALYSIS.  191 

representative  sample.  Personal  supervision  of  the  operation  of  sampling  by 
the  chemist  himself  is  always  desirable. 

In  the  analysis  of  a  complex  powder  it  must  be  seen  to  that  the  particles  are 
well  intermixed  before  weighing  out  the  portion  for  analysis,  and  that  the 
heavier  particles  have  not  sifted  to  the  bottom  of  the  bottle.  It  is  a  good  plan 
to  mix  a  powder  by  rolling  it  to  and  fro  on  a  paper,  then  flatten  the  pile  to  a  thin 
layer  and  gather  a  little  here  and  there  until  enough  is  collected  for  the 
analysis. 

The  expediency  of  grinding  an  ore  or  mineral  to  a  uniformly  fine  powder, 
whether  it  is  to  be  resolved  by  an  acid  or  fluxed,  will  soon  be  learned  by  ex- 
perience, the  rapidity  and  completeness  of  the  subsequent  decomposition  well 
repaying  the  labor  of  a  thorough  trituration.  Illustrations  are  found  in  cer- 
tain native  silicates  of  which  the  silica  separates  and  gelatinizes  at  once  on 
treatment  with  hydrochloric  acid,  and  coarse  particles  are  inclosed  by  the 
silica  and  protected  from  further  action  of  the  acid;  similarly  alloys  contain- 
ing tin,  on  treatment  with  nitric  acid  the  metastannic  acid  left  insoluble  retains 
less  of  the  other  metals  in  proportion  as  the  alloy  was  finely  divided. 

It  must  not  be  overlooked,  however,  that  grinding  changes  the  amount  of 
moisture  in  a  sample  to  a  greater  or  less  extent,  and  that  oxidation  may  occur 
even  in  apparently  stable  inorganic  compounds  —  Craig*  has  remarked  that  a 
noticeable  oxidation  attends  the  grinding  of  pyrite  in  an  agate  mortar. 

The  chemist  may  receive  for  analysis  a  solid  that  has  become  superficially 
altered  in  some  way  (as  a  bar  of  soap  containing  much  less  water  at  the  surface 
than  within),  and  in  taking  from  it  a  portion  for  the  analysis,  a  question  may 
arise  as  to  the  propriety  of  including  or  rejecting  the  parts  that  have  suffered 
alteration.  Or  a  sample  may  be  received  containing  matters  of  such  a  nature 
that  a  doubt  may  be  entertained  as  to  whether  they  formed  an  integral  part  of 
the  original  or  were  accidentally  included  during  the  collection  or  transporta- 
tion of  the  sample.  A  decision  in  matters  of  this  kind  must  be  left  to  the 
judgment  of  the  chemist. 

It  is  always  advisable  to  analyze  an  unstable  organic  mixture  for  solution 
as  early  after  receipt  as  possible.  Alterations  in  composition,  induced  by 
exposure  to  air  and  moisture,  bacilli  and  ferments,  the  escape  of  gases,  absorp- 
tion by  containers,  etc.,  may  cause  a  considerable  difference  in  the  results  of 
analyses  made  at  intervals  more  or  less  protracted.  The  same  is  true,  though 
to  a  less  extent^  of  course,  of  certain  inorganic  mixtures. f  If  it  is  imprac- 
ticable to  begin  the  analysis  at  once,  a  preservative  may  perhaps  be  com- 
pounded with  the  sample,  of  a  nature  and  in  a  quantity  that  will  not  interfere 
with  the  subsequent  analysis.  The  age,  mode  of  preservation,  material  of 
the  container  and  protection  afforded,  any  evidences  of  an  attempt  at  tam- 
pering, etc.,  and  the  general  condition  of  the  sample  should  be  recorded  on 
receipt  for  future  reference. 

In  the  preparation  for  analysis  of  chemical  compounds  one  should  guard 
particularly  against  mother-liquor  inclosed  in  cavities  of  crystals,  oxidation 
from  exposure  to  the  air,  and  adhering  moisture ;  and  the  partial  dissociation 
of  double  salts  on  crystallization  is  more  common  than  is  generally  supposed. 

2.  Imperfect  weighing  and  measuring.  If  the  analytical  balance  is  used 
by  more  than  one  operator  its  equilibrium  and  general  condition  must  be 
seen  to  before  each  weighing.  The  weights  should  be  tested  from  time  to 
time,  the  intervals  depending  on  the  frequency  of  their  use  and  the  care  given 
to  their  preservation. 


*  Journ.  Anal.  Appl.  Chem.  1892—45. 
t  Wiley,  Agricultural  Anal.  2—84. 


192  QUANTITATIVE    CHEMICAL    ANALYSIS. 

All  weighings  of  glass  or  metal  containers  are  only  to  be  regarded  as  final 
after  a  sufficient  time  has  elapsed  for  them  to  acquire  the  temperature  of 
the  balance-case.  According  to  Miller,*  on  wiping  a  small  flask  with  a  linen 
cloth  in  very  dry  weather,  an  electric  charge  was  generated  which  required 
.080  gram  in  the  opposite  pan  to  restore  equilibrium,  the  charge  requiring 
considerable  time  for  entire  dissipation. 

A  correction  is  to  be  made  in  the  most  accurate  weighings  for  the  buoyancy 
of  the  air  (page  39) ;  for  solids  and  liquids  the  loss  is  too  small  to  be  con- 
sidered in  general  work,  though  for  gases  it  cannot  be  neglected. 

The  weight  of  a  material  best  suited  for  the  determination  of  a  constitu- 
ent is  fixed  by  several  conditions.  The  general  rule  holds  that  the  greater  the 
weight  the  less  is  the  result  changed  by  the  constant  unavoidable  errors  of 
analysis.  Exceptions  where  the  use  of  large  amounts  are  restrained,  are 
some  solutions  containing  a  constituent  that  is  slowly  decomposed  at  the  heat 
of  evaporation,  and  consequently  the  evaporation  must  be  conducted  both 
with  expedition  and  at  a  low  temperature.  Similarly,  a  liquid  that  leaves  a 
residue  on  evaporation  that  consists  mainly  of  organic  matter  with  a  small 
proportion  of  inorganic  salts;  on  subsequently  burning  off  the  carbon  of  a  large 
residue  the  heat  of  combustion  may  rise  so  high  as  to  fuse  or  volatilize  the 
inorganic  matter.  These  objections  may  be  overcome  of  course  by  providing 
dishes  and  crucibles  of  a  suitable  shape  and  adequate  size. 

The  exactness  to  which  a  residue  or  precipitate  should  be  weighed  depends 
largely  on  the  intrinsic  accuracy  of  the  method,  the  skill  and  care  with  which 
the  analysis  is  prosecuted,  and,  other  things  being  equal,  the  weight  of 
substance  taken  for  analysis.  In  general,  it  is  unnecessary  to  weigh  closer 
than  one  milligram.  There  are  a  few  exceptions,  e.  g.,  in  the  fire  assay 
of  gold  ores  where  the  gold  button  must  be  weighed  with  the  greatest  accuracy ; 
on  the  other  hand,  for  the  ordinary  run  of  gold  ores,  taking  as  high  as  thirty 
grams  for  the  crucible  assay,  an  error  of  a  few  centigrams  of  the  weight  of 
the  ore  has  no  practical  significance.  , 

In  duplicate  or  triplicate  determinations  it  is  advisable  that  the  portions  of 
substance  weighed  for  a  determination  should  differ  somewhat  in  weight  as  the 
effect  of  a  constant  error  in  the  analysis  is  more  easily  detected  than  if  the 
weights  were  identical. 

The  space  occupied  by  a  precipitate  or  insoluble  residue  of  moderate  bulk  in 
a  solution  of  definite  volume  is  often  negligible  in  ordinary  analyses,  for  many 
apparently  voluminous  precipitates  are  so  attenuated  that  they  really  displace 
but  little  of  the  liquid;  if  the  bulk  is  considerable,  however,  a  correction  must 
be  made. 

In  volumetric  analysis  the  error  ordinarily  incurred  in  measuring  a  liquid  is 
undoubtedly  greater  than  in  weighing  the  substance,  but  as  it  is  not  with  the 
liquid  itself  that  we  are  concerned  but  the  reagent  dissolved  therein,  it  is  plain 
that  the  effect  of  errors  in  measurement  can  be  reduced  indefinitely  by  decreas- 
ing the  ratio  of  the  reagent  to  thevolume  of  water  holding  it  in  solution  —  in 
other  words,  the  more  dilute  the  solution  the  less  care  need  be  taken  in  m,eas- 
uring  it.  On  the  other  hand  the  more  dilute  the  solution  the  greater  the  vol- 
ume needed  to  bring  out  the  end  reaction  of  a  titration  distinctly,  so  that  there 
is  no  real  advantage  in  weakening  a  titrand  beyond  the  point  where  this  will 
be  sharply  defined  by  a  single  drop. 

In  the  measurement  of  a  gas  it  must  be  assured  that  it  is  either  perfectly  dry 
or  saturated  with  moisture.  After  standing  for  a  time  over  water  or  a  dilute 
aqueous  solution  saturation  may  be  assumed,  but  not  if  the  liquid  is  hygro- 


*  Juurn.  Amer    Chem.  Socy.  1898— 428. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  193 

scopic,  such  as  a  strong  solution  of  an  alkali,  concentrated  sulfuric  acid, 
etc. 

3.  Mechanical  losses  and  gains.  Despite  more  than  ordinary  care  there  may 
often  happen  a  loss  in  transferring  a  liquid  from  one  vessel  to  another,  in  boil- 
ing or  distilling,  igniting  a  precipitate,  etc.,  or  a  gain  from  ubiquitous  dust  or 
the  vessels  used  in  evaporations.  It  must  be  left  to  the  discretion  of  the 
operator  whether  a  mishap  of  this  kind  is  of  sufficient  consequence  to  constrain 
the  discontinuance  of  the  analysis.  Where  it  is  reasonably  certain  that  the 
amount  lost  is  so  small  a  proportion  of  the  original  as  not  to  reduce  the  weight 
of  the  predominating  constituent  appreciably,  it  is  of  course  not  imperative  that 
the  analysis  be  terminated  summarily,  for  "  here  as  elsewhere,  in  experimental 
science,  the  golden  rule  is  neither  to  strain  at  the  gnat  nor  to  swallow  the 
camel."  However,  the  student  should  strive  to  attain  such  a  mastery  over 
the  manipulations  that  even  a  comparatively  unimportant  loss  will  be  of  rare 
occurrence. 

Transferring  a  solid,  liquid  or  gas  from  one  container  to  another  is  frequently 
attended  by  loss,  and  can  often  be  avoided  by  choosing  a  vessel  of  the  proper 
size  or  shape  at  the  beginning.  As  the  loss  of  a  part  of  a  dilute  solution  is  of 
less  consequence  than  the  same  amount  of  one  more  concentrated,  it  is  ad- 
visable to  dilute  a  strong  solution  before  this  operation  provided  there  is  no 
reason  to  the  contrary. 

Some  salts  have  a  tendency  to  crystallize  above  the  surface  of  a  solution  as 
it  evaporates;  the  liquid  by  diffusion  through  the  crust  extends  until  event- 
ually it  creeps  over  the  edge  of  the  dish  or  crucible  down  the  outside.  A  pre- 
ventative  is  slightly  greasing  the  edge  with  oil  or  vaselin;  or  by  applying  the 
heat  above  the  surface  of  the  liquid . 

The  effervescence  from  the  escape  of  a  gas  when  a  metal,  sulflde  or  car- 
bonate is  dissolved  in  an  acid  may  easily  cause  a  loss  of  fluid  by  projection 
unless  the  containing  vessel  is  capacious  and  covered.  An  Erlenmeyer  flask 
is  suitable,  or  an  evaporating  dish  in  which  stands  inverted  a  wide-stemmed 
funnel  of  slightly  less  diameter.  Where  the  reaction  is  expected  to  begin 
suddenly,  accompanied  by  violent  boiling  up,  the  sample  should  be  dropped 
into  the  acid  in  several  small  portions.  This  direction  applies  particularly  to 
metals  and  nitric  acid ;  and  has  also  the  advantage  in  this  case  that  the  metal 
comes  in  contact  with  strong  acid  only,  lessening  the  danger  of  any  associated 
element  (as  sulfur)  escaping  oxidation  as  might  happen  were  the  acid  added 
in  portions  to  the  metal.  Yet  unless  some  special  oxidizing  or  other  action  is 
desired,  the  acid  should  be  so  dilute  that  solution  proceeds  slowly  and  quietly. 

Or  as  the  solution  nears  the  point  of  saturation  a  tenacious  film  of  crystals 
may  cover  the  surface  followed  by  a  vigorous  spattering  as  bubbles  of  steam 
perforate  it;  so  a  constant  stirring  or  rocking  of  the  vessel  must  be  kept  up 
until  solidification.  Salts  of  ammonia  are  especially  troublesome  in  this  way. 

Viscous  organic  bodies  containing  water  invariably  froth  in  the  retort  and 
some  of  the  foam  is  liable  to  pass  over  it  into  the  receiver.  This  may  be  pre- 
vented by  "dead -melting"  —  maintaining  the  liquid  in  the  molten  state  for 
some  time,  then  decanting  from  the  water  deposited. 

In  condensing  a  distillate  an  uninterrupted  stream  of  cold  water  must  be  kept 
running  through  the  condenser,  and  the  latter  should  expose  an  ample  surface 
to  the  vapor ;  and  be  made  of  thin  glass,  or  preferably  of  metal  (a  better  con- 
ductor of  heat)  for  distillates  containing  no  free  acid. 

The  complete  absorption  of  a  gas  in  a  liquid  may  be  insured  by  passing  it 
slowly  through  several  absorption  bulbs,  the  number  depending  on  the  solubil- 
ity -coefficient  of  the  gas.  One  bulb  may  answer  when  a  marked  chemical 
action  ensues  between  the  gas  and  absorbent. 


194  QUANTITATIVE    CHEMICAL    ANALYSIS. 

In  distillations,  organic  combustions,  etc.,  the  gas  tightness  of  the  rubber  or 
ground-glass  connections,  stoppers,  and  insealed  wires  must  be  assured  before 
the  determination  is  begun,  best  by  attaching  a  small  manometer  and  com- 
pressing the  air  within  the  apparatus.  Should  the  manometer  indicate  a  leak, 
the  location  may  be  found  by  wetting  suspected  places  with  soap  solution,  when 
a  chain  of  fine  bubbles  will  appear  whenever  air  escapes.  Smearing  with  glycerin, 
syrupy  phosphoric  acid,  or  a  mixture  of  rubber  and  beeswax  *  will  tighten  a 
loose  fitting  joint,  though  it  is  not  the  best  policy  to  permit  a  makeshift  of  this 
kind  in  an  important  analysis. 

In  the  combustion  of  an  organic  body  a  blank  determination  should  precede 
the  actual  analysis.  Usually  a  slight  fairly  constant  increase  will  be  found  in 
the  calcium  chloride  and  potash  bulbs,  but  it  should  not  be  great  enough  to 
vitiate  the  results  of  a  test. 

A  liquid  has  often  to  be  clarified  without  increasing  its  volume,  and  here  the 
filter  is  not  moistened  previous  to  filtration.  It  is  saidf  that  from  adsorption, 
the  first  few  drops  of  the  filtrate  hold  less  in  solution  than  the  original,  and 
these  should  be  rejected  if  the  fraction  first  filtered  is  to  be  a  representative  of 
the  concentration  of  the  entire  filtrate.  Contributing  to  the  dilution  is  the 
hygroscopic  moisture  of  the  paper. 

4.  Imperfect  solution.  Continuous   percolation   alone  will  readily  and  thor- 
oughly extract  a  soluble  constituent  from  a  substance  of  an  open  and  porous 
character,  not  inclined  to  swell  or  gelatinize  during  the  operation,  otherwise 
may  fail  even  when  the  percolation  is  greatly  prolonged,  for  the  reason  that 
the  usual  construction  of  the  apparatus  does  not  provide  a  convenient  way  of 
stirring  up  the  substance  at  intervals,  and  therefore  the  solvent,  following  the 
paths  of  least  resistance,    will  avoid  the  less  permeable  aggregations.    It  is 
safer  in  most  cases  to  precede  the  extraction  by  a  digestion  in  a  beaker. 

A  percolation  is  usually  considered  complete  when  the  drops  fall  uncolored, 
but  when  the  principle  to  be  extracted  is  colorless  it  may  have  passed  entirely 
into  solution  long  before  the  coloring  matter.  The  percolate  should  be  tested 
from  time  to  time  in  the  same  way  as  the  washings  from  a  filtration.  At  the 
beginning  the  percolate  may  run  cloudy  and  is  to  be  returned  to  the  percolator, 
but  care  must  be  taken  against  mechanical  loss  since  the  first  fraction  is  so 
comparatively  rich  in  extractive. 

The  solvent  employed  for  the  extraction  of  a  principle  from  a  mixture  should 
contain  no  fixed  impurities,  and  should  also  be  free  from  any  other  solvent, 
whose  presence  might  cause  small  amounts  of  other  principles  to  enter  the 
solution.  Commercial  ether  usually  contains  water  and  alcohol  and  must  be 
purified  before  using,  while  most  other  solvents  are  found  on  the  market  suffi- 
ciently pure  as  a  rule  for  immediate  use. 

In  the  lixiviation  of  one  constituent  of  a  mixture  it  must  be  remembered  that 
the  other  constituents  are  brought  in  contact  not  only  with  the  pure  solvent 
but  also  with  a  more  or  less  concentrated  solution  of  the  soluble  constituent, 
toward  which  their  deportment  may  be  quite  different.  Similarly,  undried 
vegetable  matter  may  contain  so  much  moisture  that  when  percolated  by  strong 
alcohol,  the  first  portion  of  the  percolate  contains  also  whatever  else  is  soluble 
in  weak  alcohol,  this  precipitated  in  part  by  the  stronger  spirit  that  follows. 

5.  Losses  and  gains  on  evaporation  and  ignition.     The  haloid  salts  of  a  few 
metals  are  volatilized  to  some  extent  when  their  solutions  are  boiled,  while 
many  acids  and  organic  solutions  cannot  be  evaporated  without  extensive  loss 
or  decomposition.    If  concentration  by  heat  is  unavoidable,  the  combination 


*  Journ.  Amer.  Chem.  Socy.  1898—678. 

t  Ostwald-McGowan,  Foundations  of  Anal.  Chem.  22. 


QUANTITATIVE    CHEMICAL    ANALYSIS. 

may  be  previously  changed  to  one  not  volatile,  or  the  vapor  condensed  or 
absorbed. 

With  easily  decomposed  organic  solutes,  either  a  temperature  below  the  boil- 
ing point  is  employed,  or  the  liquid  is  allowed  to  evaporate  spontaneously. 
For  an  organic  solvent  of  low  boiling  point,  a  temperature  of  30°  to  40°  is 
better,  as  spontaneous  evaporation  so  far  cools  the  liquid  that  moisture  from 
the  air  is  condensed  and  absorbed.  And  in  an  evaporation  or  distillation  the 
possibility  of  a  liquid  being  carried  off  in  the  vapor  of  one  having  a  lower  boil- 
ing point  should  not  be  overlooked. 

Precipitates  containing  a  metal  volatile  at  a  red  heat  or  below  suffer  a  certain 
loss  when  ignited  with  carbon.  When  surrounded  by  a  filter  paper,  loss  can- 
not be  entirely  prevented,  although  its  extent  is  much  less  than  commonly  be- 
lieved. Even  ignition  for  a  long  time  in  a  platinum  crucible  over  a  Bunsen 
burner  may  have  the  same  effect  through  permeation  of  reducing  gases.  By 
removing  as  much  as  possible  of  the  precipitate  from  the  paper,  moistening  the 
latter  with  solution  of  ammonium  nitrate,  and  burning  at  the  lowest  heat,  the 
loss  will  not  be  serious,  as  a  rule.  A  Gooch  crucible  is  most  suitable  for  pre- 
cipitates of  this  nature. 

Refractory  forms  of  carbon  can  be  burned  rapidly  from  associated  inorganic 
matter  by  turning  a  gentle  stream  of  oxygen  into  the  crucible.  This  scheme 
answers  well  where  the  inorganic  matter  is  practically  infusible,  as  silica,  mag- 
nesia, alumina,  but  is  not  advisable  for  residues  that  fuse  or  volatilize  at  a 
moderate  temperature,  since  the  local  heat  of  combustion  is  here  much  above 
redness. 

The  temperature  of  ignition  of  precipitates  may  usually  vary  between  wide 
limits.  Exceptions  are  where  a  certain  constituent  element  or  compound  is  to 
be  volatilized,  and  a  heat  lower  than  that  directed  in  the  method  followed  will 
fail  to  drive  it  off  completely.  Conversely,  if  the  integrity  of  the  precipitate  is 
to  be  preserved  too  high  a  heat  may  fuse  it  or  volatilize  some  constituent. 

The  loss  through  volatilization  of  a  part  of  a  constituent  of  a  precipitate  dur- 
ing ignition  may  be  restored  by  a' subsequent  process  (page  103),  but  the  better 
plan  is  to  prevent  it  if  posssible. 

Change  of  composition  through  oxidation  at  a  red  heat  can  be  avoided  by 
transmitting  a  current  of  hydrogen  or  carbon  dioxide  into  the  crucible ;  this 
plan  is  well  adapted  for  the  ignition  of  metallic  sulfldes  that  contain  or  have 
been  mixed  with  free  sulfur,  leaving  a  definite  pure  sulfide  on  ignition. 

If  a  precipitate  and  filter  have  not  been  at  least  partly  dried  before  ignition, 
a  loss  by  projection  may  be  expected  when  the  heat  of  a  burner  is  suddenly 
applied  to  the  crucible.  This  will  not  happen,  however,  after  filtration  by  the 
vacuum  pump,  or  after  the  paper  has  been  opened  on  a  porous  tile.  And  al- 
though a  pulverulent  precipitate  or  residue  has  been  thoroughly  dried,  if 
ignited  too  hastily,  the  smoke  and  gases  from  the  paper  are  apt  to  carry  off 
traces  of  the  finer  particles.  Economy  of  time  and  security  against  loss  are 
insured  by  a  close  observance  of  the  directions  given  on  page  103. 

Corrosion  of  containing  vessels.  Experiments  by  various  chemists  show 
plainly  that  discrepancies  in  accurate  analyses  are  frequently  traceable  to 
the  ingredients  of  glass  or  porcelain  glaze  acquired  by  a  solution.  The  error 
from  this  source  is  evident  when  a  large  volume  of  liquid  is  evaporated  in 
a  large  vessel  to  a  small  bulk,  and  the  liquid  transferred  to  a  smaller  tared 
dish,  evaporated  to  dryness  and  the  residue  weighed;  here  the  amount  of 
glass  dissolved  is  frequently  several  milligrams,  and  may  reach  to  a  centi- 
gram or  more.  And  in  precipitations,  any  dissolved  silica  or  calcium  silicate 
is  usually  enveloped  and  carried  down  with  the  precipitate. 


196  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Some  authorities  state  that  the  glass  and  porcelain  glaze  are  dissolved  in 
toto,  others  incline  to  the  belief  that  the  alkalies  are  extracted  disproportion- 
ally  to  the  other  constituents.  It  is  said  that  the  resistance  to  corrosion  by 
water  or  a  solution  is  at  a  minimum  with  new  vessels,  rapidly  increasing  on 
continued  contact  —  possibly  by  the  removal  of  a  superficial  film  structurally 
modified  by  contact  with  the  air  —  and  that  allowing  the  vessels  to  stand  filled 
with  water  for  several  days  will  greatly  reduce  the  amount  dissolved  during 
subsequent  use. 

In  most  cases,  porcelain  is  less  acted  on  than  glass,  and  platinum  than  either. 
According  to  Bohlig,*  100  cubic  centimeters  of  water  dissolved  every  two  sec- 
onds from  a  glass  flask  enough  free  alkali  to  neutralize  one  cubic  centimeter 
of  tenth-normal  acid;  this  glass  was,  no  doubt,  of  a  composition  exceptionally 
soluble. 

Platinum  is  usually  attacked  to  a  slight  degree  by  melted  fluxes;  traces 
of  platinum  can  often  be  discovered  in  a  solution  of  the  fusion  of  a  mineral 
with  alkali  carbonate. 

6.  Precipitation.  Too  great  an  excess  as  well  as  a  deficiency  of  the  pre- 
cipitant must  be  guarded  against,  the  former  by  calculating  the  amount  of 
reagent  required,  and  the  latter  by  testing  the  first  portion  of  the  filtrate. 

A  slight  excess  of  the  precipitant  markedly  reduces  the  amount  of  the  pre- 
cipitate retained  in  solution.  This  fact  is  well  illustrated  in  the  titration  of 
silver  nitrate  by  solution  of  sodium  chloride;  a  mixture  of  their  exact  com- 
bining weights  leaves  a  clear  supernatant  liquid  in  which  either  of  the  salts  pro- 
duces a  slight  precipitate  of  silver  chloride  by  lowering  the  solvent  power  of 
the  liquid.  As  the  precipitant  is  always  in  slight  excess  in  every  precipitation, 
the  correction  of  the  weight  of  the  precipitate  by  the  figures  of  the  published 
tables  of  the  solubilities  of  precipitates  in  water  is  inapplicable. 

Fibres  of  filter  paper,  dust  and  organic  matter  in  general,  may  impede  the 
complete  precipitation  of  a  base  by  alkalies  and  some  of  their  salts,  or  a  chem- 
ical action  may  result  in  the  reduction  of  the  per-salt  of  a  sensitive  compound 
to  a  proto-salt  or  lower,  giving  rise  to  some  perplexity.  The  addition  of  a 
few  drops  of  nitric  acid  or  a  crystal  of  potassium  chlorate  to  the  hot  acid 
solution  will  destroy  small  amounts  of  organic  matter.  If  a  volatile  organic 
compound  is  present  in  quantity,  it  may  be  expelled  by  evaporation  to  a  small 
bulk;  if  non-volatile,  by  evaporation  to  dryness,  and  gentle  ignition  to  car- 
bonization, or  fusion  with  an  oxidizing  flux. 

When  the  solution  of  a  reagent  contains  an  adjuvant  whose  purpose  is  to 
prevent  the  co-precipitation  of  other  bodies,  the  formation  of  the  precipitate  is 
somewhat  retarded  as  a  rule,  and  that  it  may  not  be  incomplete  or  include 
other  bodies  a  certain  ratio  must  be  preserved  between  the  reagent,  the 
adjuvant,  and  the  body  to  be  precipitated. 

Many  precipitates  deposit  so  slowly  that  several  hours  repose  is  required  for 
their  complete  segregation.  Generally  indicated  by  a  clear  supernatant  liquid, 
in  case  of  doubt  it  is  well  to  make  sure  by  setting  aside  the  filtrate  for  some 
time. 

Unless  otherwise  ordered  for  special  reasons,  a  precipitant  is  always  intro- 
duced into  a  solution  to  be  precipitated  in  the  form  of  a  solution,  and  never  as 
a  powder  or  crystals,  as  in  this  case  occlusion  of  the  precipitant  is  highly 
favored. 

The  valence  of  polyvalent  metals  to  be  precipitated  is  always  to  be  ascer- 
tained before  the  precipitant  is  introduced,  since  most  reactions  are  limited  to 


*  Zeits.  Anal.  23-518. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  197 

a  particular  phase.  And  generally,  attention  must  be  paid  to  the  precautions 
regarding  volume,  temperature,  reaction,  etc.,  of  solutions  laid  down  in  the 
description  of  the  method  or  left  to  the  discretion  of  the  chemist. 

7.  Defective  separation.  The  contamination    of  a    precipitate  by    another 
formed  by  some  secondary  reaction  with  the  oxygen  of  the  air,  or  fumes  of 
acids,  ammonia.,  hydrogen  sulflde,  etc.,  abundant  in  a  laboratory  occupied  by 
several  workers,  can  be  prevented   to  some  extent  by  keeping  the  solutions 
covered  as  far  as  practicable,  but  it  is  the  better  plan  to  select  another  method 
that  is  not  subject  to  this  drawback. 

A  separation  by  the  evaporation  of  a  solution  to  dryness,  causing  one  con- 
stituent to  pass  to  an  insoluble  form,  is  seldom  complete  after  but  one  evapo- 
ration, especially  where  the  solution  contains  also  much  saline  matter,  and  the 
residue  should  be  again  taken  up  with  a  little  water  or  acid  and  evaporated. 
Some  advise  heating  the  residue  to  above  100  o ,  but  it  is  not  well  to  much 
exceed  this  temperature  on  account  of  possible  inter-reactions  between  the 
constituents  of  the  residue. 

In  the  extraction  of  a  body  by  an  immiscible  solvent  (page  77),  an  emulsion 
may  be  formed  by  too  violent  shaking  in  mixing  the  solvents,  the  ease  of  emul- 
sifying the  two  liquids  being  dependent  on  the  nature  of  the  aqueous  solution 
and  its  reaction  and  that  of  the  organic  solvent.  To  induce  the  liquids  to 
ngain  separate  into  layers  there  may  be  applied  a  gentle  heat,  or  the  emulsified 
portion  drawn  off  and  shaken  with  some  of  the  organic  solvent,  or  a  little  alco- 
hol may  be  added  to  the  mixture ;  if  none  of  these  expedients  is  successful, 
nitration  or  distillation  must  be  resorted  to. 

The  decomposition  of  a  complex  precipitate  or  residue  by  a  reagent,  a  part 
becoming  transformed  to  another  insoluble  compound  and  a  part  entering  solu- 
tion, seldom  affords  a  complete  separation,  and  it  is  prudent  to  further 
test  the  insoluble  matter  for  inclosed  particles  of  the  original. 

8.  Filtration  and  washing.  The  common  grades  of  filter  paper  may  contain 
a  considerable  amount  of  soluble  impurities.     Fade"  found  in  a  sample  of  white 
filter  paper  10.4  per  cent  of  ash  of  which  7.4  per  cent  was  calcium  sulphate;  but 
such  a  large  percentage  of  ash  is  very  unusual.    The  quality  of  the  paper  sold 
for  quantitative  analysis  by  reputable  dealers  is  generally  sufficiently  pure 
for  all  ordinary  analyses.* 

The  asbestos  used  in  the  Gooch  crucible  must  always  be  previously  purified 
by  boiling  with  hydrochloric  acid,  and  in  accurate  analyses  it  is  prudent  to  dis- 
solve out  the  precipitate  after  weighing  and  reweigh  the  crucible  and  felt 
to  discover  if  any  notable  loss  has  been  sustained  during  the  filtration  and 
washing;  but  strong  acids  and  caustic  solutions  are  not  admissible  for  this 
purpose  on  account  of  their  solvent  action  on  pure  asbestos. 

In  practice  the  extent  to  which  a  precipitate  is  to  be  washed  is  governed  by 
several  considerations:  (1),  the  concentration  of  the  solution;  (2),  the  adhe- 
sion of  the  solution  to  the  precipitate;  (3),  the  adsorption  of  the  solution  in 
the  precipitate ;  (4),  the  adsorption  of  the  solution  in  the  filter  paper;  (5), 
irregularities  in  the  permeation  of  the  precipitate  by  the  wash  water;  and  (6), 
ununiform  displacement  of  the  solution  by  wash  water,  due  to  the  mixing  of 
the  two. 

Defective  washing  of  a  precipitate  is  a  most  frequent  source  of  high  results, 
especially  when  the  subject  is  flocculent  or  gelatinous,  enveloping  impurities 
and  retaining  them  with  some  tenacity.  The  precipitate,  if  at  all  bulky,  should 
be  well  roused  during  each  addition  of  wash  water,  and  care  be  taken  that  the 
edge  of  the  paper  is  not  neglected.  With  a  precipitate  difficult  to  wash,  it  is 


*  Journ.  Socy.  Chem.  Ind.  1898—70. 


198  QUANTITATIVE   CHEMICAL   ANALYSIS. 

well,  after  a  preliminary  washing,  to  disintegrate  it  by  drying  or  freezing,  then 
complete  the  washing;  or  to  transfer  it  from  the  filter  back  to  the  beaker  by  a 
jet  of  water  from  the  wash  bottle,  and  boil  for  a  short  time.  The  first  way  is 
safer  with  precipitates  that  may  slightly  dissolve  on  boiling  with  a  solution  of 
the  impurities  —  e.  g.,  traces  of  an  alkali  chromate  are  formed  when  chromic 
hydrate  is  boiled  with  a  solution  of  an  alkali  chloride. 

On  the  other  hand,  too  low  results  may  sometimes  be  attributed  to  imperfect 
washing;  as  where  a  precipitate  on  ignition  forms  a  volatile  compound  with 
the  remaining  impurity,  e.  g.t  ferric  hydrate  reacts  with  ammonium  chloride 
to  form  ferric  chloride. 

The  last  washing  should  always  be  tested  for  proof  that  everything  soluble  in 
the  wash  water  has  been  extracted.  Where  the  precipitate  itself  is  somewhat 
soluble  a  correction  may  sometimes  be  applied  for  the  amount  passing  into 
solution  in  the  wash  water,  but  no  reliance  can  be  placed  on  the  tables  of  the 
solubilities  of  precipitates  in  water  for  this  purpose,  for,  among  other  reasons, 
the  wash  water  is  usually  in  contact  with  the  precipitate  for  so  short  a  time 
that  the  solution  never  reaches  saturation. 

For  a  final  examination  as  to  the  purity  of  a  weighed  precipitate  it  may  be 
dissolved  or  lixiviated  by  a  suitable  liquid,  lixiviation  best  after  a  structural  or 
molecular  change  has  destroyed  the  absorptive  power  of  the  precipitate  or 
rendered  it  more  permeable. 

9.  Volumetric  analysis.  Although  the  graduation  of  volumetric  ware  is 
usually  accurate  enough  for  practical  purposes,  yet  the  correctness  of  any  in- 
strument should  never  be  taken  for  granted.*  Directions  for  recalibrating 
will  be  found  on  page  136. 

Aqueous  solutions,  alcohol,  and  the  like  adhere  to  the  interior  of  glass  ves- 
sels to  a  greater  or  less  extent  than  pure  water;  thus,  a  pipette  calibrated  for 
1000  grains  of  water  delivered  only  995.2  grains  of  a  strong  solution  of  zinc 
sulfate.  In  dealing  with  fairly  concentrated  solutions,  the  more  accurate  plan 
is  that  of  weighing. 

Pipettes  and  burettes  should  always  be  in  a  vertical  position  while  their  con- 
tents are  discharging;  even  an  inclination  of  ten  degrees  perceptibly  lessens 
the  volume  delivered. 

In  practice  small  changes  in  the  temperature  of  volumetric  solutions  need 
hardly  be  considered,  but  if  differing  more  than  a  few  degrees  from  that  at  the 
preceding  standardization,  should  be  raised  or  lowered  to  correspond,  or  re- 
standardized.  Corrections  for  a  change  in  temperature  are  open  to  criticism 
owing  to  the  specific  rate  of  expansion  of  solutions  even  when  dilute,  and 
scanty  and  unconfirmed  data  therefor. 

It  has  been  recommended  that  stock  solutions  be  made  of  such  a  strength 
that  equal  or  nearly  equal  volumes  shall  react  "  in  order  that  the  expansions 
and  contractions  which  the  two  liquors  undergo,  by  reason  of  changes  in  tem- 
perature of  the  laboratory,  should  be  without  influence  on  the  results  "  of  the 
titration.  But  this  direction  holds  only  when  it  has  been  ascertained  that  the 
two  equivalent  solutions  expand  by  heat  in  the  same  ratio.  However,  it  is  ad- 
visable where  a  stock  solution  is  used  from  time  to  time  to  verify  the  strength 
of  a  standard  solution,  that  the'  bottles  be  stored  side  by  side  that  they  may 
remain  at  the  same  temperature. 

The  temperature  of  the  volumetric  room  should  be  fairly  constant,  for  if  a 
solution  is  warmer  or  colder  than  the  air  of  the  room,  when  poured  into  a  nar- 
row burette  the  solution  will  gradually  contract  or  expand  during  a  titration. 

The  depth  of  color  to  be  considered  as  the  end-point  of  a  titration  Is  to  some 


*  Analyst.  1898—3. 


QUANTITATIVE    CHEMICAL    ANALYSIS.  199 

extent  a  matter  of  individual  choice.  Some  accept  the  least  indication  of  a 
change,  while  others  prefer  an  unmistakable  deep  tint,  but  in  either  case  there 
should  be  an  endeavor  at  uniformity  in  the  standardization  and  the  titration  of 
the  sample  analyzed.  A  common  fault  of  beginners  is  that  of  introducing  too 
great  a  proportion  of  indicator  to  the  volume  of  the  titrate,  the  pronounced 
color  unduly  extending  the  transition  tint. 

The  error  due  to  the  excess  of  titrate  required  to  produce  the  end  reaction 
is  generally  negligible.  Where  otherwise,  a  correction  is  to  be  found  by  com- 
pounding a  blank  solution  of  the  same  volume  as  the  titrate,  and  of  about  the 
same  composition,  including  the  products  of  the  reaction.  The  fraction  of  a 
cubic  centimeter  of  the  titrand  required  to  plainly  show  the  end-point  in  the 
solution  is  to  be  deducted  from  the  actual  determination  and  the  same  correc- 
tion be  made  in  the  standardization. 

When  the  end -point  is  observed  by  testing  a  drop  of  the  titrate,  the  amount 
removed  for  this  purpose  in  the  course  of  a  titration  undoubtedly  occasions  a 
loss.  But  considering  the  comparatively  great  bulk  of  the  titrate  and  that  the 
drops  are  mostly  withdrawn  toward  the  end  of  the  operation  (when  the  con- 
centration as  regards  the  reacting  body  has  been  largely  reduced),  the  deficit 
is  seldom  of  consequence. 

A  persistent  tenacious  froth  may  be  thrown  up  on  stirring  certain  titrates  of 
an  organic  nature  that  intercepts  and  retains  drops  of  the  titrand.  The  ten- 
dency may  be  diminished  by  the  addition  of  alcohol  —  sometimes  the  vapor  of 
alcohol  alone  will  dissipate  the  foam. 

10.  Colorimetry.  The  depth  of  color  of  a  liquid  increases  with  the  thickness 
of  the  layer  through  which  light  is  transmitted  to  the  eye ;  and  when  a  colored 
solution  is  diluted  with  one  that  is  colorless  the  shade  lightens.    But  in  neither 
case  can  it  be  taken  for  granted  that  the  increase  or  reduction  is  strictly  in 
proportion  to  the -depth  or  dilution.    The  variation  may  be  greater  or  less, 
according  to  the  nature  of  the  chromogen  and  solvent,  nevertheless  it  is  always 
safest  to  approximate,  as  nearly  as  may  be,  the  standard  to  the  sample  both  in 
shade  and  general  composition,  the  former  by  tentative  trials.    The  rule  applies 
whether  the  liquids  are  viewed  axially  or  transversely,  in  round  tubes,  flat 
cells,  or  glass  wedges. 

The  depth  of  color  of  some  organic  bodies  increases  for  a  time  after  being 
brought  into  solution,  and  nearly  all  fade  on  long  exposure  to  daylight. 
However,  if  the  maximum  color  is  reached  within  an  hour  and  the  fading  does 
not  begin  to  be  apparent  for  a  day  or  more,  a  comparison  is  not  interfered  with. 

It  should  be  observed  that  both  standard  and  sample  are  in  clear  solution 
and  at  a  uniform  moderate  temperature;  that  the  glass  of  the  comparison  tubes 
is  clear  and  colorless,  and  their  internal  diameter  the  same;  and  that  the 
source  of  light  is  directly  in  line  with  the  eye  and  tubes,  and  no  side-lights 
interefere.  The  background  to  the  tubes  should  be  white  paper  or  unglazed 
porcelain  since  a  polished  surface  may  reflect  images. 

Before  making  a  final  comparison  it  is  well  to  rest  the  eyes  for  a  short  time 
on  a  screen  of  the  color  complementary  to  that  of  the  tubes. 

11.  Impurities  in  reagents.    No  chemical  can  be  purchased  absolutely  pure, 
nor  is  this  necessary  as  a  rule,  for  a  distinction  is  to  be  made  between  such 
impurities  as  are  objectionable  for  a  particular  analysis  and  others  that  cannot 
affect  its  accuracy.    Thus,  in  the  analysis  of  pyrite,  even  a  considerable  amount 
of  sulfuric  acid  In  the  aqua  regia  used  for  solution  will  have  no  effect  on  the 
determination  of  the  iron;  while  for  the  estimation  of  the  sulfur,  of  course  the 
acid  and  every  other  reagent  should  be  free  from  this  element.    So  it  is  well 
before  commencing  an  analysis  to  consider  what  impurities  may  be  in  the 
reagents  without  detriment  and  what  must  be  absent,  and  test  for  the  latter. 


200  QUANTITATIVE    CHEMICAL    ANALYSIS. 

The  proportion  of  an  interfering  impurity  in  a  reagent  is  best  found  by  a 
blank  or  parallel  determination.  Another  plan  is  to  make  two  determinations 
on  the  sample  to  be  analyzed,  in  one  case  taking  double  the  weight  of  the  other, 
but  with  equal  quantities  of  the  reagent  in  both.  The  difference  between  the 
results,  calculated  in  percentages,  is  that  due  to  the  impurities  in  the  reagent 
and  can  be  used  as  a  factor  for  the  correction  of  future  analyses  made  under 
like  conditions. 

The  use  of  a  volatile  solvent  that  has  been  recovered  from  an  organic  analy- 
sis by  distillation  is  often  open  to  criticism  since  it  may  retain  other  volatile 
bodies  difficult  to  remove  by  the  ordinary  process  of  purification,  and  that  may 
interfere  in  future  analyses. 

As  to  the  quality  of  the  chemicals  to  be  used  in  analysis  it  may  be  affirmed 
that  the  purest  are  ultimately  the  cheapest.  The  impurities  in  a  crude  chemical 
are  apt  to  be  of  the  most  varied  nature,  and  there  is  always  an  uncertainty  as 
to  whether  some  unsuspected  one  may  not  tend  to  vitiate  or  at  least  disturb 
the  normal  conduct  of  a  determination. 

12.  Calculation.  The  most  recent  official  tables  of  atomic  weights  are  doubt- 
less nearest  correct  and  should  be  employed  exclusively.  The  table  is  revised 
annually  and  is  usually  published  in  two  divisions;  in  one  the  basis  is  hydro- 
gen as  unity,  oxygen  being  15.976,  and  in  the  other  the  basis  is  oxygen  at  16, 
hydrogen  becoming  1.008.  Since  one  is  the  counterpart  of  the  other  there 
will  be  no  difference  whatever  in  results  calculated  entirely  by  either  table,  or 
at  most  an  inconsiderable  variation  from  the  rounding  off  of  the  values  of  the 
different  elements  at  the  first  or  second  decimal  place. 

It  may  seem  superfluous  to  recommend  that  every  computation  should  be 
verified  by  a  repetition,  yet  it  is  surprising  how  frequently  a  simple  arithmeti- 
cal problem  is  missolved.  One  should  early  acquire  the  habit  of  corroborat- 
ing every  observation  —  twice  noting  the  label  of  a  reagent  bottle  or  the 
reading  of  a  burette  or  the  rider- scale;  reckoning  a  weight  from  the  gaps  in 
the  weight-box,  then  from  the  weights  on  the  pan ;  in  short,  checking  up  his 
work  wherever  possible. 

A  common  oversight  is  that  of  not  taking  into  account  all  the  atoms  when 
dividing  a  molecule ;  thus,  a  molecule  of  ferric  oxide  (Fe2O3,  mol.  wt.  112) 

contains  two  atoms  of  iron  or  - —   of  its  weight.     After  writing  out  an  equa- 

160 

tion  one  should  check  off  the  right-hand  side  against  the  left,  to  make  sure 
that  every  atom  appears  in  both. 

Of  a  mixture  that  is  mainly  organic,  certain  inorganic  constituents  may  be 
soluble  in  water  and  therefore  be  included  in  both  the  aqueous  extract  and 
the  ash.  With  such  material  the  residue  from  the  watery  extract  should  be 
calcined  and  the  residue  deducted  from  the  weight  of  the  ash  of  the  material 
itself. 


Of  all  the  foregoing,  the  only  errors  that  cannot  be  avoided  in  obvious  ways, 
at  least  to  a  great  extent,  are  the  solubility  of  precipitates  and  their  alteration 
on  heating,  for  which  the  method  should  have  made  provision,  and  the  solu- 
bility of  glass  and  porcelain  in  the  analytical  solutions,  which  should 
not  greatly  influence  the  results  provided  the  vessels  are  of  a  resisting 
quality. 

The  student  will  be  fortunate  if  he  progresses  far  without  encountering 
difficulties  and  anomalies  quite  enough  to  tax  his  patience.  Through  neglect 
of  a  seemingly  unimportant  detail,  a  simple  operation,  ordinarily  successful, 


QUANTITATIVE    CHEMICAL    ANALYSIS.  201 

may  display  a  perverse  inclination  to  go  wrong;  in  spite  of  attempts  at  rectifi- 
cation the  trouble  persists  or  increases  until  he  may  be  tempted  to  affirm  that 
the  laws  of  chemistry,  commonly  believed  immutable,  have  been  temporarily 
reversed  expressly  for  his  personal  annoyance.  Yet  if  the  study  is  begun  with 
the  more  simple  analyses  where  the  conditions  admit  of  considerable  modifica- 
tion without  impairing  their  efficiency  and  the  manipulations  are  such  that  no 
great  expertness  is  demanded,  he  will  soon  acquire  such  a  familiarity  with  the 
details  of  working  as  to  foresee  at  what  points  and  under  what  circumstances 
errors  are  liable  to  occur,  and  so  forewarned,  forearmed;  or  when  an  unex- 
pected difficulty  presents  itself,  to  know  and  apply  the  proper  means  to  over- 
come it.  Given  a  well-tried  method,  a  fair  acquaintance  with  the  principles 
of  analysis,  and  reasonable  dexterity  in  manipulation,  a  result  that  is  erroneous 
or  doubtful  should  be  an  exception. 


PART   2. 


EXEECISES, 


BEAGENTS. 

The  reagents  employed  in  the  following  analyses  are  now  manufactured  by 
several  European  and  American  firms,  of  quite  sufficient  purity  for  analytical 
work;  yet  it  is  never  the  part  of  prudence  to  omit  their  examination,  for  in 
the  chemical  market  the  designation  "C.P."  is  so  elastic  as  to  cover  every 
grade  from  the  crude  to  the  most  refined.  When  it  is  difficult  to  purchase  a 
chemical  of  the  proper  quality,  the  student  will  find  it  instructive  to  prepare  a 
quantity  synthetically  or  purify  the  commercial  article. 

Most  of  the  salts  described  below  are  to  be  dissolved  in  distilled  water  and 
the  solutions  diluted  to  the  strength  specified.  No  strictly  uniform  system  of 
moderate  concentration  can  be  adopted  since  the  solubilities  of  the  various 
reagents  differ  so  greatly.*  A  decimal  system  is  probably  as  convenient  as  any  — 
one  part  of  the  reagent  in  ten  parts  of  water,  and  where  the  solubility  will  not 
permit  of  this  strength,  in  twenty,  thirty,  etc.,  parts  water.  Sparingly  soluble 
reagents  are  conveniently  made  upas  saturated  solutions,  a  layer  of  the  reagent 
covering  the  bottom  of  the  bottle. 

The  solutions  are  preserved  in  chemical -glass  (lead-free)  bottles  provided  with 
glass  stoppers  and  protected  from  dust  and  fumes  by  caps  or  inverted  beakers. 
The  labels  are  best  molded  or  etched  in  the  glass,  though  paper  ones  will  answer 
if  given  a  coat  of  white  varnish  or  washed  with  a  solution  of  paraffin  in 
gasoline.  The  stoppers  and  bottles  should  be  numbered  conjointly  to  prevent 
interchange,  as  no  two  ground  stoppers  will  accurately  fit  the  neck  of  one 
bottle.  Solutions  of  caustic  potash  or  soda  will  firmly  cement  the  stopper  to 
the  neck  if  the  bottle  is  left  unopened  for  some  time;  interposing  a  strip  of  thin 
platinum  foil  will  prevent  this,  or  a  closely  fitting  glass  cap  may  be  substituted 
for  the  stopper. 

Hydrofluoric  acid  and  solutions  of  the  caustic  alkalies  and  their  carbonates 
are  best  kept  in  bottles  of  platinum,  silver  or  ceresin.  Black  glass  protects 
silver  solutions,  etc.,  from  decomposition  by  light;  amber  glass  is  less  efficient. 
Reagents  used  in  the  solid  form  are  kept  in  wide-mouth  (salt-mouth)  bottles 
and  dispened  with  a  horn  spoon,  while  insoluble  precipitants,  such  as  certain 
metallic  oxides  and  carbonates,  are  kept  for  use  suspended  in  water. 

Of  solutions  acting  strongly  on  glass  and  those  that  decompose  or  ferment 
on  standing  it  is  better  to  dissolve  about  the  required  weight  of  the  compound 
just  before  using.  The  concentration  of  a  saturated  aqueous  solution  of 
hydrogen  sulfide  is  so  small  and  it  so  early  decomposes  on  keeping  that  it  is 
usually  better  to  transmit  a  current  of  the  gas  through  the  liquid  to  be 
precipitated  than  to  depend  on  the  hydrogen  sulfide  water. 

In  most  cases  the  reagent  alone  is  dissolved  in  water  or  other  simple  solvent, 
but  a  vehicle  in  the  form  of  another  chemical  is  sometimes  incorporated  in  the 
solution  for  specific  reasons.  If  the  adjuvant  is  to  take  part  in  the  reactions 
of  the  principal,  the  object  of  its  introduction  may  be  to  assist  in  solution  or 
precipitation;  to  change  the  composition  of  a  precipitate  to  one  more  stable  or 
of  an  aggregation  easier  to  filter  and  wash;  to  cause  or  aid  in  the  solution  or 
precipitation  of  an  associated  body,  or  to  prevent  its  solution  or  precipitation, 


*  Chem.  News,  1890— 1-245. 

(205) 


206  QUANTITATIVE    CHEMICAL   ANALYSIS. 

etc.  If  taking  no  direct  part  in  the  reactions,  the  purpose  may  be  that  of 
effecting  the  solution  of  the  insoluble  or  sparingly  soluble  principal ;  to  diminish 
the  solubility  of  a  precipitate  or  residue;  to  induce  an  alteration  of  the  physical 
structure  of  a  precipitate;  to  prevent  decomposition,  coagulation,  putrefaction, 
or  crystallization  of  the  principal;  or  by  its  superior  affinity  for  oxygen  or 
carbon  dioxide  to  preserve  it  from  alteration  through  contact  with  the  air,  etc. 
Many  of  the  reagents  in  common  use  are  extremely  poisonous,  not  only  when 
taken  internally,  but  even  on  coming  in  contact  with  an  abrasion  of  the  skin  or 
a  mucous  membrane,  notably  potassium  cyanide,  arsenic  compounds,  chlor- 
acetic  and  hydrofluoric  acids.  The  vapors  of  hydrocyanic  and  hydrofluoric  acids, 
arsine,  the  halogen,  hydrogen  sulfide,  etc.,  undiluted  by  air  are  active  poisons, 
and  very  deleterious  to  the  respiratory  organs  even  when  considerably  diluted. 
As  many  serious  and  not  a  few  fatal  accidents  have  come  about  through  care- 
lessness, the  student  is  urged  to  be  continually  on  his  guard  in  this  respect. 
And  great  care  should  be  taken  when  using  ether,  gasoline,  and  such  light 
volatile  bodies,  as  their  vapors  are  very  diffusive  and  may  catch  fire  at  a  con- 
siderable distance  from  the  liquids. 


To  prepare  a  volume  F  of  a  solution  whose  specific  gravity  shall  be  d,  by 
dilution  of  a  volume  X  whose  specific  gravity  is  a,  the  specific  gravity  of 
water  being  b.  The  volume  of  the  original  solution  is  found  from  the  equation 

X—  V  d~~b   ,  and  the  volume  of  water  to  be  added  to  Xis  V— X. 
a  —  b 

Conversely,  to  dilute  a  certain  volume  F  of  a  liquid  of  a  specific  gravity  a  to 
one  of  a  lower  gravity  d,  the  equation  reads  X==  V  a  ~~  ?  X  being  the 

volume  of  the  diluted  solution  and  X —  Fthe  volume  of  water  to  be  added. 
If  a  grams  of  a  solid  is  contained  in  b  grams  of  a  solution,  and  X  is  the 

o,     # 
weight  of  the  solid  in  one  Cc.  of  the  solution,  then  X=  ^ * 


Alcohol.   CjHeo. 

1.  Absolute  alcohol.  Prepared  by  the  fermentation  of  sugar;  rectified,  and  freed  from 
water  by  distillation  from  a  highly  hygroscopic  solid.    It  is  a  colorless  volatile  liquid  of 
agreeable  odor,  and  mixes  in  all  proportions  with  water  and  ether.    When  anhydrous  it 
has  a  specific  gravity  of  .794,  solidifies  at  —130°,  and  boils  at  78.4°. 

2.  Commercial.  Sold  as  containing  95  per  cent,  but  is  often  found  to  assay  only  90  to  94 
per  cent,  with  6  to  10  per  cent  of  water,  and  traces  of  organic  acids,  aceton,  fusel  oil, 
aldehyd,  etc. 

3.  Proof  spirit.  Has  a  gravity  of  .920  and  contains  49.24  per  cent  by  weight  or  57.06  per 
cent  by  volume  of  alcohol . 

The  commercial  95  per  cent  article  is  sufficiently  pure  and  strong  for  most  analytical 
purposes ;  it  should  completely  volatilize  without  an  unpleasant  odor,  and  dissolve  pure 
caustic  potash  without  acquiring  at  the  time  more  than  a  light  yellow  color.  Fusel  oil  is 
detected  by  the  silver  nitrate  test. 

Commercial  alcohol  invariably  contains  a  little  free  acid,  aceton,  and  aldehyd.  No 
process  is  known  by  which  the  last  two  can  be  entirely  removed;  the  process  of  Waller  f 
giving  a  fair  purification  is  as  follows.  Powdered  potassium  permanganate  is  slowly 
stirred  In  the  alcohol  until  the  color  has  become  a  deep  purple;  after  standing  until  all 
the  manganese  has  been  deposited  as  hydrated  peroxide,  the  alcohol  is  decanted  and  dis- 


*  Journ.  Amer.  Chem.  Socy.  1897  —  587. 
t  Chem.  News,  1890—1—53. 


REAGENTS.  207 

tilled,  with  the  addition  of  a  little  calcium  carbonate,  from  a  flask  with  a  fractionating 
column.    The  distillation  should  be  slow  and  the  first  and  last  fractions  rejected. 

Alcohol  strictly  neutral  in  reaction  is  required  for  dissolving  fatty  acids  to  be  titrated 
by  a  standard  alkali.  ,To  commercial  alcohol  is  added  a  few  drops  of  phenol -phthalein  or 
turmeric  solution  and  dilute  solution  of  caustic  potash  dropped  in  until  the  reaction  has 
become  distinctly  alkaline,  then  more  alcohol  is  dropped  in  until  the  red  color  has  almost 
disappeared. 

Ammonium  chloride.    NH-iCl,  53.522. 

(Sal-ammoniac.)  White  crystals,  soluble  in  three  parts  of  cold  and  1.4  parts  of  boiling 
water.  As  purchased  it  generally  contains  some  insoluble  matters,  though  yielding  a  rea- 
sonably pure  solution.  Recrystallization  is  advisable  when  the  salt  is  to  be  used  for  the 
determination  of  the  alkalies  In  silicates ;  the  filtered  solution  is  evaporated  up  to  the  point 
of  crystallization,  and  while  cooling  is  constantly  stirred  that  small  crystals  may  form. 

The  salt  should  volatilize  completely  when  heated  on  platinum  foil,  and  be  free  from 
sulfate. 

Ammonia.    NHs,  17.064. 

A  solution  of  gaseous  ammonia  in  water,  one  volume  of  water  at  zero  and  760  Mm.  of 
mercury  absorbing  about  1100  volumes  of  the  gas. 

Ammonic  hydrate,  NH4OH,  35.08.  As  purchased,  the  usual  specific  gravity  is  .JL  .  The 
solution  should  be  colorless,  leave  no  residue  on  evaporation,  and  give  no  precipitate  with 
nitric  acid  and  silver  nitrate  (chlorine),  barium  chloride  (sulfuric  acid),  or  hydrogen 
snlfide  (lead,  etc.). 

One  gram  of  the  solution  of  .900  at  15  o  Cent,  contains  .283  gram  of  NHs,  or  .581  gram  of 
NH4OH ;  one  Cc.  contains  .255  gram  of  NHs,  or  .524  gram  of  NHiOH,  aud  neutralizes  .545 
gram  of  HC1,  .942  gram  of  HNOs,  or  .733  gram  of  H2SO4. 

Used  to  neutralize  free  acids  and  to  precipitate  the  hydroxides  of  weaker  bases.  Both 
the  concentrated  solution  and  the  tenth  dilution  are  needed. 

Ammonium  carbonate. 

As  found  in  the  market  it  approaches  the  formula  3NH3.2CO2.H2O  or  NsHiiCgOs.  It 
should  volatilize  completely  when  heated  on  platinum  foil,  and  after  dissolving  in  an 
excess  of  dilute  hydrochloric  acid  give  no  turbidity  with  barium  chloride. 

Used  for  neutralization,  and  to  precipitate  the  carbonates  of  calcium,  barium,  lead,  etc. 
The  solution  is  made  up  just  before  use  by  dissolving  in  water  containing  a  little  ammonia 
and  filtering. 

Ammonium  oxalate.    (NH4)2C2O4.H2O,142.116. 

Colorless  needles  soluble  in  about  25  parts  of  cold  water.  Generally  quite  pure, 
but  may  contain  a  trace  of  lead  or  calcium ;  may  be  purified  by  recrystallization  from  hot 
water.  Should  leave  no  residue  on  ignition. 

Solution,  one  part  in  25  of  water.  Used  to  precipitate  calcium,  lead,  and  a  few  other 
metals,  and  as  a  standard  reducing  agent.  One  gram  of  calcium  oxide  is  precipitated  by 
2.535  grams,  and  one  gram  of  potassium  permanganate  is  reduced  by  2.2483  grams  of  the 
crystallized  salt. 

Barium  chloride.    BaCl2.2H2O,  .44.332. 

Colorless  transparent  crystals  soluble  in  2.4  parts  of  cold,  and  1.3  parts  of  hot  water. 
Prepared  by  dissolving  barium  carbonate  in  an  insufficiency  of  hydrochloric  acid  for 
entire  solution  and  crystallizing  after  filtration ;  or  may  be  precipitated  as  a  powder  by 
adding  alcohol  or  concentrated  hydrochloric  acid  to  the  strong  aqueous  solution. 
Used  almost  exclusively  for  the  determination  of  sulfuric  acid.  Solution,  one  part  of 
the  crystallized  salt  in  ten  parts  of  water.  One  gram  of  H2SO4  is  precipitated  by  2.491 
grams  of  the  crystallized  salt;  one  gram  of  SOs  by  3.052  grams;  and  one  gram  of  sulfur 
(converted  into  sulfuric  acid)  by  7.619  grams. 

Barium  hydrate.    Ba(OH)2.H2O,  315.544. 

The  crystals  are  soluble  in  about  20  parts  of  cold  water.  Used  as  an  absorbent  for 
carbon  dioxide  from  a  mixture  of  gases,  as  a  precipitant  for  the  weaker  bases  in  sepa- 
rations, and  to  remove  carbon  dioxide  from  volumetric  solutions  of  the  caustic  alkalies. 
One  gram  of  sodium  carbonate  (Na2COs)  is  rendered  caustic  by  2.974  grams  of  the  crystals 
and  one  gram  of  K2CO3,  by  2.283  grams. 

Battery  Fluid. 

The  liquid  for  exciting  the  battery  described  on  page  249  is  made  by  adding  to  2000  Cc. 
of  cold  water,  400  Cc.  of  commercial  oil  of  vitriol,  and  after  cooling,  dissolving  300  grams 
of  commercial  sodium  chromate  in  the  mixture. 


208  QUANTITATIVE    CHEMICAL   ANALYSIS, 


Bromine.     Br,  79.95. 

A  dark-red,  corrosive,  volatile  liquid  of  specific  gravity  2.99,  solidifying  at  — 25  o  Cent. 
Soluble  in  about  29  parts  of  cold  water,  and  freely  in  alcohol,  ether,  and  carbon  disulfide. 
It  is  used  as  an  oxidizing  agent.  A  saturated  solution  is  prepared  by  pouring  some  of  the 
commercial  article  into  a  glass -stoppered  bottle  and  washing  with  water  once  or  twice ; 
the  bottle  is  then  filled  with  water  and  after  standing  for  a  time,  with  occasional  shaking 
up,  is  ready  for  use. 

The  bottle  in  which  bromine  is  purchased  should  be  opened  with  care  in  the  open  air. 
If  the  stopper  cannot  be  removed,  a  cord  may  be  tied  round  the  neck,  saturated  with 
alcohol,  and  lighted ;  the  heat  will  usually  crack  the  neck  so  that  the  stopper  can  be  broken 
out  and  the  bromine  poured  into  another  bottle. 

Calcium  carbonate.    CaOOs. 

Used  in  J.  Lawrence  Smith's  method  for  alkalies  In  silicates.  It  must  be  free  from 
potassium  and  sodium  salts,  frequent  contaminations  of  the  commercial  product. 

Prepared  by  dissolving  calcite  or  chalk  in  dilute  hydrochloric  acid  leaving  a  portion 
nndissolved ;  then  precipitating  any  magnesia  or  other  base  by  milk  of  lime.  The  filtered 
liquid  is  heated  nearly  to  boiling,  precipitated  by  solution  of  ammonium  carbonate,  washed 
With  hot  water,  and  dried. 

Chloroform.   CHCls. 

Trichlormethane.  A  colorless,  volatile  liquid,  specific  gravity  1.51  at  15°,  boiling  at  61°, 
and  having  a  sweet  taste  and  agreeable  odor.  It  is  soluble  in  about  200  parts  of  water,  and 
mixes  in  all  proportions  with  alcohol  and  ether.  It  should  be  neutral  to  litmus  paper  and 
leave  no  residue  on  evaporation.  The  commercial  article  (U.  S.  Pharmacopoeia  1900)  is 
generally  pure  enough  for  most  purposes;  if  not  it  is  to  be  redistilled. 

Chloroplatinic  acid.    H2PtCl6.6H2O,  517.712. 

A  brown  deliquescent  solid  very  soluble  in  water  and  alcohol.  If  purchased  it  should 
be  tested  for  sulfuric  acid;  also  for  sodium  chloride  by  ignition  and  extraction  of  the 
residual  platinum  by  dilute  nitric  acid,  which  after  filtering  should  leave  no  residue  on 
evaporation. 

May  be  prepared  as  follows.  Boil  soft  scrap  platinum  cut  in  very  small  pieces,  in  con- 
centrated hydrochloric  acid  to  clean  them;  weigh,  and  boil  with  aqua  regia  (three  volumes 
of  hydrochloric  to  one  of  nitric)  until  dissolved.  Evaporate  to  dryness,  dissolve  in  con- 
centrated hydrochloric  acid,  and  again  evaporate.  Dissolve  the  residue  in  water,  ten  cubic 
centimeters  for  each  gram  of  platinum,  and  filter.  The  product  is  sufficiently  pure  for 
analytical  purposes;  it  should  be  preserved  in  a  glass-stoppered  bottle  away  from  dust  and 
ammonia  fumes. 

Used  as  a  precipitant  for  potassium  and  ammonia  chlorides.  Of  a  solution  of  the  above 
strength,  one  gram  of  potassium  chloride  is  precipitated  by  13.1  Co.;  of  potassium  as 
chloride  by  25  Cc. ;  of  ammonium  chloride  by  18.2  Cc. ;  and  of  ammonia  (NHs)  as  chloride 
by  57.2  Cc.  A  large  excess  is  always  needed  to  lessen  the  solubility  of  the  precipitates. 

Ferrous  sulfate,  crystallized,  FeSO4.7H2O,  278.182. 

Transparent  light-green  crystals  soluble  in  1.64  parts  of  cold  and  .3  part  of  hot  water, 
Insoluble  in  alcohol. 

The  principal  impurity  of  the  commercial  salt  is  ferric  snlfate,  removed  by  dissolving 
the  crystals  in  water  containing  sulfurous  acid  and  crystallizing,  or  by  precipitating  the 
concentrated  aqueous  solution  by  alcohol.  Both  the  solution  and  the  crystals  are  slowly 
perduced  on  exposure  to  the  air. 

Used  as  a  reducing  agent,  and  in  volumetric  analysis  for  standardizing  permanganate 
solution.  One  gram  of  the  crystallized  salt  contains  .201365  gram  of  iron,  and  8.7983  grams 
reduce  one  gram  of  potassium  permanganate. 

Hydrochloric  acid .    HC1 ,  36 . 458 . 

(Muriatic  acid,  chlorhydric  acid,  spirits  of  salt.)  A  colorless  gas  soluble  in  l-500th  of  its 
weight  or  water  at  zero  and  760  Mm .  The  specific  gravity  of  the  commercial  acid  is  usually 
1.200.  It  should  be  colorless,  leave  no  residue  on  evaporation,  and  after  concentration 
with  the  addition  of  a  crystal  of  potassium  nitrate,  not  be  precipitated  by  a  dilute  solution 
of  barium  chloride.  Free  chlorine  is  shown  by  the  blue  color  developed  by  dilute  solution 
of  potassium  iodide  and  starch -paste,  and  arsenic  by  a  brown  color  with  stannons  chloride 
and  sulfuric  acid. 

It  is  of  general  use  as  a  solvent  for  minerals,  ores,  oxides,  etc  Of  the  gravity  of  1.200, 
one  gram  contains  about  .400  gram  of  HOI;  one  Cc  contains  480  gram.  One  Cc.  neutral- 
lizes  .739  gram  of  potassium  hydrate;  .527  gram  of  sodium  hydrate;  and  .225  gram  of 
ammonia  (NHs). 


REAGENTS.  209 


Hydrogen  peroxide.    H2O2,  34.016. 

The  commercial  solution  of  the  gas  is  usually  stated  to  be  of  ten  or  twelve  volume 
strength,  meaning  that  one  volume  of  the  solution  contains  five  or  six  volumes  of  the  gas 
and  evolves  ten  or  twelve  volumes  of  oxygen  when  decomposed  by  potassium  permangan- 
ate. It  slowly  loses  strength  on  keeping,  less  readily  if  the  solution  contains  a  small 
amount  of  an  acid  or  a  metallic  salt. 

Iron.    Fe,  56. 

The  grade  of  Iron  wire  known  to  the  trade  as  "  malleable  wire  "  is  made  from  the  best 
quality  of  refined  iron  and  may  be  considered,  without  sensible  error,  to  contain  99.9  per 
cent  of  metallic  iron  and  .1  per  cent  of  slag,  carbon,  etc.  Wire  of  this  purity  of  a  diameter 
of  No.  21  American  wire  gauge,  cut  in  six-inch  lengths,  is  sold  by  dealers  in  chemicals.  It 
should  be  cleaned  before  weighing  by  drawing  between  a  fold  of  emery-paper. 

When  dissolved  In  hydrochloric  or  dilute  sulfurlc  acid  without  access  of  air  there  is 
yielded  a  ferrous  solution  useful  in  standardizing  volumetric  solutions  of  an  oxidizing 
nature.  One  gram  of  potassium  permanganate  is  reduced  by  1.7716  grams  of  metallic 
iron. 

Lead  carbonate.    PbCOs,  266.92. 

A  heavy  white  powder  prepared  by  dissolving  commercial  lead  acetate  in  water,  adding 
a  few  drops  of  ammonia,  and  filtering.  The  filtrate  is  poured  slowly  and  with  constant 
stirring  into  a  dilute  solution  of  sodium  carbonate,  and  the  precipitate  filtered,  washed 
with  hot  water,  and  dried  at  100  o.  This  compound  must  not  be  confounded  with  painter's 
white  lead  which  contains  also  the  hydrate. 

Nitric  acid .    HNOs,  63.048. 

Usual  specific  gravity  1.42.  The  colorless  acid  turns  yellow  on  exposure  to  light,  lower 
oxides  of  nitrogen  being  formed.  It  is  a  powerful  oxldizer;  used  as  a  solvent  for  metals 
and  to  destroy  organic  matter.  It  boils  at  121  o. 

Fixed  bases  are  tested  for  by  evaporation  to  dryness ;  chlorine  by  very  dilute  solution 
of  silver  nitrate ;  sulf uric  acid  by  dilute  solution  of  barium  chloride,  after  concentration 
with  a  little  potassium  nitrate. 

One  gram  of  the  acid  of  1.42  contains  about  .700  gram  of  HNOs. 

One  Cc.  contains  .994  gram  of  HNOs,  and  neutralizes  .883  gram  of  potassium  hydrate; 
.631  gram  of  sodium  hydrate;  and  .553  gram  of  ammonium  hydrate. 

Phenol-phthaleln.    (C6H4OH)2.C6H4CO  :C  :O. 

(Pron.  fee"-nole-thal'-a-in,  Stand.  Diet.,  but  variant.)  Made  by  combining  phthallc 
acid  with  phenol,  removing  a  molecule  of  water  by  anhydrous  phosphoric  acid.  Soluble 
in  alcohol  and  ether,  insoluble  in  dilute  acids.  Used  as  an  indicator  in  alkalimetry, 
being  colorless  in  acid  and  deep  red  in  alkaline  solutions.  It  Is  affected  by  carbonic 
acid,  and  unsuited  for  ammonia  except  with  certain  precautions. 

Solution,  one  gram  in  50  Cc.  of  alcohol ;  a  drop  or  two  suffices  for  a  titration. 

Potassium  ferricyanide,    KeFe2(CN)2. 

Dark  red  crystals  soluble  in  2.5  parts  of  cold  and  1.3  parts  of  hot  water  to  an  intense 
yellow  solution.  It  is  made  by  the  oxidation  of  potassium  ferrocyanide  by  chlorine. 

Used  as  an  indicator  in  volumetry,  producing  strongly  colored  precipitates  with  sev- 
eral metals.  The  solution  should  give  no  blue  color  with  ferric  compounds. 

Solution  for  the  spot  indication  of  ferrous  iron,  about  .050  gram  in  50  cubic  centimeters 
of  water,  to  be  freshly  prepared  before  use.  It  is  well  to  rinse  the  crystals  with  water 
before  making  up  the  solution  in  order  to  wash  off  the  surface  which  may  have  become 
reduced  to  ferrocyanide  by  dust  or  fumes. 

Potassium  hydrate .    KOH,  58 . 118 . 

A  white  deliquescent  solid  containing  variable  proportions  of  water  up  to  30  per  cent 
or  more.  In  the  market  are  found  three  grades:  "White"  in  sticks  or  granulated; 
4t  Purified  by  alcohol ",  and  "  Purified  by  baryta  ".  The  first  contains  considerable  carbonic 
acid  and  sodium  hydrate,  with  some  silica,  alumina,  potassium  sulf  ate,  potassium  chloride, 
etc.,  while  the  two  latter  are  pure  enough  for  ordinary  use.  The  solution  of  one  part  of 
potassium  hydrate  in  ten  of  water  is  to  be  preserved  in  ceresine  or  silver  bottles.  One 
gram  of  the  anhydrous  compound  neutralizes  .650  gram  of  hydrochloric  acid ;  1.124  gram 
of  nitric  acid ;  and  .874  gram  of  sulf  uric  acid. 

Potassium  iodide.    KI.165.96. 

Crystallizes  in  milk-white  cubes  soluble  in  .8  part  of  cold  water  and  .5  part  of  hot 
water.  Used  in  volumetric  analysis  to  aid  the  solution  of  iodine  in  water,  and  as  an  inter - 

14 


210  QUANTITATIVE    CHEMICAL   ANALYSIS. 

mediate  in  the  titratlon  of  free  chlorine  by  thiosulfate.    The  pharmaceutical  article  is 
pure  enough  for  most  purposes. 

Potassium  permanganate.    K2Mn2O8,  316.22. 

Dark  purple  prisms  with  a  bronze  iridescence,  soluble  in  16  parts  of  cold  and  3  parts  of 
hot  water.  Purified  by  recrystalllzation  from  an  aqueous  solution,  after  filtration  through 
asbestos  or  gun  cotton.  The  aqueous  solution  slowly  decomposes  on  standing;  e.g.,  a 
solution  of  four  grams  in  a  liter  of  water  lost  two  per  cent  of  its  oxidizing  power  in  30 
days. 

Used  in  volumetric  analysis,  the  decinormal  solution  containing  3.162  grams  per  liter. 
One  gram  contains  .263  gram  of  oxygen  in  excess  of  that  forming  protoxides  with  the 
potassium  and  manganese.  One  gram  of  iron  is  converted  from  the  ferrous  to  the  ferric 
state  by  .56444  gram  of  permanganate;  one  gram  of  crystallized  ferrous  sulphate  by  .11366 
gram ;  one  gram  of  crystallized  oxalic  acid  is  decomposed  by  .5017  gram ;  one  gram  of  crys- 
tallized ammonium  oxalate  by  .44479  gram ;  and  one  gram  of  C2O4  by  .7185  gram. 

Potassium  sulfocyanide.    KCNS,  97.22. 

(Potsssinm  thiocyanate  or  rhodanate.)  Transparent  colorless  crystals,  very  hygroscopic 
and  soluble  in  water.  The  clear  solution  after  acidification  by  nitric  acid  should  not  be 
troubled  by  silver  nitrate,  nor  be  colored  red  on  acidification  by  pure  hydrochloric  acid 
(FeaCle)*.  Used  as  a  precipitant  for  silver  and  copper,  and  as  a  volumetric  solution  for  the 
determination  of  the  former.  Solution,  one  part  in  ten  of  water. 

One  gram  precipitates  .654  gram  of  copper  (in  presence  of  a  reducing  agent),  and  1.109 
grams  of  silver. 

Pyrogallol.    GsH3(OH)3. 

(Trioxy benzol,  pyrogallic  acid,  pyrogallin,  pyrrol.)  The  pharmaceutical  product  is 
usually  pure  enough  for  the  purpose  here  desired  —  the  absorption  of  oxygen  from  a  mix- 
ture of  gases.  Five  grams  is  dissolved  in  15  Cc.  of  water  and  the  solution  mixed  with  80 
Co.  of  water  containing  80  grams  of  potassium  hydrate.  It  should  be  made  up  just  before 
using. 

Ferrous  potassium  tartrate  has  been  proposed  as  a  substitute. 

Silver  nitrate.    AgNOs,  169.96. 

Crystallizes  in  colorless  anhydrous  plates  soluble  in  less  than  its  own  weight  of  water. 
The  pharmaceutical  salt  is  sufficiently  pure.  Used  to  precipitate  the  halogens,  phosphoric 
and  arsenic  acids.  Solution,  one  part  in  ten  of  water,  to  be  kept  in  a  dark  place  free  from 
dust.  One  gram  precipitates  .208  gram  of  chlorine. 

Sodium  ammonium  phosphate.    NaXH4HPO4.4HzO,  209.194. 

Soluble  in  6  parts  of  cold  and  one  part  of  hot  water;  the  usual  Impurities  are  a  little 
caleium  or  magnesium  phosphate ;  to  eliminate  these,  the  salt  is  dissolved  in  20  parts  of 
water  containing  a  little  ammonia,  and  filtered  after  standing  for  an  hour  or  so.  As  the 
solution  strongly  attacks  glass  it  should  only  be  made  up  shortly  before  use. 

The  salt  serves  to  introduce  phosphoric  acid  and  ammonia  into  a  solution  for  the  pre- 
cipitation of  magnesium,  manganese,  or  zinc  salts  as  their  ammonium  phosphates.  One 
gram  of  the  crystallized  salt  precipitates  .193  gram  of  magnesia;  .339  gram  of  manganese 
protoxide ;  and  .389  gram  of  zinc  oxide.  With  the  two  latter  a  large  excess  is  needed  to 
induce  crystallization . 

Sodium  carbonate.    Na2COs.lOaq,  286.26.    NasCOs,  106.10. 

The  anhydrous  salt  is  most  used  in  analysis  and  may  be  prepared  quite  pure  by  the 
process  of  Reinitzer .  f  He  dissolves  sodium  bicarbonate  in  water  at  80  o  to  saturation  and 
filters.  On  cooling  to  10  o  to  15 o  the  salt  Na2COs  +  NaHCOs  separates ;  this  is  washed  by 
a  little  cold  water  by  decantatlon,  dried,  and  ignited  gently  in  a  platinum  crucible. 

As  purchased,  the  so-called  "  dry  sodium  carbonate"  is  really  of  the  grade  known  as 
"mono-hydrated  ",  containing  about  85  per  cent  of  Na2CO3,  and  15  per  cent  of  carbon 
dioxide  and  water.  The  usual  impurities  are  silica,  alumina,  chlorine  and  sodium  sulfate, 
for  which,  in  the  determination  of  any  of  these  compounds,  the  reagent  must  be  tested  and 
a  correction  made  if  found  present  in  more  than  traces. 

Used  as  a  precipitant  for  many  metals,  for  neutralizing  free  acids,  and  as  a  flux  to  de- 
compose minerals  insoluble  in  acids,  notably  the  silicates.  One  gram  of  anhydrous 
sodium  carbonate  neutralizes  .687  gram  of  hydrochloric  acid,  1.188  of  nitric  acid,  and  .924 
of  snlfnric  acid:  and  unites  with  .569 gram  of  silica. 


*  Chem.  News,  1891—1—150. 
t  Chem.  News,  1895—1—31. 


REAGENTS.  211 


Sodium  chloride.    NaCl,  58.5. 

Common  salt  crystallizes  in  cubes  with  recessed  faces.  It  is  soluble  in  2.78  parts  of 
water  at  15 o,  and  2  53  parts  at  100®,  the  increase  in  solubility  with  rise  of  temperature 
being  less  than  in  most  other  salts.  It  is  insoluble  in  alcohol  and  strong  acids. 

Ordinary  table  salt  is  purified  by  dissolving  to  saturation  in  cold  water  with  the  addi- 
tion of  a  few  drops  of  sodium  carbonate  solution  to  precipitate  the  earthy  salts,  filtered, 
and  the  filtrate  compounded  with  half  its  volume  of  strong  hydrochloric  acid.  The  liquid 
is  decanted  from  the  precipitated  salt,  which  is  drained  and  dried  on  the  water  bath. 
The  mass  is  then  powdered  and  heated  to  dull  redness  in  a  platinum  dish. 

The  solution  should  give  no  turbidity  with  barium  chloride  or  sodium  carbonate,  and 
not  more  than  a  faint  red  color  with  potassium  sulfocyanide  and  hydrochloric  acid.  Its 
principal  use  is  as  a  volumetric  solution  for  the  assay  of  silver  compounds. 

Sodium  hydrate.    NaOH,  40.058 . 

(Sodium  hydroxide,  caustic  soda.)  In  most  respects  this  compound  is  similar  to  po- 
tassium hydrate  and  a  corresponding  grade  of  one  may  usually  be  substituted  for  the 
other.  A  very  pure  article  is  for  sale  at  a  moderate  price,  made  by  oxidizing  metallic 
sodium  by  water  and  evaporating  the  solution  in  a  silver  vessel.  Wollney  states  that  the 
usual  impurities  of  the  commercial  article,  sodium  carbonate,  nitrate  and  sulfate,  are 
insoluble  in  a  50  per  cent  solution  of  the  hydrate. 

Sodium  hydrate  has  about  1.4  times  the  neutralizing  power  of  potassium  hydrate  toward 
a  given  acid.  One  gram  of  anhydrous  sodium  hydrate  neutralizes  .910  gram  of  hydro- 
chloric acid,  1.573  gram  of  nitric  acid  (HNOs),  and  1.224  gram  of  sulfuric  acid. 

Sulfuric  acid.    H2SO4,  98.086. 

(Oil  of  vitriol.)  According  to  Pickering*  the  specific  gravity  of  100  per  cent  acid  is 
1.833937  at  17.90  Cent.  An  acid  containing  10  per  cent  of  H2SO4hasa  gravity  at  15.50  of 
about  1.07.  The  usual  gravity  of  the  acid  as  purchased  is  1.84,  a  heavy  oily  liquid  boiling 
at  3380,  very  corrosive,  deliquescent,  and  evolving  great  heat  on  dilution  with  water. 
Should  be  colorless  or  only  slightly  brown.  The  diluted  acid  must  give  no  color  to  potas- 
sium iodide  and  starch  paste  (HNOa).and  no  precipitate  with  hydrogen  sulfide  (lead,  etc.). 
Nitric  acid  is  shown  by  a  brown  ring  at  the  junction  with  a  strong  solution  of  ferrous 
sulfate;  sulfurous  acid  by  diluting  and  adding  a  drop  of  a  dilute  solution  of  potassium 
permanganate  which  should  not  be  immediately  decolorized;  iron,  by  testing  with  dilute 
solutions  of  potassium  sulfocyanide  and  ferricyanide ;  and  arsenic,  by  the  brown  colora- 
tion of  the  diluted  acid  when  treated  with  stannous  chloride  and  tin. 

Used  as  a  solvent  for  some  metals  and  their  oxides,  indigo,  cellulose,  casein,  etc.,  and 
to  precipitate  lead  and  the  earths. 

Of  the  specific  gravity  of  1.84  it  contains  about  95  per  cent  of  H2SO4  (an  acid  of  99.6 
per  cent  has  the  same  gravity) ;  one  gram  contains  about  .950  gram  of  HaSO4,  and  one  Cc. 
1.748  grams.  One  Cc.  neutralizes  2.000  grams  of  potassium  hydrate;  1.428  grams  of 
sodium  hydrate,  .608  gram  of  ammonia  (NHg);  and  precipitates  2.735  grams  of  baryta; 
1.000  gram  of  lime;  and  3.687  grams  of  lead. 

By  adding  58  cc.  of  the  acid  of  1.84  gravity  to  one  liter  of  cold  water  is  furnished  a 
solution  containing  about  ten  per  cent  of  HaSO4. 

Sulfurous  acid.    HaSOs,  82.092. 

Sulfurous  anhydride  (SOa)  is  a  colorless  gas,  specific  gravity  2.23.  One  volume  of 
water  at  zero  dissolves  79.8  volumes,  and  at  200  39.4  volumes.  The  solution  loses 
strength  and  oxidizes  on  standing. 

The  solution  is  prepared  by  leading  into  cold  water  the  gas  generated  from  sodium  bi- 
sulfite and  dilute  sulfuric  acid,  or  copper  turnings  heated  with  concentrated  snlfuric  acid, 
The  solution  should  smell  strongly  of  the  gas  and  leave  no  residue  on  evaporation. 
Used  as  a  reducing  agent. 

Water.    HaO,  18.016. 

Few  natural  waters  are  so  free  from  suspended  or  dissolved  matters  as  to  be  fit  for 
quantitative  analysis,  so  recourse  is  had  to  the  simple  mode  of  purification  known  as 
distillation.  The  still  for  boiling  the  water  and  the  condenser  for  liquefying  the  steam 
as  shown  in  Fig.  136  are  too  well  known  to  need  an  extended  description. 

The  body  of  the  still  is  a  copper  tank  surmounted  by  a  beak  or  capital  which  is  fitted 
to  the  neck  of  the  still  by  friction,  or  the  two  are  provided  with  flanges  and  may  be 


Chem,  News,-1892-l-14. 


212 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


bolted  together,  Interposing  a  rubber  gasket.    The  water  in  the  still  is  boiled  by  a  gas 
burner  or  other  source  of  heat,  or  by  a  coil  of  steam  pipe  in  the  interior.    From   the 

beak  the  steam  en- 
ters a  coil  of  block- 
tin  pipe  wherein  it 
Is  condensed,  the 
water  flowing  out 
from  the  lower  end 
of  the  coil  project 
ing  from  the  tank 
Surrounding  the 
coil  Is  a  metal  tank 
containing  cold 
water,  which  is 
continuously  re- 
newed as  it  absorbs 
the  heat  of  the 
steam,  entering 
Fig.  136.  through  a  pipe  at 

the  bottom  and  leaving  near  the  top. 

The  still  and  head  are  coated  interiorly  with  tin,  and  a  tin  pipe  forms  the  worm  since 
this  metal  is  practically  unaffected  by  water,  which  is  not  the  case  with  other  common 
metals  and  glass. 

It  will  be  seen  that  this  apparatus,  economically  considered,  is  very  imperfect,  and 
several  improved  apparatus  are  now  on  the  market,  so  modified  as  to  operate  automatically 
and  produce  a  greater  -yield  of  distillate  in  proportion  to  the  volume  of  gas  burned. 
Usually  the  still  is  broader  and  shallower  and  the  coiled-pipe  worm  is  replaced  by  a  thin 
wide  tube  Incased  in  a  box  of  the  same  shape.  The  cooling  water  circulates  between  the 
tube  and  box  entering  at  the  bottom,  its  temperature  increasing  as  it  rises,  until  at  the  top 
It  Is  almost  at  the  boiling  point.  From  thence  a  part  flows  into  the  still  through  a  constant 
level  (page  27),  saving  the  time  and  fuel  needed  to  heat  cold  feed- water  to  the  boiling 
point  as  in  the  ordinary  form  of  still. 

If  the  top  be  closed  and  the  hot  water  near  the  surface  be  raised  quite  to  the  boiling 
point  the  condenser  Itself  may  be  utilized  as  an  auxiliary  still  by  directing  the  steam  gen- 
erated into  its  worm.  This  is  accomplished  in  the  "  Domestic  Still".  The  steam  as  it 
leaves  the  still  passes  to  the  worm  through  an  aspirator  (on  the  principle  of  the  vacuum 
pump,  page  93),  drawing  in  with  it  steam  from  the  condenser;  this  is  generated,  not  by 
raising  the  water  to  full  100  o,  but  through  the  lowering  of  its  boiling  point  by  the  creation 
of  a  partial  vacuum  in  the  condenser  by  virtue  of  the  aspirator. 

The  first  portion  of  the  distillate  is  rejected  as  containing  the  gases  always  present  in 
natural  water,  etc.,  and  the  distillation  is  stopped  before  all  the  water  has  evaporated. 

Distilled  water  should  be  neutral,  tasteless,  colorless,  and  odorless;  leave  no  residue  or 
only  a  trace  on  evaporation;  and  not  be  clouded  by  solutions  of  barium  chloride,  silver 
nitrate,  or  hydrogen  sulflde.  Minute  traces  of  organic  matter,  carbon  dioxide,  nitric  acid, 
etc.,  are  of  no  consequence  In  general  analysis;  for  the  few  special  determinations  where 
no  organic  matter  whatever  may  be  present,  the  ordinary  distilled  water  is  rectified  over 
potassium  permanganate  or  chromic  acid  and  sulfuric  acid.  Ammonia-free  water  Is 
easiest  obtained  by  boiling  ordinary  distilled  water  in  an  open  wide-mouth  flask  until  a 
test  shows  that  all  free  ammonia  has  passed  off. 

Distilled  water  is  always  to  be  preserved  in  chemical -glass  bottles,  never  in  stoneware 
jugs. 

Whenever  water  is  mentioned  in  the  following  exercises,  distilled  water  is  to  be  under- 
stood. 

Zinc.    Zn.  65.40. 

The  bluish -white  metal  is  soluble  in  most  acids,  giving  hydrogen- gas  with  hydrochloric 
and  dilute  sulfuric  acids.  Some  commercial  varieties  are  remarkably  pure.  For  analytical 
use  it  should  contain  very  little  carbon,  iron,  lead,  or  arsenic.  The  first  two  are  tested  for 
by  dissolving  the  zinc  in  dilute  sulfuric  acid  and  titrating  by  a  weak  solution  of  potassium 
permanganate ;  lead  remains  undissolved,  and  arsenic  is  volatilized  as  arsine,  shown  by 
Marsh's  or  other  test. 

A  convenient  foliated  form  is  left  when  the  zinc  is  melted  in  a  clay  (not  graphite) 
crucible  and  poured  into  water.  The  metal  in  stick,  granulated,  and  powdered  form  Is 
now  on  the  market. 

Used  to  reduce  per-  to  proto-salts  in  acid  solution,  and  to  decompose  some  metallic 
compounds  with  separation  of  their  bases  in  the  metallic  form,  as  those  of  lead,  silver  or 
copp.  r.  It  is  also  of  general  use  as  a  reducing  agent. 


ALCOHOL.  213 


EXEECISES  FOR  PRACTICE. 

The  following  exercises  illustrate  many  of  the  principles  and  expedients 
employed  by  the  analyst  and  have  been  chosen  from  the  simpler  problems  he 
is  called  on  to  solve.  The  methods  have  been  selected  primarily  with  reference 
to  their  requirement  of  but  little  skill  and  experience  on  the  part  of  the  oper- 
ator and  of  few  special  or  complicated  forms  of  apparatus;  for  this  reason, 
there  may  be  found  in  the  standard  works  other  methods  that  are  superior  in 
some  respects  to  those  here  given,  and  it  will  be  well  for  the  student,  after 
acquiring  some  familiarity  with  the  physical  and  chemical  properties  of  the 
substance  treated,  to  essay  one  or  more  of  them  as  circumstances  permit. 

Directions  have  been  given  in  detail,  though  it  is  not  to  be  inferred  that  any 
deviation  therefrom  is  unallowable  —  on  the  contrary,  they  are  intended  as  a  guide 
rather  than  a  rule,  except  where  it  is  obvious  that  they  must  be  strictly  adhered  to. 

The  student  is  advised  to  make  every  analysis  in  triplicate  as  the  time 
consumed  does  not  greatly  exceed  that  for  a  less  number,  and  confidence  in  his 
work  is  engendered  by  a  close  agreement  of  three  results;  moreover,  should 
an  accident  happen  one,  the  other  two  may  be  carried  forward  without  the 
delay  incident  to  starting  the  analysis  afresh. 

A  knowledge  of  the  capabilities  of  the  balance  used  and  the  limits  of  its 
sensibility  under  different  loads,  the  closeness  of  the  agreement  of  the 
weights  one  with  another,  and  a  verification  of  the  graduation  of  the  volumet- 
ric vessels,  must  be  secured  before  any  analytical  work  is  undertaken;  as 
otherwise  there  is  always  a  feeling  of  uncertainty  and  a  temptation  to  charge 
unsatisfactory  results  to  their  inaccuracies.  The  student  should  personally 
acquire  this  information,  as  at  the  same  time  he  becomes  familiar  with  the 
manipulation  of  the  various  instruments. 

Wherever  two  exercises  are  included  under  one  number,  the  principles  of 
the  two  are  similar  and  the  one  most  convenient  may  be  chosen  for  analysis. 

EXERCISE  1- ALCOHOL. 

Determination  by  Specific  Gravity. 

The  density  of  absolute  alcohol  at  15.50  is  .7946,  rising  as  it  is  diluted  with 
water.  As  a  contraction  in  volume  takes  place  when  the  two  liquids  are  mixed, 
the  published  tables  of  the  percentages  of  alcohol  corresponding  to  different 
specific  gravities  have  been  compiled  from  direct  experiments  on  mixtures  of 
known  volumes. 

The  commercial  grade  of  alcohol  sold  as  '  95  per  cent '  contains  from  90  to  95 
per  cent  by  weight.  Its  specific  gravity  is  found  from  the  weight  of  a  con- 
venient volume  as  compared  with  that  of  an  equal  volume  of  water,  at  a  stand- 
ard temperature,  usually  15.5°  Cent.*  As  the  temperature  of  the  laboratory 
is  usually  not  less  than  20  o ,  dew  is  condensed  on  a  vessel  holding  a  liquid  at  a 
lower  temperature  and  prevents  an  exact  weight  being  taken;  so  instead  of  the 
pyknometer  or  specific  gravity  flask  it  will  be  found  more  convenient  to  note 
the  loss  in  weight  suffered  by  an  immersed  solid  as  compared  with  its  weight 
in  air,  this  being  the  weight  of  an  equal  volume  of  the  surrounding  liquid. 


Allen,  Coml.  Org.  Anal.  1—92. 


214 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


Over  the  left-hand  pan  of  the  balance  is  placed  the  wooden  bridge,  Fig.  137, 
supporting  as  wide  a  beaker  as  will  easily  pass  between  the  wires  without 
touching  either.  In  the  beaker,  about  half  way  down,  there  is  hung  by  a  silk 
thread  or  a  fine  wire  a  clean  and  smooth  piece  of  metal  weighing  from  50  to 
100  grams.  The  weight  is  accurately  observed,  taking  care  that  the  metal  does 
not  touch  the  beaker,  nor  the  pan  wires  the  beaker  or  bridge. 

Two  flasks  of  about  the  same  capacity  as  the  beaker  are  filled,  one  with  the 
alcohol  to  be  tested,  the  other  with  distilled  water;  they  are  immersed  in  ice- 
water  until  the  temperature  of  the  liquids  has  fallen  below  15°.  The  flask 
containing  the  alcohol  is  removed  from  the  bath,  and  when 
the  temperature  of  the  alcohol  has  risen  to  about  14°,  the 
beaker  is  filled,  and  the  plummet  weighed,  to  within  a  milli- 
gram only,  since  the  vibrations  of  the  beam  are  impeded  by 
the  density  of  the  liquid. 

The  beaker  is  emptied,  tho  plummet  dried,  and  its  weight 
in  the  water  found  under  the  same  conditions  that  obtained 
with  the  alcohol,  but  care  must  be  taken  that  any  air- 
bubbles  are  removed  (by  a  camels-hair  pencil  when  in  the 
water)  that  may  adhere. 

Calculation.  Let  the  weight  of  the  plummet  in  air  be 
A;  in  alcohol,  B;  and  in  water,  C.  Then  A—B  is  the 
weight  of  a  volume  of  alcohol,  and  A — C,  that  of  a  volume 
of  water  equal  to  the  volume  of  the  plummet;  therefore 
A — C  :  A — B  :  :  1  :  x,  the  specific  gravity  of  the  alco- 
hol. In  the  table  belcw,  x  will  probably  be  found  to  lie 
between  two  of  the  gravities  there  given.  The  one  next 
lower  is  subtracted  from  it  and  the  difference  divided  by 
the  corresponding  number  in  the  column  headed  ' Decrease 
in  specific  gravity.'  The  quotient  is  subtracted  from  the 
percentage  given  in  the  second  column,  the  result  being 
the  percentage  of  alcohol  by  volume.  The  percentage  by  weight  may  be  ob- 
tained by  multiplying  the  percentage  by  volume  by  .7946  and  dividing  the 
product  by  the  observed  specific  gravity. 

Example.  A  brass  cylinder  weighed  in  air  99.840  grams;  in  alcohol  at  15.5° , 
90.091 ;  and  in  water  at  15.5® ,  87  941  grams. 

99.840  —  97.941  :  99.840  —  90.091  :  :  1.0000  :  x.  x  =  .8193.  From  the  table, 
.8193  — .8164  =  .0029;  and  .0029 -*-  .0035  =  .83  Hence  95.00— .83  =  94.17  per 
cent  by  volume,  or  91.33  per  cent  by  weight. 


Fig.  137. 


Table  of  the  percentage  of  absolute  alcohol  in  commercial  alcohol  by  volume 
corresponding  to  specific  gravities  at  15.5°  Cent,  against  water  at  15.5° 
(Squibb). 

Per  Cent  of 
Alcohol. 
93 


Specific 
Gravity. 
.8496 

Per  Cent  of 
Alcohol. 
85 

.8466 

86 

.8434 

87 

.8408 

88 

.8373 

89 

.8340 

90 

.8305 

91 

.8272 

92 

Decrease 

Specific 

in  Sp.  Gr. 

Gravity. 

.... 

.8237 

.0030 

.8199 

.0032 

,8164 

.0026 

.8125 

.0035 

.8084 

.0033 

.8041 

.0035 

.7995 

.0033 

.7946 

94 
95 
96 
97 
98 
99 
100 


Decrease 
in  Sp.  Gr. 
.0035 
.0038 
.0035 
.0039 
.0041 
.0043 
.0046 
.0049 


FERROUS    SULFATE.  215 


EXERCISE  2  — A.  LEAD  CARBONATE. 

Determination  of  Carbon  Dioxide. 

Pure  lead  carbonate  (page  209)  is  a  white  powder,  unaltered  at  100°, 
but  converted  into  the  protoxide  at  a  temperature  approaching  redness, 
carbon  dioxide  escaping  —  PbCO3  =  PbO  -f-  CO2. 

Lead  protoxide  is  a  white  powder,  yellow  while  hot.  It  melts  at  a  bright 
red  heat,  at  which  temperature  it  is  reduced  to  metallic  lead  by  carbon,  carbon 
monoxide,  or  gaseous  hydrocarbons. 

The  determination  of  carbon  dioxide  is  made  by  igniting  the  carbonate, 
the  loss  in  weight  being  that  of  the  carbon  dioxide  escaping. 


Clean  a  small  porcelain  crucible  and  cover,  and  heat  gently  over  a  Bunsen 
burner.  Transfer  to  a  desiccator,  and  when  cold  weigh  to  within  one 
milligram. 

Remove  the  cover  and  place  it  on  the  scale- pan  beside  the  crucible.  Add 
a  two-gram  weight  to  those  in  the  pan,  and  with  a  spatula  or  horn  spoon 
drop  into  the  crucible  a  little  more  of  the  powder  than  will  restore  equilibrium ; 
cover  the  crucible  and  dry  in  the  water  oven  or  air-bath  at  100  °  for  half  an 
hour.  Cool  in  the  desiccator  and  weigh  accurately. 

Heat  the  covered  crucible  over  a  small  flame  gradually  increased,  until  the 
bottom  of  the  crucible  appears  slightly  red,  and  the  powder  turns  brown, 
then  yellow  or  nearly  so.  Slowly  diminish  the  heat,  cool  in  the  desiccator 
and  weigh.  Repeat  the  ignition  and  weighing  until  two  consecutive  weights 
do  not  differ  by  more  than  one  milligram. 

The  heat  applied  must  not  be  so  great  as  to  fuse  the  lead  oxide,  and  the 
crucible  must  be  heated  and  cooled  slowly  to  prevent  fracture.  The  point  of 
the  blue  flame  should  never  reach  the  bottom  of  the  crucible. 


Calculation.  The  weight  of  the  lead  carbonate  less  the  weight  of  the  lead 
oxide  is  the  weight  of  the  carbon  dioxide  expelled.     Also  —  Weight  of  the  lead 
carbonate  :  weight  of  carbon  dioxide  :  :  100  :  percentage  of  carbon  dioxide. 
Example. 

Weight  of  Grams. 

•     Crucible  and  lead  carbonate 25.574 

Crucible  alone .  23.038 


Lead  carbonate 


Crucible  and  lead  oxide 25. 154 

Crucible  alone 23  038 

Lead  oxide.... 2.116 

Carbon  dioxide 420 

Percentage  of  carbon  dioxide 16.56 

Theoretical  percentage 16.49 

B.  FERROUS  SULFATE. 

Determination  of  Iron. 

On  heating  crystallized  ferrous  sulfate  (FeSO47H2O),  it  loses  first  its  water 
of  crystallization,  then  sulfurous  and  sulfuric  anhydrides,  and  ferric  oxide 
remains  — 

2FeS04  7ET20  -f  hear  —  14H20  -f  -SO2  -f  80s  +  Fe2Os. 


216  QUANTITATIVE    CHEMICAL    ANALYSIS. 

From  the  weight  of  the  residual  ferric  oxide  is  calculated  that  of  the  iron  it 
contains  and  the  percentage  of  iron  in  the  crystallized  salt. 


Select  small  dry  unoxidized  cry^ials  prepared  according  to  the  directions  on 
page  208.  Heat  a  covered  platinum  crucible  to  redness,  cool  in  a  desiccator 
and  weigh.  Introduce  about  two  grams  of  the  crystals  and  weigh  accurately. 
Heat  the  covered  crucible  over  a  very  low  flame  until  the  water  is  expelled, 
then  support  the  crucible  as  shown  in  Fig.  94  and  gradually  increase  the  heat 
to  dull  redness.  The  expulsion  of  the  sulfur  oxides  is  known  to  be  complete 
when  the  color  has  changed  to  a  uniform  dark  red.  It  is  well  to  stir  the 
powder  occasionally  with  a  thin  platinum  wire.  Cool  the  crucible  in  the  desic- 
cator and  weigh.  Again  ignite  the  uncovered  crucible,  cool  and  weigh  as 
before,  repeating  If  the  two  weights  do  not  agree. 

Calculation.  One  molecule  of  ferric  oxide  (160)  contains  two  atoms  of  iron 
(112) ;  hence  160  :  112  :  :  weight  of  ferric  oxide:  weight  of  iron  it  contains. 

Also,  Weight  of  crystallized  salt  :  weight  of  iron  contained  :  :  100  :  Y,  the 
percentage  of  iron  contained. 

Example : 

Grams.  Grams. 

Crucible  and  salt 18.775  Crucible  and  ferric  oxide 17.347 

Crucible  empty 16.774  Crucible  empty 16.774 

Ferrous  sulf ate 2.001  Ferric  oxide 573 

160  :  .112  :  :  573   :  X.    X=  .4011  gram  iron. 

2.001  :  .4011  :   :  100  per  cent:    Y.     Y  =  20.04,  the  percentage  of  iron   in  the 

crystallized  ferrous  sulfate. 

Theoretically,  278.182  (ferrous  sulfate)  :  56  (iron)  :  :  100  :  Z.  Z  =  20.13 
per  cent. 

EXERCISE  3 -SODIUM  CHLORIDE. 

Determination  of  the  Atomic  Weight  of  Chlorine. 

On  evaporation  of  a  solution  of  sodium  chloride  with  an  excess  of  nitric  acid 
it  is  converted  into  sodium  nitrate  — 

3NaCl  +  4HN03  =  3NaNO3  +  NOC1  -f  C12  +  2H2O. 

Sodium  nitrate  is  a  crystalline  transparent  salt,  easily  soluble  in  water, 
slightly  hygroscopic,  and  melting  at  330  °  .  It  decomposes  at  a  dull  red  heat 
evolving  oxygen. 


Clean  and  dry  a  crystallizing  dish  about  three  inches  in  diameter,  cover 
with  a  watch-glass,  and  weigh.  Introduce  about  four  grams  of  pure  sodium 
chloride  (page  211),  heat  in  the  air  bath  to  110°  for  a  half  hour,  cool  in  the 
desiccator,  and  weigh  exactly. 

Place  the  dish  on  the  water  bath,  remove  the  watch-glass,  and  add  25  Cc.  of 
water  and  10  Cc.  of  concentrated  nitric  acid.  When  the  solution  has  evapo- 
rated to  a  small  volume  add  10  Cc.  of  water  and  evaporate  to  complete  dryness. 

Wipe  the  exterior  of  the  dish,  heat  in  the  air  bath  to  about  120°  for  an 
hour,  cover  with  the  watch-glass,  cool  in  the  desiccator  and  weigh.  Dissolve 
the  residue  in  water  and  be  as»ured  that  the  conversion  is  complete  by  test- 
ing for  chlorine  by  silver  nitrate. 


COFFEE.  217 

Calculation.  Taking  the  atomic  weights  of  oxygen  as  16,  sodium  as  23.05, 
nitrogen  as  14.04,  and  chlorine  as  X,  since  one  molecule  of  sodium  chloride  is 
transformed  to  one  molecule  of  sodium  nitrate  we  have  the  proportion, 

Weight  of  sodium  nitrate  found  :  weight  of  sodium  chloride  :  :  85.09  (mol. 
wt.  of  NaN03)  :  X+  23.05  (mol.  wt.  of  NaCl). 

Example.  A.  B.  C. 

Weight  of  dish  and  NaCl 45.7579        34.4927        47.0140 

Weight  of  dish 41.7531         304227        43.0206 

Weight  of  NaCl ! 4.0048          4.0700          3.9934 

Weight  of  dish  .and  NaNOa 47.5819        36.3398        48.8283 

Weightofdish 41.7531        30.4227        43.0206 


Weightof  NaNOs 5.8288          5.9171          5.8077 

Atomic  weight  of  chlorine , 35.41  35.48  85.46 

EXERCISE  4  — A.  COFFEE. 

Determination  of  the  Extractive  Matter. 

Coffee  is  the  seeds  of  any  species  of  the  genus  Coffea,  especially  the  Coffta 
Arabica  Nat.  Ord.  Cinchonaceae.  The  seeds  have  the  form  of  plano-couvex 
pyrenes,  and  before  exportation  are  decorticated  and  dried.  The  imports  into 
the  United  States  come  chiefly  from  Brazil  and  Sumatra,  of  the  Rio  and  Pedang 
varieties  and  a  small  proportion  of  genuine  Java. 

The  berry  is  made  up  of  a  cellular  structure  of  cellulose  inclosing  a  complex 
mixture  of  an  oil  or  fat,  the  alkaloid  caffeine,  tanLic  and  caffeo  tanmc  acids, 
gum,  and  smaller  amounts  of  sugar,  inorganic  salts,  etc.    Ti.e  proportions 
vary  considerably  as  shown  in  the  following  analyses  made  on  dried  material. 

Maximum.  Minimum. 

Gummy  matter  and  sugar 27.40  20.60 

Caffeine 1.53  .64 

Fat 21.79  14.76 

Tannic  and  caffeo- tannic  acids 23.10  19.50 

Ash 4.90  3.80 

Cellulose 36.40  29.90 

By  roasting,  the  water  contained  is  reduced  from  about  11  to  3  per  cent, 
part  of  the  caffeine  is  driven  off,  and  a  part  of  the  sugar  changed  to  caramel, 
while  a  fragrant  aromatic  body  is  developed. 

When  the  powder  of  roasted  and  ground  coffee  is  exhausted  by  boiling  water 
there  are  dissolved  caramel,  caffeic  acid,  caffeine,  legumine,  a  volatile  oil 
(caffeone),  a  little  fatty  matter,  and  some  inorganic  bodies  mainly  potassium 
phosphate.  On  evaporation  the  filtered  decoction  leaves  a  residue  of  about 
22  to  28  per  cent  of  the  coffee,  the  average  being  about  24  per  cent.* 

The  lower  grades  of  coffee  are  sometimes  adulterated  with  the  dried  and 
powdered  root  of  the  chicory  (Cichorium  Intybus),  and  wholly  factitious  beans 
have  been  manufactured  on  a  large  scale.  On  extraction  as  above^  however, 
chicory  leaves  a  much  higher  percentage  of  residue,  approximating  70  per  cent, 
so  that  any  percentage  greater  than  about  24  in  a  sample  of  ground  coffee 
would  indicate  an  admixture  of  chicory  or  other  foreign  matter.  Assuming 
chicory  to  be  the  only  adulterant,  its  proportion  could  be  roughly  calculated 

from  the  formula  X=  100  —HI — ,  where  X  is  the  percentage  of  chicory  in  the 

a  —  o 

*  Analyst,  1898—226. 


218  QUANTITATIVE    CHEMICAL    ANALYSIS. 

mixture;  a,  the  percentage  of  residue  from  chicory  (70);  6,  the  average  per- 
centage of  residue  from  coffee  (24)  ;  and  d,  the  percentage  of  residue  found  in 
a  mixture. 


About  an  ounce  of  the  roasted  berries  is  ground  to  pass  through  a  20- mesh 
seive;  five  grams  is  quickly  weighed  and  brushed  into  a  large  porcelain  dish 
containing  200  Cc.  of  boiling  water.  The  mixture  is  boiled  for  15  minutes, 
allowed  to  settle,  and  the  turbid  liquid  decanted  from  the  coarse  fragments  into 
a  250  Cc.  measuring  flask.  The  residue  is  again  boiled  for  five  minutes  with  50 
Cc.  of  water  and  decanted  into  the  flask. 

The  decoction  is  cooled  to  the  temperature  of  the  room  in  a  stream  of  water, 
made  up  to  the  mark  with  cold  water  and  mixed  well.  It  is  then  filtered 
througli  a  dry  triple  filter  into  a  dry  beaker,  returning  the  filtrate  until  it  passes 
clear  or  nearly  so. 

Three  crystallizing  dishes  of  about  three  inches  diameter  are  weighed  and  50 
Cc.  of  the  extract  transferred  to  each  by  a  pipette.  After  evaporation  to  dry- 
ness  on  the  water  bath,  they  are  heated  f^r  30  minutes  in  the  water  oven, 
cooled  in  the  desiccator  and  weighed. 

Calculation.   The  increase  in  weight  of  each  dish  corresponds  to  the  soluble 

50 

matter  in —  of  5  grams  of  coffee  (  =  1  gram),  hence  the  increase  multiplied  by 
250 

100  is  the  percentage  of  soluble  matter. 
Example. 

A.  B.  C. 

Weight  of  dish  and  residue 280750        224945        26.0965 

Weightofdish 27.8360         22.2580         25.8575 

Weight  of  residue 2390  .2365  .2390 

Percentage  of  soluble  matter 23.90  23.65  23.90 

B.  GINGER. 

The  rhizome  of  the  Zingiber  officianalis  nat.  ord.  Zingiberaceae.  The  plant  is 
a  native  of  tropical  countries  and  several  varieties  are  found  in  commerce  — 
the  Jamaica,  African,  East  Indian,  Cochin,  etc.  The  rhizome  is  composed 
principally  of  starch,  woody  fiber,  resin,  volatile  and  fixed  oils,  and  mineral 
matter  with  ten  to  fifteen  per  cent  of  moisture.  The  powder  is  sometimes 
adulterated  with  rice-starch,  chalk,  etc.,  but  more  frequently  is  fraudulently 
deprived  of  a  portion  of  its  essential  oil  by  steeping  in  water,  the  residue 
being  dried  and  sold  for  the  unsophisticated  article.  A  positive  statement 
that  a  sample  of  ginger  has  undergone  this  degradation  is  admissible  only  when 
indicated  by  several  different  tests.  One  of  these  is  the  extraction  of  the  pow- 
dered ginger  by  alcohol,  evaporating  and  weighing  the  residue  which  should 
not  fall  below  4.8  per  cent  of  the  undried  ginger. 

The  alcoholic  extract  contains  the  pungent  or  active  principle  gingerol,  a 
viscid,  odorless  liquid  of  the  consistency  of  treacle;  extractive  matter  soluble 
in  water;  neutral  and  acid  resins;  small  quantities  of  a  red  fat;  wax;  etc. 
Since  a  large  part  of  the  extractive  is  volatile  even  at  ordinary  temperatures, 
the  evaporation  should  be  conducted  at  as  low  a  heat  as  possible  and  the 
residue  weighed  without  delay.  For  this  reason  also,  the  results  of  the 
determinations  may  differ  to  a  considerable  extent. 


UNREFINED    IRON.  219 

The  closed  end  of  a  test-tube  three -fourths  of  an  inch  in  diameter  is  softened 
in  the  flime  of  a  Bunsen  burner  and  drawn  out  to  a  small  orifice.  Absorbent 
cotton  is  tightly  picked  in  to  the  height  of  about  an  inch,  and  a  little  alcohol 
run  through  to  clear  it  of  any  soluble  matter.  A  glass  crystallizing  dish  or  bot- 
tom of  a  beaker  is  dried  and  weighed.  The  tube  is  clamped  to  a  retort  stand 
aain  figure  45,  the  dish  beneath  it. 

Powdered  ginger  is  put  into  a  wide-mouth  bottle  until  half  full,  and  the 
bottle  corked  and  shaken  until  all  lumps  are  broken  up  and  the  powder  thor- 
oughly mixed.  Five  grams  is  weighed  to  within  a  milligram,  and  transferred 
to  the  tube  by  a  square  of  glazed  paper  and  a  camels-hair  pencil. 

A  narrow-necked  flask  holding  50  Cc.  is  filled  to  the  brim  with  95  per  cent 
alcohol,  then  stopped  with  the  finger  and  inverted  over  the  tube,  and  the  neck 
quickly  lowered  into  it.  The  level  of  the  alcohol  in  the  tube  slowly  falls  to  the 
mouth  of  the  flask  and  remains  there  until  the  flask  is  empty  —  this  should  take 
several  hours,  and  it  is  best  to  nearly  cover  the  dish  with  a  watch-glass  and 
allow  the  percolation  to  proceed  over  night. 

The  tincture  is  then  evaporated  to  dryness  on  the  water  bath,  taking  care 
that  the  alcohol  does  not  boil.  The  dish  is  wiped  dry,  allowed  to  stand  in  the 
desiccator  until  cool,  and  weighed. 

i  on 

Calculation.  The  weight  of  the  residue  times  —  gives  the  percentage  of  the 

5 

alcoholic  extract. 

Example.  Five  grams  of  Jamaica  ginger  treated  us  above  — 

Weight  of  dish  and  residue 30.764        23.986         26796 

Weight  of  dish 30502        23.722        26.544 


Weight  of  residue 262  .264  .252 

Percentage  of  extract 5.24  5.28  5.04 

References.  Uniteii  Stat<  s  and  American  Dispensatories;  Amer.  Journ.  of 
Pharm.  1879-519.  Bulletin  No.  13,  U.  S.  Dept.  of  A^r. culture,  1887. 

EXERCISE  5  — UNREFINED  IRON. 

Determination  of  Silicon. 

Pig-  or  cast-iron  contains  from  3.5  to  4.5  per  cent  of  carbon,  partly  dis- 
solved in  the  iron  or  combined  with  it,  and  partly  free  in  the  form  of  graphitic 
plates;  small  and  variable  proportions  of  sulfur,  phosphorus,  and  manganese; 
and  silicon  up  to  3  per  cent  or  more,  usually  less  in  "  white  iron  "  than  «« gray 
iron  ". 

When  the  metal  is  dissolved  in  dilute  sulfuric  acid  most  of  the  combined 
carbon,  sulfur  and  phosphorus  unite  with  nascent  hydrogen  and  pass  off  as 
odorous  gases;  the  iron  and  manganese  with  the  remainder  of  the  phosphorus 
dissolve;  the  graphite  is  unaffected;  while  the  silicon  is  hydrated,  the  major 
portion  dissolving.  If  this  solution  be  treated  with  nitric  acid,  the  ferrous  is 
changed  to  ferric  sulfate,  and  the  remainder  of  the  combined  carbon  and  sulfur 
dissolve.  On  evaporation,  nitric  acid  and  water  are  expelled ;  the  excess  of  the 
sulfuric  acid  becomes  concentrated  and  abstracts  the  water  from  the  hydrated 
silica  (leuocone);  and  on  treating  the  residue  with  dilute  hydrochloric  acid  the 
ferric  and  manganous  sulfates  are  dissolved  leaving  anhydrous  silica  mixed 
with  graphite.  After  filtering,  the  graphite  is  burned  away,  leaving  the  silica 
ready  to  be  weighed.  From  its  weight,  the  percentage  of  silicon  is  calculated.* 


Drown,  Trans.  Amer.   Inst.  Mining  Engineers,  7—346;  Journ.  Amer.   Chem.   Socy. 

8-547. 


220  QUANTITATIVE    CHEMICAL    ANALYSIS. 

The  iron  should  be  in  the  form  of  drillings  or  turnings,  well  mixed  and  free 
from  oil,  rust  and  dirt.  Weigh  one  gram  on  a  small  watch-glass  and  brush  it 
into  a  small  beaker.  Dissolve  in  25  Cc.  of  dilute  sulfuric  acid  (1  to  10),  and 
when  effervescence  ceases,  oxidize  with  5  Cc.  of  concentrated  nitric  acid.  Re- 
move the  watch-glass  covering  the  beaker  and  evaporate  on  the  hot-plate 
until  white  fumes  of  sulfuric  acid  arise.  Cool,  add  10  Cc.  of  concentrated 
hydrochloric  acid  and  50  Cc.  of  hot  water  and  boil  until  all  the  white  ferric 
sulfate  is  taken  up.  Filter  through  a  9  Cm.  paper  and  wash  alternately  with 
water  and  hot  dilute  hydrochloric  acid  until  the  washings  show  no  red  color 
with  potassium  sulfocyanide. 

Heat  a  covered  platinum  crucible  to  redness,  cool  and  weigh.  Partially  dry 
the  filter  on  a  tile  or  blotting  paper,  and  place  it  in  the  crucible.  Heat  gently 
until  the  paper  is  charred,  then  burn  it  and  the  graphite  as  directed  on  page  103. 
The  graphite  requires  upward  of  a  half  hour  for  its  combustion  at  a  red  heat, 
stirring  occasionally  with  a  platinum  wire.  The  remaining  silica  should  be 
pure  .white. 

Calculation. 

SiO2    (60.40)    :  Si  (28.40)   :   :  weight  of    SiO2  :  weight  of   Si.      Weight    of 
iron  :  weight  of  silicon  :   :  100  :  per  cent  of  silicon. 

Example.  Three  portions  of  one  gram  each  treated  as  above ; 

A.  B.  C. 

Weight  of  crucible  and  silica 12.1365  12.9985  15.4811 

Weight  of  crucible 12.1005  12.9622  15.4453 

Weight  of  silica 0360  .0363  .0358 

Percentage  of  silicon 1.69  1.71  1.68 

EXERCISE  6  — ETHER. 

Determination  of  Alcohol. 

Pure  ethyl  ether  C2H5.O.C2Hs  is  a  colorless,  inflammable  and  very  volatile 
liquid  boiling  at  34.6°  and  of  a  specific  gravity  at  15/15°  of  .719.  It  mixes  in 
all  proportions  with  alcohol,  is  soluble  in  about  12  parts  of  water,  and  dis- 
solves about  1/35  its  weight  of  water,  more  freely  when  containing  alcohol. 

The  '  sulfuric  ether '  of  commerce  always  contains  a  certain  proportion  of 
alcohol  and  water  as  impurities.  There  are  three  grades  on  the  market;  one 
prepared  according  to  the  U.  S.  Pharmacopoeia  of  1880  and  containing  about 
26  per  cent  of  alcohol  and  varying  amounts  of  water;  another  according 
to  the  U.  S.  P.  of  1890  containing  about  four  per  cent  of  alcohol  and  one 
or  two  of  water;  and  'ether  for  anaesthesia,'  as  pure  or  purer  than  the 
latter. 

1.  Commercial  ether  parts  with  its  alcohol  when  shaken  with  a  sufficient  pro- 
portion of  water ;  the  water  also  dissolves  a  portion  of  the  ether,  but  none,  of 
course,  if  previously  saturated  with  ether. 

2.  If  a  measured  volume  of  commercial  ether  be  agitated  with  sufficient  ether- 
saturated  water,  the  diminution  in  volume  of  the  former  corresponds  to  the 
volume  of  alcohol  extracted.    The  increase  in  volume  of  the  water  is  not  equal 
to  the  volume  of  alcohol  entering  it,  on  account  of  the  contraction  of  the 
mixture. 


Cold  distilled  water  is  tinted  to  a  light  violet  by  a  weak  solution  of  Hoff- 
mann's violet  in  water,  and  about  30  Cc.  is  poured  into  a  burette  containing 
about  5  Cc.  of  ether.  The  burette  is  closed  by  the  thumb  and  briskly  shaken 


ACIDIMETRY.  221 

for  a  few  minutes.    On  standing  for  a  short  time  the  excess  of  ether  will  float 
on  the  surface  of  the  saturated  colored  water. 

Into  a  clean  graduated  measuring-tube  or  burette  is  tapped  about  20  Cc.  of 
the  colored  water  and  measured  after  standing  for  a  short  time.  Then  20  to  25 
Cc.  of  the  sample  of  ether  to  be  examined  is  run  in  from  a  pipette,  holding  the 
orifice  against  the  side  of  the  tube  to  prevent  the  liquids  mixing  to  any  extent. 
The  tube  or  burette  is  closed  by  a  smooth  soft  cork,  and  the  dividing  line  and 
the  surface  of  the  ether  are  read.  The  dividing  line  is  more  easily  discernible 
by  the  contrast  in  color. 

The  ether  is  emulsified  by  vigorous  shaking,  and  the  tube  or  burette  stood  up- 
right for  15  minutes,  and  the  volume  of  ether  again  read.  The  ether  should  be 
colorless  —  if  tinted  there  is  too  great  a  proportion  of  alcohol  to  water,  and 
another  experiment  should  be  made,  previously  diluting  the  sample  with  an 
equal  volume  of  alcohol-free  ether. 

It  is  important,  of  course,  that  the  temperatures  of  the  liquid  at  the  times  of 
reading  are  practically  the  same,  and  in  a  room  subject  to  draughts  it  is  well 
to  immerse  the  tube  in  a  jar  of  water  for  a  few  minutes  previous  to  each 
observation.  Results  are  usually  fairly  accurate,  the  error  minus  and  not 
exceeding  one  per  cent.* 

Calculation.  The  diminution  in  volume  of  the  ether  divided  by  the  volume  of 
the  sample,  and  the  quotient  multiplied  by  100  is  the  volume-percentage  of 
alcohol  contained.  The  product  is  to  be  doubled  in  cases  where  the  original 
sample  was  diluted  with  an  equal  volume  of  pure  ether. 

Examples. 

No.  1.  No.  2. 

Volume  of  water 19.8  Cc.  20.9  Cc. 

Volume  of  ether 25.0  Cc.  25.0  Cc. 

After  extraction,  volume  of  ether 24  2  Cc.  22.8  Cc. 

Diminution  in  volume  of  the  ether 8  Cc.  2.2  Cc. 

Percentage  V/V  of  alcohol  contained 3.2  8.8 

EXERCISE  7— ACIDIMETRY. 

Preparation  of  Standard  Acid  and  Alkali. 

Standard  Acid.  An  approximately  normal  solution  of  sulfuric  acid  is  made 
by  diluting  the  concentrated  pure  acid  of  commerce.  Into  a  one-liter  measuring 
flask  about  three-quarters  filled  with  cold  water  is  poured  29  Cc.  of  the  acid  of 
1.84  specific  gravity.  When  the  mixture  has  cooled  to  the  temperature  of  the 
room  it  is  made  up  to  the  mark  with  water  and  well  mixed.  The  solution  will 
be  slightly  stronger  than  the  normal  (49.043  grams  of  H2S04  per  liter). 

Standardizing.  The  exact  strength  or  titre  of  the  acid  may  be  found  in 
several  ways  of  which  one  is  here  given. 

A  measured  volume  of  the  acid  is  neutralized  by  ammonia  and  evaporated  to 
dryness.  From  the  weight  of  the  residual  ammonium  sulfate  is  calculated  that 
of  the  sulfuric  acid.  Of  course  any  non-volatile  impurities  in  the  acid  or 
ammonia  will  vitiate  the  result. 

Select  a  glass  crystallizing  dish  (or  bottom  of  a  beaker,  page  60)  about  three 
inches  in  diameter  and  rub  the  upper  edge  slightly  with  vaseline ;  cover  with  a 
light  watch-glass  and  weigh.  Note  the  temperature  of  the  acid,  and  pipette  25  Cc, 
into  the  dish.  Slowly  add  dilute  ammonia  until  a  narrow  strip  of  litmus  paper 
just  turns  blue.  Rinse  the  paper  and  evaporate  to  dryness  on  the  water- bath. 


Analyst,  2—98. 


222  .  QUANTITATIVE    CHEMICAL    ANALYSIS. 

As  soon  as  a  film  of  crystals  forms  on  the  surface  there  is  danger  of  loss  by 
spattering,  and  one  must  continually  agitate  the  dish  until  the  salt  solidifies. 

Now  add  a  few  drops  of  ammonia,  evaporate  to  complete  dryness,  and  heat 
in  the  air-bath  to  105°  for  a  half-hour.  Cover  with  the  watch-glass,  cool  in 
the  desiccator  and  weigh.  The  increase  is  (NH4)2SC>4  from  which  the  £[2804  is 
calculated  by  the  proportion 

Weight  of  (NH4)2SO4  :  weight  of  H2SO4  :  :  132.214  :  98.086. 
and  the  weight  of  H2SO4  divided  by  25  gives  the  weight  in  one  Cc. 

Example.  Twenty-five  Cc.  of  acid,  diluted  as  above,  evaporated  with 
ammonia; 

A.  B.  C. 

Weight  of  ammonium  sulfate  and  dish 31.145        33.863        31.146 

Weight  of  dish 29.419        32.136        29.417 

Weight  of  ammonium  sulfate  1.726          1.727          1.729 

One  Cc.  of  the  acid  contains 05122        .05125        .05131 

The  average  of  the  three  results  is  .05126.  The  neutralizing  power  of  this 
acid  for  potassium  and  sodium  hydrates  is  found  by  multiplying  this  number 

"1  I  O    OQC  Qf\    1  1  ft 

by  98  086  and  98*086  resPectivelv-     Tne  acid  ls  Poured  into  a  dry  glass -stop- 
pered bottle  labeled  about  as  follows:  "Approximately  normal  sulfuric  acid. 

At degrees  Cent,  one  Cc.  contains gram  of  H2SO4  and 

neutralizes gram  of  KOH,  and gram  of  NaOH, 

with as  indicator.    Date  of  standardization <...." 

The  neutralizing  power  of  normal  acid  decreases  with  rise  of  temperature  * ; 
if  100  at  15°,  it  Will  be  99.92  at  17.5°  ;  99.85  at  20°  ;  99.77  at  22.5°  ;  99.69 
at  25  c  ;  99.61  at  27.5  °  ;  and  99.52  at  30  <=> . 

Standard  Potassium  Hydrate.  A  solution  is  made  up  from  the  grade  of 
caustic  known  as  '  purified  by  alcohol,'  and  standardized  by  titration  against  the 
standard  acid.  Since  the  commercial  potash  contains  a  considerable  propor- 
tion of  water  (  often  30  per  cent),  a  solution  stronger  than  normal  is  made  and 
tested,  then  diluted  to  the  proper  strength.  About  100  grams  of  potash  is  dis- 
solved in  cold  water  and  diluted  to  one  liter;  50  Cc.  of  this  solution  is  trans- 
ferred to  a  beaker,  diluted  with  water,  and  a  few  drops  of  phenol-phthalein  added. 
A  burette  is  rinsed  with  the  standard  acid  and  filled  to  the  zero  mark.  The 
acid  is  cautiously  run  into  the  red  titrate  until  the  color  is  just  discharged; 
the  volume  of  acid  required  is  V  cubic  centimeters. 

Since  of  strictly  normal  solutions  equal  volumes  react,  we  will  calculate  to 
what  extent  the  alkali  solution  must  be  diluted  to  make  it  approximately 
normal.  From  the  proportion  50  :  V  :  :  x  :  1000,  we  find  that  x  Cc.  of  the 
alkali  would  be  neutralized  by  1000  Cc.  of  the  acid;  hence  by  diluting  x  Cc.  to 
1000  Cc.  with  water,  the  acid  and  alkali  will  react  in  equal  volumes. 

Having  measured  oft  x  Cc.  of  the  alkali,  diluted  to  one  liter,  and  mixed  well,  we 
repeat  the  titration  of  50  Cc.  to  ascertain  the  exact  content  of  alkali.  Let  there 
be  required  V  Cc.  of  the  acid;  then  if  one  Cc.  of  the  acid  contains  a  gram  of 
H2SO4,  the  weight  of  H2SO4  neutralizing  the  alkali  in  50  Cc.  of  the  caustic  solu- 
tion is  a.  V.  Hence  the  proportion 

2KOH  (112.236)  :    :  H2SO4  (98.086)  :    :    T  :  a  V. 

And  -  is  the  weight  of  potassium  hydrate  in  one  Cc.  of  the  caustic    solution 

50 

determined  by  the  aid  of  phenol-phtbalein  as  indicator. 


*  Analyst,  1894-100. 


CHLORAL    HYDRATE.  223 

Example.  100  grams  of  '  potash  by  alcohol '  was  dissolved  to  one  liter:  50  Cc. 
required  65.1  Cc.  of  standard  acid  of  which  one  Cc.  contained  .05126  gram  of 
HaSO*.  Therefore  x  =  768  Cc. :  and  this  volume  diluted  to  one  liter  formed  an 
approximately  normal  solution.  Fifty  Cc.  was  titrated  and  required  49.9Cc.  of 

Y 

acid,  hence,     -    =  .05854,  the  weight  of  KOH  in  one  Cc. 

EXERCISE  8-  A.  ACIDITY  OF  VINEGAR. 

"  Vinegar  is  a  more  or  less  colored  liquid  consisting  essentially  of  impure 
dilute  acetic  acid,  obtained  by  the  oxidation  of  wine,  beer,  cider,  or  other  alco- 
holic liquor."  Wine  vinegar  has  a  specific  gravity  of  1.014  to  1.022,  while 
that  from  malt  is  1.017  to  1.019.  Vinegar  contains  usually  from  three  to  six 
per  cent  of  free  acetic  acid  by  weight;  In  some  States  the  legal  minimum 
is  four  per  cent.  It  is  frequently  fortified  by  sulf  uric  or*  acetic  acid,*  rarely  by 
hydrochloric. 

On  titration  by  standard  alkali  the  free  acid  or  acids  are  neutralized  — 
HCaH3O2  (acetic  acid)  -+-  KOH  =  KC2H3O2  (potassium  acetate)  -f  H2O.  From 
the  volume  of  alkali  solution  may  be  calculated  the  percentage  of  free  acid, 
assumed  to  be  acetic. 

From  a  pipette  add  50  Cc.  of  vinegar  to  100  Cc.  or  more  of  cold  water  con- 
taining a  few  drops  of  phenol-phthalein  solution,  and  titrate  to  faint  redness 
by  standard  potassium  hydrate. 

Calculation.  Assuming  the  acid  to  be  acetic, 

HC2H8O2  (60.032)  :  KOH  (56.118)  :  :  weight  of  HC2H302  :  weight  of  KOH; 
and  weight  of  HC2HsO2  X  W®  -*-  50  =  Per  cent  °'  acetic  acid  by  weight  i  n  one 
volume  of  vinegar. 

Example.  Required  for  50  Cc.  of  vinegar  41.1  Cc.  of  potassium  hydrate  solution 
containing  .05854  gram  of  KOH  per  Cc.  The  percentage  of  acetic  acid  is  there- 
fore 5.14. 


B.  FREE  ACIDS  IN  CITRUS  FRUITS. 

The  grateful  acid  taste  of  fruits  of  the  citrine  genus  is  due  to  free  organic 
acids  mostly  citric;  in  lemon  pulp  it  averages  from  eight  to  ten  per  cent.f 

Strain  the  pulp  of  a  large  lemon  through  brass  wire  gauze  to  remove  seeds 
and  fiber.     Weigh  a  small  beaker,  introduce  about  25  grams  of  the  juice,  and 
again  weigh.    Rinse  with  cold  water  into  a  large  beaker  and  titrate  by  stand- 
ard potassium  hydrate  and  phenol-phthalein. 
Calculation. 

HsCeHsOr  (192.064)  +3KOH  (168.354)  =3K3C6H6O7+  3H2O. 
Citric  acid.  Potassium  citrate. 

EXERCISE  9  -CHLORAL  HYDRATE. 


Chloral  ((^HClsO)  is  an  oily  colorless  liquid  of  a  pungent  odor.  It  is  formed 
on  passing  dry  chlorine  into  absolute  alcohol  for  a  long  time,  the  principal 
reaction  being  C2H5OH+4C12  =  C2HC13O  +  5HC1.  However,  by  a  secondary 
reaction  between  the  alcohol  and  hydrochloric  acid  there  is  formed  water  — 
C2H6OH  +  HCl  ==  (C2H5)C1  -H  H2O  —  which  immediately  unites  with  the  chloral 
to  form  chloral  hydrate,  so  that  chloral  itself  is  not  obtained  by  this  process. 

Chloral  hydrate,  CC]3.CH(OH)2,  is  a  colorless  compound  crystallizing  in  the 


*  Analyst,  1893—180  and  1894-89. 
t  Journ.  Ohem.  Socy.  28—937. 


224  QUANTITATIVE    CHEMICAL    ANALYSIS. 

monoclinic  system,  melting  at  57  °  and  boiling  at  97.5  o  .  It  is  soluble  in  water 
alcohol  and  ether.  As  found  in  pharmacy  in  the  form  of  clusters  of  small 
crystals  or  crusts,  it  is  usually  quite  pure,  though  occasionally  containing  a 
small  amount  of  free  organic  acid.  Employed  in  medicine  as  a  sedative,  its 
action  on  the  human  economy  is  said  to  depend  on  its  decomposition  into  chlo- 
roform. It  is  deliquescent,  has  a  faint  odor,  and  may  be  sublimed  without 
decomposition. 

When  an  aqueous  solution  of  chloral  hydrate  is  mixed  with  a  caustic  alkali 
it  is  decomposed  into  chloroform,  an  alkali  formate,  and  water,  the  mixture 
becoming  turbid  from  the  separation  of  minute  globules  of  the  chloroform  — 
CCla.CH(OH)2  (165.374)  +  KOH  (56.118)  =  CHC13+  KCHO2  -f  H2O. 

A  determination  may  be  made  by  dissolving  a  weighed  quantity  of  the 
hydrate  in  water,  decomposing  it  by  an  excess  of  standard  potassium  hydrate, 
and  titrating  the  excess  of  alkali  by  standard  acid. 


Dissolve  a  few  grams  of  the  commercial  medicinal  chloral  hydrate  in  water 
and  test  the  reaction  with  blue  litmus  paper. 

Weigh  quickly  (to  avoid  absorption  of  moisture  from  the  air)  about  five 
grams,  brush  into  a  12-ounce  beaker,  and  dissolve  in  about  150  Cc.  of  cold 
water.  Should  the  above  test  show  the  presence  of  free  acid,  stir  in  a  little 
precipitated  calcium  carbonate,  filter  and  wash  with  cold  water.  Add  from  a 
pipette  50  Cc.  of  standard  potassium  hydrate,  stir  well,  and  titrate  by  standard 
sulfuric  acid  and  phenol-phthalein.  Also  titrate  50  Cc.  of  the  alkali  by  the 
acid. 

Calculation.  In  the  above  equation  56.118  parts  of  potassium  hydrate 
decompose  165.374:  parts  of  chloral  hydrate,  or  one  gram  decomposes 

i^Zi  =2.947  grams. 

56.118 

Let  a  be  the  volume  of  sulfuric  acid  neutralizing  50  Cc.  of  the  alkali,  and  6 
the  volume  of  sulfuric  acid  required  in  the  residual  titration  ;  then  a—  b  is  the 
volume  of  acid  equivalent  to  the  alkali  decomposing  the  chloral  hydrate. 

Let  c  be  the  weight  of  potassium  hydrate  neutralized  by  one  Cc.  of  the  acid, 
then  (a  —  6)  X  c  is  the  weight  of  potassium  hydrate  required  to  decompose 
the  chloral  hydrate;  and  since  one  gram  of  potassium  hydrate   decomposes 
2.947  grams  of  chloral  hydrate,  then 
(a  —  6)  X  c  X  2-9*7  is  the  weight,  and 

(a  -  6)  X  c  X  2.947  X  sample  i8  the  Percenta§e  of  chloral 


in  the  sample.* 

Example.  Fifty  Cc.  of  the  potassium  hydrate  solution  required  49  .3  Cc.  of 
the  sulfuric  acid  for  neutralization.  One  Cc.  of  the  acid  neutralized  .057119 
gram  of  KOH. 

Weight  of  Residual                         Percentage  of 

sample.  titration.                        chloral  hydrate. 

5.146  grams.  19.4  Cc.                                   97.82 

5.003      "  20.1    "                                        98.25 

5.237      "  18.8    "                                      98.03 


*  Allen,  Ooml.  Org.  Anal.  1—229. 


ACETIC    ACID.  225 


EXERCISE  10  —  ACETIC  ACID. 

Determination  of  the  Kate  of  Distillation  from  an  Aqueous  Solution. 

When  a  dilute  solution  of  acetic  acid  is  fractionally  distilled  the  acid  accom- 
panies the  water,  not  uniformly,  however,  but  in  a  constantly  increasing  ratio. 
That  is,  of  a  number  of  fractions  of  equal  volume,  each  contains  more  acid  than 
its  predecessor.  Richmond*  states  the  following  laws  of  the  volatility  of  the 
higher  fatty  acids  (page  320)  from  a  dilute  solution. 

1.  Each  acid  on  distillation  behaves  as  a  perfect  gas  and  conforms  to  Henry's 
law  [that  when  equilibrium  is  established  between  a  gas  and  a  liquid  in  con- 
tact, the  ratio  of  the  concentration  of  the  gas  to  that  dissolved  in  the  liquid  is 
a  constant  for  any  given  pressure]. 

2.  Each  acid  has  a  fixed  rate  of  distillation  which  is  an  inverse  function  of 
its  solubility  in  water  and  is  quite  independent  of  the  properties  of  the  pure 
acids. 

3.  The  apparent  rate  of  distillation  may  be  modified  by  condensation  in  the 
retort. 
He  proposes  the  following  formula 


loo**-1  ioo«-i 

in  which  x  is  any  percentage  of  the  total  volume  distilled  ;  y,  the  corresponding 
percentage  of  the  total  acid  distilled;  and  a,  the  ratio  between  the  acid- 
content  of  the  vapor  and  that  of  the  liquid  in  the  retort  —  for  acetic  acid  it  is 
about  .667. 

In  distilling  from  an  ordinary  retort  or  flask  more  or  less  condensation  of 
aqueous  vapor  takes  place  by  contact  with  the  upper  air-cooled  part  of  the 
distilling  vessel,  the  drops  formed  running  back  into  the  liquid  ;  the  amount 
of  this  air-condensation  depends  on  the  extent  of  the  surface  of  the  unoccupied 
part  of  the  still,  the  rapidity  of  boiling,  temperature  of  the  air,  draughts,  etc. 
A  little  consideration  of  Henry's  law  will  show  that  the  acid-content  (con- 
centration) of  the  air-condensed  liquid  is  intermediate  between  those  of  the 
vapor  and  the  liquid  in  the  retort;  and  as  in  distilling  dilute  acetic  acid  the 
acidity  of  the  vapor  is  always  less  than  that  of  the  liquid  in  the  retort 
(a  =.667),  the  air-condensed  drops  are  always  stronger  in  acid  than  the 
remaining  vapor.  Hence  the  greater  the  air-  condensation  the  weaker  will  be 
the  vapor  as  it  enters  the  condenser,  and,  of  course,  the  distillate  as  well. 
Obviously,  therefore,  if  concordant  results  are  to  be  expected  in  several  dis- 
tillations, either  the  operation  must  be  conducted  under  rigidly  uniform  con- 
ditions, or  a  correction  deduced  and  applied.  Richmond  (loc.  cit)  adopts  an 
approximate  correction  expressed  as  a  logarithmic  function  of  the  volume  of 
the  distillate,  namely,  K~*  (supra),  K  varying  with  the  amount  of  air-condens- 
ation ;  in  the  original  experiments  of  Duclauxf  it  is  nearly  unity. 


A  weak  aqueous  solution  of  acetic  acid  is  prepared  by  diluting  about  20  Cc.  of 
the  glacial  acid  to  500  Cc.  Two  portions  of  110  Cc.  each  are  withdrawn  by  pi- 
pettes, one  into  a  beaker  and  titrated  by  potassium  hydrate  and  phenol -phthalein, 


*  Analyst,  1895—195  et  seq. 
t  Anil.  Chim.  Phys.  5—2—233. 


226 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


the  other  into  a  flask  of  about  250  Cc.  capacity. 
Into  the  latter  a  few  pieces  of  pumice-stone  about 
the  size  of  coffee-beans  are  thrown  to  secure  reg- 
ular boiling,  and  the  flask  is  supported  on  a  sand- 
bath  and  connected  with  a  condenser  by  a  rubber 
stopper,  as  shown  in  Fig.  138.  Below  the  con- 
denser is  placed  a  10  Cc.  measuring  jar  for  a 
receiver.  After  seeing  that  the  stopper  and 
connection  with  the  condenser  are  steam-tight, 
the  burner  is  lighted  and  the  liquid  gently  boiled. 
The  distillation  proceeds  slowly,  and  when  ten 
Cc.  has  come  over  another  measuring  jar  is  substi 
tuted  for  the  first.  The  distillate  is  carefully  poured 
into  a  large  beaker  and  the  jar  rinsed  by  once 
filling  with  water  and  pouring  into  the  beaker. 

After  titration  by  standard  alkali  and  phenol-phthalein,  the  jar  is  dried  by  a 
roll  of  filter  paper  and  substituted  for  the  second  jar  when  that  has  received 
ten  Cc.  The  second  fraction  is  poured  into  the  beaker  containing  the  first 
distillate,  the  jar  rinsed  and  the  titration  continued.  Ten  fractions  in  all  are 
collected  and  titrated,  leaving  about  ten  Cc.  remaining  in  the  flask. 

Calculation.  The  volume  of  alkali  required  for  each  fraction  times  100  di- 
vided by  the  volume  required  for  110  Cc.  of  the  acid  gives  the  percentage  of 
acid  in  each  fraction. 

The  percentage  of  acid  divided  by  that  calculated  from  the  formula  y  =  100  — 
* 
gives  a  factor  of  correction  for  air-condensation  which  should  be 


Fig.  138. 


100 

fairly  constant  for  all  the  fractions  of  any  one  distillation  provided  the  condi- 
tions were  uniform  throughout.    The  results  may  be  plotted  as  in  Fig.  171. 

Example.  In  the  following  table  x  represents  the  percentages  of  the  volumes 
of  successive  fractions  of  the  total  distillate  on  the  basis  of  110  Cc.  equaling 
ICO  per  cent;  column  y  the  percentage  of  acid  in  each  volume  as  calculated  by 
the  above  formula;  columns  A,  B,  and  C,  the  results  of  three  experiments 
conducted  as  described  above;  and  a,  b}  and  c,  the  respective  factors  of  cor- 
rection, or  the  ratio  of  A,  B,  and  C,  to  y. 


A 

A 

B 

B 

C 

C 

X 

y 

Cc. 

Per 
cent. 

Cc. 

Per 
cent. 

Cc. 

Per 
cent. 

a          b 

c 

9.09 

6.2 

4.6 

6.3 

4.2 

5.8 

4.8 

6.5 

.98 

1.07 

.95 

18.18 

12.5 

9.4 

12.8 

8.7 

11.9 

9.8 

13.3 

.98 

1.05 

.94 

27.27 

19.1 

14.4 

19.6 

13.2 

18.0 

15.0 

20.3 

.97 

.06 

.94 

36.36 

26.0 

19.5 

26.6 

18.1 

24.7 

20.4 

27.6 

.98 

.05 

.94 

45.45 

33.3 

24.9 

34.0 

23.2 

31.7 

25.9 

35.1 

.98 

.05 

.95 

54.54 

40.9 

30.5 

41.6 

28.5 

38.9 

31.7 

43.0 

.98 

.05 

.95 

63.64 

49.0 

36.6 

49.9 

34.2 

46.7 

37.9 

51.4 

.98 

.05 

.95 

72.73 

57.9 

43.1 

58.8 

40.5 

55.2 

44.5 

60.4 

.98 

.05 

.96 

81.82 

67.9 

50.7 

69.2 

47.9 

65.3 

51.9 

70.4 

.98 

.04 

.96 

90.91 

79.8 

59.6 

81.3 

57.0 

77.8 

60.3 

81.8 

.98 

.03 

.97 

110  Cc. 

required  of  standard  alkali, 

in  A,  73 

3  Cc.  ; 

in  B,  73.3 

Cc.  ;  in  C,  73. 

7Cc. 

The  effect  of  air-condensation  is  plainly  shown  in  B  and  C;  in  both  the  dis- 
tillation was  made  from  the  same  flask.  In  B  the  amount  of  air- condensation 
was  considerable  and  the  distillation  correspondingly  slow  (129  minutes)  and 
therefore  the  distillate  weaker  throughout  than  in  y,  while  in  C  the  flask  was 
closely  covered  by  a  non-conductor  of  heat  (sheet  asbestos) ,  hence  the  air- 
condensation  was  small,  the  distillation  rapid  (42  minutes),  and  the  distillate 
stronger  than  in  y. 


HYDRASTIS-GUARANA.  227 

EXERCISE  11-HYDRASTIS. 

Determination  of  Berberine. 

Hydrastis,  the  dried  rhizome  and  roots  of  the  Hydrastis  Canadensis  nat.  ord. 
Eanunculaceae  (U.  S.  Pharmacopoeia).  It  is  extensively  used  as  a  medicine 
and  dispensed  by  pharmacists  in  the  form  of  a  fine  powder  or  its  fluid 
extract.* 

The  drug  contains  at  least  three  alkaloids  constituting  the  active  principle  — 
berberine,  hydrastine,  and  canadine.  The  first  named,  considered  by  some  to 
be  the  most  important  medicinally,  assays  upward  of  four  per  cent  in  the  com- 
mercial powder.  It  is  a  yellow  or  brown  crystalline  solid  (C2oHi7NO4),  inodor- 
ous and  of  a  bitter  taste.  Soluble  in  hot  water  and  alcohol,  and  unites  with 
acids  to  form  crystalline  salts. 

Berberine  hydrochloride  (CsjoHirNC^HCl.il^O) crystallizes  in  yellow  prisms 
sparingly  soluble  in  water  (100  parts),  cold  alcohol,  and  dilute  acids,  and  very 
slightly  in  a  mixture  of  alcohol  and  ether. 

The  assay  of  the  drug  is  made  by  extracting  the  alkaloids  by  hot  alcohol; 
precipitating  the  berberine  by  hydrochloric  acid  and  ether;  drying  the  precipi- 
tate, when  it  loses  its  water  of  crystallization ;  and  weighing  as 


Dry  the  powdered  hydrastis  at  100  ° ,  and  weigh  one  portion  of  ten  grams  and 
transfer  to  an  8-oz.  beaker  with  60  Cc.  of  alcohol  (95  per  cent) ;  cover  with  a 
watch-glass  and  heat  on  the  water  bath  for  a  half  hour,  stirring  now  and  then. 
Run  through  a  12.5  Cm.  filter  into  a  100  Cc.  measuring  flask;  without  washing, 
drop  back  the  residue  into  the  beaker,  add  50  Cc.  more  alcohol,  and  heat  again  for 
a  half  hour.  Filter  through  the  same  paper,  and  wash  with  hot  alcohol  until 
the  mark  on  the  flask  is  reached.  After  cooling,  make  up  to  exactly  100  Cc. 
with  alcohol  and  mix  well. 

Draw  out  three  lots  of  25  Cc.  into  four-ounce  Erlenmyer  flasks.  Add  to 
each,  one  Cc.  of  concentrated  hydrochloric  acid,  three  drops  of  concentrated 
sulfuric  acid,  and  15  Cc.  of  ether.  Close  the  flasks  by  smooth  corks  (not  rub- 
ber stoppers),  and  shake  the  flasks  until  crystals  appear.  Let  stand  over 
night  in  a  cool  place. 

Filter  each  on  a  smooth  9-Cm.  paper  and  wash  a  few  times  with  a  mixture  of 
equal  volumes  of  alcohol  and  ether.  A  yellow  coloring  extract  stains  the 
filter  and  cannot  be  washed  away  without  dissolving  some  of  the  precipitate  — 
its  weight  is  inconsiderable.  Dry  the  fllter  first  on  blotting  paper,  then  in 
the  air-bath  at  110°  to  115  o  for  an  hour.  Open  the  fllter  and  transfer  the 
precipitate  to  a  tared  watch-glass  with  the  aid  of  a  camels-hair  brush  and 
weigh. 

Calculation.     C2oHi7NO4HCl  :  C^HirNO*  :  :  371.634  :  335.176. 

Example.  One  gram  of  the  dried  powder  treated  as  above  gave  .0740,  .0735, 
and  .0725  grams  of  the  anhydrous  hydrochloride,  equivalent  to  2.67,  2.65,  and 
2.62  per  cent  of  the  alkaloid  in  the  drug. 

EXERCISE  12-GUARANA. 

Determination  of  Caffeine. 

The  guarana  (Paullinia  Cupana  nat.  ord.  Sapindaceae)  is  a  climbinsj  shrub 
indigenous  to  Brazil.  The  seeds  are  dried  and  powdered,  moistened  with 


*  Proc.  Michigan   Pharm.  Assn.  1893;  Prescott,  Organic  Anal.  71 ;  Lloyd,  Drugs  and 
Medicines    of  N.    A.    76;    Amer.    Journ.    Pharm.  71—257;    Journ.    Amer.    Chem.    Socy. 

1899—732. 


228  QUANTITATIVE    CHEMICAL    ANALYSIS. 

water,  and  kneaded  to  a  stiff  paste  which  is  dried,  formed  into  rolls,  and  ex- 
ported. The  commercial  article  contains  starch,  gum,  a  green -colored  fat, 
tannin,  and  from  two  to  four  per  cent  of  the  active  principle  the  alkaloid  caffeine 
(guaranine).  The  official  medicinal  fluid  extract  is  a  clear  liquid  of  a  deep  red. 
brown  color,  peculiar  odor,  and  astringent  taste ;  it  is  made  by  the  process  of 
cold  repercolation  of  the  powdered  guarana  by  diluted  alcohol,  the  percolate 
being  diluted  until  one  cubic  centimeter  represents  one  gram  of  the  drug  and 
contains  from  30  to  40  milligrams  of  caffeine  in  a  menstruum  of  about  three 
parts  of  alcohol  to  one  part  of  water.  It  may  be  found  at  any  pharmacy. 

The  assay  of  the  fluid  extract  is  made  by  diluting  it  with  a  magma  of  magne- 
sium carbonate  in  water,  thus  neutralizing  it  and  precipitating  resin  and  vari- 
ous extractives.  After  filtering,  the  caffeine  is  absorbed  or  "  shaken  out "  by 
chloroform  from  an  aliquot  part  of  the  filtrate.  On  evaporation  of  the  chloro- 
form the  anhydrous  alkaloid  is  left  nearly  pure.  Obtained  in  this  way,  caffeine 
(CsHioN^)  is  a  crystalline  mass  of  stellate  tufts  of  colorless  needles,  perma- 
nent in  the  air,  but  subliming  a  little  above  100  °  ,  and  soluble  in  about  80  parts 
of  water,  7  of  chloroform,  and  33  of  alcohol.  It  may  be  identified  by  the  '  mur- 
exoin  reaction.'* 


Into  a  100  Cc.  measuring-flask  is  introduced  10  Cc.  of  the  fluid  extract,  allow- 
ing the  pipette  to  drain  for  two  minutes;  this  is  diluted  with  about  80  Cc.  of 
cold  water,  and  about  a  gram  of  finely  powdered  commercial  magnesium  car- 
bonate added.  The  mixture  is  shaken,  made  up  to  the  mark  with  water,  and 
well  mixed.  The  precipitate  and  the  excess  of  the  carbonate  are  separated  by 
filtration  through  a  dry  11  Cm.  paper  into  a  dry  beaker. 

Two  portions  of  the  filtrate  of  40  Cc.  each  are  accurately  measured  from  a 
tall  measuring-jar  or  burette,  and  each  extracted  as  follows:  after  pouring  into 
a  50  Cc.  burette  with  glass  stopcock  (or  a  small  separatory  funnel),  10  Cc.  of 
chloroform  is  added,  the  burette  stopped  by  the  thumb  and  shaken  vigorously 
to  emulsify  the  liquids;  then  fixed  in  its  stand,  and  the  chloroform  allowed  to 
segregate  below  the  aqueous  stratum. 

A  glass  dish  (page  60) ,  about  three  inches  in  diameter  is  weighed  and  nearly 
all  the  chloroform,  containing  a  large  proportion  of  the  alkaloid,  drawn  into  it, 
being  careful  not  to  allow  any  water  or  mucilaginous  matter  at  their  junction 
to  follow.  To  withdraw  what  alkaloid  remains,  the  aqueous  solution  is  again 
shaken  out  with  four  successive  portions  of  chloroform  of  5  Cc.  each. 

The  30  Cc.  of  chloroform  in  the  dish  is  now  evaporated  to  dryness  on  the 
water  bath  at  a  heat  insufficient  to  boil  it.  The  dish  is  wiped  dry  and  weighed 
after  standing  in  the  balance  case  for  15  minutes.  The  caffeine  should  be 
white  or  nearly  so  and  distinctly  crystalline ;  it  is  often  fragrant,  especially  on 
heating,  though  the  puro  alkaloid  is  odorless. 

Calculation.  The  volume  of  the  magnesium  carbonate  and  the  precipitate  it 
forms  is  not  considered,  as  the  error  introduced  is  less  than  those  from  other 

40 
sources.     Since  the  alkaloid  is  obtained  from  ^QQ  of  10  Cc.  (or  4  Cc.)  of  fluid 

100 
extract,  its  weight  times  -7-  is  the  percentage  W/V  of  caffeine  in  the  extract. f 

Example.  Ten  Cc.  treated  as  above  gave  .153  and  .152  gram  of  caffeine, 
equal  to  3.82  and  3.80  per  cent  of  the  alkaloid  in  one  volume  of  the  fluid  ex- 
tract. 


*  Prescott,  Organic  Anal.  79. 

t  U.  S.  Dispensatory,  Guarana.    Journ.  Araer.  Ohem.  Socy.  1896—978. 


M  lEHMAiNGANATE-POTAssiuM  CHLOKATE.       229 


EXERCISE  13  -  POTASSIUM  PERMANGANATE. 

Preparation  of  a  Standard  Solution. 

The  formula  of  this  compound  may  be  expressed  as  K2O.2MnO.05.  When 
brought  in  an  acid  solution  in  contact  with  a  reducing  agent  the  acid  is  decom- 
posed, the  hydrogen  uniting  with  the  five  atoms  of  available  oxygen  to  form 
water,  while  the  potassium  and  manganese  unite  with  the  halogen  or  acid- rest 
to  form  proto- salts;  thus,  with  nitric  acid  and  hydrogen  peroxide  — 

K2O.2MnO.O5  +  6HNO3  -f  5H2O2  =  2KNO3  +  2Mn(NO3)2  +  502  +  8H2O. 

The  purple  color  of  permanganic  acid,  perceptible  in  even  extremely  dilute 
solutions,  is  a  well  marked  indication  of  the  least  excess,  the  proto-salts  of 
potassium  and  manganese  being  colorless. 

No  great  amount  of  hydrochloric  acid  may  be  present  in  the  sulfuric  or  nitric 
solution  of  the  compound  to  be  titrated,  especially  if  it  be  hot,  lest  the  per- 
manganate react  with  it  also;  e.  g.,  K2O.2MnO.Os  +  16HC1  =  2KC1  -f  2MnCl? 
+  5C12  +  6H2O. 


Four  grams  of  the  clean  crystals  is  dissolved  in  a  liter  of  pure  water ;  after 
standing  several  hours  the  solution  is  decanted  from  any  sediment.  The  so- 
lution slowly  decomposes  with  age  and  immediately  with  most  organic  matter. 
The  strength  is  ascertained  before  each  series  of  analyses  by  titrating  a  known 
weight  of  an  oxidizable  salt,  such  as  ferrous  iron  in  a  strongly  acid  solution  — 

10FeSO4  +  K2Mn2O8  +  8H2SO4  =  5Fe2(SO4)3  +  K2SO4  +  MnSO4  -f  8H2O, 
and  is  generally  expressed  as  the  weight  of  metallic  iron  oxidized  by  one  cubic 
centimeter  of  the  permanganate  solution. 

Weigh  accurately  about  five  grams  of  bright  iron  wire  (page  209)  and  trans- 
fer to  a  one-liter  measuring  flask ;  add  about  300  Cc.  of  water  and  50  Cc  of  concen  - 
trated  sulfuric  acid,  cover  the  flask  with  a  watch-glass  and  heat  gently  until  the 
iron  is  dissolved.  Cool,  add  a  few  grains  of  metallic  zinc  (to  reduce  any  ferric 
sulfate  formed  by  oxidation) ,  and  when  the  zinc  is  dissolved,  dilute  to  the  mark 
with  water  and  mix  well.  The  solution  slowly  oxidizes  on  keeping. 

Fill  a  burette  with  the  permanganate  solution,  pipette  60  Cc.  of  the  ferrous 
solution  to  a  large  beaker  containing  about  200  Cc.  of  cold  water,  and  titrate 
to  faint  redness  with  constant  stirring.  The  color  persists  for  a  few  minutes 
only,  the  slight  excess  of  permanganic  acid  reacting  with  the  manganous  salt 
present. 

Calculation.  The  weight  in  grams  of  iron  in  50  Cc.  of  the  ferrous  solution 
divided  by  the  volume  of  permanganate  solution  used  equals  the  weight  in 
grams  of  iron  oxidized  by  one  cubic  centimeter  of  the  permanganate. 

Example.  Iron  wire  weighed  5.013  grams.  Deducting  .005  gram  for  impuri- 
ties leaves  5.008.  For  50  Cc.  was  required  35.5, 35.5  and  35.6  Cc.  of  permangan- 
ate; therefore  one  Cc.  oxidizes  .007053  gram  of  iron. 

EXERCISE  14 -A.    POTASSIUM  CHLORATE. 

Determination  of  Oxygen. 

The  salt  crystallizes  in  colorless  monoclinic  plates  soluble  in  16  parts  of  cold 
and  2  parts  of  hot  water.  The  most  common  impurity  is  potassium  chloride,, 
removable  by  recrystallization  from  hot  water. 

The  determination  is  made  by  a  residual  titration.  Sulfuric  acid  is 
decomposed  on  heating  with  a  ferrous  salt  and  a  chlorate  — 

KClOs  +  4H2S04  +  6FeSO4  =  KHSO4  -f  3Fe2(S04)3  +  HC1  +  3H20. 


230  QUANTITATIVE    CHEMICAL    ANALYSIS. 

If  a  known  weight  (an  excess)  of  ferrous  sulfate  be  dissolved,  and  what 
remains  unoxidized  be  titrated  by  potassium  permanganate,  the  weight  of 
oxygen  can  be  calculated. 


Powder  pure  potassium  chlorate,  weigh  one  gram,  and  dissolve  in  exactly 
500  Cc.  of  cold  water  in  a  half -liter  measuring  flask.  Draw  out  with  the  pipette 
three  portions  of  50  Cc.  into  pint  Erlenmyer  flasks  and  add  to  each  exactly  100 
Cc.  of  the  ferrous  solution  (page  209),  and  about  75  Cc.  of  dilute  sulfuric  acid 
(one  acid  to  ten  water).  Cover  the  flasks  by  watch-glasses  and  heat  to  about 
60°  to  70°  (not  higher,  lest  the  free  hydrochloric  acid  act  on  the  ferrous  sul- 
fate) ;  then  cool  the  flasks  in  a  stream  of  water,  dilute  with  100  Cc.  of  cold 
water,  and  titrate  by  standard  permanganate. 

Calculation.  From  the  above  equation,  three  atoms  of  oxygen  (48)  react  with 
six  atoms  of  iron  (336);  hence  the  weight  of  iron  in  100  Cc.  of  the  ferrous 

JQ  i 

solution,  less  that  oxidized  by  the  permanganate,  multiplied  by  — —  or  _  is  the 

336       7 

weight  of  oxygen  in  .1  gram  of  the  chlorate. 

Example.  Ferrous  solution,  5.008  grams  of  iron  wire  in  one  liter.  Per- 
manganate, 4  grams  per  liter.  Fifty  Cc.  of  the  former  was  oxidized  by  35.5  Cc. 
of  the  latter,  hence  one  Cc.  of  permanganate  oxidized  .007053  gram  of  iron.  In 
the  determination  were  required  32.1,  32.1,  and  32.2  Cc.  of  permanganate, 
giving  respectively  39.20,  39.20,  and  39.10  per  cent  of  oxygen.  Theory  requires 
39.16  per  cent. 


C.  FORGE  SCALE. 

Determination  of  Iron. 

The  scales  detached  when  hot  iron  is  hammered  are  composed  of  a  mixture 
of  iron  protoxide  and  sesquioxide,  with  small  amounts  of  metallic  iron,  oxide 
of  manganese,  etc.  The  ratio  of  the  protoxide  to  the  sesquioxide  is  quite 
variable,  though  approaching  the  proto-sesquioxide,  FeaO*. 

As  the  scale  is  but  slowly  acted  on  by  sulfuric  acid,  and  a  hydrochloric 
solution  is  inadmissible  in  a  titration  by  permanganate,  the  solution  is 
made  first  in  the  latter  and  this  evaporated  with  an  excess  of  the  former, 
leaving  a  residue  of  ferric  and  ferrous  sulfates  soluble  in  hot  water. 

The  iron  is  determined  by  finding  the  volume  of  standard  permanganate 
required  to  convert  it  from  the  ferrous  to  the  ferric  state,  so  that  previous 
to  the  titration,  ferric  sulfate  must  be  reduced  to  ferrous  by  some  reagent 
whose  excess  (used  to  insure  complete  reduction)  can  be  easily  removed 
or  so  altered  as  not  to  affect  permanganate.  Metallic  zinc  answers  this 
requirement,  as  zinc  sulfate  does  not  react  with  permanganate  and  the  excess 
of  zinc  may  be  decanted  or  filtered  off  or  allowed  to  dissolve.  The  reaction 
involved  in  the  reduction  is  Zn  -f  H2SO4  =  ZnSO4 -f  2H;  and  Fe2(SO4)8  +  2H 
(nascent)  =  2FeSO4  +  H2SO4. 


Grind  a  few  grams  of  the  clean  scale  to  an  impalpable  powder  in  an  agate 
mortar  and  dry  at  100°.  Weigh  about  one-half  gram  into  a  No.  2  beaker,  add 
20  Cc.  of  concentrated  hydrochloric  acid,  stir  well  and  digest  in  a  warm  place 
until  all  or  nearly  all  seems  to  be  dissolved.  Slowly  heat  to  near  boiling,  add- 


CHROME   YELLOW.  231 

ing  a  crystal  of  potassium  chlorate  to  destroy  any  carbonaceous  matter  origi- 
nally present  as  carbon  in  the  iron. 

Dilute  the  solution  with  50  Cc.  of  hot  water  and  filter  through  a  small  paper 
into  a  20- oz.  beaker;  wash  the  filter  alternately  with  hot  water  and  dilute  hy- 
drochloric acid  until  no  ferric  chloride  remains  in  the  paper.  Add  to  the  fil- 
trate about  5  Cc.  of  concentrated  sulfuric  acid  and  evaporate  on  the  hot  plate 
until  white  fumes  of  sulfuric  acid  appear. 

Dissolve  the  ferric  sulfate  in  a  little  hot  water,  dilute  with  200  Cc.  of  cold 
water,  and  drop  in  about  5  grams  of  granulated  or  powdered  zinc.  When  the 
solution  has  become  colorless,  add  more  sulfuric  acid  to  complete  the  solution 
of  the  zinc,  and  test  a  drop  of  the  liquid  by  potassium  sulf ocyanide  —  only  a  faint 
red  color  should  appear.  When  all  the  zinc  has  dissolved,  the  watch-glass  is 
rinsed  and  the  solution  immediately  titrated  by  standard  permanganate. 

Zinc  usually  contains  carbon  and  iron  In  quantities  that  will  reduce  a 
perceptible  volume  of  permanganate,  and  for  a  correction  five  grams  is 
dissolved  in  dilute  sulfuric  acid  and  the  solution  titrated,  the  volume  of  per- 
manganate used  to  be  deducted  from  the  former  reading. 

Calculation.  Let  the  volume  of  the  permanganate  used  be  V;  the  volume 
used  for  the  blank  titration  of  five  grams  of  zinc  be  v ;  the  weight  of  iron  oxi- 
dized by  one  Cc.  of  the  permanganate  be  F;  and  the  weight  of  scale  taken  for 

100 
analysis  be  W;  then  (F—  »)  X  **X  ~pp  is  tne  percentage  of  metallic  iron  in 

the  scale. 

Example.  A  weight  of  .496  gram  of  scale  treated  as  above  required  51.0  Cc. 
of  permanganate,  of  which  one  Cc.  oxidized  .007053  gram  of  iron.  Deducting 
.1  Cc.,  the  volume  required  for  five  grams  of  zinc,  the  percentage  of  iron  in  the 
scale  was  72.38. 


If  there  were  contained  in  the  scale  only  ferrous  and  ferric  oxides  the  pro- 
portion of  each  could  be  calculated  according  to  the  formula 
/Percentage  of  iron  V  160  \       1** 

\ Tl2 —  10V  X  "16  =  Percentage  of  FeO. 

Problem.  Given  a  dry  powder  consisting  of  a  mixture  of  FeaO4  and  Fe2Os,  to 
determine  by  a  chemical  process  the  percentages  of  each  without  the  use  ol 
the  balance  or  standardized  volumetric  solutions. 

EXERCISE  15 -CHROME  YELLOW. 

Determination  of  Lead  Chromate. 

The  pigment  known  to  the  trade  as  chrome  yellow,  lemon  chrome,  etc.,  is 
essentially  normal  chromate  of  lead,  PbCrO4,  and  is  sold  as  a  fine  powder  or 
ground  in  oil.  To  produce  the  lighter  shades  a  suitable  proportion  of  a  white 
pigment,  such  as  barium  sulfate,  lead  sulfate,  china  clay,  etc.,  is  admixed  during 
the  manufacture,  so  that  the  commercial  pigment  often  contains  less  than  half 
its  weight  of  lead  chromate. 


(1).  On  heating  the  powder  with  dilute  sulfuric  acid  the  lead  chromate  is 
decomposed  with  the  formation  of  chromic  acid  and  insoluble  lead  sulfate : 

2PbCrO4  +  2H2SO4  =  2H2CrO4-r-2PbSO4 (1) 

any  carbonates  present  are  also  converted  into  sulfates. 

(2).  Chromic  acid  reacts  with  a  ferrous  salt,  in  presence  of  a  free  mineral 


232  QUANTITATIVE    CHEMICAL   ANALYSIS. 

acid,  to  form  a  chromous  and  a  ferric  salt;  thus,  with  ferrous  sulfate  and  sul- 

furic  acid  — 

2H2CrO4  +  6FeSO4  +  6H2SO4  =  3Fe2(SO4)3  +  Cr2(SO4)3  +  8H2O (2) 

(3),  Ferric  salts  are  not  precipitated  by  a  ferricyanide,  while  a  ferrous  salt 

produces  a  blue  precipitate,  or  if  very  dilute,  a  blue  coloration.    Lead  sulfate 

and  insoluble  matter  do  not  interfere  with  this  reaction. 


The  determination  is  done  by  digesting  the  pigment  with  sulfuric  acid,  when 
chromic  acid  is  liberated  according  to  equation  (1).  The  chromic  acid  is  then 
titrated  by  a  standard  solution  of  ferrous  sulfate  according  to  equation  (2) . 
The  end-point  is  shown  by  ferricyanide.  The  solutions  needed  for  the 
determination  are  the  standard  solution  of  ferrous  sulfate,  page  229,  containing 
about  .005  gram  of  iron  per  cubic  centimeter,  and  one  of  potassium  ferri- 
cyanide, page  209.  

The  finely  powdered  pigment  is  dried  at  100° ,  and  four  portions  of  one  gram 
each  weighed  and  transferred  to  eight-ounce  beakers.  Into  each  is  poured 
about  60  Cc.  of  cold  water  followed  by  five  Cc.  of  concentrated 
sulfuric  acid,  and  the  mixture  stirred  until  the  yellow  leadchromate  has  wholly 
passed  to  the  white  sulfate.  The  turbid  yellow  liquid  is  then  diluted  with 
about  an  equal  volume  of  cold  water  and  titrated  by  the  ferrous  solution. 

The  end-point  is  found  by  placing  several  separate  drops  of  the  ferricyanide 
solution  on  white  drawing  paper  or  a  glazed  white  tile,  and,  after  each  addi- 
tion of  the  titrand  and  stirring  well,  letting  fall  a  drop  of  the  titrate  from  a 
glass  rod  into  one  of  the  drops  on  the  paper.  When  a  distinct  blue  or  greenish- 
blue  color  appears,  the  ferrous  sulfate  is  in  excess  and  the  titration  is  finished. 

The  first  titration  is  made  by  running  in  the  titrand  in  volumes  of  five  Cc.  at 
a  time,  this  showing  the  volume  required  to  within  that  limit.  To  the  second 
is  added  at  once  a  volume  less  by  five  Cc.  than  that  of  the  first  titration,  and 
concluded  by  additions  of  .2  Cc.  The  third  and  fourth  may  be  treated  at  once 
by  one  Cc.  less  than  the  second  titration,  and  finished  by  .1  Ccs. 

A  measurable  volume  of  the  ferrous  solution  is  needed  to  produce  a  blue 
color  with  ferricyanide,  and  this  volume  should  be  deducted  from  every  read- 
ing. It  is  found  by  titrating  as  above  a  mixture  of  100  Cc.  of  water  with  5  Cc. 
of  sulfuric  acid,  but  adding  the  titrand  in  drops  only. 


Calculation.  From  the  equations  under  (1)  and  (2),  we  see  that  two 
molecules  of  lead  chromate  react  with  six  molecules  of  ferrous  sulfate  or  with 
six  atoms  of  iron. 

Let  a  be  the  weight  of  the  sample  of  chrome  yellow;  6,  the  weight  of  iron 
in  one  Cc.  of  the  ferrous  solution;  c,  the  weight  of  lead  chromate  reduced  by 
one  Cc.  of  the  ferrous  solution;  d,  the  volume  of  ferrous  solution  used  for  the 
titration,  and  d',  the  volume  used  In  the  blank ;  and  x,  the  percentage  of  lead 
chromate  in  the  sample:  then  336  (6Fe)  :  646.04  (2PbCrO4)  :  :  b  :  e; 
646.046  .  _  ,  _  __  ,,xs/  100 

-«ox  — • 


27.33 
27.04 
27.23 


whence  c  =  - 

336 

-  =  1.9227 

b.    Then  x 

=  cX  (<*-< 

o: 

Example. 

a 

b 

c 

d 

d' 

A. 

1.000 

.005006 

.009623 

25  —  30 

A 

B. 

(I 

tt 

ft 

28.8 

if 

C. 

(I 

i< 

ti 

28.5 

« 

D. 

(4 

u 

« 

28.7 

If 

V 

METOL.  233 


EXEECISE  16 -METOL. 

Determination  of  Sulfuric  Acid. 

Metol  is  the  trade-name  of  an  organic  compound  largely  used  as  a  photo- 
graphic developer.  Chemically  it  is  a  phenol  derivative,  the  sulfate  of  mono- 

methyl-paraamidophenol,  CH3.NH.C6H4OH.?^2f  (172.155;.    It  is  found  in  the 

market  in  the  form  of  minute  colorless  needles,  very  soluble  in  water,  and 
slightly  soluble  in  alcohol  and  ether.  The  aqueous  solution  darkens  on 
standing  through  oxidation  by  the  air. 

On  treating  an  aqueous  solution  of  metol  with  barium  chloride  the  sulfnric 
acid  is  precipitated  as  barium  sulfate,  leaving  the  chloride  of  monoraethyl- 
paraamidophenol  in  solution  — 

2(CH3NH.C6H4OH.5??2.4)  +BaCl2=2  (CH8.NH.C6H4OH.HCl)  +  BaSO4. 

Barium  snlfate  is  a  white  powder,  infusible  at  a  white  heat,  insoluble  in 
water,  and  but  slightly  soluble  in  dilute  hydrochloric  acid.  It  is  unaltered  on 
ignition,  though  a  small  amount  may  be  reduced  to  barium  sulflde  when  heated 
with  carbon. 

A  weighed  quantity  of  metol  is  dissolved  in  water,  acidulated  by  hydro- 
chloric acid,  and  precipitated  by  barium  chloride.  The  liquid  is  filtered  and 
the  barium  sulfate  washed  with  hot  water,  ignited  and  weighed.  From  the 
weight  is  calculated  the  proportion  of  sulf  uric  acid  in  the  metol. 


Weigh  accurately  three  portions  of  metol  of  about  three  grams,  and  trans- 
fer to  12-ounce  Griffen  beakers.  Dissolve  each  in  about  200  Cc.  of  hot 
water  and  acidify  by  ten  Cc.  of  concentrated  hydrochloric  acid.  Precipitate 
by  a  small  excess  of  barium  chloride  solution,  stir  well,  and  let  stand  until 
the  supernatant  liquid  is  clear.  Filter  through  a  close  double  9-Cm.  paper 
(or  one  of  Dreverhoffs  No.  400),  and  wash  thoroughly  with  hot  water. 

Weigh  a  clean  platinum  crucible,  fold  the  filter  paper  around  the  precipitate, 
and  put  into  the  crucible.  Wipe  off  any  barium  sulfate  adhering  to  the  funnel 
by  a  small  piece  of  filter  paper.  Heat  the  crucible  gently  until  no  more  smoke 
appears,  incline  the  crucible  as  in  Fig.  94,  and  burn  the  charred  paper  at  a  low 
heat.  Cover  the  crucible,  heat  to  redness  for  a  few  minutes,  cool  and  weigh. 

Put  a  few  drops  of  water  in  the  crucible  and  one  drop  of  dilute  sulfuric  acid, 
this  to  convert  any  barium  sulflde  to  sulfate.  Evaporate  the  water  on  the 
water  bath,  and  gently  ignite  the  covered  crucible,  finally  to  redness.  Cool 
and  reweigh. 


Calculation.  Weight  BaSO4  :  weight  H2SO4  :  :  233.47  :  98.086. 

98.086  ,    100 

Hence.  Weight  BaSO,  X  ^jX  We,ght  Qf  ^  -  Per    cent   of    snltoic 

acid  in  metol. 
Example.  Sample  of  Hauff's  manufacture. 

A.  B.  C. 

Weight  of  metol 3.025  3.264  8.538  grams. 

Weight  of  barium  sulfate 2.056  2.219  2.408 grams. 

Percentage  of  sulfuric  acid 28.55  28.56  28.60 

Theoretical  percentage 28.49 


234  QUANTITATIVE    CHEMICAL    ANALYSIS. 


EXERCISE    17—  A.  SODIUM  THIOSTJLFATE. 

Sodium  thiosulfate,  Na2S2O3.5H2O  (formerly  called  sodium  hyposulflte) 
crystallizes  in  clear  colorless  prisms  slowly  efflorescing  in  the  air.  Like  many 
other  commercial  salts,  the  article  sold  as  "commercially  pure"  contains  but 
little  impurity.  The  most  common  impurities  are  other  sodium  salts,  some- 
times calcium  sulfate.  It  may  be  purified  by  recrystallization  from  hot  water. 

The  crystals  dissolve  in  about  an  equal  weight  of  cold  water.  On  acidifying 
an  aqueous  solution  by  a  mineral  acid,  thiosulf uric  acid  is  set  free  —  Na2S2O3  + 
H2SO4  =  Na2SO4  +  H2S203.  But  after  a  short  time  the  thiosulf  uric  acid  spon- 
taneously decomposes  into  sulf urous  acid  and  free  sulfur  —  H2S2C>3  =  H2SOs 
-|-  S  —  the  separated  sulfur  making  the  liquid  milky  white.  The  more  concen- 
trated and  warmer  the  solution,  the  more  quickly  does  the  decomposition  set  in. 


1.  Potassium  permanganate  reacts  with  potassium  iodide  in  an  acid  solu- 
tion, liberating  iodine  — 

10KI  -f-  K2Mn208  +  8H2SO4  =  5I2  -j-  6K2S04  +  2MnSO4  +  8H2O. 

2.  Free  iodine  reacts  with  sodium  thiosulfate  to  form  sodium  tetrathionate  — 

2Na2S2O3.5H2O  +  I2  =  Na2S4O6  +  2NaI  -f  5H2O. 

3.  In  an  acid  solution,  free  iodine  combines  with  dissolved  starch  to  form 
the  intensely  blue  starch  iodide ;  this  compound  is  decomposed  by  thiosulf  uric 
acid. 


For  the  determination  a  weighed  amount  of  the  crystals  is  dissolved  in  water, 
an  excess  of  potassium  iodide  and  a  little  starch-paste  are  added,  and  the  solu- 
tion acidified  by  sulf  uric  acid.  Standard  permanganate  is  run  in  from  a 
burette;  as  it  enters  the  titrate  the  reaction  (1)  above  takes  place,  then  the 
liberated  iodine  immediately  reacts  with  the  thiosulfate  as  in  equation  (2) ; 
finally,  when  all  of  the  tniosulfate  has  passed  to  tetrathionate,  the  least  excess 
of  free  iodine  unites  with  starch  and  the  solution  becomes  permanently  blue, 
showing  the  end-point. 

Four  solutions  are  used  in  the  determination. 

1.  Potassium  permanganate  of  a  concentration  of  about  four  grams  per  liter, 
prepared  and  standardized  as  described  on  page  229. 

2.  Potassium  iodide,  made  by  dissolving  about  20  grams  of  the  compound  in 
100  Cc.  of  water. 

3.  Starch- paste,  made  by  stirring  up  about  one-half  a  gram  of  starch  powder 
in  a  little  cold  water  and  pouring  into  100  Cc.  of  boiling  water. 

4.  Dilute  sulf  uric  acid  —  about  one  volume  concentrated  acid  to  two  volumes 
of  water. 


A  weight  of  about  ten  grams  of  the  crystallized  thiosulfate  is  dissolved  in 
cold  water  in  a  500  Cc.  measuring  flask  and  the  solution  made  up  to  the  mark. 

Preliminary  titration.  Into  a  large  beaker  is  poured  about  300  Cc.  of  cold 
water,  20  Cc.  of  the  potassium  iodide  solution,  a  few  Cc.  of  the  starch- 
paste,  and  one  Cc.  of  dilute  sulfuric  acid.  The  beaker  is  placed  under  a 
burette  filled  with  permanganate,  60  Cc.  of  the  thiosulfate  solution  run  in  from 
a  pipette,  and  the  liquid  immediately  titrated  until  blue. 

The  volume  of  permanganate  used  in  this  titration  will  be  a  few  cubic 
centimeters  greater  than  corresponds  to  the  equations,  due  principally  to  the 


MANGANESE    IN   STEEL.  235 

decomposition  of  apart  of  the  thiosulfuric  acid  as  before  mentioned  (one  mole- 
cule of  sulfurous  acid  is  oxidized  to  sulfuric  acid  by  two  molecules  of  iodine). 
To  eliminate  this  error,  the  titration  is  repeated  with  the  modification  of  add- 
ing the  thiosulfate  to  a  solution  containing  a  quantity  of  free  iodine  nearly  suffi- 
cient for  its  entire  conversion  to  tetrathionate. 

Titration.  A  large  beaker  is  charged  with  about  300  Cc.  of  cold  water,  20  Cc. 
of  the  potassium  iodide  solution,  a  few  Ccs.  of  starch- paste,  and  one  Cc.  of 
dilute  sulfuric  acid.  Into  this  is  run  from  the  burette  a  volume  of  permangan- 
ate less  by  three  or  four  Cc.  than  that  used  in  the  preliminary  titration. 
Then  50  Cc.  of  the  thiosulfate  solution  is  run  in  from  a  pipette  with  constant 
stirring,  and  the  titration  immediately  continued  to  the  end  point. 


Calculation  1.  From  the  equation  (4)  below,  we  see  that  one  molecule  of 
permanganate  oxidizes  ten  atoms  of  iron  from  the  ferrous  to  the  ferric  state. 
From  the  equation  (1)  above,  we  see  that  one  molecule  of  permanganate 
liberates  ten  atoms  of  iodine  from  potassium  iodide.  The  atomic  weights 
of  iron  being  56,  and  of  iodine,  126.85,  it  follows  that  one  Cc.  of 
permanganate  solution  will  oxidize  iron  and  set  free  iodine  in  the  ratio 
of  10  X  56  to  10  X  126.85;  that  is,  if  one  Cc.  of  permanganate  solution 

1268. 5a 
oxidizes  a  grams  of  iron,  it  will  liberate     fi6Q     grams  of  iodine. 

2.  From  equation  (2),  we  note  that  one  atom  of  iodine  (126.85)  reacts  with 
one  molecule  (248.32)  of  crystallized  sodium  thiosulfate.    Hence  the  weight  of 

248  ^2 

iodine  used  in  the  reaction  times  •      '  „•  will  equal  the  weight  of  thiosulfate. 

Ub.oo 

3.  Combining  the  above,   "  56'Q  a  X  126  35  *  volume  of  permanganate  solu- 
tion used  equals  the  weight  of  thiosulfate. 

4.  If  V represents  the  volume  of  permanganate  used  in  the  titration;   Wt  the 
weight  of  the  thiosulfate ;  and  a,  the  weight  of  iron  oxidized  by  one  Cc.  of  the 
permanganate  solution ;  then  the  percentage  of  crystallized  sodium  thiosulfate 

in  the  sample  is  4.4344  a.  V.^£ 

W 

Example.  A  commercial  article  of  fair  quality.  Weight  of  thiosulfate  10.861 
grams.  One  Cc.  of  permanganate  oxidized  .007049  gram  of  iron. 

Preliminary  titration 33.8  Cc.  =  101 .98  per  cent. 

First  titration .30.0  +  2.7    Cc.  =    98.66        " 

Second      "       31.0  +  1.7    Cc.  =   98.66        " 

Third        u       32.0+    .6     Cc.  =    98.36        " 

B.  MANGANESE  IN  STEEL. 

All  varieties  of  steel  contain  a  small  proportion  of  manganese  which  is  in- 
corporated during  the  manufacture  for  the  purpose  of  counteracting  the  effect 
on  the  strength  and  ductility  of  the  metal  of  the  various  impurities  unavoid- 
ably present.  The  proportion  ranges  from  .1  to  over  one  per  cent,  depending 
on  the  character  of  the  steel  and  the  use  for  which  it  is  intended. 

1.  When  heated  with  moderately  dilute  nitric  acid  steel  dissolves  completely, 
the  iron  and  manganese  becoming  respectively  ferric  and  manganous  nitrates, 
while  the  carbon  is  converted  to  a  hydrated  compound  passing  completely  into 
solution  with  a  brown  color. 

2.  If  from  this  solution  the  water  be  evaporated,  the  residue  mixed  with  hot 


23<5  QUANTITATIVE    CHEMICAL    ANALYSIS. 

concentrated  nitric  acid,  and  crystals  of  potassium  chlorate  added,  first  the 
carbon  is  oxidized  to  carbon  dioxide,  then  the  manganese  precipitated  as 
binoxide  ;  e.  g.  — 

5Mn(N03)2  +  2KC103  +  4H20  =  5Mn02  +  2KN03  +  8HN03+2Cl  .........  (1). 

in  the  form  of  a  fine  black  powder;  the  iron  remains  almost  entirely  in 
solution. 

3.  la  a  dilute  acid  solution  hydrogen  peroxide  dissolves  manganese  binoxide 
to  a  manganous  salt  with  evolution  of  oxygen,  both  compounds  yielding  an 
atom  of  oxygen  to  form  a  molecule  — 


4.  Hydrogen  peroxide  reacts  with  potassium  permanganate  in  a  similar 
manner  — 

K2Mn208  +  5H2O2  -f  GHNOa  =  2KNO3  +  2Mn(NO3)2  +  8H2O  +5O2  .........  (3). 

6.  The  reaction  between  ferrous  sulfate  and  perraanginate  is 
K2Mn2O8  +  10FeSO4  +  8H2SO4  =  5Fe2(SO4)3  -f  K2SO4  +  2MnSO4  -f  8H2O  ....  (4)  . 

The  determination  is  made  by  dissolving  a  weighed  amount  of  steel  in  dilute 
nitric  acid,  concentrating  the  solution,  compounding  with  concentrated  nitric 
acid,  and  precipitating  the  manganese  by  potassium  chlorate.  The  (unflltered) 
solution  is  diluted  with  water  and  the  precipitate  dissolved  by  a  known  volume 
of  standard  solution  of  hydrogen  peroxide.  The  excess  of  the  hydrogen 
peroxide  is  titrated  back  by  standard  permanganate,  and  the  percentage  of 
manganese  in  the  steel  calculated  from  the  volume  required.* 


Three  standard  solutions  are  required  — 

Potassium  permanganate.  Made  by  dissolving  .8  gram  in  one-half  liter  of 
water;  or  by  mixing  200  Cc.  of  the  standard  solution,  page  229,  with  300  Cc.  of 
water.  The  solution  is  standardized  by  titrating  50  Cc.  of  the  ferrous  solution 
following. 

Ferrous  sulfate.  Made  by  dissolving  about  .500  gram  of  iron  wire  in  dilute 
snlf  uric  acid  and  diluting  to  250  Cc.  with  cold  water. 

Hydrogen  peroxide.  The  commercial  medicinal '  ten-volume '  article  is  as- 
sayed and  diluted  to  a  convenient  strength.  Exactly  two  cubic  centimeters  is 
run  into  about  200  Cc.  of  water,  ten  Cc.  of  dilute  sulfuric  acid  added,  and 
titrated  by  the  permanganate  solution.  The  quotient  of  700  divided  by  the 
volume  of  permanganate  solution  required  is  the  number  of  cubic  centimeters 
of  the  hydrogen  peroxide  to  be  diluted  with  water,  plus  a  little  sulfuric  acid, 
to  500  Cc.  This  solution  slowly  decomposes  on  keeping. 


The  steel  for  the  analysis  may  be  drillings  or  chippings  of  a  Bessemer  steel 
rail,  perfectly  free  from  dirt,  oil  or  rust.  The  percentage  of  manganese  con- 
tained will  probably  be  between  .75  and  1.25.  Three  portions  of  about  two 
grams  each  are  weighed  and  transferred  to  eight-ounce  Griffen  beakers,  and 
each  dissolved  in  a  mixture  of  20  Cc.  concentrated  nitric  acid  with  25  Cc.  of 
water.  When  action  ceases,  the  solution  is  boiled  for  a  few  minutes,  then 
evaporated  until  a  thick  scum  forms  on  the  surface.  The  residue  is  taken  up 
in  25  Cc.  of  concentrated  nitric  acid,  and  the  beaker,  covered  with  a  watch- 
glass,  is  heated  to  boiling  on  a  hot  plate.  While  briskly  boiling,  a  crystal  of 
potassium  chlorate  (say  .1  gram)  is  thrown  in;  yellow  fumes  appear,  which 
may  suddenly  vanish  after  a  short  time ;  if  not,  a  small  crystal  is  added  to  the 


*  Journ.  Socy.  Chem.  Ind.  1898—185. 


GALENA.  237 

boiling  solution  every  few  minutes  until  this  occurs.  Finally  another  crystal 
is  added  and  the  solution  boiled  for  a  few  minutes  longer,  then  set  aside  to 
cool. 

Cold  water  is  poured  in  until  the  beaker  is  half  filled,  then  50  Cc.  of  the 
hydrogen  peroxide  solution  run  in  from  a  pipette.  After  stirring  until  clear, 
the  liquid  is  immediately  titrated  by  permanganate  to  a  faint  pink  —  the  color 
fades  rapidly. 

The  relation  of  the  permanganate  to  the  peroxide  solution  is  found  by 
diluting  60  Cc.  of  the  latter  with  200  Cc.  of  water,  adding  25  Cc.  of  con- 
centrated colorless  nitric  acid  and  titrating  by  the  former.  A  reaction  lag 
may  often  be  observed  at  the  beginning  of  the  titration,  but  after  a  trace  of 
manganous  nitrate  is  formed  the  reaction  proceeds  regularly. 


Calculation.  Let  a  be  the  weight  of  iron  oxidized  by  one  Cc.  of  the  stan- 
dard permanganate;  6,  the  weight  of  steel;  c,  the  volume  of  permanganate 
reducing  50  Cc.  of  the  peroxide  solution ;  d,  the  volume  of  permanganate  used 
in  the  titration;  and  Xt  the  percentage  of  manganese  in  the  steel. 

1.  The  difference  between  the  volumes  of  permanganate  solution  required 
for  50  Cc.  of  the  peroxide  and  for  the  titration,  (c  —  d),  is  the  volume  equal 
in  oxidizing  power  to  the  precipitate  of  MnC>2. 

2.  From  equations  (3)  and  (4)  we  see  that  KsMngOg  (316.22)   oxidizes  lOFe 
(560),  and  likewise  5H2O2  (170.08).     Hence,  if  one  Cc.  of  permanganate  solu- 

170  08 
tion  oxidizes  a  grams  of  iron,  it  will  also  oxidize    56()      a  or  ,3037  a  grams  of 

hydrogen  peroxide. 

3.  From    equation    (2),    one  molecule  of  MnO£,    containing  one  atom  of 
manganese  (55),  is  reduced  by  one  molecule  of  H2O2  (34.016).     Hence  one  gram 

of  H2O2  corresponds  to  — — —  =  1.6169  grams  of  Mn;  and  one  Cc.  of  perman- 

o-l.Ulo 

ganate  solution  correspouds  to  .3037  a  X  1-6169  =  .491  a  grams  of  Mn. 

4.  The  volume  of  permanganate  solution  c  —  d  is  that  volume  which  would 
reduce  the  same  weight  of  H2O2  as  does  the  Mn02.     Hence  (c  —  d}  .491  a  is  the 
weight  of  manganese  in  the  sample  of  steel  dissolved  for  analysis. 

5.  Since  b  is  the  weight  in  grams  of  the  steel,  (c  ~d^  '491  a  X  100  =  X,  the 

6 
percentage  of  manganese  in  the  steel.     More    conveniently    the    formula  is 

expressed  as  49.1  a  =  F,  and  X=  F(c  —  d) 

Example.  Drillings  of  rail-steel.  Permanganate  solution,  .8  gram  in  500  Cc. 
of  water.  Ferrous  solution,  .5100  gram  of  iron  in  250  Cc.  of  dilute  sulfuric 
acid.  Titration  of  60  Cc.  of  the  ferrous  solution  required  36.1Cc.  of  per- 
manganate; hence  one  Cc.  of  permanganate  oxidizes  .002826  gram  of  iron. 
Of  the  hydrogen  peroxide  solution,  50  Cc.  was  reduced  by  35.4  Cc.  of  per- 
manganate. 

a  b  c  d  F  X 

A.  .002826  2.023  36.4  18.6  .1386  1.16 

B.  "  2.014  «  19.1  «  1.12 

C.  "  2.009  "  18.9  •«  1.14 

EXERCISE  18  —  GALENA. 

Determination  of  Lead. 

The  mineral  is  a  sulfide  of  lead,  formula  PbS,  crystallizing  in  the  isometric 
system  with  an  eminent  cubic  cleavage. 


238  QUANTITATIVE    CHEMICAL   ANALYSIS. 

1.  If  the  powder  be  treated  with  an  excess  of  metallic  zinc  and  a  dilute  acid 
a  replacement  occurs,  the  nascent  hydrogen  abstracting  the  sulfur; 

PbS  +  Zn  +  2HC1  =  Pb  +  ZnCl2  -f  H2S. 

The  lead  remains  as  a  dark  coherent  mass  but  slightly  soluble  in  dilute  hy- 
drochloric acid,  and  completely  insoluble  during  the  evolution  of  hydrogen. 

2.  Metallic  lead  dissolves  readily  in  dilute  nitric  acid  leaving  as  a  residue  any 
quartz,  etc.,  that  may  be  present  in  the  gangue. 

3.  If  sulf uric  acid  be  added  to  a  solution  of  lead,  a  precipitate  of  lead  sulfate 
will  fall, 

Pb(N03)2  +  H2S04  =  PbS04  +  2HNO3. 

It  is  a  white  powder  soluble  in  22800  parts  of  cold  water,  more  readily  in 
nitric  acid,  and  less  so  in  alcohol  and  dilute  sulf  uric  acid.  The  presence  of 
salts  of  zinc  does  not  interfere.  On  ignition  it  is  unchanged,  except  when  in 
contact  with  reducing  agents  which  convert  it  to  metallic  lead. 


Select  pure  cleavage  cubes  and  grind  to  a  fine  powder  in  an  agate  mortar. 
Weigh  about  two  grams  and  brush  into  a  tall  beaker.  Add  150  Cc.  of  cold 
water,  about  four  grams  of  powdered  or  granulated  zinc,  and  20  Cc.  of 
hydrochloric  acid.  Cover  with  a  watch-glass  and  let  stand  until  the  liquid 
has  become  clear  and  no  longer  smells  of  hydrogen  sulfide. 

Dilute  with  an  equal  volume  of  water,  stir  and  allow  to  settle.  Decant  care- 
fully nearly  all  the  solution  from  the  lead  and  excess  of  zinc,  and  dissolve  the 
metals  by  pouring  in  100  Cc.  of  hot  water  and  15  Cc.  of  nitric  acid.  Pour  the 
solution  through  a  small  filter  and  dissolve  any  lead  sulfate  in  the  residue  of 
gangue  with  a  little  hot  dilute  hydrochloric  acid,  finally  washing  the  filter  witk 
hot  water. 

Add  to  the  filtrate  25  Cc.  (an  excess)  of  dilute  sulfuric  acid  and  evaporate  on 
the  water-bath  until  the  nitric  acid  is  entirely  expelled,  known  by  the  absence 
of  its  odor.  Cool  the  beaker  and  dilute  the  excess  of  sulfuric  acid,  now  con- 
centrated, with  about  50  Cc.  of  cold  water,  stir  well  and  filter  through  a  9  Cm. 
paper,  washing  with  dilute  alcohol  (equal  volumes  of  strong  alcohol  and 
water)  until  sulfuric  acid  is  removed.  Dry  the  filter  at  100  o . 

Remove  the  precipitate  from  the  paper  and  burn  the  latter  in  a  weighed 
porcelain  crucible.  Moisten  the  ash  with  a  few  drops  each  of  water  and  nitric 
acid,  and  one  drop  of  dilute  sulfuric  acid.  Evaporate  to  dryness  and  heat 
until  the  excess  of  acid  is  driven  off.  Introduce  the  lead  sulfate,  heat  gently 
and  weigh.* 

Calculation.  (1).  PbSO4  :  Pb  :  :  302.99  :  206.92. 

(2).  Wt.  of  galena  :  wt.  of  lead  :  :  100  :  per  cent  of  lead. 
(3).  Theoretically,  PbS  :  Pb  :  :  238.99  :  206.92. 

Example.  Two  grams  of  galena  gave  2.534  grams  of  PbS04,  equal  to  1.7305 
grams  of  lead,  equal  to  86.53  per  cent.  Theory  requires  86.69. 

EXERCISE  19  — BARIUM  CHLORIDE. 

Complete  Analysis. 

The  crystallized  salt  (BaCl2.2H2O)  is  purified  as  directed  on  page  207. 

1 .  From  the  aqueous  solution  barium  is  precipitated  by  ammonium  carbonate — 
BaCl2+(NH4)2CO3  =  BaCO3  +  2NH4Cl  — as  white  granular  BaCO8  soluble  in 
acids  and  slightly  so  in  water,  but  insoluble  in  neutral  or  alkaline  solutions 


Crookes,  Select  Methods,  350;  Fresenius  Quant.  Anal.  300. 


BARIUM    CHLORIDE.  239 

of  ammonium  salts.  It  may  be  ignited  alone  without  alteration,  but  is  reduced 
to  oxide  by  carbon ;  the  oxide  can  be  reconverted  by  heating  with  ammonium 
carbonate,  ammonia  being  liberated. 

2.  If  silver  nitrate  be  added  to  a  solution  of  a  metallic  chloride  curdy  white 
silver  chloride  is  precipitated  —  BaCl2  +  2AgNO8  =  Ba(NO3)2  +  2AgCl.     Expo- 
sure of  the  precipitate  to  actinic  light  results  in  a  superficial  decomposition 
with  the  formation  of  silver  subchloride,  chlorine  escaping,  and  loss  of  weight. 

Silver  chloride  is  insoluble  in  water  and  dilute  acids,  and  may  be  heated  to 
incipient  fusion  without  change  except  in  presence  of  carbon  or  reducing  gases 
which  transform  it  to  metallic  silver. 

3.  On  heating  crystallized  barium  chloride  to  redness  the  water  is  expelled ; 
usually  also  a  little  chlorine  is  lost  but  can  be  restored  by  heating  with  am- 
monium chloride  —  e.  g.,  BaO  +  2NH4C1  =  BaCl2  -f  2NH3  +  H2O. 


Barium.  Weigh  about  one  gram  of  the  crystals  into  a  12-ounce  beaker; 
dissolve  in  about  200  Cc.  of  hot  water,  and  precipitate  by  an  excess  of 
solution  of  ammonium  carbonate  in  dilute  ammonia.  Allow  to  settle 
until  the  supernatant  fluid  is  clear,  filter  and  wash  with  a  dilute  solu- 
tion of  the  precipitant  until  no  reaction  for  chlorine  is  found  in  the  wash- 
ings when  tested  with  silver  nitrate.  Burn  the  filter  and  contents  in  a  platinum 
crucible.  Moisten  the  residue  with  a  few  drops  of  the  reagent,  dry,  heat  to  dull 
redness  and  weigh. 

Chlorine.  Dissolve  about  one  gram  in  a  12-ounce  beaker  in  300  Cc.  of  hot 
water ;  add,  while  stirring,  a  solution  of  silver  nitrate  until  no  further  precipi- 
tation occurs,  then  about  5  Cc.  of  strong  nitric  acid  and  allow  to  settle.  Filter 
and  wash  with  hot  water  until  silver  is  removed,  breaking  up  the  clumps  of 
precipitate  with  a  glass  rod.  Cover  the  funnel  with  a  filter  paper  and  dry  in 
the  water  oven.  All  these  operations  are  to  be  conducted  with  as  little  ex- 
posure to  the  light  as  possible. 

Remove  the  precipitate  as  completely  as  can  be  done  by  shaking  and  rubbing 
the  filter,  fold  the  latter  tightly  and  burn  it  in  a  small  weighed  porcelain  cruci- 
ble. Moisten  the  ash  with  a  few  drops  of  dilute  nitric  acid  and  heat  for  a 
moment,  then  with  a  drop  of  hydrochloric  acid  and  evaporate  to  dryness  on  the 
water -bath;  bring  in  the  precipitate,  heat  till  the  edges  begin  to  melt,  and 
weigh  as  AgCl. 

Water  of  crystallization.  Heat  one  to  three  grams  to  dull  redness  in  a 
platinum  crucible  for  15  minutes.  Cool,  add  a  few  grains  of  pure  ammonium 
chloride  and  heat  gently  until  the  excess  is  expelled.  Cool  and  weigh ;  the  loss 
is  water. 

Calculation. 

Weight  of  BaCO3  :  weight  of  Ba  :  :  197.40  :  137.40. 

Weight  of  AgCl  :  weight  of  Cl  :  :  143.37  :  35.45. 

Weight  of  BaCl2.2H2O  :  weight  of  2HgO  :  :  100.00  :  per  cent  of  H2O.  * 

Example. 

.993  gram  gave  .803  gram  of  BaCOs  =  56.29  per  cent  of  Ba. 

1.002  "  1.171       "          AgCl     =28.90         "          «  Cl. 

2.671  "  .398       "         H20      =  14.90         »          "  H2O. 


*  Fresenius,  Quant.   Anal.  791.   Chem.  News,  1894—29  and  64.  Crookea,  Select  Meth- 
ods, 571. 


240  QUANTITATIVE   CHEMICAL   ANALYSIS. 


EXERCISE  20  —  LARD. 

Pure  lard  is  a  mixture  in  somewhat  variable  proportions  of  tri-olein 
CCi8H33O2)3,  tri-stearin  CaHsCCigHsAOs,  and  tri-palmitin  CsRstCwUaQfia.  The 
commercial  article  is  sometimes  adulterated  with  water  (up  to  25  per  cent  or 
more),  cotton- seed  stearin  or  beef -tallow;  a  little  salt  may  be  legitimately 
incorporated  to  preserve  it.  Good  lard  is  pure  white  in  color,  and  nearly 
tasteless  and  odorless. 

A.  The  water  contained  is  determined  by  drying  at  a  temperature  somewhat 
above  100  o ,  as  at  this  temperature  all  the  water  may  not  be  driven  off. 

B.  The  non-drying  oils  and  fats  on  exposure  to  light  and  air  acquire  an  acid 
reaction  due  to  the  conversion  of  a  small  portion  of  the  glycerides  into  free 
fatty  acids,  and  this  change  is  accompanied  to  a  certain  extent  by  the  alteration 
in  odor  and  taste  known  as  rancidity.    In  fresh  lard  the  free  acid  should  not 
exceed  a  small  fraction  of  one  per  cent.    As  the  change  takes  place  more  readily 
with  olein  than  with  stearin  or  palmitin,  the  free  acid  is  generally  expressed  as 
so  much  oleic  acid.    It  is  determined  by  heating  the  fat  with  neutral  alcohol 
which  dissolves  the  fatty  acids,  and  titrating  to  neutrality  by  standard  acid. 

C.  On  heating  an  animal  or  vegetable  fat  with  a  solution  of  a  caustic  alkali 
it  is  saponified,  i.  e.,  the  constituent  glycerides  are  successively  converted  into 
fatty  acid  salts  of  the  alkali  with  the  production  of  glycerol,  the  radical  CsH« 
exchanging  with  the  alkali  metal;  thus  in  the  case  of  stearin  — 
(C3H6)(Ci8H35O2)3  (stearin) +3KOH  =  K3CCi8H35O2)3    (potassium    stearate)  + 

(C3H6)(OE1)3  (glycerol). 

So  that  for  890.88  grams  of  pure  stearin  there  is  required  168.354:  grams  of 
potassium  hydrate;  or  one  liter  of  normal  solution  of  potassium  hydrate 
(56.118  grams  KOH  per  liter)  will  saponify  890  88  X  56.118-5-168.354  =  296.96 
grams  of  stearin,  and  by  a  similar  calculation,  294.91  grams  of  olein,  and  268.93 
of  palmitin. 

The  number  of  grams  of  a  fat  or  oil  saponified  by  one  liter  of  normal  potas- 
sium hydrate  is  called  its  saponiflcation  equivalent;  for  pure  anhydrous  lard  it 
lies  between  286  and  292,  for  cotton-seed  stearin  from  285  to  294,  cocoanut  oil, 
209  to  228,  butter-fat,  241  to  253,  etc.  The  Koettstorffer  Number  is  the  number 
of  milligrams  of  KOH  required  to  saponify  one  gram  of  anhydrous  fat  —  it  is 
simply  another  way  of  expressing  the  saponiflcation  equivalent.  In  both  cases 
there  is  required  an  equivalent  of  alkali  for  both  the  decomposition  of  the  fats 
and  the  neutralization  of  the  associated  free  fatty  acids. 

The  saponiflcation  is  effected  by  boiling  the  fat  with  an  excess  of  a  standard 
solution  of  caustic  potash  in  alcohol  (the  alcohol  takes  no  direct  part  in  the 
reaction  but  attacks  the  fat  much  more  energetically  than  an  aqueous  solution) 
and  determining  the  uncombined  alkali  by  titration  with  a  standard  acid  and 
phenol-phthalein,  this  indicator  being  unaffected  in  a  cold  solution  by  the 
potassium  fatty  acid  salts.  The  weight  of  alkali  taking  part  In  the  reaction  is 
found  by  difference. 

D.  After  titration,  the  fatty  acids  combined  with  potassium  are  set  free  by 
the  addition  of  a  mineral  acid,  e.  g.,  hydrochloric  — 

KCisHsAz  +  HC1  =  CisHasO.OH  -f  KC1. 
Potassium  oleate.  Oleic  acid. 

The  mixed  fatty  acids  (insoluble  in  water  and  mineral  acids)  are  filtered, 
washed  with  water,  dried  and  weighed,  one  gram  of  dry  lard  giving  about  .962 
grams,  and  of  cottonseed  stearin,  .955  grams. 


LARD.  241 

A.  Water.  A  large  beaker  is   weighed  and  about  50  grams  of  fresh  lard 
introduced.    The  beaker  is  heated  to  105°  for  an  hour,  cooled  and  reweighed; 
the  loss  is  water. 

B.  Free  acid.  Fifty  grams  of  the  undried  lard  is  weighed  into  a   beaker, 
covered  with  50  Cc.  of  neutral  (page  207)  alcohol,  and  heated  to  boiling.    The 
mixture  is  then  titrated  with  standard  potassium  hydrate  and  phenol-phthalein, 
taking  care  that  the  red  color  persists  after  vigorous  stirring.    The  presence 
of  the  undissolved  lard  does  not  interfere.    The  reaction  is  assumed  to  be 

H3(Ci8H3302)3  (846.816)  +3KOH(168.354)  =  ^(CisHgsOsOs -{- 3H2O. 
Oleic  acid.  Potassium  oleate. 

C.  Saponiflcation  equivalent.  An  alcoholic  solution  of  potassium  hydrate 
containing  about  15  grams  of  KOH  in  300  Cc.  is  prepared  as  follows:  About 
3  grams  of  stick  potash  is  weighed  and  dissolved  in  water  and  titrated  with 
standard  sulfuric  acid,  and  the  following  proportion  solved, 

Cubic  centimeters  of  acid  required,  times  the  weight  of  KOH  neutralized  by 
one  Cc.  :  weight  of  potash  taken  :  :  15  grams  :  x. 

Then  x  grams  of  the  potash  is  dissolved  in  300  Cc.  of  strong  alcohol  and  the 
solution  filtered  into  a  glass-stoppered  bottle. 

Three  No.  3  beakers  *  are  weighed  and  about  8  grams  of  the  dried  lard  from 
A  introduced  in  each  and  accurately  weighed.  Into  each  beaker  and  two 
others  of  the  same  size,  is  run  50  Cc.  of  the  potash  solution,  allowing  five  drops 
to  drain  from  the  pipette  to  secure  a  uniform  measure.  The  five  beakers  are 
then  covered  with  watch-glasses  and  boiled  gently  for  fifteen  minutes.  After 
cooling,  the  beakers  are  three-fourths  filled  with  cold  water,  stirred  until 
clear,  and  cautiously  titrated  with  standard  sulfuric  acid  and  phenol-phthalein 
until  the  red  color  has  just  changed  to  yellow. 

D.  Fatty   acids  by  weight.    Each  of  the  three  solutions  is  treated  as  fol- 
lows: To  expel  the  alcohol  the  solution   is  evaporated  nearly  to  dryness  on 
the  water  bath,  and  after  diluting  with  hot  water  to  dissolve  the  soap  dried 
"on  the  surface,  hydrochloric  acid  is  poured  in  until  the  reaction  is  decidedly 
acid  and  the  fatty  acids  clot  and  form  one  mass  on  stirring.    The  beaker  is 
then  heated  on  the  water  bath  until   the  fatty  acids  melt,  and  digested  in  a 
warm  place,  preferably  over  night,  until  the  solution  of  glycerol  and  potas- 
sium salts  is  clear. 

A  close  filter  of  12.5  Cm.  diameter  is  inclosed  in  a  small  beaker,  covered 
with  a  watch-glass,  dried  at  100°  and  weighed;  the  filter  is  fitted  to  a  funnel, 
half  filled  with  hot  water,  and  the  solution  filtered.  The  paper  must  never  be 
much  more  than  half  filled  at  any  time,  and  the  operation  should  continue  un- 
interruptedly. The  beaker  is  rinsed  and  the  filter  washed  with  boiling  water 
a  few  times,  and  the  funnel  lowered  into  cold  water  to  congeal  the  fatty 
acids. 

The  ring  of  fat  adhering  to  the  beaker  and  rod  is  dissolved  in  a  little  hot 
alcohol  which  is  poured  into  the  small  beaker  and  evaporated  to  dryness.  The 
filter  is  removed  from  the  funnel  and  opened  on  blotting  paper  to  partially  dry 
it.  The  cone  of  fatty  acids  is  then  dropped  in  the  small  beaker,  followed  by  the 
filter.  After  drying  at  100  °  for  an  hour,  the  beaker  is  covered  with  the  watch- 
glass,  cooled  and  weighed.  The  drying  and  weighing  are  repeated,  and  if  the 
loss  does  not  exceed  a  few  milligrams,  the  fatty  acids  may  be  considered  free 
from  water. 

Calculation.  The  difference  between  the  volume  of  acid  used  for  the  lard, 
and  that  for  the  average  of  the  blanks,  times  the  KOH  equivalent  of  1000 


*  Chem.  News,  1891-1-82. 


242  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Cc.  of  the  standard  acid,   divided  by  the  weight  of  dry  lard  taken,  gives  the 
Koettstorffer  Number;  and  56118  divided  by  this  number  gives  the  saponiflca- 
tion  equivalent.    One  gram  of  KOH  neutralizes  5.033  grams  of  oleic  acid. 
Example. 

A.  Weight  of  lard 49.163  grams. 

Loss  on  drying 052       " 

Percentage  of  water 11 

B.  Weight  of  lard 50.000  grams. 

Volume  of  standard  KOH  for  neutralization .5  Cc. 

One  Cc.  contains  of  KOH 05882  gram. 

Free  fatty  acids  expressed  as  oleic .30  per  cent. 

A.  B.  C. 

C.  Weight  of  dry  lard 8.314  9.620  9.010 

Volume  of  standard  acid 19.1  Cc.        14.8  Cc.        16.7  Cc. 

For  50  Cc.  alcoholic  potash 46.2  Cc.        46.2  Cc 

KOH  equivalent  of  acid 0593 

Koettstorffer  Number 193.3  193.5  194.1 

Saponiflcation  equivalent 290.3  290.0  289.1 

D.  Weight  of  fatty  acids 7.969  9.233  8.632 

Percentage  of  fatty  acids 95.85  95.98  95.80 

EXERCISE  21— POTASSIUM  PERMANGANATE. 

Complete  Analysis. 

On  heating  potassium  permanganate  with  hydrochloric  acid,  the  potassium 
and  manganese  become  chlorides,  the  other  products  of  the  reaction  being 
chlorine  and  water  — 

K2Mn2O8  +  16HC1  =  2KC1  +  2MnCl2  -f  5C12  +  8H2O. 

The  potassium  is  determined  by  precipitation  as  potassium  platinchloride , 
and  the  manganese  by  precipitation  as  manganous  ammonium  phosphate ;  the 
oxygen  by  its  reaction  with  a  ferrous  salt. 

1 .  Potassium  chloride  forms  with  chloroplatinic  acid  a  yellow  or  red  crystalline 
compound  —  2KC1  +  H2PtCl6  =  K2PtCle  +  2HC1.     This  precipitate  is  soluble  in 
100  parts  of  cold  water,  about  4000  parts  of  alcohol  of  80  per  cent,  and  12000  parts 
of  absolute  alcohol.    Manganous  chloride  does  not  combine  with  this  reagent, 
and  is  easily  soluble  in  both  water  and  alcohol ;  the  two  metals  may  therefore 
be  separated  in  this  way,  using    alcohol  to  decrease   the   solvency   of   the 
precipitate. 

On  igniting  the  precipitate  there  remains  2KC1  -f-  Pt,  chlorine  escaping.  The 
precipitate  may  be  dried  at  105°  without  change. 

2.  After  precipitating  the  potassium  the  manganese  could   be  determined 
in  the  filtrate,  but  as  the  excess  of  platinic  chloride  is  troublesome  to  remove, 
it  is  preferable  to  operate  on  another  portion  of  the  permanganate. 

Manganous  chloride  is  precipitated  by  ammonium  phosphate  as  manganous 
ammonium  phosphate  — 

MnCl2  +  NaNH4HP04  +  NH4OH  =  MnNH4PO4  +  NaCl  +  NH4C1  -f  H20. 

First  appearing  in  white  flocks,  but  condensing  on  boiling  or  long  standing 
in  presence  of  a  large  excess  of  the  precipitant,  to  rose-colored  scales.  The 
precipitate  is  soluble  in  acids  but  insoluble  in  water  and  dilute  ammonia.  On 
ignition  there  is  left  manganese  pyrophosphate,  in  the  form  of  a  white  crust 
fusible  at  a  bright  red  heat  —  2MnNH4PO4  +  heat  =  Mn2P2O7  -f  2NH3  -f  H2O. 

3.  The  five  atoms  of  oxygen  are  determined  volumetrically  by  their  power  of 
decomposing  sulf  uric  acid  in  presence  of  a  ferrous  compound  — 

K2Mn2O8  -f  8H2SO4  +  10FeSO4  =  5Fe2(SO4)3  +  K2SO4  +  2MnSO4  +  8  H2O. 


POTASSIUM    PERMANGANATE.  243 

An  excess  of  a  standard  solution  of  ferrous  sulfate  is  oxidized  by  a  given 
weight  of  the  sample,  and  the  remaining  ferrosum  found  by  titration  by 
standard  permanganate. 


Select  a  few  grams  of  small,  clean,  well-formed  crystals,  break  to  a  coarse 
powder  in  a  mortar,  and  preserve  in  a  stoppered  tube. 

1.  Determination  of  potassium.  Weigh  exactly  about  .5  gram  and  transfer 
to  a  two-ounce  beaker.    Dissolve  in  20  Cc.  of  hot  water,  cover  the  beaker  and 
slowly  add  5  Cc.  of  concentrated  hydrochloric  acid.    Boil  gently  until  the 
solution  is  nearly  colorless,  shaking  the  beaker  to  wash  down  any  manganic 
hydrate  that  may  collect  on  the  sides. 

Precipitate  the  potassium  by  a  volume  of  solution  of  chloroplatinic  acid 
containing  about  one  gram  of  platinum.  Evaporate  on  the  water  bath  just 
to  dryness. 

Moisten  ths  residue  with  a  few  drops  of  water,  add  20  Cc.  of  alcohol  of 
about  85  per  cent,  and  stir  well.  Decant  on  a  7-Cm.  filter  keeping  the  pre- 
cipitate in  the  beaker.  Wash  a  few  times  by  decantation  with  alcohol  con- 
taining a  few  drops  of  the  reagent,  then  transfer  to  the  filter  and  wash  with 
alcohol  alone.  After  drying  on  the  water-bath,  shake  and  brush  the  pre- 
cipitate into  a  tared  watch-  glass  or  weighing-bottle,  dry  for  a  half  hour  at 
105  ° ,  and  weigh.  The  filter  retaining  a  little  precipitate  is  burned  in  a 
platinum  crucible  and  the  ash  weighed. 

A  Gooch  crucible  may  be  used  with  advantage  for  this  determination. 

The  results  are  apt  to  be  a  trifle  low  from  the  solubility  of  the  precipitate  in 
alcohol.  Care  should  be  taken  to  prevent  access  of  ammonia  fumes  to  the 
solution  as  ammonium  platinchloride  may  be  formed. 

2.  Determination    of  manganese.  Weigh    about  one    gram    of  the  sample, 
transfer  to  an   eight-ounce   beaker    and   dissolve   in  50  Cc.   of  hot  water. 
While   gently  boiling  add    cautiously   about  15  Cc.  of  concentrated  hydro- 
chloric acid  and  boil  until  the  liquid  is  a  clear  yellow. 

Dissolve  in  a  porcelain  dish  about  ten  grams  of  sodium  ammonium  phos- 
phate in  100  Cc.  of  warm  water  plus  a  few  drops  of  ammonia.  Filter  the  solu- 
tion into  the  manganese  solution,  and  heat  the  latter  to  boiling.  Now  add  a 
few  cubic  centimeters  of  sulfurous  acid,  then  ammonia  to  decided  alkaline 
reaction.  Stir  for  a  few  minutes  and  set  aside  in  a  warm  place  until  the  pre- 
cipitate becomes  entirely  crystalline,  which  may  require  an  hour  or  more .  Filter 
through  a  12.5  Cm.  paper,  wash  thoroughly  with  cold  water  containing  a  few 
drops  of  ammonia,  burn  the  filter  containing  the  precipitate  in  a  platinum  cru- 
cible, ignite  to  dull  redness,  and  weigh  as  manganese  pyrophosphate. 

3.  Determination  of  oxygen.  Weigh  between  .200    and   .250  gram  of  the 
sample  and  transfer  to  an  eight-ounce  beaker.    Add  about  100  Cc.  of  cold 
water,  exactly  100  Cc.  of  the  standard  ferrous  solution  (page  229),  and  25  Cc. 
of  dilute  sulfuric  acid;  stir  until  dissolved,  and  immediately  titrate  the  excess 
by  standard  permanganate. 

Calculation. 

1.  Potassium  oxide  — 

f  In  the  precipitate,     K2PtCl6  :  K20  :  :  485.82  :  94.22. 
\  With  the  filter,        2KC1  -f  Pt.  :  K2O  :  :  344.02  :  94.22. 

2.  Manganese  oxide.  Mn2P2Oz  :  2MnO  :  :  284  :  142. 

3.  Available  oxygen.  This  may    be  computed   in  several  ways.    In  the  fol- 
lowing we  find  not  the  percentage  of  oxygen  in  the  sample  itself,  but  that  in 


244  QUANTITATIVE    CHEMICAL   ANALYSIS. 

such  a  volume  of  the  standard  solution  of  permanganate  as  exactly  equals  it  in 
action  on  ferrous  sulfate. 

Let  a  be  the  weight  of  the  sample,  and  6  the  weight  of  iron  oxidized  by  one 
cubic  centimeter  of  the  standard  permanganate.  From  the  equation  ante  we 
see  that  10  atoms  of  iron  (560)  react  with  5  atoms  of  oxygen  (80),  or  in  the 

ratio  of  7  of  iron  to  1  of  oxygen,  hence  —  is  the  weight  of  oxygen  in  one 

cubic  centimeter  of  the  standard  permanganate. 

Let  c  be  the  volume  of  standard  permanganate  used  in  titrating  100  Co.  of 
the  ferrous  solution;  and  <Z,  the  volume  used  for  the  excess  in  the  determina- 
tion. Then  c  —  d  is  the  volume  of  standard  permanganate  of  equal  oxidizing 

power  to  a,  and  (c  —  d)   X  ~y~  is  tne  weight  of  oxygen  in  c  —  6. 
Therefore  (c  —  d)  X  ~~~  X is  tne  percentage  of  available  oxygen  in  a* 


Example.  In  an  exceptionally  pure  commercial  article  was  found  — 

1.  Weight  of  sample  .5014  gram.    The  potassium  platinchloride   weighed 
.7715  gram,  and  the  residue  in  the  filter  .001  gram.    Therefore  the  percentage 
of  potassium  oxide  was  29.89. 

2.  Weight  of  sample  1.0155  grams.    The  manganese  pyrophosphate  weighed 
.9137  gram.    Therefore  the  manganous  oxide  was  44  98  per  cent. 

3.  Weight  of  sample  .2624  gram.     One  hundred  Cc.  of  the  ferrous  solution 
contained  .5098  gram  of  iron,  and  required  72.3  Cc.  of  the  standard  permangan- 
ate.   In  the  determination  the  excess  of  ferrous  solution  required  6.8  Cc.  of 
permanganate.    Therefore  the  available  oxygen  was  25.14  per  cent. 

Found.          In  theory. 

Potassium  oxide 29.89  29.80 

Manganese  oxide 44.98  44.90 

Availiable  oxygen 25.14  25.30 

100.01  100.00 

EXERCISE  22— A.  DETERMINATION  OF  NITROGEN  IN  AIR. 

The  atmosphere  contains  by  volume  from  20.96  to  20.99  per  cent  of  oxygen, 
the  remainder  being  nitrogen  with  a  little  water  vapor  and  carbon  dioxide  and 
traces  of  ammonia,  nitrous  acid,  etc.  The  normal  proportion  of  carbon  diox- 
ide is  from  .03  to  .06  per  cent  but  in  a  confined  space  may  be  considerably 
augmented  by  combustion,  respiration  or  fermentation. 

Oxygen  and  carbon  dioxide  are  simultaneously  absorbed  by  a  solution  of 
pyrogallin  in  potassium  hydrate,  leaving  nitrogen  as  a  residue.  A  measured 
volume  of  air  is  brought  in  contact  with  this  reagent  and  the  residual  nitrogen 
measured;  the  operation  may  be  performed  in  any  form  of  gas-absorption 
apparatus,  of  which  the  one  devised  by  Bunte  is  here  employed.  It  is  made 
entirely  of  glass  and  consists  of  a  tube  A,  Fig.  139,  of  such  a  size  that  it  con- 
tains 100  Cc.  from  the  plug  of  the  stopcock  B  to  the  zero  C,  and  is  divided  into 
cubic  centimeters  and  tenths.  The  stopcock  B  is  of  the  form  known  as  a 
"three-way,"  since  according  to  the  position  of  the  plug  it  opens  a  passage 
from  A  to  D  or  from  A  to  E,  or  closes  all  three.  An  ordinary  stop-cock  F 


*0rookes  Select  Methods,  1  etseq.;  Amer.  Journ.  Science,  1899—206;    School   of  Mines 
Quart.  11—355;  Journ.  Amer.  Chem.  Socy.  1895—453. 


AMMONIUM   SULFATE. 


245 


terminates  the  burette  below.  An  aspirator  bottle  G,  filled  with  water,  is  con- 
nected with  F  by  a  long  rubber  tube  H.  The  burette  is  held  vertically  in  two 
clamps  fixed  on  th6  rod  of  a  retort  stand. 


Fig.  139. 


Filling  the  burette.  The  stopcocks  are  opened  and  G  raised  until  the  burette 
is  filled  with  water ;  G  is  lowered  until  the  surface  of  water  it  contains  is  at  a 
level  with  the  zero  mark  C.  The  stop-cock  B  is  turned 
so  as  to  close  all  the  exits;  then  F  is  closed  and  the 
tube  H  removed.  The  burette  now  contains  100  Cc.  of 
moist  air  at  the  temperature  of  the  laboratory  and  the 
pressure  of  the  surrounding  atmosphere. 

Into  the  funnel  E  is  poured  25  Cc.  of  the  pyrogallate 
solution  (page  210) ;  the  stopcock  B  is  turned  so  that  a 
little  of  the  solution  enters  the  burette ;  as  the  oxygen 
is  absorbed  more  of  the  solution  enters  to  replace  it. 
After  closing  B  the  burette  is  removed  from  the  stand, 
held  horizontally,  and  rotated  to  bring  any  unabsorbed 
oxygen  in  contact  with  the  reagent. 

The  burette  is  returned  to  the  support  and  B  filled  with 
water;  the  cock  B  is  opened  slightly,  and  when  no  more 
water  enters  the  burette,  F  is  also  partly  opened.  The 
slow  stream  of  water  flowing  through  the  burette  washes 
out  the  reagent,  E  being  replenished  with  water  until  that 
leaving  F  is  nearly  colorless.  B  is  then  closed  and  the 
tube  H  slipped  over  the  tube  of  F  taking  care  that  H  is 
entirely  filled  with  water  before  so  doing.  F  is  opened 
and  G  raised  until  the  surfaces  of  water  in  it  and  the 
burette  are  at  a  level.  After  standing  for  15  minutes, 
the  volume  of  residual  nitrogen  is  read. 

Care  must  be  taken  that  no  air  enters  or  gas  escapes  from  the  burette  during 
these  operations,  and  that  the  temperature  of  the  laboratory  does  not  mate- 
rially change  during  the  analysis. 

Calculation.  Let  N be  the  percentage  of  nitrogen  in  the  air;  Vt  the  original 
volume  of  air;  v}  the  volume  of  moist  nitrogen  remaining  after  absorption  of 

100 
the  oxygen  and  carbon  dioxide;  then  N  =  — y~.    No  account  is  taken  of  the 

temperature,  pressure,  or  tension  of  aqueous  vapor,  since  these  are  identical 
or  nearly  so  in  both  readings. 

Example.  Three  experiments  on  100  Cc.  of  pure  air  gave  79.3,  79.3  and  79.4 
per  cent  of  nitrogen. 

A  substitute  for  the  Bunte  burette  may  be  contrived  by  closing  the  top  of  an 
ordinary  burette  by  a  cork  holding  a  funnel-tube  with  a  glass  stopcock.  The 
volume  of  the  space  between  the  plug  of  the  stopcock  and  the  zero  of  the 
burette  is  ascertained  by  filling  the  burette  with  water  and  drawing  what  is 
contained  between  these  two  points  into  a  small  measuring-jar;  this  volume 
is  to  be  included  in  the  calculation  of  the  result  of  a  determination. 

B.  AMMONIUM  SULFATE. 

Determination  of  Nitrogen. 

Ammonium  sulfate  crystallizes  in  anhydrous  colorless  needles  soluble  in  1.3 
parts  of  cold  and  an  equal  weight  of  hot  water.  The  commercial  salt  may  be 
purified  by  stirring  100  grams  in  100  Cc.  of  boiling  water,  filtering  hot,  and 


246 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


allowing  to  cool  slowly.  The  crystals  deposited  are  drained  and  pressed  in 
filter  paper  until  apparently  dry,  and  preserved  in  a  desiccator  over  sulfuric 
acid.  The  pure  salt  contains  21.24  per  cent  of  nitrogen. 

The  determination  is  made  by  setting  free  the  nitrogen  measuring  its  volume 
and  calculating  its  weight. 

1.  When  bromine  is  mixed  with  a  strong  solution  of  sodium  hydrate  there 
are  formed  sodium  bromide  and  sodium  hypobromite  — 

2NaOH  +  Br2  =  NaBr  +  NaOBr  -f  HgO. 

2.  Ammonium  sulfate  is  decomposed  when  brought  in  contact  with  sodium 
hydrate,  forming  free  ammonia,  sodium  sulfate,  and  water  — 

(NH4)2SO4  +  2NaOH  =  Na2SO4  +  2NH3  -f  2H2O. 

3.  Sodium  hypobromite   reacts  with   ammonia  to   form  nitrogen-,    which 
being  nearly  insoluble  in  water,  is  evolved  — 

2NH3  +  3NaOBr  =  N2  -f  3NaBr  +  3H2O. 

4.  The  volume  of  nitrogen  is  found  by  the  increase  in  volume  of  a  fixed, 
though  unmeasured,  volume  of  air  when  the  two  are  united;  the  increase, 
measured  in  cubic  centimeters  when  saturated  with  aqueous  vapor  and  at  the 
temperature  and  pressure  of  the  atmosphere  at  the  time  of  the  experiment,  is 
reduced  to  standard  conditions  (page  183)  and  multiplied  by  the  weight  of  one 
cubic  centimeter  of  nitrogen  under  these  conditions  (.0012562  gram). 

6.  Since  the  nitrogen  Is  evolved  into  a  certain  volume  of  air,  the   amount 
retained  by  the  solution  generating  it  is  proportional  to  the  volume  of  nitrogen 
given   off.     Under   the   conditions  detailed  below  the 
volume  retained  amounts  to  about  .5  Cc. 

The  apparatus  for  the  test  is  fitted  up  as  shown  in  Fig.  140. 
A  is  a  wide-mouth  bottle  of  about  175  Cc.  capacity;  B,  a 
test  tube  of  %  inch  diameter  cut  off  to  such  a  length  as 
will  stand  in  the  bottle  in  an  inclined  position  as  shown. 
A  rubber  stopper  closes  A  and  holds  a  bent  glass  tube  con- 
nected by  a  short  rubber  tube  C  to  the  tube  D  which  fits 
in  a  stopper  inserted  into  the  top  of  a  50  Cc.  burette  E. 
The  bottom  of  the  burette  is  joined  to  a  100  Cc.  pipette 
F  by  a  long  rubber  tube  G. 

The  burette  and  pipette  are  held  firmly  in  clamps  of  a 
burette  stand,  and  A  is  provided  with  a  rest  at  the  proper 
height. 

A  solution  of  sodium  hydrate  is  prepared  by  dissolving 
40  grams  of  commercial  caustic  soda  in  100 Cc.  of  water; 
after  cooling  10  Cc.  of  bromine  is  added  and  stirred  until 

dissolved.    This  solution  is  kept  in  a  stoppered  flask:  it  retains  its  activity  for 
a  few  days  only. 

The  room  in  which  the  analysis  is  made  should  not  be  liable  to  sudden 
changes  of  temperature.  F  is  supported  close  to  the  burette,  at  such  a  height 
that  the  zero  mark  is  opposite  the  middle  of  the  bulb  and  water  is  poured  into 
E  until  it  rises  to  the  zero  mark.  Twenty-five  Cc.  of  the  hypobromite  solution 
is  poured  into  A. 

The  test-tube  B  is  dried  and  supported  in  the  stand  Fig.  30,  and  the  two 
weighed  together ;  about  .200  gram  of  the  ammonium  sulfate  is  placed  in  the 
tube  and  the  stand  and  tube  again  weighed.  About  ten  Cc.  of  water  is  care- 
fully poured  into  the  tube  and  shaken  around  until  the  salt  is  dissolved,  then 
the  tube  lowered  into  the  bottle  A.  The  stoppers  are  firmly  inserted  in  the 
bottle  and  burette  and  the  apparatus  left  undisturbed  for  a  half-hour  that  the 
liquids  may  come  to  the  room  temperature. 


Fix.  140. 


NICKEL-  COPPER  ALLOY.  247 

On  returning,  the  operator  adjusts  the  water  levels,  reads  the  burette,  and 
records  the  temperature  and  barometric  pressure  of  the  air  of  the  room.  Hold- 
ing the  bottle  in  the  left  hand  and  F  in  the  right,  he  inclines  A  until  some  of 
the  solution  in  the  tube  flows  into  the  bottle  ;  at  the  same  time  he  lowers  F 
so  as  to  keep  the  levels  of  water  in  the  burette  and  pipette  about  the  same, 
this  tending  to  prevent  leakage  of  air  or  gas  from  connections  not  perfectly 
tight.  When  all  the  solution  has  run  from  B,  A  is  quickly  turned  upright  in 
order  that  B  may  partly  fill  with  the  hypobromite  solution. 

F  is  now  supported  at  such  a  height  that  the  levels  of  water  are  the  same 
and  the  apparatus  left  to  cool  for  an  hour.  The  levels  of  water  are  then 
accurately  adjusted  and  the  volume  read,  and  the  barometer  and  thermometer 
again  noted  to  discover  any  variation  from  the  former  reading. 

Calculation.  The  following  formula  embodies  the  principles  stated  on 
page  183. 


(T-+.SCC.)  X  (*-  10  X  760  X 


in  which  N  is  the  percentage  of  nitrogen  in  the  salt;  V,  the  observed  volume 
of  gas;  .5  Cc,,  the  correction  for  absorption  of  nitrogen  in  the  liquid;  B,  the 
height  of  the  barometer  in  millimeters  ;  F,  the  tension  of  aqueous  vapor  at  the 
observed  temperature  ;  t,  the  temperature  of  the  room  in  degrees  Centigrade  ; 
and  St  the  weight  of  the  sample.  The  computation  may  be  facilitated  by  the 
tables  at  the  end  of  the  volume. 

Example. 

A.  B.  C. 

Weight  of  ammonium  sulf  ate  .....................  2016         .2047         .1996 

Reading  of  burette  before  evolution  .............  6  Cc.        .6Cc.        A  Cc. 

Reading  of  burette  after  evolution  ...............    37.7  Cc.    39.0  Cc.     37.9  Cc. 

Volume  of  nitrogen  (uncorrected)  ...........  ....     37.1  Cc.     38.4=  Cc.     37.5  Cc. 

Height  of  barometer  in  millimeters  ..............     762  760  762 

Temperature  of  room  in  degrees  Cent  ............    23.0  24.5  26.0 

Percentage  of  nitrogen  ..........................    21.07          21.25         21.17 

Results  are  usually  somewhat  below  the  theoretical  (21.24)  by  reason  of  the 
formation  of  small  amounts  of  nitrogen  compounds  not  decomposed  by  the 
hypobromite  solution. 

EXERCISE   23  —  NICKEL  —  COPPER  ALLOT. 

A.  Determination  of  the  Metals  Gravimetrically. 

Both  nickel  and  copper,  as  well  as  the  small  amount  of  iron  generally  pres- 
ent, are  readily  soluble  in  nitric  acid,  leaving  the  sulfates  on  evaporation  with 
sulf  uric  acid.  In  the  following  method  the  copper  is  precipitated  as  cuprous 
sulfocyanide,  roasted  to  cupric  oxide;  and  the  nickel  in  the  filtrate  as  nickelic 
hydrate,  ignited  to  nickelous  oxide. 

1.  Cupric  sulfocyanide,  Cu2(CNS)4,  is  a  black  precipitate  thrown  down  by 
potassium  sulfocyanide  ;  in  presence  of  a  reducing  agent  it  is  transformed  to 
cuprous  sulfocyanide  — 

2CuS04  +  2KCNS  +  H2SO3  +  H20  =  2CuCNS  +  K2SO4  +  2H2SO4. 

The  cuprous  sulfocyanide  is  a  white  or  pale  rose-colored  powder,  insoluble 
in  very  dilute  acids,  but  slightly  decomposed  by  water.  On  ignition  with  free 
access  of  oxygen,  there  remains  cupric  oxide  with  a  little  cuprous  sulfide;  this 
is  converted  into  sulf  ate  by  solution  in  nitric  and  sulf  uric  acids,  which  leaves 
pure  cupric  oxide  on  evaporation  and  strong  heating. 

*  Allen,  Coml.  Org.  Anal,  3—3—263. 


248  QUANTITATIVE    CHEMICAL   ANALYSIS. 

2.  Nickel  is  not  precipitated  by  a  sulfocyanide,  but  gives  green  nickelous 
hydrate  with  the  fixed  alkalies;  if  chlorine  or  bromine  be  present,  the  precipitate 
comes  down  mainly  as  black  nickelic  hydrate,  insoluble  in  alkalies  and  water  — 

2NiSO4  +  6KOH  -+-  C12  =  Ni2(OH)6  +  2K2SO4  -f  2KC1. 

On  ignition  there  pass  off  water  and  oxygen,  leaving  nickelous  oxide  (NiO). 
This  contains  the  iron  of  the  alloy  and  is  also  liable  to  be  contaminated  with 
silica  coming  from  the  alkali  or  dissolved  from  the  vessels  used. 

3.  Copper  and  nickel  form  soluble  double  salts  with  excess  of  ammonia, 
while  ferric  iron  is  precipitated  as  a  hydrate,  Fe2(OH)e.     On  ignition  this  is 
converted  into  ferric  oxide, 


Clean  one  five-cent  coin  of  recent  issue,  and  after  weighing  dissolve  in  a 
large,  tall  beaker  or  flask  in  a  mixture  of  100  Cc.  of  water  and  50  Cc.  of  con- 
centrated nitric  acid.  Cool  the  solution  and  add  100  Cc.  of  dilute  (ten  per 
cent)  sulfuric  acid,  and  500  Cc.  of  cold  water.  Pour  into  a  one-liter  measur- 
ing flask  and  make  up  to  the  mark  with  water,  rinsing  the  beaker  with  water. 

Copper. 

Pipette  50  Cc.  of  the  solution  into  a  12 -oz.  beaker,  heat  to  near  boiling  and 
add  dilute  solution  of  sodium  carbonate  until  a  slight  permanent  precipitate  of 
copper  hydrate  is  formed ;  dissolve  this  in  75  Cc.  of  strong  sulf urous  acid 
water,  and  precipitate  by  adding  a  slight  excess  of  potassium  sulfocyanide. 
Let  stand  in  a  warm  place  for  half  an  hour,  occasionally  stirring,  and  pass 
through  a  double  filter,  receiving  the  filtrate  in  a  large  beaker.  Wash  with  a 
mixture  of  one  volume  of  sulf  urous  acid  water  with  nine  volumes  of  water. 
Inclose  the  precipitate  in  the  paper  and  heat  gently  in  a  weighed  platinum 
crucible.  After  the  paper  has  charred,  incline  the  crucible,  and  roast  at  a  dull 
red  heat  until  no  more  sulfurous  acid  escapes. 

Cool  the  crucible,  add  one  or  two  Cc.  of  water,  five  or  six  drops  of  concen- 
trated sulfuric  acid  and  a  few  drops  of  nitric  acid.  Heat  until  solution  takes 
place,  evaporate  to  dryness  on  the  water  bath  and  drive  off  the  excess  of  acid 
by  heat  gradually  increased  to  redness.  Weigh  quickly,  as  the  copper  oxide  is 
somewhat  hygroscopic. 

Nickel. 

Transfer  the  filtrate  and  washings  to  a  casserole  or  porcelain  dish,  add  five 
Cc.  of  concentrated  nitric  acid,  cover,  and  boil  under  the  hood  until  efferves- 
cence ceases.  This  decomposes  the  excess  of  potassium  sulfocyanide  which, 
.would  interfere  with  the  precipitation  of  the  nickel. 

After  the  addition  of  50  Cc.  of  bromine  water,  make  slightly  alkaline  with 
pure  potash  solution,  freshly  made.  Boil  for  a  few  minutes,  allow  to  settle, 
filter,  and  wash  thoroughly  with  hot  water.  Burn  the  filter  with  the  precipi  - 
tate  in  a  platinum  crucible,  heat  to  bright  redness  and  weigh  as  NiO.  If  a 
hydrogen  generator  is  at  hand,  ignite  in  a  current  of  the  gas,  reducing  the 
oxide  to  the  metal. 

The  oxide  is  brushed  into  a  small  beaker,  dissolved  by  heating  with  concen- 
trated hydrochloric  acid,  and  evaporated  to  dryness  with  a  little  sulfuric  acid. 
The  residue  is  dissolved  in  hot  water  and  filtered  from  any  silica  that  may  re- 
main. The  silica  is  ignited  and  weighed  and  the  weight  subtracted  from  that 
of  the  nickel  oxide,  as  is  also  that  of  one-fourth  of  the  ferric  oxide  found  be- 
low. 


NICKEL- COPPER    ALLOY. 


249 


Iron. 

Draw  200  Cc.  from  the  flask  to  a  porcelain  dish,  heat  to  boiling  and  precipi- 
tate by  a  slight  excess  of  ammonia.  After  filtering  through  a  small  paper  and 
washing  with  hot  water,  the  ferric  hydrate  is  dissolved  into  a  small  beaker  by 
washing  the  filter  with  hot  dilute  hydrochloric  acid,  and  again  precipitated  by 
ammonia  to  free  it  entirely  from  the  other  metals.  After  filtration  and 
thorough  washing,  it  is  ignited  with  the  filter  in  a  platinum  crucible,  and 
weighed  as  Fe2O3. 
Calculation. 

Fe2O3  :  Fe2  :  :  160.00  :  112.00. 
CuO     :  Cu    :  :  79.60     :  63.60. 
NiO     :  Ni    :  :  74.70     :  58.70.      • 
Example. 

Weight  of 

Coin 4.886  grams; 

Ferric  oxide 0008     " 

Cupric  oxide 2300    * ' 

Nickel  oxide 0785      " 

(less  SiO2  .0013,  and  Fe2O3,  .0002) 


Per  cent. 

Iron 06 

Copper 75.22 

Nickel 24.77 


B.  Electrolytic  determination. 

On  connecting  plates  of  platinum  with  the  poles  of  a  galvanic  battery,  and 
immersing  them  in  an  acid  solution  of  copper  sulfate,  the  copper  will  be  slowly 
deposited  on  one  of  them  as  a  closely-adhering  film,  while  oxygen  separates  at 
the  other.  Nickel  is  not  deposited  from  an  acid  solution,  but  when  this  is 
made  ammoniacal  it  separates  in  the  same  manner  as  copper. 

The  battery  may  be  any  form  giving  a  continuous  constant  current  propor- 
tioned to  the  amount  of  metal  to  be  deposited,  resistance  of  the  solution,  etc* 
There  may  be  used  either  three  '  gravity, '  or  two  Edison -Lalande  cells  (or  a  cur- 
rent from  an  incandescent  light  wire  reduced  by  a  resistance  coil  or  incandes- 
cent bulbs,  or  a  storage  battery").  A  very  effective  form  is  a  one-gallon  Bunsen 
cell  consisting  of  a  carbon  rod  standing  In  a  porous 
clay  cup  surrounded  by  a  cylinder  of  zinc,  the 
whole  set  in  a  glass  jar  about  six  inches  diameter 
by  eight  inches  high,  One  of  these  will  precipitate 
the  copper  from  one  or  two  solutions,  and  runs 
for  24  hours  with  one  charge  of  battery  fluid  — 
essentially  a  solution  of  chromic  acid,  page  207. 

The  electrodes  may  be  in  the  form  of  small 
cylinders,  Fig.  141,  with  heavy  wires  fused  to  them 
to  connect  to  the  battery  wires.  The  deposition 
is  made  in  a  tall  12-oz  beaker.  To  prevent  a 
partial  re-solution  of  the  deposited  metal,  the 
current  must  continue  until  the  cathode  has  been 
removed  from  the  solution. 


Heat  the  larger  cylinder  to  redness  and  weigh 
when  cool.  Hang  them  concentrically,  about  one- 
eighth  inch  from  the  bottom  of  the  beaker,  con- 
necting their  wires  by  binding- screws  to  the  wires 
from  the  battery  as  shown  in  Fig.  142,  taking  care 


^.  m 


250 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


Fig.  H2. 


that  the  ends  entering  the  binding-screws  and  the  interior  of  the  latter  are 
clean  and  bright. 

In  the  evening  withdraw  100  Cc.  of  the  solution  of 
the  coin  and  run  it  into  the  beaker.  Add  water  until 
the  cylinders  are  covered  and  mix  well.  Charge  the 
battery  by  nearly  filling  the  porous  cup  with  the 
battery  fluid,  and  the  glass  jar  to  the  same  height 
with  water.  Observe  that  the  platinum  cylinders 
are  not  in  contact  and  that  the  connections  are 
screwed  tightly  together.  Cover  the  beaker  with 
two  plates  of  glass,  and  the  whole  apparatus  with  a 
sheet  of  paper  to  exclude  dust. 

On  the  following  morning  the  copper  will  be  found 
deposited,  probably  completely,  on  the  outer  cylin- 
der; it  should  have  a  clean  bright  red  color.  Add  a 
little  water  and  mix  with  a  glass  rod,  so  as  to  ex- 
pose an  uncoated  surface  of  platinum,  and  if  no 
copper  is  deposited  thereon  in  a  half  hour,  the 
separation  is  complete. 
Two  beakers,  large  enough  to  contain  the  cathode,  are  filled  with  distilled 
water;  the  cathode  is  transferred  as  rapidly  as  possible  from  the  solution  to 
one  of  the  beakers,  then  to  the  other;  drained,  dried  in  the  oven  for  a  few 
minutes  at  100°,  cooled  and  weighed.  Or  it  may  be  finally  rinsed  with 
strong  alcohol,  drained  and  touched  with  a  flame ;  when  the  alcohol  has  burned 
the  cylinder  is  weighed. 

Nickel. 

Unite  the  waters  used  for  washing  the  cylinders  with  the  solution  of  the 
nickel  sulfate,  evaporate  the  whole  to  dryness  to  expel  nitric  acid,  and  heat  till 
sulfuric  fumes  arise.  Cool,  dissolve  in  100  Cc.  of  hot  water,  and  transfer  to 
the  beaker  used  in  precipitating  the  copper. 

The  battery  is  emptied,  and  the  copper  dissolved  from  the  cathode  by  nitric 
acid.  The  cathode  is  weighed,  the  two  cylinders  connected  up  as  before,  and 
the  battery  recharged.  Add  to  the  nickel  solution  30  Cc.  of  concentrated 
ammonia,  and  hot  water  until  the  cylinders  are  covered.  The  remainder  of  the 
operation  is  conducted  substantially  as  directed  for  the  copper. 

Iron. 

The  iron  is  determined  colorimetrically.  It  is  separated  from  the  nickel  and 
copper  by  precipitatioa  of  the  former  by  ammonia,  filtered,  and  the  precipitate 
dissolved  in  a  little  dilute  nitric  acid.  The  iron  is  converted  into  sulfocyanide 
by  an  excess  of  potassium  sulfocyanide,  and  the  depth  of  the  red  color  of  the 
ferric  sulfocyanide  is  matched  by  one  containing  a  known  weight  of  iron  in  an 
approximately  equal  volume  of  solution. 

Prepare  a  solution  of  ferric  nitrate  of  known  concentration  by  dissolving  .010 
gram  of  iron  wire  in  a  little  dilute  nitric  acid,  and  boiling  for  a  few  minutes; 
cool  and  transfer  to  a  measuring-jar  and  dilute  to  100  Cc.  Rinse  and  fill  a  bu- 
rette with  the  solution. 

Measure  100  Cc0  of  the  solution  of  the  coin  into  a  small  beaker  and  precipi- 
tate the  iron  exactly  as  directed  on  page  249.  Dissolve  the  second  ferric  hy- 
drate precipitate  on  the  filter  by  running  through  a  little  dilute  nitric  acid, 
letting  the  solution  run  into  a  comparison  tube ;  wash  the  filter  with  water, 
and  dilute  the  filtrate  and  washings  to  30  to  40  Cc.  Into  the  other  comparison 
tube  put  about  as  much  water  and  add  to  each  two  Cc.  of  solution  of  potassium 


SILICATES.  251 

sulfocyanide.  Immediately  run  into  the  second  tube  sufficient  iron  solution  to 
cause  the  tints  to  appear  equal,  or  each  tube  darker  when  held  to  the  left  of  the 
other.  The  comparison  may  be  made  by  holding  the  tubes  close  together,  at 
an  angle  of  45  °  over  a  sheet  of  white  paper  facing  a  window,  viewing  them 
against  a  clear  sky,  or  otherwise  as  is  found  most  suitable  for  the  vision  of  the 
comparer.  Read  the  volume  of  iron  solution  used  and  the  volume  in  each  of 
the  comparison  tubes. 

Calculation.  Let  A  be  the  volume  of  solution  in  the  first  tube;  B,  the  volume 
in  the  second  tube;  C,  the  volume  of  iron  solution  added  to  B;  W,  the  weight 
of  the  coin;  6,  the  weight  of  iron  in  A;  and  X,  the  percentage  of  iron  in  the 
alloy.  Then  .0001  O  is  the  weight  of  iron  in  B,  and  A  :  B  :  :  b  :  .0001  C. 

Hence  b  =  -0001  CA  and X=  -0001  C'A^  100° X  100 
B  100  B.  W 

Example. 

Weight  of  coin 4.990  grams. 

Volume  of  solution  in  first  comparison  tube 41.0  Cc. 

Volume  in  second  tube 47.0  Cc. 

Volume  of  iron  solution  added  to  second  tube 4.4  Cc. 

Percentage  of  iron  in  the  coin 08 

Weight  of  cylinder  and  copper 22. 0885 grams. 

Weight  of  cylinder 21.7135    " 

Weight  of  copper 3750    " 

Weight  of  cylinder  and  nickel 21  8378    •* 

Weight  of  cylinder 21.7133    " 

Weight  of  nickel 1245    te 

Gravimetrically.  Electrolytically 

Copper 75.22  75.15 

Nickel 24.77  24.95 

Iron .06  .08 


100.05  100.18 

References.  Journ.  Anal.  Chem.  1889—342.  Journ.  Anal.  Appl.  Chem.  6—183  and  184. 
Zeits.  Anal.  19—314.  Chem.  News,  1894—1—17.  Idem,  1892—2—215. 

EXERCISE  24  — SILICATES. 

A.  Decomposed  by  Hydrochloric  Acid  —  Wollastonite. 

Wollastonite  is  a  silicate  of  calcium,  CaO.SiCk,  crystallizing  in  the  oblique 
system ;  color  white  or  light  shades.  Usually  it  contains  as  impurities  small 
amounts  of  the  oxides  of  iron,  aluminum,  manganese  and  magnesium,  some- 
times alkalies  and  combined  water. 

In  the  following  scheme  the  hydrochloric  solution  of  the  mineral  is  first  freed 
from  silica  by  evaporation,  then  each  base  successively  thrown  down  by  an 
appropriate  precipitant.  Alkalies  and  water  are  determined  in  separate  por- 
tions of  the  mineral. 

1.  The  powder  is  decomposed  by  dilute  hydrochloric  acid,  the  silica  mainly 
passing  into  solution,  and  the  bases  forming  soluble  chlorides.  On  evapora- 
tion of  this  solution  to  dryness  the  chlorides  remain  unchanged,  while  the  silica 
becomes  practically  insoluble  in  acids  and  water.  On  filtering  and  drying  the 
silica  it  appears  as  a  light,  loose,  white  powder,  infusible  and  unaffected  by 
ignition.  As  a  small  portion  of  the  mineral  may  have  resisted  the  acid,  after 


252  QUANTITATIVE   CHEMICAL   ANALYSIS. 

•weighing  the  silica  it  is  evaporated  with  hydrofluoric  and  sulfuric  acids,  the 
silica  passing  off  as  the  gaseous  compound  hydrofluosilicic  acid,  while  the  resi- 
due is  a  mixture  of  sulfates  of  the  bases.  This  residue  is  weighed,  dissolved 
in  hydrochloric  acid,  and  added  to  the  nitrate  from  the  silica;  the  weight  is  also 
subtracted  from  that  of  the  impure  silica  —  a  slight  error  is  here  introduced 
from  the  presence  of  the  sulfuric  radical. 

2.  The  addition  of  an  excess  of  ammonia  to  a  solution  of  ferric  chloride 
precipitates  the  metal  as  ferric  hydrate  insoluble  in  water  and  ammonia,  but 
easily    soluble    in  acids  —  Fe2Cl6  -f  6NH4OH  =  Fe2(OH  )6  +  6NH4C1.    The  hy- 
drate loses   water    when    ignited    and  becomes  ferric    oxide;  if   ammonium 
chloride  be  not  completely  washed  out  there  is  a  slight  loss  from  formation 
of  volatile  ferric  chloride. 

Aluminum  chloride  shows  a  similar  behavior  to  ferric.  In  either  case  any 
traces  of  silica  in  the  solution  are  carried  down  mechanically  with  the  precip- 
itate. 

3.  Ferric  and  aluminum  oxides  are  slowly  but  completely  soluble  in  concen- 
trated sulfuric  acid  when  hot,  silica  remaining.    After  filtering,   the  ferric 
sulfate  is  reduced  by  zinc  and  titrated  by  weak  standard  permanganate.    The 
iron  found  is  calculated  'to  ferric  oxide,  the  weight  of  the  residual  silica  added, 
and  the  sum  subtracted  from  the  original  weight;  the  remainder  is  the  alumina. 

4.  Manganous,    calcium,    and   magnesium    chlorides   are   transformed    to 
acetates  on  the  addition  of  an  alkali  acetate.    Manganous  acetate  is  decomposed 
when  heated  with  bromine  and  ammonia,  insoluble  (hydrated)  manganic  oxide 
being  precipitated  —  e.  g. 

2Mn(C2H302)2  -f  5Br2  +  16NH3  +  4H2O  =  2Mn02  +  10NH4Br  -f  4NH4CaH3O2-|-N2. 
Ou  ignition  the  precipitate  loses  water  and  oxygen  and  becomes  trimanganic 
tetroxide  —  3MnO2.H2O  =  Mn8O4  +  O2  +  H2O. 

5.  Neither  calcium  nor  magnesium  acetate  is  precipitated  by  these  reagents. 
Calcium  is  precipitated  as  calcium  oxalate  by  ammonium  oxalate ;  magnesium 
oxalate  remains  in  solution  nearly  completely;  if  in  large  quantity  usually  a 
small  part  comes  down  with  the  calcium  oxalate,  but  can  be  removed  by  redis- 
solving  the  filtered  precipitate  and  reprecipitating  with  ammonia. 

On  igniting  calcium  oxalate  below  redness  there  escapes  carbon  monoxide 
leaving  calcium  carbonate;  at  bright  redness  carbon  dioxide  is  driven  off  from 
the  carbonate  leaving  calcium  oxide  as  a  white  powder,  quite  hygroscopic. 

6.  In  the  filtrate  from  the  calcium  oxalate  the  magnesium  is  precipitated  by 
ammonium  phosphate  as  magnesium  ammonium  phosphate,  it  slowly  deposit- 
ng  in  a  crystalline  or  semi-crystalline  powder  which  tends  to  adhere  closely 
to  the  sides  of  the  vessel  especially  where  rubbed  with  a  glass  rod.    The  pre- 
cipitate is  soluble  in  mineral  acids,  insoluble  in  dilute  ammonia,  and  on  igni- 
tion is  converted  into  magnesium  pyrophosphate  with  evolution  of  water  and 
ammonia  — 

2MgNH4PO4  +  heat  =  Mg2P2O7  -f  2NH3  +  H2O. 

The  pyrophosphate  is  left  as  a  crisp  white  mass,  soluble  in  mineral  acids 
With  reconversion  to  the  orthophosphate. 

7.  A  silicate  decomposes  when  ignited  with  calcium  chloride  and  oxide  to 
form  silicates  of  calcium  and  other  bases  insoluble  in  water,  and  chlorides  of 
the  alkalies.    On  lixiviation  with  water,  the  chlorides  are  extracted,  together 
with  calcium  chloride  and  a  little  calcium  hydrate.     In  the  filtrate,  the  calcium 
is  precipitated  by  ammonium  carbonate,  leaving  only  alkali  salts  in  solution , 
on  evaporation  and  ignition  the  ammonium  salts  escape.    The  chlorides  of 
potassium  and  sodium  are  weighed  as  such,  and  may  be  separated  by  precipita- 
tion of  the  former  by  platinic  chloride  (page  243) ;  or  the  chlorine  may  be 


SILICATES.  253 

determined  in  the  mixture,  and  the  proportions  of  the  metals  calculated  there- 
from (page  179). 

8.  Water  of  combination  is  found  from  the  loss  in  weight  on  ignition  to  red- 
ness. This  will  also  include  any  organic  matter  or  carbon  dioxide  contained 
in  the  mineral. 


1.  Silica.  Select  pure  fibers  of  the  mineral  and  grind  a  few  grams  to  a  very 
fine  powder  in  an  agate  mortar.     Dry  at  100  °  and  weigh  one  gram  and  transfer 
to  a  porcelain  or  platinum  dish  or  a  casserole.     Cover  with  100  Cc.  of  boiling 
water  and  decompose  by  25  Cc.  of  concentrated  hydrochloric  acid  added  slowly 
with  constant  stirring.    When  no  more  grit  can  be  felt  with  a  glass  rod,  evap- 
orate on  the  water-bath  to  dryness;  moisten  with  concentrated  hydrochloric 
acid  and  again  evaporate  to  complete  dryness. 

The  residue  is  moistened  with  a  little  concentrated  hydrochloric  acid  and  a 
few  drops  of  nitric  acid,  diluted  with  100  Cc.  of  hot  water,  and  filtered  into  a 
porcelain  dish,  washing  the  silica  on  the  filter  with  hot  water  until  the  wash- 
ings show  no  cloud  with  silver  nitrate.  The  filter  is  opened  on  a  porous  tile 
and  then  wrapped  around  the  precipitate,  burned  in  a  platinum  crucible  and 
ignited  to  a  white  heat. 

The  purity  of  the  silica  is  tested  by  moistening  it  with  water  in  the  crucible, 
then  adding  about  5  Cc.  of  pure  hydrofluoric  acid  and  a  drop  of  sulfuric  acid; 
the  liquid  is  evaporated  to  dryness  and  the  crucible  heated  to  redness.  If 
there  remains  only  a  few  milligrams  of  residue  it  is  again  evaporated  with  a 
little  hydrofluoric  acid,  weighed  and  dissolved  in  hydrochloric  acid  and  the 
solution  added  to  the  original  filtrate.  But  if  the  residue  is  considerable  it  is 
evident  that  the  mineral  was  not  ground  finely  enough,  and  the  analysis  is 
recommenced  with  a  finer  powder. 

2.  Iron  and  aluminum  oxides.     Heat  the  filtrate  to  boiling  and  precipitate  by 
ammonia,  using  but  a  slight  excess.    Boil  until  the  smell  of   ammonia  is 
gone,  allow  to  settle,  and  pass  through  a  small  filter;  to  prevent  access  of  car- 
bon dioxide  from  the  air  —  which  would  precipitate  calcium  —  keep  the  funnel 
and  dish  closely  covered.     Wash  a  few  times  with  hot  water,  then  dissolve  the 
precipitate  by  washing  the  filter  alternately  with  hot  dilute  hydrochloric  acid 
and  hot  water,  letting  the  solution  run  into  the  porcelain  dish.    The  solution 
is  now  precipitated  by  ammonia  as  before,  filtered  and  washed  thoroughly. 
The  precipitate  is  wrapped  up  in  the  paper  and  the  latter  burned,  finally 
igniting  the  precipitate,  not   higher   than    dull  redness.    It   is  weighed   as 


3.  Iron  oxide.  The  precipitate  is  brushed   into  a  small  beaker  and  20  Cc. 
of  dilute  (10  per  cent)  sulfuric  acid  added;  then  evaporated  until  fumes  of 
sulfuric  acid  arise.    After  dilution  with  water  and  repeating  the  evaporation 
and  dilution,  usually  all  the  residue  will  dissolve  except  light  flocculent  silica; 
if  not,  the  evaporation  is    again  repeated.    The  silica  is  now  filtered  and 
weighed,  and  the  result  added  to  the  weight  of  the  silica  in  (1).    A  little  zinc 
is  dropped  into  the  filtrate,  and  when  dissolved,  the  ferrous  sulfate  is  titrated 
by  standard  permanganate   (page  229)  diluted  with  nine  volumes  of   water. 
The  iron  is  calculated  to    ferric  oxide  and  with  the  silica,  subtracted  from 
the  weight  of  (2),  giving  the  alumina  by  difference. 

4.  Manganous  oxide.    The  united  filtrates  from  (2)  is  acidified  with  five  Cc. 
of  glacial  acetic  acid,  allowed  to  cool  completely,  and  colored  yellow  with 
bromine  water.    Ammonia  is  then  added  till  alkaline  and  the  solution  heated 
to  boiling.    The  precipitated  manganic  hydrate  (if  any)  is  filtered  and  washed 


254  QUANTITATIVE    CHEMICAL    ANALYSIS. 

with  hot  water,  and  purified  from  calcium  by  redissolving  in  hydrochloric  acid 
and  reprecipitating  as  before.  It  is  calcined  at  a  bright  red  heat  in  a  platinum 
crucible,  and  weighed  as  MnsO*. 

5.  Calcium  oxide.  First  method.  The   united  filtrates  from  (4)  is  concen- 
trated to  about  200  Cc.  and  precipitated  by  ammonium  oxalate  and  ammonia. 
When  the  liquid  clears,  which  requires  some  hours,  it  is  decanted  through  a. 
close  filter  and  washed  with  hot  water.    The  filter  inclosing  the  precipitate  is 
burned  and  the  calcium  compound  ignited,  at  first  gently,  finally  for  one-half 
hour  at  a  bright  red  heat.    Weigh  quickly  and  repeat  the  ignition  until  no 
further  loss  (greater  than  one  milligram)  is  sustained. 

Second  method.  The  solution  is  evaporated  to  about  20  Cc.  and  the  calcium 
precipitated  by  about  ten  Cc.  of  dilute  sulfuric  acid.  Fifty  Cc.  of  alcohol 
is  added  and  after  standing  in  the  cold  for  an  hour  or  more,  the  liquid  is  filtered 
and  the  calcium  sulfate  washed  by  dilute  alcohol  until  free  from  chlorides. 
The  filter  and  precipitate  are  dried,  the  filter  separated  and  burned  in  a  plati- 
num crucible,  and  the  ash  moistened  with  a  few  drops  of  water  and  a  drop  of 
dilute  sulfuric  acid  to  revert  to  sulfate  any  calcium  sulflde  formed,  and  dried 
on  the  water  bath.  The  precipitate  is  transferred  to  the  crucible,  heated  to 
redness,  and  weighed  as  CaSO* 

6.  Magnesium  oxide.  The  filtrate  from  either  of  the  above  methods  is  con- 
centrated  to  100  Cc.  or  less  and  after  cooling  precipitated  by  an  excess  of 
sodium  ammonium  phosphate  and  about  20  Cc.  of  concentrated  ammonia. 
After  stirring  for  a  few  minutes,  not  touching  the  sides  of  the  beaker  with  the 
glass  rod,  let  stand  for  some  hours  that  the  supernatant  liquid  may  become 
clear;  filter  and  wash  with  dilate  ammonia  (one  part  concentrated  ammonia 
to  five  parts  of  water).    The  filter  with  the  precipitate  inclosed  is  burned,  the 
crucible  heated  to  redness,  and  the  precipitate  weighed  as  M.S2Pz07- 

7.  Alkalies.    Mix  one  gram  of  the  mineral  in  a  mortar  with  five  grams  of 
calcium  carbonate  and  one-half  gram  of  ammonium  chloride.    Transfer  the 
mixture  to  a  platinum  crucible   (if  not  large  enough  for  the  entire  quantity, 
make  two  ignitions),  and  heat  to  redness  for  a  half  hour.    Turn  the  sinter 
into  a  porcelain  or  wedgewood  mortar  and  grind  it  to  a  coarse  powder.    Brush 
this  into  a  beaker  and  moisten  with  water;  when  the  lime  has  slaked  to  a  soft 
paste,  rinse  the  crucible  and  its  cover  with  water  into  the  beaker,  stir  well, 
and  filter,  washing  with  hot  water.    Precipitate  the  calcium  from  the  filtrate  by 
ammonium  carbonate  (page  207)  and  a  drop  of  ammonium  oxalate  solution. 
When  the  calcium  carbonate  has  become  granular  it  is  filtered  and  washed  with 
cold  water,  and  the  filtrate  tested  for  complete  separation  of  calcium  by  a  drop 
of  ammonium  oxalate.    If  it  remains  clear   (or  after  filtration   if    cloudy), 
it  is  evaporated  to  dryness  and  the  residue  heated  until  all  ammonium  salts  are 
driven  off.    The  residue  is  dissolved  in  a  little  water  and  the  solution  filtered 
through  a  very   small    paper  into  a    weighed  crucible;    a  drop  of   hydro- 
chloric acid  is  added  and  evaporated  to  dryness  on  the  water  bath.     The 
residue  is  heated  gently  and  weighed  as   KCl  +  NaCl;   it  should  dissolve  in 
water  to  a  clear  solution,  except  perhaps  a  trace  of  black  carbonaceous  matter 
practically  negligible. 

If  the  alkali  chlorides  are  present  in  sufficient  amount  to  admit  of  a  practical 
separation  —  say  in  a  quantity  of  over  ten  milligrams  —  the  solution  in  dilute 
alcohol  is  precipitated  by  platinic  chloride  and  the  potassium  platinchloride 
weighed,  or  the  chloride  determined  and  the  bases  calculated.  If  too  small 
for  separation  the  residue  is  reported  as  a  mixture  of  the  two  alkalies, 
conventionally  as  KNaCk. 

8.  Combined  water.  Determined  by  igniting  about  one  gram  of  the  powder 
in  a  platinum  crucible  to  bright  redness,  repeating  to  constant  weight. 


SILICATES.  255 

Calculation. 

Fe2  :     Fe2O3     :   :     112.         :     160. 

Mn3O4       :     3MnO     :  :     229.         :     213. 
CaSO4       :     CaO         :  :     136.17     :       56.1 
Mg2P207   :     2MgO     :  :     222.60     :       80.6 
KNaCl      :     KNaO     :  :     133.06     :       78.16 

Example.  Weight  taken  for  analysis,  one  gram. 

Silica,  etc.,  weighed  .5054  gram.  Less  .0030  gram  of  residue,  plus  .0026 
gram  of  silica  recovered  =  .5050  gram.  Silica  =  50.50  per  cent. 

Alumina,  ferric  oxide,  and  silica  together  weighed  .0056  gram.  The  silica 
weighed  .0026,  leaving  alumina  plus  ferric  oxide  =  .0030.  Titration  required 
1.1  Cc.  of  permanganate  of  which  one  Cc.  oxidized  .0007  gram  of  iron;  this 
gives  iron  .00077  gram  =.0011  gram  of  ferric  oxide.  Ferric  oxide  =  .11  per 
cent. 

Alumina  plus  ferric  oxide  .0030;  subtracting  .0011  of  ferric  oxide  leaves 
alumina  .0019  gram.  Alumina  =  .19  per  cent. 

Manganic  oxide  too  small  in  amount  to  be  weighed.  Manganese  oxide  = 
trace. 

Calcium  oxide  weighed  .4750  gram.    Calcium  oxide  =  47.50  per  cent. 

Magnesium  pyrophosphate  weighed  .0055  gram.  Magnesium  oxide  =  .20  per 
cent. 

Potassium  sodium  chloride  weighed  .0045  gram.    Alkalies  =  .26  per  cent. 

Loss  on  ignition  .0114  gram.    Combined  water  =  1.14  per  cent. 

The  analysis  is  then 

Silica 50.50    Magnesium  oxide 20 

Ferric  oxide 11    Alkalies 26 

Alumina 19    Combined  water 1.14 

Manganese  oxides trace 

Calcium  oxide 47.50  Total 99.90 

The  directions  formulated  for  the  separation  of  the  bases  apply  to  the  analy- 
sis of  silicates  in  general,  with  such  modifications  as  the  presence  of  other 
bases  may  necessitate,  or  a  few  changes  where  their  proportion  varies  from 
those  in  the  example  given.  Thus,  should  there  be  considerable  ferric  oxide  or 
alumina  it  is  best  to  precipitate  them  by  the  basic  acetate  process  since  the 
calcium  and  manganese  are  more  perfectly  separated  than  by  ammonia.  With 
much  manganese  it  is  preferable  to  dissolve  the  manganic  oxide  in  hydrochloric 
acid  and  precipitate  as  ammonium  manganese  phosphate  (page  243);  etc. 
Silicates  containing  much  alumina,  or  those  associated  with  titanic  acid 
present  many  difficulties  for  an  accurate  analysis. 


B.  Silicates  insoluble  in  hydrochloric  acid. 

Quartz,  glass,  feldspar,  mica,  steatite,  etc. 

The  general  course  of  the  analysis  differs  from  the  preceding  mainly  in 
that  the  silicate  is  made  soluble  by  fluxing  with  sodium  carbonate,  the  melt 
becoming  double  silicates  of  the  bases  and  sodium,  carbon  dioxide  escaping. 
Silicates  containing  lead  (e.  g.,  table-glass)  cannot  be  decomposed  in  this 
way  since  there  is  danger  of  metallic  lead  forming  and  alloying  with  the 
crucible  —  they  are  best  dissolved  in  hydrofluoric  acid  and  the  silica  found  by 
difference. 

One  gram  of  the  fine  powder  is  mixed  on  glazed  paper  with  about  five 
grams  of  pure  dry  sodium  carbonate,  and  the  mixture  turned  into  a  capacious 


256  QUANTITATIVE   CHEMICAL    ANALYSIS. 

platinum  crucible  and  heated  over  a  large  Bunsen  burner.  When  the  fusion 
becomes  tranquil  the  crucible  is  inclined,  and  after  solidification,  the  lump 
dropped  out  into  a  large  porcelain  dish  or  casserole.  The  crucible  is  washed 
out  and  the  lump  digested  with  a  large  volume  of  hot  water  until  entirely  dis- 
integrated. The  dish  is  covered  by  a  watch-glass  and  the  liquid  cautiously 
acidified  by  hydrochloric  acid;  the  watch-glass  is  rinsed  and  the  solution 
further  proceeded  with  substantially  as  for  a  soluble  silicate.* 


For  those  desiring  more  extended  practice  in  analysis  selections  may  be 
made  from  the  following,  with  especial  attention  to  Numbers  1  to  6.  Detailed 
directions  will  be  found  in  the  general  treatises  on  quantitative  analysis  and  in 
technical  works  on  the  special  subjects,  and  need  not  be  included  here. 

1.  Elementary  analysis  of  an  organic  or  semi-organic  body,  e.  g.,  sugar, 
oxalic  acid,  an  alkaloid,  etc. 

2.  Gold  or  silver  ore  by  the  fire  assay. 

3.  Sugar  in  cane  or  beet  by  the  saccharimeter  and  Fehlings  solution. 

4.  Natural  water;  industrial  and  sanitary  analysis. 

5.  Fertilizer;  for  nitrogen,  alkali  and  phosphoric  acid. 

6.  Illuminating  gas;  complete  analysis. 

7.  Soap;  complete  analysis. 

8.  Pig  lead ;  complete  analysis. 

9.  Iron  ore ;  iron,  phosphorus,  silica,  manganese. 

10.  Textile  fabric;  wool,  cotton,  silk. 

11.  Nickel  matte;   complete  analysis. 

12.  Manganese  ore ;  manganese  and  available  oxygen. 

13.  Indigo;  indigotin  and  indirubin. 

14.  Urine ;  urea,  uric  acid,  and  inorganic  bases,  and  tests  for  albumin  and  sugar. 

15.  Licorice  mass.     (Analyst,  1898—94). 

16.  Meat  extract.     (Chem.  News,  1890—1—290  and  303). 

17.  Creosote.     (Amer.  Journ.  Pharm.  1899 — 409). 

18.  Opium;  morphine. 


*  References.  Chem.  News,  54—242;  60—14;  62—6;  69—171.    Journ.  Amer.   Chem.  Socy. 
1889—421  and  1890—159.    Crookes,  Select  Matnods,  28.     Fresenius  Quant.  Anal.  426. 


PART  3. 


SPECIAL  AND  TECHNICAL  METHODS. 


COLORIMETRY.  259 


COLORIMETRY. 

The  practice  of  colorimetry  has  as  a  basis  the  principle  that  the  depth  of 
color  conferred  to  a  liquid  by  a  chromogenous  constituent  body  in  solution,  as 
viewed  by  transmitted  white  light,  or  the  depth  of  color  of  a  solid  or  powder 
viewed  by  reflected  light,  is  in  a  direct  ratio  to  the  weight  of  the  body  in 
solution  or  admixture.  In  all  the  methods  there  is  attained  an  equality  of  depth 
of  color  between  the  solution  of  the  substance  to  be  assayed  and  an  arbitrary 
1  standard/  this  usually  a  solution  of  the  same  or  a  similar  chromogen  as  exists 
in  the  substance  to  be  assayed. 

The  depth  of  color  of  a  liquid  or  solid  is  the  extent  of  the  departure  from 
pure  white,  while  the  density  or  tone  of  a  given  color  is  expressed  by  desig- 
nating the  various  primary  colors  that  unite  to  produce  the  given  shade,  and 
their  relative  proportions.*  The  two  are  independent,  as  the  depth  of  any  given 
color  may  vary  indefinitely,  and  conversely,  several  different  colors  may  be  of 
equal  depth.  Colorimetric  determinations  are  based  on  the  depth  of  color,  and 
are  applied  for  the  most  part  to  solutions  and  liquids,  less  often  to  powders. 

The  plan  of  « comparison  by  dilution »  is  usually  adopted  for  occa- 
sional determinations,  and  is  as  follows:  There  is  weighed  of  the  sub- 
stance to  be  assayed  such  an  amount  as  will  presumably  yield  a  solution 
of  a  suitable  depth  of  color  for  comparison,  and  of  the  pure  colorific 
constituent  or  a  compound  thereof  (the  *  standard')  as  will  produce  a 
solution  of  approximately  equal  depth.  After  dissolving  both  in  equal 
volumes  of  the  solvent,  the  darker  of  the  two  is  diluted  with  water  or  other 
colorless  liquid  until  the  depth  is  the  same.  The  concentration  of  the  solution 
of  the  sample  is  then  deduced  by  a  simple  calculation  from  the  volumes  of  the 
solutions  and  the  weights  of  the  sample  and  standard  (page  183),  the  concentra- 
tions being  inversely  as  the  volumes.  It  is  assumed,  of  course,  that  any  colorific 
constituents  of  the  sample  other  than  the  one  determined  have  been  removed 
or  changed  to  some  colorless  combination  before  the  comparison  is  made. 

It  is  a  fundamental  rule  of  all  accurate  determinations  that  the  two  solutions 
shall  be  so  made  up  that  the  relative  concentrations  as  regards  the  chromogen 
shall  be  fairly  equal  before  proceeding  to  dilute  them.  The  reasons  for  this 
direction  are  (a)  that  generally  the  depth  of  color  of  a  solution  is  not  reduced 
in  strict  proportion  to  the  dilution;  and  (b)  the  color  itself  is  often  modified 
by  dilution.  By  preparing  the  solutions  as  directed,  the  error  of  (a)  is  kept 
within  a  negligible  limit,  and  (b)  the  comparison  of  the  depth  of  color  is 
faciliated. 

Other  requisites  are  that  both  solutions  be  perfectly  free  from  suspended 
matter  and  at  the  same  temperature,  and  that  the  comparison  tubes  be  of  clear 
white  glass  and  equal  in  bore,  and  uniformly  illuminated  by  one  source  of  light. 

Usually  a  solid  substance  is  prepared  for  comparison  by  treatment  with  a 
solvent  and  filtering,  but  it  is  not  uncommon  that  a  constituent  that  dissolves 
to  a  colorless  or  light  colored  solution  may  be  oxidized,  reduced,  or  otherwise 
chemically  transformed  to  a  highly  colored  combination.  For  example,  a  free 
acid  in  a  liquid  may  on  digestion  with  an  insoluble  hydroxide  of  a  metal  (e.  g.y 
copper)  dissolve  an  equivalent  of  the  hydroxide  to  form  a  salt  of  a  color  suit- 


*  Journ.  Socy.  Dyers  &  Col.  1896-166. 


260 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


able  for  comparison;  or  a  base  maybe  precipitated  in  combination  with  a 
colored  acid  radical   (chromic,  picric,  permanganic),  the  precipitate  filtered, 
redissolved  and  the  colored  solution  compared  with  the   corresponding  chro- 
mate,  etc.    This  scheme  has  an  occasional  use  where  certain  associates  of  the 
constituent  to  be  determined  make  other  methods  difficult  of  application. 

Solutions  of  a  hue  unsuited  for  direct  comparison  can  possibly  be  com- 
pounded with  a  suitably  colored  liquid  that  will  reduce  the  hue  to  a  compa- 
rable simple  color.  Thus,  an  aqueous  extract  of  logwood  when  mixed  with  an 
ammoniacal  solution  of  potassium  chromate  becomes  of  a  pure  blue. 

The  usual  illuminant  is  diffused  sunlight  reflected  from  a  dull  white  surface 
or  transmitted  through  a  ground  or  opal  glass  plate,  tracing  cloth,  or  other 
translucent  white  material.  Of  artificial  lights,  as  a  rule  the  whiter  the  flame  the 
better,  and  the  intensity  should  be  ample  to  allow  the  tubes  to  be  placed  for 
comparison  at  a  distance  of  several  feet  to  be  certain  that  the  illumination  is 
equal.  Monochromatic  light  permits  a  sharper  comparison  of  liquids  of  a 
compound  color.  For  fluorescent  solutions  and  those  impossible  to  clarify,  it 
is  best  to  surround  the  tube  with  opaque  material  and  arrange  for  the  light  to 
pass  upward  through  the  bottom. 

In  practice  the  two  solutions  are  held  in  tubes  of  colorless  glass,  identical 
in  bore  and  thickness  of  wall,  and  graduated  in  cubic  centimeters  and  tenths. 
The  tubes  are  either  provided  with  glass  stoppers,  or 
the  upper  end  beyond  the  graduation  is  bent  to  an 
angle  of  about  45°  for  convenience  in  mixing  with 
the  diluent.  In  order  to  cut  off  reflections  from  near 
by  objects  and  shield  the  tubes  from  side-lights,  the 
tubes  may  stand  close  together  itf  a  "  camera  "  near 
the  posterior  end.  The  camera  is  a  long  shallow  box 
of  blackened,  wood .  the  anterior  end  open  for  viewing 
the  tubes,  and  the  posterior  end  closed  by  a  sheet  of 
white  paper  or  opal  glass  illuminated  by  daylight  or  a 
monochromatic  flame.  Since  the  shade  of  a  liquid 
when  viewed  in  a  round  tube  is  deepest  at  the  center, 
Fig.  143.  a  flat  tube  or  cell  is  preferable,  or  the  modification  of 

the  camera  shown  in  horzoutal  section  in  Fig.  143  —  only  narrow  bands  at  the 
center  of  the  tubes  are  seen,  the  remainder  being  screened  by  the  wooden  casing, 
A.  To  most  observers  the  left  hand  tube  appears  the  darker  when  the  solu- 
tions are  identical  in  depth  of  color,  and  the  point  of  equality  is  known  when 
either  tube  appears  darker  when  at  the  left  of  the  other. 

Where  periodical  tests  are  made  of  certain  commercial  raw  materials  or  prod- 
ucts in  which  the  percentages  of  the  coloring  constituent  vary  only  between 
narrow  limits,  a  number  of  devices  are  in  use  to  avoid  the  preparation  of  a 
standard  for  each  test  or  batch  of  tests. 

1.  Of  a  sample  containing  the  lowest  percentage  expected,  there  is  dissolved 
a  standard  weight  in  a  standard  volume  of  solvent,  in  a  test  tube  of  standard 
diameter.  A  solution  of  a  stable  tinctorial  body  (as  a  salt  of  copper,  iron,  or 
cobalt,  or  an  organic  body  like  caramel  or  an  aniline  dye)  is  diluted  or  mixed 
with  others  in  such  ratios  as  will  produce  the  same  color  and  an  equal  tint 
when  contained  in  a  similar  tube;  the  tube  is  hermetically  sealed,  and  each 
sample  to  be  tested,  prepared  as  above,  is  diluted  until  identical  to  it  in  tint. 
As  the  colors  of  all  these  artificial  standards  fade  in  course  of  time,  more 
quickly  if  organic,  there  has  been  proposed  as  a  substitute  colored  glass  rods, 
selecting  from  a  series  the  proper  diameter  and  shade  for  use.  A  few  solutions 
act  on  the  glass  of  the  containing  tubes  and  alter  in  shade  by  the  constituents 


COLORIMETRT. 


261 


dissolved,  e.  g  ,  picric  acid  dissolving  alkali  from  glass  with  a  deepening  in 
color. 

2.  The  tentative  dilution  of  the  sample  — at  best  a  tedious  operation  —  may 
be  dispensed  with  by  preparing  a  series  of  standards  whose  concentrations 
vary  by  regular  inter- 
vals—  say  one-tenth  of 
one  per  cent  for  the  lighter 
and  one  per  cent  for  the 
darker  tints.  A  standard 
weight  of  each  is  dissolved 
and  the  series  arranged 
progressively  in  a  rack, 
Fig.  144,  spaced  to  allow 
a  similar  tube  containing 

the  solution  of  the  sample  to  be  placed  between  any  two.  Across  the  back  of 
the  frame  is  stretched  a  sheet  of  white  paper  for  a  background.  Whan  the 
sample  is  placed  between  two  of  the  tubes,  being  intermediate  in  tint,  the  eye 
can  readily  estimate  the  percentage  to  within  one-fourth  of  the  difference 
between  the  contiguous  tubes.  Or  the  series  may  be  made  up  by  diluting 
measured  portions  of  the  artificial  liquid  of  (1)  made  of  such  a  strength  as 
corresponds  to  the  highest  percentage  expected  in  the  samples,  with  the  cal- 
culated volumes  of  water  or  alcohol.  The  series  is  contained  in  sealed  tubes, 
the  rack  shielded  from  the  light  when  not  in  use. 

Following  the  general  rule,  a  series  made  up  of  samples  containing  the 
chromogen  in  progressive  ratios,  will  afford  more  accurate  results  than  where 
one  sample  of  a  high  percentage  is  diluted  by  the  calculated  volumes  of  water 
or  other  liquid. 

Of  certain  commercial  materials  that  contain  varying  amounts  of  a  soluble 
adjective  —  a  filler,  make-weight,  preservative,  etc. — the  series  is  best  com- 
pounded of  the  pure  chromogen  plus  corresponding  inverse  weights  of  the 
adjective.  For  example,  in  extract  of  logwood  where  the  active  principle 
haematein  is  associated  with  a  mixture  of  one-third  of  chestnut  extract  and 
two-thirds  treacle;  the  diluent  of  the  standard  should  be  a  mixture  in  this  pro- 
portion. 

The  '  tintometer '  of  Lovibond  *  is  designed  on  strict  scientific  principles  and 
is  suited  to  a  great  variety  of  substances  transmitting  or  reflecting  colored 
light.  The  color-standards  are 
thin  glass  plates  of  three  pure 
colors,  red,  blue  and  yellow.  A 
full  set  of  each  color  comprises 
over  150  glasses,  graded  in  degrees 
of  depth  of  color.  The  intervals 
between  the  degrees  are  the  small- 
est differences  that  the  normal 
vision  can  differentiate;  as  the 
shades  become  lighter,  and  the 
ability  to  discriminate  greater,  the 
intervals  are  subdivided  into 
tenths  and  hundredths.  Fig.  146. 

The  standard  of  the  system  is  the  quantity  of  normal  white  light  absorbed 
by  the  superposition  of  one  red,  blue,  and  yellow  glass  of  unit  depth.  On 


*  Journ.  Socy.  Dyers  &  Col.  1887—186  and  1894—3,  22,  and  206. 


262  QUANTITATIVE   CHEMICAL   ANALYSIS. 

increasing  the  number  of  these  combinations,  the  emergent  white  light  grows 
dimmer  and  eventually  is  extinguished,  and  the  luminous  intensity  of  the  light 
so  absorbed  is  denoted  by  the  number  of  combinations  of  glasses  interposed. 
The  standardizing  is  done  by  the  maker  against  powders  or  definite  solutions 
of  pure  chemical  compounds. 

The  apparatus,  Fig.  145,  is  a  long  narrow  box  B  open  at  both  ends  and 
divided  longitudinally  by  a  thin  partition.  The  box  is  supported  at  a  con- 
venient angle  and  points  to  a  white  reflector  F,  a  layer  of  compressed  pre- 
cipitated calcium  sulfate.  On  one  side  of  the  partition  at  J  is  placed  a  glass 
cell  or  bottle  containing  the  liquid  to  be  tested,  and  on  the  other  side  are 
superimposed  such  of  the  colored  glass  plates  as  will  together  transmit  exactly 
the  same  hue  and  depth  as  the  cell.  From  the  sura  of  the  plate  numbers  is 
determined  the  "  visual  composition."  The  dimension  of  the  cell  selected  for 
a  test  is  governed  by  the  intensity  of  the  color  and  shade  of  the  inclosed  liquid, 
ranging  from  one-eighth  of  an  inch  for  dark  liquids  up  to  24  inches  for  natural 
potable  waters.  Opaque  bodies  are  illuminated  by  the  same  light  as  is  reflected 
from  F. 

( *  The  system  of  nomenclature  is  based  on  the  fact  that  under  ordinary  day- 
light conditions,  the  vision  is  only  simultaneously  sensitive  to  two  of  the  six 
colour  rays  which,  when  united,  produce  normal  white  light,  the  colour  sensations 
of  the  remaining  four  being  lost  in  what,  for  want  of  a  better  expression,  we 
term  the  luminous  intensity  of  the  accompanying  daylight.  These  two  colours 
are  always  adjacent  in  the  solar  spectrum ;  for  violet,  the  spectrum  colours  must 
here  be  considered  as  lying  in  a  circle,  the  red  and  blue  having  violet  between 
them."  Red  glass  absorbs  yellow,  green  and  blue,  and  transmits  violet,  red  and 
orange;  blue  glass  absorbs  red,  orange  and  yellow,  and  transmits  green,  blue 
and  violet;  and  yellow  glass  absorbs  red,  violet  and  blue,  and  transmits  yellow, 
orange  and  green. 

For  example,  a  powder  of  commercial  Prussian  blue  required  standard  glasses, 
viz.,  red  9.0,  yellow  1.0,  and  blue  6. 8.  The  yellow  being  numerically  the  small- 
est in  the  combination  is  taken  to  unite  with  1.0  of  red  and  1.0  of  blue  to  com- 
pose 1.0  of  black.  This  leaves  8.0  of  red  and  5.8  of  blue.  Now,  red  glass  ab- 
sorbs blue  rays,  and  blue  glass  red  rays,  while  both  transmit  violet  rays,  here  to 
the  extent  of  5.8;  consequently  the  excess  of  red  is  8.0  —  5.8  =  2.2,  and  the 
visual  composition  is  black  1.0,  red  2.2  and  violet  5.8. 

The  factors  of  a  complex  color  sensation  may  be  obtained  by  drawing  three 
parallel  horizontal  lines  of  lengths  corresponding  to  the  unit  values  of  the  match- 
ing glasses,  then  drawing  vertical  lines  through  the  termini  of  the  two  shorter. 
The  three  sections  equal  to  the  shortest  of  the  horizontal  lines  represent  the  ab- 
solute chromatic  units ;  the  two  interlinear  sections,  the  monochromatic  color 
developed;  and  the  extension  of  the  longest  line,  the  proportion  of  the  domin- 
ant ray. 

Thus  a  five  per  cent  solution  of  litmus  required  standard  glasses  to  match, 
red  18.8,  olue  21.2.  The  color  sensation  transmitted  is  therefore  violet  18.8  -f- 


YEUOW2.3 

RED       3.1 
SLUE     8.0 


blue  2.4;  for  18.8  units  of  violet  is 
developed  by  the  mutual  absorption 
of  18.8  units  of  red  and  18.8  of  blue, 


••2.3-— *•**• ~i8. ....... ..4    leaving  2.4  units  of  unaltered  blue. 

Fig.  146.  Again,  a  cloth  dyed  with  a  one  per 

cent  solution  of  Victoria  blue  matched  3.1  red  +  2.3  yellow  -}-  8.0  blue.    From 

the  diagram,  Fig.  146,  the  color  sensation  transmitted  is  2.3  black  -j-  .8  violet  4- 

4.9  blue. 

The  tintometer  finds  use  in  dyeing  and  calico-printing,  malting  and  brewing, 


COLORIMETRY . 


263 


the  manufacture  of  sugar  and  caramel ;  for  the  examination  of  oils,  waxes  and 
varnishes,  the  standardizing  and  comparing  of  pigments,  flours,  enamels, 
lards,  etc.,  and  for  colorimetric  determinations  of  carbon  in  steel,  ammonia  in 
potable  water 3  etc.* 


S   A 


A  variation  of  the  usual  practice  of  finding  the  point  of  equality  in  depth  of 
color  of  two  solutions  is  that  of  determining  the  thickness  of  a  layer  of  a  liquid 
of  a  complementary  color  that  will  extinguish  the  first.  In  this  case  a  glass 
wedge  is  filled  with  a  solution  complementary  to  that  in  the  sample  tube  and 
one  beam  of  light  is  passed  through  both,  the  extinction  point  being  shown 
when  the  wedge  is  in  such  a  position  that  a  neutral  tint  is  transmitted,  neither 
the  color  of  the  tube  or  that  of  the  wedge  predominating. 

On  this  principle  is  the  '  chromometer '  of  Eoenig.  He  fluxes  a  minute 
weight  of  an  ore,  of  copper  for  example,  with  a  weighed  amount  of  borax-glass 
in  a  small  platinum  cylinder,  and  views  a  ray  of 
light  passing  through  it  and  a  wedge  of  amethyst 
glass  which  is  complementary  in  color  to  the 
blue  of  the  borax -bead.  The  wedge  is  made  by 
grinding  a  narrow  strip  of  body- 
glass  to  an  edge  at  one  end,  and 
is  mounted  in  a  frame  divided 
longitudinally  into  millimeters 
and  moved  transversely  to  the 
light-ray  by  a  micrometer  screw. 
The  wedge  is  standardized  by 
borax-beads  containing  known 
weights  of  copper. 

3.  In  dealing  with  liquids  of  a  very  light  color  a  more  satis- 
factory comparison  can  be  made  by  viewing  the  tubes  vertically 


Fig.  147. 


F 

Fig.  148. 


instead  of  horizontally,  the  depth  of  color     «.• 


being  greatly  increased.  Since  dilution 
has  here  no  direct  effect  on  the  depth  of 
color,  various  arrangements  have  been 
devised  for  altering  the  heights  of  the 
liquids  in  the  tubes  at  will.  A  simple 
plan  is  to  provide  the  comparison  tubes 
with  taps  situated  near  the  bottom,  and 


by  drawing  out  part  of  the  liquids  alternately,  equality 
in  color-depth  is  finally  secured.  Another  apparatus 
is  shown  in  Fig.  147,  the  height  of  liquid  in  the  tube  A 
adjusted  by  raising  and  lowering  a  third  tube  C  connected 
to  A  by  rubber  tubing. 

In  the  Leeds  color-comparator,  f  Fig.  148,  the  flat  bot- 
tomed tubes  C  rest  on  a  shelf  D  over  a  screen  E  pro-, 
vided  with  narrow  longitudinal  slits, one  directly  beneath 
each  tube ;  daylight  is  reflected  from  the  inclined  mirror 
B  down  through  the  tubes  to  a  second  inclined  mirror 
F  on  which  appear  images  of  the  slits  bearing  the  colors 
of  the  respective  tubes.  Or  a  fluid  of  the  same  color  is 
inclosed  in  a  long  thin  glass  wedge  resting  on  E, 

*  Journ.  Amer.  Chem.  Socy.,  1896—269, 
t  Journ.  Arner.  Chem.  Socy.  1896—301. 


Fig.  149. 


264 


QUANTITATIVE    CHEMICAL,    ANALYSTS, 


divided  longitudinally  into  millimeters,  the  shade  transmitted  deepening  uni- 
formly as  the  wedge  is  moved  from  apex  to  base  beneath  a  slit.  The  percentage 
corresponding  to  the  shades  at  a  division  near  each  end  of  the  wedge  is  found 
by  comparison  with  samples  previously  analyzed,  and  the  values  of  inter- 
mediate divisions  interpolated. 

In  most  colorimeters  the  images  of  the  colors  to  be  compared  are  separated 
to  a  greater  or  less  distance,  whereas  a  nuance  is  most  evident  when  the  two 
abut  or  at  most  are  separated  by  a  line  only.  This  condition  is  obtained  in 
some  instruments  by  the  aid  of  mirrors  fixed  at  such  angles  that  will  reflect  one 
image  to  the  side  of  the  other. 

In  the  apparatus  of  Kruess,  the  principle  illustrated  in  Fig.  149,  refracting 
prisms  are  employed  for  the  purpose.  The  liquids  to  be  compared  are  held  in 
two  similar  cylinders  B  and  C,  each  covered  by  a  hard  rubber  plate  perforated 
at  the  center;  light  is  reflected  upward  through  the  cylinders  from  a  white  por- 
celain plate  A.  The  beam  emerging  from  B  enters  a  calcite  prism  D  which  has 
been  cut  diagonally  and  again  cemented  together  at  d  d.  The  beam  is  polar- 
ized, the  ordinary  ray  is  reflected  out  of  the  field  of  vision  by  the  plane  d  d, 
while  the  extraordinary  ray  proceeds  into  the  Nicol  prism  E.  The  light  emerg- 
ing from  the  cylinder  C  meeting  the  prism  F  is  also  polarized,  the  extraordi- 
nary ray  passing  out,  while  the  ordinary  ray  is  reflected  to  d  d  and  thence  to  E. 
Entering  E  then,  side  by  side,  are  the  extraordinary  ray  from  B  and  the 
ordinary  ray  from  C.  The  Nicol  E  Is  so  mounted  that  it  can  be  rotated  on  a 
vertical  axis  and  the  angular  deviation  measured;  as  it  is  turned  the  apparent 
brightness  of  one  ray  increases  and  that  of  the  other  diminishes  until  at  a  cer- 
tain position  a  one  ray  vanishes  the  other  being  at  its  maximum  brightness, 
while  at  90°  from  a  the  reverse  is  true.  Midway  between  these  two  points 
the  fields  are  equally  clear,  whether  the  cylinders  are  empty  or  filled  with  a 
colorless  liquid  or  with  equal  volumes  of  a  colored  liquid. 

But  if  the  liquid  in  one  cylinder  is  of  a  darker  color  than  that  in  the  other,  the 
heights  being  the  same,  the  angle  of  equal  brightness  will  not  be  at  45  °  from 
A  B  a  but  at  another  angle  as  at  <£,  and  if 

the  positions  of  the  cylinders  be 
reversed,  at  <£>'.  Denoting  the  depths 
of  color  or  concentrations  of  the  solu- 
tions as  c  and  c';  then 

c  :  c'  :  :  tan  <J>  :  tan  <j>' 
Proctor's  colorimeter  is  shown  in 
Fig.  150.    A  rectangular  box  open  at 
O  and  facing  a  uniform  light  has  two 
E  partitions  AA  and  BB  each  with  two 

Fig.  150.  orifices.    In  a  line  with  these  orifices 

are  placed  the  glass  cells  C  and  D  containing  respectively  the  solutions  of  the 
sample  and  standard,  or  D  may  be  one  or  more  sheets  of  colored  glass.  Mi 
and  M2  are  two  mirrors  inclined  to  the  longitudinal  axis  of  the  box  at  45  ° , 
reflecting  the  pencils  of  light  emerging  from  the  orifices  of  AA  to  the  eye-piece 
E.  At  the  center  of  M2  the  amalgam  is  removed  leaving  a  star- shape  of  clear 
glass :  through  this  passes  light  reflected  from  Mi.  If  the  depth  of  color  in  C 
is  greater  or  less  than  in  D,  the  eye  will  see  through  E  a  star  darker  or  lighter 
than  the  field;  when  the  depths  are  equal  the  star  disappears. 


/ 
/ 

c 

D 

LJ 


The  color-intensity  of  a  solution  or  translucent  solid  can  be  expressed  by 
the  difference  between  the  number  of  standard  colored  glass  plates  required  to 


COLORIMETRY.  265 

extinguish  a  given  light  and  the  number  required  to  extinguish  the  same  light 
after  emerging  from  a  layer  of  standard  thickness  of  the  solution.  The  opacity 
of  a  solution  due  to  finely  divided  suspended  matter  is  measured  by  the  depth 
of  a  layer  of  the  solution  which  will  conceal  a  white  object. 

Mills'  chromometer,  Fig.  151,  is  a  glass  jar  graduated  into  100  divisions,  and 
is  closed  at  the  top  by  a  metal  cap  which  reaches  to  below  the  surface  of  the 
liquid.  Through  the  cap  slides,  with  easy  friction,  a  glass  rod  B 
supporting  at  the  bottom  a  white  disk  C.  The  jar  is  filled  with  the 
liquid  to  be  tested  and  the  rod  depressed  until  the  disk  is  just  lost  to 
sight,  when  the  depth  of  the  disk  in  the  jar  as  read  on  the  scale  is  the 
measure  of  the  opacity  of  the  liquid.  This  instrument  is  adapted  to 
the  determination  of  suspended  precipitates  or  turbidities;  where 
the  suspended  matter  is  white  or  of  a  light  color,  e.  g.}  the  fat- 
globules  of  milk,  a  black  disk  replaces  the  white  one. 

Hinds  *  measures  the  opacity  produced  by  barium  chloride  in 
very  dilute  solutions  of  sulfates  (e.  g.,  natural  waters),  by  the  extinc- 
tion of  a  ray  of  transmitted  light  by  the  turbid  solution.  The  appa- 
ratus is  simply  a  flat-bottomed  graduated  test-tube  held  above  a 
candle-flame;  the  turbid  liquid  is  poured  in  until  the  image  of  the 
flame  just  disappears,  viewing  the  tube  axially.  He  finds  that  the 
product  of  the  percentage  of  sulfuric  acid  by  the  depth  of  the  liquid 
in  centimeters  is  a  constant,  viz.,  .059,  hence  .059  divided  by  the  Fig.  151. 
depth  of  liquid  equals  the  percentage  of  sulfuric  acid  in  solution.  In  a  similar 
way  calcium  chloride  is  precipitated  by  ammonium  oxalate;  here  the  product 
of  the  calcium  oxalate  by  the  depth  of  liquid  is  not  a  constant  but  of  the  form 
of  a  hyperbolic  series.  For  such  dilute  solutions  the  method  is  claimed  to  be 
as  exact  as  a  volumetric  one. 

A  similar  scheme  is  applied  by  Vogel  for  the  examination  of  albuminous 
urine.  After  acidifying  the  urine  and  boiling  it  to  coagulate  the  albumin,  the 
opacity  is  measured  by  noting  the  thickness  of  a  layer  that  will  cause  the  dis- 
appearance of  the  outline  of  a  candle -flame.  Oliver  modifies  the  test  by  sub- 
stituting for  the  candle- flame  black  lines  of  different  widths  ruled  on  white 
paper. 

A  modification  of  colorimetry,  adapted  to  a  compound  which  dissolves  in 
water  to  a  colorless  solution  but  develops  an  intense  color  on  contact  with  a 
reagent,  is  that  of  tentatively  diluting  the  aqueous  solution  with  water  until  a 
small  definite  volume  withdrawn  and  mixed  with  the  reagent  shows  no  per- 
ceptible color  when  compared  with  water  viewed  under  the  same  conditions. 
For  a  standard,  a  suitable  weight  of  the  pure  compound  is  treated  in  the  same 
manner.  

The  following  examples  may  be  of  interest  as  illustrating  the  great  variety 
of  technical  work  to  which  the  principles  of  colorimetry  can  be  applied.  From 
their  simplicity  and  ease  of  manipulation,  colorimetric  methods  are  regarded 
with  great  favor  by  technical  chemists  for  determinations  that  do  not  call  for 
any  high  degree  of  accuracy. 

A  low -grade  copper  ore  is  quickly  assayed  by  dissolving  a  fixed  weight  of 
the  ore  in  a  certain  volume  of  nitric  acid  held  in  a  test-tube.  The  solution  is 
filtered  and  the  clear  blue  liquid  containing  nitrate  of  copper  is  compared 
against  a  series  made  up  of  different  weights  of  pure  copper  dissolved  in  nitric 
acid.  Some  operators  prefer  to  deepen  the  colors  by  the  addition  of  an  excess 
of  ammonia. 


Journ.  Amer.  Chem.  Socy.  1896—661. 


266  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Steel  manufacturers  follow  the  conversion  of  pig-iron  to  steel  and  grade  the 
products  by  the  aid  of  a  similar  scheme  for  determining  the  carbon  contained. 
When  steel  drillings  are  dissolved  in  dilute  nitric  acid  the  carbon  is  oxi- 
dized to  flocks  of  some  compound  of  unknown  composition  which  on  heating 
dissolve  to  a  greenish-brown  solution.  For  a  standard  is  taken  a  steel  of  about 
the  same  composition  in  which  the  carbon  has  been  determined  gravimetri- 
cally. 

Paper  is  tested  for  wood  fiber  by  moistening  it  with  a  solution  of  dimethyl- 
paraphenylene-diamine  which  has  no  action  on  cotton  or  linen  but  colors  wood 
fiber  red.  As  standards,  papers  containing  known  proportions  of  wood  fiber 
are  stained  by  the  reagent. 

The  hydrogen  sulflde  evolved  by  the  solution  of  a  sulfide  in  a  non-oxidizing 
acid,  when  brought  in  contact  with  a  polished  silver  plate  produces  a  tarnish  of 
silver  sulfide,  which  is  yellow,  brown,  or  blue,  in  proportion  to  the  amount  of 
gas  impinging  on  the  plate ;  or  if  the  gas  be  passed  through  a  cloth  impregnated 
with  a  salt  of  cadmium  there  is  developed  a  more  or  less  deep  yellow  stain. 
This  method  is  applied  to  some  commercial  metals  that  contain  a  minute  amount 
of  sulfide. 

Wurstur  determines  the  "  active  oxygen  "  in  air  (nitrous  acid,  ozone,  etc.)  by 
fastening  a  paper  impregnated  with  tetramethyl-paraphenylene-diamine  over 
the  end  of  a  glass  tube  .6  Mm.  in  bore,  and  drawing  the  air  through  by  suction. 
Active  oxygen  in  limited  amount  causes  the  paper  to  become  blue,  The  color 
is  compared  with  a  scale  of  colors  prepared  by  the  action  of  iodine  solutions 
of  progressive  concentrations  on  the  '  tetra- paper.' 

Pfeiffer  determines  the  oxygen  of  coal-gas  by  passing  the  gas  through  an 
aqueous  solution  of  pyrogallol.  The  standard  is  made  by  acting  on  cane-sugar 
by  hydrochloric  acid  and  adding  the  brown  solution  drop  by  drop  to  distilled 
water. 

To  determine  the  combined  carbon  of  steel,  Peipers,*  paraphrasing  the  well- 
known  touchstone  test  for  gold  alloys,  would  rub  the  steel  on  a  white  unglazed 
porcelain  plate  to  abrade  the  metal,  then  treat  the  streak  with  a  solution  of 
copper  ammonium  chloride  which  dissolves  the  iron  and  manganese  and 
leaves  the  carbon,  the  streak  remaining  more  or  less  black  according  to  the 
percentage  of  carbon  in  the  steel.  For  comparison,  steels  of  known  percent- 
ages of  carbon  are  similarly  treated  beside  the  sample. 

Fritschef  proposes  to  determine  the  suspended  carbonaceous  matter  in  chim- 
ney-gas by  drawing  ten  to  twenty  liters  of  the  gas  through  a  small  glass 
filtering-tube  loosely  packed  with  cellulose.  The  cellulose  is  then  transferred 
to  a  flask  and  shaken  up  with  200  Cc.  of  water  to  a  uniform  gray  pulp.  The 
pulp  is  compared  with  standards  made  by  mixing  .005  to  .030  gram  of  soot 
with  cellulose  and  treating  as  above.  Permanent  standards  can  be  produced 
by  washing  cardboard  disks  with  dilute  india-ink. 

Struve  determines  iodine  in  urine  (coming  from  medicine  exhibited)  by  mix- 
ing with  fuming  nitric  acid  and  carbon  disulflde,  the  former  reagent  liberating 
the  iodine  from  its  combinations,  and  the  latter  dissolving  it  with  the  acquire- 
ment of  a  violet  hue.  The  standards  are  made  up  of  different  weights  of  potas- 
sium iodide  dissolved  in  water  and  treated  with  the  above  reagents. 

Bosanilin  dyes  added  to  red  wine  to  heighten  the  color,  are  extracted  by 
ether  from  the  wine  made  alkaline  by  ammonia.  A.  part  of  the  ethereal  solution 
is  evaporated  in  contact  with  a  thread  of  white  wool  of  a  certain  size.  The 


*  Zelts.  angew.  1895—321  and  466;  Stahl  u.  Elsen  1895—199. 
t  Zeits.  anal.  1898—92. 


COLORIMETRY.  267 

colored  thread  is  compared  with  a  series  of  similar  threads  prepared  from 
alcohol-ether  solutions  of  magenta. 

The  yellow  tone  developed  in  an  alkaline  solution  of  mercuric  iodide  by 
traces  of  free  ammonia  is  a  valuable  method  for  the  determination  of  ammonia 
or  its  salts  when  in  so  dilute  a  solution  that  other  methods  cannot  be  applied. 

The  comparative  money-values  of  several  brands  of  a  dye-stuff  or  dye-ex- 
tract are  approximately  fixed  by  a  colorimetric  determination.  Such  weights 
as  are  inversely  proportional  to  the  vendors'  prices  are  dissolved  in  equal 
volumes  of  a  solvent,  and  it  is  noted  with  what  additional  volume  each  is  to  be 
diluted  that  it  shall  equal  the  lightest  one  taken  as  a  standard. 

Jean  has  proposed  a  method  for  the  approximate  determination  of  tannin  in 
extracts  of  tanning  materials,  on  the  principle  of  the  opacity  of  a  precipitate. 
A  beaker  of  a  certain  diameter  is  placed  over  a  small  disk  of  white  paper  lying  on 
a  black  cloth.  Into  the  beaker  is  poured  a  definite  volume  of  weak  standard  solu- 
tion of  acidified  ferric  chloride.  The  aqueous  extract  of  a  certain  weight  of  bark 
is  then  run  in  slowly  from  a  burette  until  from  the  formation  of  green  or  blue 
iron  tannate,  the  paper  disk  becomes  invisible.  Another  experiment  is  made 
with  a  standard  solution  of  the  purest  tannic  acid  obtainable,  and  from  the 
relation  between  the  two  is  calculated  the  percentage  of  tannin  In  the  extract. 

Blunt  determines  minute  amounts  of  silver  combined  as  nitrate  or  sulfate  by 
dividing  the  solution  into  two  equal  parts;  the  first  (a)  is  treated  with  a  drop 
(an  excess)  of  hydrochloric  acid  and  filtered  from  the  silver  chloride.  The 
filtrate  and  second  portion  (6)  in  equal  sized  beakers  are  placed  before  a  black 
cloth ;  a  drop  of  hydrochloric  acid  is  added  to  (6),  and  to  (a)  small  volumes 
of  a  weak  standard  solution  of  silver  nitrate  until  the  turbidities  are  the  same. 
Double  the  weight  of  silver  in  the  silver  nitrate  added  during  the  titration  is 
the  weight  of  silver  in  the  original  solution.  The  above  procedure  nullifies  the 
effect  of  any  lead,  etc.,  present  in  the  original  solution. 

And  generally,  the  color  of  many  commercial  articles  or  solutions  is  an  indi- 
cation, to  a  greater  or  less  degree,  of  purity  or  the  fitness  of  their  application 
to  some  practical  purpose.  Examples  are  found  in  natural  waters,  beverages, 
edible,  burning  and  lubricating  oils,  animal  products,  sugars,  flours,  etc.,  etc. 

The  color  intensity  of  a  pigment  in  powder  is  compared  with  that  of  an  arbi- 
trary standard  by  intimately  mixing  equal  weights  of  each  with  a  large  propor- 
tion of  a  white  powder  such  as  china-clay.  When  the  tints  are  identical,  the 
weights  of  the  diluent  are  in  direct  proportion  to  the  tinctorial  power  of  the 
pigments.  The  comparison  of  the  standard  and  sample  is  done  by  laying  a 
little  pile  of  each  adjacent  on  a  sheet  of  matt-surfaced  paper,  white  for  the 
darker  pigments,  and  black  for  the  lighter.  The  piles  are  pressed  down  with  a 
spatula,  coming  together  with  a  sharp  line  of  demarcation,  and  any  difference 
can  easily  be  seen. 

Mills  and  Buchanan*  propose  a  photometric  method  for  expressing  or  deter- 
mining the  shades  of  a  color.  They  proceed  by  dyeing  equal-sized  pieces  of 
white  cashmere  in,  baths  of  the  same  dye  of  different  concentrations.  The 
pieces,  now  of  graded  tints,  are  arranged  in  rows  on  a  vertical  surface,  and  the 
whole  covered  with  a  cardboard  perforated  with  circular  holes  of  an  equal 
diameter;  there  is  exposed  to  view  a  circle  of  each  of  the  dyed  pieces. 

Of  this  a  photographic  negative  is  taken,  and  a  print  is  made  on  bromide  of 
silver  photographic  paper.  After  'fixing*  the  latter  to  dissolve  out  all 
unacted-on  silver  bromide,  the  disks  are  cut  out,  each  incinerated  and  the 
silver  in  the  ash  determined  volumetrically.  The  darker  the  shade  of  the 


Jouin.  Socy.  Chem.  Ind.  7—309. 


268  QUANTITATIVE    CHEMICAL    ANALYSIS. 

cashmere,  the  more  silver  (reduced  from  silver  bromide)  is  contained  in  the 
ash  of  the  corresponding  disk.  Since  one  print  would  afford  too  little  silver 
for  an  accurate  determination,  several  prints  are  made  and  corresponding 
disks  burnt  together. 

The  observation  of  the  color  of  a  natural  water  is  made  on  a  column  of 
not  less  than  eight  inches.  For  standards  have  been  proposed  Nesslerized 
ammonia  solution,  dilute  solution  of  platinic  chloride,  Lovibond's  plates, 
etc.  Tidy  recommends  two  hollow  glass  wedges  filled  respectively  with  a 
brown  solution  of  a  mixture  of  ferric  and  cobalt  chlorides,  and  a  blue  solution 
of  cupric  sulfate,  both  of  a  definite  concentration.  On  superimposing  the 
wedges,  a  position  will  be  found  where  the  combination  exactly  matches  in 
color  and  depth  the  water  under  examination.  The  wedges  carry  scales,  each 
division  indicating  the  breadth  in  millimeters  at  that  point.  The  color  of  the 
water  is  reported  to  equal  a  divisions  of  the  brown  solution  plus  b  divisions 
of  the  blue  as  viewed  through  a  twenty-four  inch  tube.  Various  other  iso- 
chromes  have  been  proposed. 


THE    FIRE    ASSAY.  2G9 


THE  FIRE  ASSAY. 

Pyro-chemical  methods  are  available  for  metalliferous  ores,  slags  and  mattes 
and  some  alloys,  extracting  the  valuable  constituent  and  leaving  it  in  the 
metallic  state  in  a  pure  or  nearly  pure  condition  and  ready  for  weighing.  They 
are  applied  to  ores  of  gold,  silver,  platinum  and  allied  metals,  copper,  tin, 
lead  andiron,  and  to  bullion,  copper  and  lead  mattes  and  speisses. 

GOLD   AND    SILVER. 

It  is  possible  to  determine  the  percentage  of  the  precious  metals  in  their 
ores  by  the  usual  gravimetric  methods,  but  from  the  fact  that  ordinarily  the 
metals  or  their  mineral  compounds  are  disseminated  through  a  silicious  or 
earthy  gangue  in  extremely  minute  proportions,  an  inconveniently  large  weight 
of  the  ore  would  have  to  be  operated  on  to  extract  even  a  weighable  quantity 
of  the  metals,  so  that  the  fire-assay  is  found  advantageous  in  the  way  of 
accuracy,  economy,  and  time  consumed. 

The  process  comprises  five  stages. 

1.  Roasting  —  heating  in  a  current  of  air  to  expel  or  oxidize  certain  elements 
in  the  gangue  of  the  ore. 

2.  Crucible  fusion  or  Scoriflcation  —  collecting  all  the  gold  and  silver  into  an 
alloy  with  metallic  lead.    The  character  and  richness  of  the  ore  under  consid- 
eration decides  which  of  the  two  processes  will  be  the  most  suitable  for  its  assay. 

3.  Cupellation  —  oxidizing  the  lead  of  the  alloy  and  absorbing  the  oxide  in  a 
cup  of  porous  earth  leaving  a  practically  pure  alloy  of  gold  and  silver. 

4.  Quartation  —  diluting  the  alloy  from  (3)  with  silver  to  assist  in 

5.  Parting  —  dissolving  the  silver  in  nitric  or  sulfuric  acid  leaving  the  gold. 
The  roasting  and  quartation  are  often  unnecessary  and  omitted. 

On  pulverizing  some  ores,  particles  of  native  gold  or  silver  or  their  malleable 
minerals  are  flattened  into  disks.  These  are  retained  on  the  finer  sieves  and 
must  be  assayed  separately,  since  they  cannot  be  united  with  the  powder  with 
any  assurance  that  the  mixture  is  so  homogeneous  that  it  can  safely  be  further 
subdivided  down  to  the  weight  taken  for  the  assay. 

Crucible  Fusion. 

For  the  crucible  process  is  weighed  from  one-fifth  to  two  or  more  e  assay- 
tons'  (page  41)  of  the  powdered  ore,  the  amount  depending  on  the  supposed 
content  of  gold  or  silver.  The  weighed  amount  is  heated 
for  some  time  to  dull  redness  (roasted)  with  free  access  of 
air  until  no  more  fumes  escape.  The  roasting  is  done  in  a 
shallow  iron  pan  or  clay  dish  coated  with  iron  oxide  to  pre- 
vent the  ore  sticking  to  the  dish.  The  roasted  ore  is  mixed 
with  various  fluxes  and  carbon,  and  the  whole  turned  into  a 
large  crucible  made  of  fire-clay.  Fig.  152.  Over  the  surface 
of  the  charge  is  spread  a  layer  of  common  salt,  and  the 
covered  crucible  is  heated  in  a  coke  or  gas  furnace  to  a 
bright  red  heat  until  the  charge  and  salt  have  come  to  a 
state  of  tranquil  fusion.  The  crucible  is  allowed  to  cool 
until  the  melt  has  solidified,  then  broken  and  the  small 
spheroid  of  lead  containing  the  gold  and  silver  of  the  ore 
freed  from  adhering  slag  and  hammered  into  a  cube  for  the 
process  of  cupellation.  The  reactions  are  Fig.  152. 

1.  The  roasting  oxidizes  or  volatilizes  elements  like  sulfur 
and  arsenic  which  would  interfere  in  the  subsequent  fusion.    Powdered  char- 


270  QUANTITATIVE    CHEMICAL    ANALYSIS. 

coal  is  often  mixed  with  the  ore  to  transiently  reduce  some  of  the  compounds' 
of  the  volatile  metals,  which  immediately  afterward  burn  to  oxides,  the  greater 
part  passing  off  as  fume.  It  is  unnecessary  to  roast  ores  free  from  reducing 
elements  or  carbonaceous  matter. 

2.  The  general  run  of  ores  are  made  up  of  one  or  more  of  the  commonly 
occurring  minerals  —  quartz,  silicates,  calc-spar,  dolomite,  and  the  like,  with 
more  or  less  pyrite,  galena,  iron  oxides,  etc.,  and  as  a  rule  are  infusible  at 
ordinary  furnace  temperatures.    But  the  silicates  of  lead  and  alkalies,  and  the 
double  silicates  of  these  and  other  bases  (alumina,  lime,  magnesia,  etc.)  are 
easily  melted.    So  the  first  step  in  an  assay  is  to  ascertain  the  approximate 
composition  ol  the  ore  in  hand,  either  by  a  qualitative  examination  or  simply 
by  inspection  of  the  lumps;  then  to  make  up  the  charge  of  ore  and  fluxes  of 
such  a  composition  as  will  be  fusible  at  redness  to  a  mobile  slag.    If  the 
gangue  be  principally   quartz  or  the  more  silicious  silicates,  th£  fluxes  are 
litharge  (lead  protoxide),  sodium  carbonate,  and  borax;  if  of  the  more  basic 
silicates,  some  powdered  silica  is  added  to  the  above;  while  if  decidedly  basic 
in   character    (dolomite,  hematite,  manganese    superoxides,  etc.)  the  flux  is 
largely  made  up  of  silica  and  litharge.    The  litharge  should  be  free  from  gold, 
and  the  minute  amount  of  silver  present  determined  by  a  previous  assay —  it  is 
seldom  found  entirely  free  from  this  element. 

The  silver  in  the  ore  is  sometimes  free  (native  silver)  more  often  as 
chloride,  sulfide,  etc.,  but  whatever  the  combination,  metallic  silver  is  pro- 
duced either  by  reaction  with  an  oxidizer  (litharge),  or  a  reducer  (sodium 
carbonate  or  metallic  lead)  or  by  dissociation  by  heat  alone.  Gold  is  nearly 
always  present  in  an  ore  in  the  elementary  state,  free  or  alloyed  with  silver, 
tellurium,  or  other  metal. 

3.  Through  the  reaction  with  powdered  carbon  or  a  proportional  weight  of 
an  organic  compound,  an  equivalent  of  lead  oxide  is  reduced  to  the  metal,  the 
lead  being  generated  throughout  the  charge  in  the  form  of  minute  granules. 
The  reaction  between  the  silica  of  the  ore  and  the  sodium  carbonate  of  the  flux 
evolves  carbon  dioxide,  which,  as  it  escapes,  causes  a  violent  and  protracted 
boiling  of  the  semi-fluid  mass.    The  particles  of  lead,  gold,  and  silver  are 
thereby  brought  into  contact  and  readily  alloy.    Finally  the  effervescence  ceases, 
the  slag  becomes  a  quiet  mobile  fluid,  and  the  alloy  sinks  to  the  bottom  of  the 
crucible  where  the  drops  coalesce  to  one  globule  beneath  the  lighter  slag. 

From  the  equation  2PbO  +  C  =  2Pb  -J-  CO2,  it  is  easy  to  calculate  what  weight 
of  carbon  is  to  be  admixed  in  the  charge  to  give  a  convenient  weight  of  lead 
(about  15  grams)  for  the  subsequent  cupellation.  As  many  unoxidized  elements 
that  may  be  present  in  the  ore  will  also  reduce  lead  oxide,  it  is  assumed  that 
the  ore  contains  none  of  these,  or  that  they  have  been  driven  off  or  oxidized 
during  the  previous  roasting.  Some  assayers  invariably  dispense  with  roast- 
ing, and  instead  make  a  preliminary  test  of  the  reducing  power  of  the  ore 
itself  by  melting  an  assay  ton  with  an  excess  of  litharge  and  the  other  fluxes. 
The  weight  of  lead  produced  is  subtracted  from  fifteen  grams  and  enough  car- 
bon added  in  the  assay  to  make  up  the  difference ;  but  if  the  ore  should  be  so 
rich  in  reducing  elements  that  the  preliminary  test  gives  more  than  fifteen 
grams  of  lead,  carbon  is  omitted  from  the  charge  for  the  assay,  and  such  a 
weight  of  niter  is  substituted  as  will  reoxidize  the  excess  of  lead.  Metallic 
iron,  in  the  form  of  large  nails,  is  often  introduced  in  the  charge,  it  combining 
with  refractory  pyrite  to  form  easily  fusible  ferrous  sulflde  —  FeS2  +  Fe  =  2FeS. 

In  making  up  a  charge,  some  assayers  prefer  to  omit  the  oxidizer  entirely  and 
diminish  the  quantity  of  litharge  to  such  an  amount  as  will  yield  on  reduction 
the  proper  weight  of  lead  for  cupellation,  but  as  there  is  always  left  undecom- 
posed  more  or  less  of  the  minerals  of  a  reducing  tendency  —  sulfides,  arsenides, 


THE    FIRE    ASSAY.    .  271 

etc.,  —  there  is  room  for  suspicion  that  some  of  the  gold  and  silver  may  be 
retained  therein. 

The  layer  of  common  salt  melts  and  floats  above  the  slag.  It  has  no  chemical 
functions,  serving  only  to  protect  the  charge  against  reducing  gases  from  the 
furnace  and  to  wash  down  any  slag  thrown  up  against  the  sides  of  the  crucible. 
The  following  is  a  well-tried  general  formula  for  ores  that  either  contain  no  re- 
ducing constituents  or  have  been  previously  roasted.  The  proportion  of  silica  is 
to  be  varied  according  to  the  acid  or  basic  nature  of  the  ore  under  examination. 

Ore One  assay-ton  (29.167  grams). 

Litharge 60  grams. 

Sodium  carbonate 30  grams. 

Powdered  silica . .  20  grams. 

Anhydrous  borax 10  grams. 

Pulverized  sugar 1  gram. 

Salt  to  cover. 

Where  it  is  known  that  a  gold  ore  contains  little  or  no  silver  it  is  the  custom 
to  add  to  the  charge  a  small  amount  of  metallic  silver  in  the  shape  of  foil,  the 
object  being  to  assist  in  collecting  the  gold  during  fusion  and  dispense  with  the 
operation  of  quartation. 

Scoriflcation. 

In  this  process  the  unroasted  ore  is  mixed  with  metallic  lead  and  a  trifle  of 
borax  and  strongly  heated  for  some  time  with  free  access  of  air.  The  ore  floating 

on  the  surface  of  the  melted  lead  is  roasted  to  some 
extent,  while  the  lead  is  continually  being  oxidized 
by  the  air  passing  over  it.  A  part  of  the  lead 
oxide  transfers  its  oxygen  to  any  antimony,  arse- 
nic, copper,  etc.,  that  may  be  in  the  ore,  and  these 
oxides,  together  with  the  borax  and  the  silica 
and  bases  of  the  gangue  combine  with  the  re- 
mainder of  the  fluid  lead  oxide  or  dissolve  in  it, 
Fig.  153.  V  -1/  forming  a  complex,  easily  fusible  slag  of  silico- 

borates  of  the  various  bases,  while  the  gold  and 

silver  alloy  with  the  unoxidized  lead.  Finally  when  the  major  portion  of  the 
lead  has  been  converted  to  oxide,  what  remains  is  detached  from  the  slag  for 
cupellation,  or  a  rescorification  if  necessary. 

The  scorification  is  done  in  a  small  dish  of  baked  fire- clay,  Fig.  153,  termed 
a  "scarifier  ",  and  as  the  success  of  the  operation  depends  primarily  on  an 
ample  supply  of  air  to  oxidize  the  lead,  it  is 
heated  by  radiation  from  the  walls  of  an 
incandescent  muffle,  Fig.  154;  this  is  a  thin 
semi-cylinder  of  hard-burned  fire-clay,  open  in 
front,  but  closed  at  the  rear  where  a  narrow 
slit  or  small  hole  provides  a  vent  for  the  fumes  Fig.  154.  1/5  -  l/l& 

arising  from  the  scorifier  and  creates  a  cur- 
rent of  air  through  the  muffle.  It  is  supported  horizontally  in  a  special 
muffle-furnace,  Fig.  155,  made  of  clay  or  iron  lined  with  fire-brick,  and  is 
surrounded  with  burning  fuel,  usually  coke.  For  occasional  assays,  a  smaller 
furnace  heated  by  a  gas  blowpipe  or  gasoline  burner  will  be  found  cleaner 
and  more  economical. 

The  charge  varies  with  the  character  of  the  gangue  and  is  made  up  of  from 
one-tenth  to  one-half  an  assay-ton  of  ore,  five  to  fifteen  times  its  weight  of 
granulated  lead  (test-lead)  and  one -tenth  to  five-tenths  gram  of  borax  glass 
(anhydrous  sodium  biborate).  Half  of  the  lead  is  strewn  over  the  bottom  of 
the  scorifler  and  covered  with  a  mixture  of  the  ore,  borax,  and  the  remainder 


272 


QUANTITATIVE    CHEMICAL    ANALYSIS, 


of  the  lead.     The  muffle  being  at  a  white  heat,  the  scorifler  is  gradually  moved 
back  to  the  hottest  part. 

The  lead  melts  at  once  to  a  globule  with  a  convex  surface;  the  air  covers  it 
with  a  film  of  oxide  which  immediately  floats  to  the  edge,  again  exposing  a 
bright  surface.  In  this  way  the  oxi- 
dation goes  on  continuously  until  the 
size  of  the  globule  has  been  so  far 
reduced  that  it  disappears  beneath 
the  accumulated  slag.  This,  of 
oourse,  terminates  the  operation. 
The  remaining  lead  and  slag  are 
poured  out  into  a  hemispherical  or 
conical  depression  in  a  sheet  of  cop- 
per, Fig.  156,  the  lead  sinking  to  the 
bottom.  When  cold  the  two  are 
broken  apart,  and  if  the  scoriflcation 


<=S^--;:^^^^ — ^. 


Fig.  155.     1/20 

has  been  successful  the  lead-button  is  malleable,  and  the  slag  homogeneous  and 
free  from  undecomposed  particles  of  ore .  Frequently,  however,  the  buttons  from 
this  or  the  crucible  fusion  are  too  large  for  cupellation,  or  they  may  be  brittle  from 
the  presence  of  copper,  sulfur,  etc.,  and  must  be  rescorifled  one  or  more  times. 
Of  the  two,  the  crucible  process  is  best  adapted  for  low-grade  ores  and 
tellurides,  and  those  reasonably  free  from  arsenic,  antimony,  zinc,  etc.  An 

objectionable  feature  of  the  cru- 
cible fusion  is  that  the  slag  from 
some  ores  may  be  so  viscid  that 
particles  of  lead  holding  silver 
or  gold  may  remain  scattered 
through  it,  to  be  recovered  only 
Fig.  156.  /4  by  a  second  fusion.  It  is  seldom 

that  a  scoriflcation-slag  is  too  viscid,  and  if  so  can  usually  be  thinned 
by  proper  additions.  On  the  other  hand,  the  small  weight  of  ore  that 
may  be  treated  in  a  scorifier  of  moderate  size  (not  over  one-half  an  assay  ton) 
limits  the  scoriflcation  process  to  the  richer  ores,  though  of  course  buttons 
from  several  scoriflcation s  can  be  united  for  cupellation. 

From  an  ore  containing  much  copper  the  lead  button  is  found  alloyed  with 
metallic  copper  and  cannot  be  directly  cupelled.  The  copper  can  be  oxidized 
and  slagged  off  by  repeated  scoriflcations  with  lead.  For  cupriferous  ores  and 
auriferous  copper  mattes,  a  recent  process*  that  combines  the  wet  and  dry 
methods  has  come  into  use.  The  sample  is  treated  with  moderately  concen- 
trated nitric  acid,  decomposing  the  sulfldes,  arsenides,  etc.,  and  dissolving  all 
the  metals  except  gold,  tin,  and  antimony.  A  strong  solution  of  lead  acetate 
is  added,  followed  by  enough  sulfuric  acid  to  throw  down  a  considerable  pre- 
cipitate of  lead  sulfate;  this  in  falling  envelops  and  carries  down  all  the 
suspended  gold,  and  on  filtering,  there  is  left  in  the  paper  a  mixture  of  lead 
sulfate,  insoluble  silicates  and  silica,  and  any  insoluble  compounds  of  silver, 
but  practically  free  from  copper,  arsenic,  etc.,  and  which  can  be  scorified  with 
ease.  The  filtrate  is  compounded  with  sodium  bromide  and  sulfuric  acid,  pre- 
cipitating the  silver  in  solution  and  part  of  the  lead  as  bromides,  and  the  re- 
mainder of  the  lead  as  sulfate.  The  precipitate,  also  free  from  copper,  etc., 
is  filtered  and  scorified. 

Cupellation. 

This  process  is  based  on  the  resistance  of  the  precious  metals  to  oxidation 
by  the  air,  even  at  high  temperatures,  and  the  ready  oxidation  of  other  metals. 


*  Journ.  Anal.  Appl.  Chem.  1892—262. 


THE    FIRE    ASSAY.  273 

The  cupel,  Fig.  157,  is  a  small  cup  made  by  compressing  moist  coarsely  pow- 
dered bone  ashes  (calcium  phosphate  and  carbonate)  in  a  brass  mold;  after 
drying,  it  has  a  porous  granular  texture  and 
absorbs  liquids  with  great  facility.  It  is 
heated  in  the  muffle  to  bright  redness  to 
expel  moisture  and  combined  water  and  the 
clean  lead  button  dropped  in.  The  button 
melts  and  "clears  "  of  the  scum  of  oxide, 
after  which  the  oxidation  proceeds  rapidly, 
most  of  the  lead  oxide  being  absorbed  in  the 
cupel,  the  remainder  volatilizing.  When  all 
but  a  trace  of  lead  is  gone,  the  spheroid  of 
gold  and  silver  iridesces  (u  brightens  ",  "  ful- 
gurates ",  or  "  blicks  ")  from  the  refraction  of 
light  by  the  thin  film  of  lead  oxide  envelopingit.  Fig.  157. 

Shortly  after  the  iridescence  the  button  solidifies,  now  practically  free  from  lead. 

Although  it  is  presumed  that  silver  is  not  oxidized  in  this  process  nor  vola- 
tilized at  the  moderate  temperature  of  cupellatiou,  nevertheless  so  much  may 
pass  off  as  vapor  or  react  with  lead  oxide  and  be  absorbed  in  the  cupel  that  a 
perceptible  loss  will  be  incurred.  The  minimum  loss  is  sustained  when  the 
cupel  is  at  such  a  heat  that  lead  oxide  fumes  are  just  visible  as  they  rise,  so  the 
assayer  endeavors  to  maintain  this  temperature  up  to  the  point  of  iridescence, 
when  it  is  raised  to  favor  the  removal  of  the  last  traces  of  lead.  Tables  of 
corrections  for  this  loss  may  be  found  in  works  on  assaying. 

Alloys  of  gold,  silver  and  copper,  such  as  coins,  bullion  or  jewelry,  are 
wrapped  in  thin  sheet  lead  and  cupelled  directly  unless  they  contain  too  great 
a  proportion  of  copper.  A  specific  correction  for  cupellation  loss  is  found 
by  cupelling,  side  by  side,  the  alloy  and  an  equal  weight  of  a  •'  proof".  The 
proof  is  made  up  from  chemically  pure  gold,  silver  and  copper  in  the  same  rel- 
ative proportions  as  compose  the  alloy,  this  having  been  ascertained  by  a  pre- 
liminary assay  or  volumetric  determination  of  the  silver  and  copper. 

Quartation. 

After  weighing  the  button  on  an  assay  balance,  the  two  metals  are  to  be 
separated  by  dissolving  the  silver  in  an  acid  which  will  not  affect  the  gold. 
The  separation  will  not  be  complete  unless  the  former  predominates,  so  if  a 
yellow  tinge  is  perceptible  in  the  alloy,  there  is  incorporated  with  it  by  fusion 
on  charcoal  before  the  blowpipe,  enough  pure  silver  that  the  relative  weights 
shall  be  at  least  two  of  silver  to  one  of  gold.  The  alloy  is  then  rolled  or  ham- 
mered into  foil  and  coiled  into  a  spiral,  and  is  ready  for  the  operation  of  parting. 

Parting. 

The  "cornet"  is  heated  in  a  small  flask  with  dilute  nitric  acid,  the  solution 
of  silver  nitrate  poured  off,  and  the  residual  gold  washed  several  times  with 
water  by  decantation.  Finally  the  flask  is  filled  with  water,  covered  with  a 
bisque  clay  dish,  and  inverted;  when  the  gold  has  fallen  through  the  water  into 
the  dish,  the  flask  is  cautiously  removed.  After  the  water  has  been  absorbed 
by  the  clay,  the  gold  is  transferred  to  a  platinum  capsule,  heated  to  redness 
and  weighed.  If  the  ratio  of  silver  to  gold  has  not  been  in  excess  of  three  or 
four  parts  to  one,  the  gold  is  left  in  the  form  of  a  spongy  mass  with  some  co- 
herence, but  with  a  greater  ratio,  as  a  black  powder.  The  washing  and  trans- 
ference of  the  latter  is  more  difficult  to  perform  without  mechanical  loss.  On 
heating  to  near  redness,  the  black  allotropic  modification  is  transformed  to  the 
familiar  yellow  of  the  massive  state,  acquiring  also  a  considerable  degree  of 
cohesion. 


274  QUANTITATIVE    CHEMICAL,    ANALYSIS. 

Volmetrlc  methods. 

The  volumetric  method  of  precipitation  by  sodium  chloride  is  now  universally 
adopted  for  the  determination  of  silver  in  alloys  on  account  of  the  greater 
accuracy  and  convenience  as  compared  with  the  fire  assay;  the  reaction  is 
AgNO3-J-NaCl  =  AgCl  +  NaNO3.  Other  reagents,  such  as  sodium  bromide, 
and  barium  chloride  with  zinc  sulphate,  have  been  proposed  and  certain  ad- 
vantages claimed,  but  sodium  chloride  still  remains  in  common  use. 

The  salt  solutions  are  of  two  strengths  —  the  standard  (misnamed  '  normal') 
containing  5.4207  grams  of  NaCl  per  liter  of  water  at  15  ° ,  and  a  weaker  one  of 
one- tenth  this  concentration.  Exactly  one  gram  of  silver  is  precipitated  by 
100  Cc.  of  the  former,  and  1  milligram  by  1  Cc.  of  the  latter.  They  are 
prepared  by  dissolving  the  above  weight  of  salt  in  water  and  making 
up  to  one  liter,  then  withdrawing  10  Cc.  and  diluting  to  100  Cc. 
To  accurately  standardize  them,  one  gram  plus  a  few  milligrams  of  fine 
silver  is  weighed  and  dissolved  in  nitric  acid  in  a  small  flask  and  the  solu- 
tion cooled  and  diluted  with  water.  One  hundred  Cc.  of  the  stronger  salt 
solution  is  run  in  from  a  pipette  and  the  flask  shaken  until  the  precipitate  has 
clotted,  leaving  the  liquid  clear.  The  small  amount  of  silver  remaining  unpre- 
cipitated  is  determined  by  dropping  in  the  decimal  salt  solution,  shaking  after 
each  addition,  until  finally  no  opalescence  is  produced;  here  much  is  left  to 
the  expertness  of  the  assayer  in  deciding  the  point  where  precipitation  ceases. 

From  the  volumes  required  for  the  test  there  can  be  calculated  to  what 
extent  the  stronger  solution  is  to  be  fortified  or  weakened  to  be  exactly  stand- 
ard ;  when  this  has  been  done,  a  portion  is  diluted  with  nine  volumes  of  water 
to  form  the  decimal  solution.  But  since  the  strengths  of  the  solutions  vary 
slightly  from  day  to  day,  from  changes  in  temperature,  evaporation,  etc.,  it  is 
customary  to  leave  the  concentrations  unchanged  and  correct  in  the  calculation 
for  the  variation  from  the  strict  standard. 

In  examining  an  alloy  containing  silver,  a  preliminary  assay  is  made  either 
by  cupelling  with  lead,  or  by  titrating  a  nitric  solution  by  standard  potassium 
sulfocyanide  with  ferric  sulfate  as  indicator  (AgNOs  4-  KCNS  =s  AgCNS  -f- 
KNO3,  and  Fe2(SO4)3  -f  6KCNS  ==:  Fe2  (CNS)6  -f  3K2S04).  Calculating  from 
this  datum,  a  weight  of  the  alloy  which  will  contain  a  few  milligrams  over  one 
gram  of  silver  is  dissolved  in  dilute  nitric  acid  and  titrated  as  above  by  the 
standard  and  decimal  solutions.  Access  of  actinic  light,  which  would  reduce 
the  silver  chloride  to  subchloride,  may  be  prevented  by  wrapping  the  flasks  in. 
black  cloth  or  inserting  them  in  pasteboard  boxes,  or  by  glazing  the  windows 
of  the  assay  room  with  orange  or  red  panes.  No  other  metals  present  in  alloys 
will  interfere  with  the  titration  except  mercury,  and  this  is  easily  expelled  by 
a  previous  fusion  of  the  alloy  or  otherwise. 

For  the  determination  of  silver  in  alloys  where  gold  or  platinum  predomi- 
nates, .500  gram  is  heated  in  a  porcelain  crucible  with  potassium  cyanide  and 
three  grams  of  pure  cadmium.  When  the  metals  have  melted  to  an  alloy  the 
fusion  is  cooled,  and  the  button,  freed  from  adhering  cyanide,  is  treated  with 
dilute  nitric  acid ;  the  silver,  copper,  and  cadmium  dissolve  leaving  the  gold 
and  platinum.  The  silver  in  solution  is  titrated  by  salt  as  above. 

For  the  assay  of  gold  bullion,  samples  are  cut  from  the  top  and  bottom  of 
the  ingot  or  bar.  To  a  weight  of  .500  gram  is  added  enough  pure  silver  to 
make  a  ratio  of  two  of  silver  to  one  of  gold,  and  if  no  copper  be  already  con- 
tained, a  weight  of  .050  gram  of  pure  copper  which  has  the  effect  of  toughen- 
ing the  silver  button  and  insuring  smooth  edges  on  the  cornet;  the  whole  is 
wrapped  up  in  a  small  sheet  of  lead.  At  the  same  time  a  (  proof '  or  *  witness  * 
is  made  up  from  pure  gold,  silver,  copper,  and  any  other  metals  contained  in 
the  bullion,  as  nearly  identical  with  it  in  proportions  as  possible,  and  a  sheet 
of  lead.  The  two  are  cupelled  side  by  side  in  a  hot  muflle. 


THE   FIKE   ASSAY.  275 

The  buttons  of  alloyed  gold,  silver  and  copper  are  flattened  under  a  hammer, 
annealed,  rolled  to  foil,  again  annealed,  and  coiled  into  a  cornet.  The  parting 
acid  is  nitric  of  about  1.27  sp.  gr.  in  which  the  cornet  is  boiled  for  ten  min- 
utes; the  container  may  be  a  test  tube,  porcelain  crucible,  or  platinum 
cup.  The  boiling  with  acid  is  repeated  to  remove  the  last  traces  of  silver 
and  copper,  and  the  residual  gold  washed,  dried,  annealed  and  weighed. 
Usually  the  weight  of  the  gold  from  the  proof  is  slightly  higher,  sometimes 
lower,  than  the  original  weight,  and  the  assay  is  corrected  accordingly. 

Assay  by  the  Blowpipe  —  Pyritology. 

Assays  accurate  enough  for  prospecting  and  the  exploitation  of  mines  may  be 
obtained  by  means  of  the  mouth  blowpipe.    The  process  is  essentially  a  repro- 
duction in  miniature  of  the  furnace  scoriflcation  and  cupellation. 
The  charges  for  the  blowpipe  assay  are  made  up  about  as  follows. 

A.       B.       C.       D.        E.        F. 

Ore  (in  fine  powder) 100     .100     .100    .100    .100    .100 

Borax-glass 050    .050    .050    .050    .050    .100 

Sodium  carbonate  (anhydrous) 050    

Sulfur 050    .... 

Lead  (in  powder) 500  1.0001.500  1.000  1.000  1.000 

A  is  suitable  for  pure  galenas  whose  gangue  is  not  highly  acid  or  basic;  B, 
for  general  ore  mixtures  not  highly  acid  or  basic;  C,  for  refractory  sulfldes, 
arsenopyrite,  pyrite,  copper  sulfides,  and  ores  containing  much  copper;  D, 
for  highly  siliceous  ores;  E,  for  ores  containing  much  iron;  and  F,  for  highly 
acid  or  basic  ores. 

1.  Melting  down.  The  charge  is  poured  into  a  cylinder  made  by  rolling  a  slip 
of  paper  around  a  pencil;  the  paper  may  be  impregnated  by  sodium  carbonate 
for  the  purpose  of  delaying  combustion  until  the  charge  has  melted  down. 
The  cylinder  is  put  into  a  deep  capsule  made  of  carbon,  the  capsule  resting  in 
the  end  of  a  light  holder.    The  paper  is  burned  in  the  oxidizing  flame  of  the 
blowpipe,  then  the  charge  heated  in  the  reducing  flame  until  the  slag  has  be- 
come homogeneous  and  the  lead  has  collected  to  one  mass.    The  silver  sulflde, 
sulfarsenide,  and  chloride  that  may  be  in  the  ore,  here  react  with  lead  forming 
e.  g,t  lead  sulflde  and  chloride,  and  arsenic  sulflde,  the  silver  alloying  with  lead. 

2.  The  fusion  is  allowed  to  cool  and  the  lead  button  detached.     If  the  lead 
is  dark  in  color  or  crystalline,  it  is  to  be  refined  by  melting  with  borax  in  the 
reducing  flame,  then  heating  in  the  oxidizing  flame  to  remove,  in  great  measure, 
the  sulfur  and  antimony. 

3.  The  lead  button  is  cleaned  from  borax  and  scorified  in  a  thin  clay  cup  in 
the  oxidizing  flame.    The  oxidation  is  carried  so  far  that  only  a  small  lead 
button  remains. 

4.  A  cupel  is  prepared  by  ramming  finely  powdered  bone  ash  into  a  depression 
in  an  iron  disk.    The  button  is  heated  so  that  all  the  litharge  is  absorbed  in  the 
bone  ash,  until  the  brightening  takes  place  or  it  no  longer  reduces  in  size.* 

The  button  of  silver  finally  obtained  is  too  small  for  weighing,  even  on  the 
assay  balance,  so  that  its  weight  is  deduced  from  its  macro -diameter,  it  having 
been  proved  that  the  volume  of  the  oblate  spheroids  of  gold  and  silver  of  this 
minute  size  bear  practically  constant  ratios  to  the  volumes  of  spheres  of  the 
same  transverse  diameter.  The  measuring  is  done  on  an  ivory  plate  on  which 
have  been  drawn  two  fine  lines  AB  and  AC  diverging  at  such  an  angle  that  at  a 
distance  from  A  of  six  inches,  B  and  C  are  exactly  .04  inch  apart.  From  A  to 
BC  is  divided  into  100  equal  parts,  the  divisions  marked  with  the  corresponding 
weights  of  buttons.  The  button  to  be  measured  is  moved  up  and  down  between 
the  lines  until  at  some  one  division  they  are  tangential  to  its  circumference. f 

*  Crookes,  Select  Methods,  355. 

\  Chcm.  News,  15-281;  Journ.  Amer.  Chem.  Socy.  1901—203. 


27G  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Gold  ores  may  be  tested  along  the  same  lines,  but  unless  very  rich,  must  first 
be  concentrated  by  washing  away  part  of  the  gangue. 

The  blowpipe  assay  has  also  been  proposed  for  ores  of  some  of  the  common 
metals.* 

ORBS   OP   THE  BASE   METALS. 

The  principle  of  the  assay  of  the  ores  of  lead,  tin,  iron,  etc.,  is  simply  that  of 
reducing  the  metallic  compound  by  melting  the  ore  in  a  clay  crucible  with  some 
reducing  agent  and  a  suitable  flux,  including  a  desulfurizer  if  the  ore  be  a 
sulfide,  and  weighing  the  more  or  less  pure  metallic  button  produced.  It  is 
assumed,  of  course,  that  no  other  reducible  metallic  compound  than  the  one 
sought  is  present  in  a  weighable  amount.  The  results  are  generally  too  low 
owing  to  volatilization  of  the  reduced  metal  or  its  retention  in  the  slag,  though 
the  impurities  in  the  button  may  offset  these  losses  to  some  extent. 

The  so-called  e  dry  '  methods  for  the  base  metals  are  still  given  the  preference 
by  some  of  the  older  metallurgists,  one  reason  assigned  being  that  as  the 
methods  follow  to  some  extent  the  practice  of  smelting  and  refining,  the  results 
will  indicate  the  returns  that  may  be  expected  from  the  latter  when  carried  on 
under  favorable  circumstances.  The  fallacy  of  this  position  appears  when  it  is 
considered  that  the  losses  in  the  ordinary  fire-assay  are  never  so  constant  as  in 
a  well  regulated  metallurgical  process,  and  there  is  not  afforded  the  opportun- 
ity for  comparing  the  merits  of  one  practice  against  another,  or  of  variations  in 
the  detail  of  the  routine  of  any  one.  And  so  while  the  time-honored  principles 
of  the  fire-assay  are  yet  almost  universally  retained  for  the  ores  of  gold  and 
silver  in  default  of  better  processes,  for  other  metals,  with  few  exceptions,  the 
*  wet '  gravimetric  and  volumetric  methods  have  supplanted  the  *  dry  »  methods 
until  the  latter  possess  little  more  than  a  historical  interest. 

For  the  assay  of  lead,  antimony,  bismuth  or  tin  f  ores,  potassium  cyanide,  at 
once  a  reducer,  desulfurizer,  and  flux  for  the  gangue,  is  the  most  satisfactory 
reagent;  others  are  mixtures  of  sodium  carbonate,  borax,  potassium  bitartrate, 
flour,  etc.,  with  the  addition  of  iron  wire  where  the  ore  is  a  sulfide,  and  of 
hematite  or  cryolite  to  fix  the  silica  of  a  tin  ore.  A  layer  of  salt  covers  the 
charge.  The  closed  crucible  is  rapidly  heated  to  above  the  fusing  point  of  the 
metal  and  the  heat  maintained  until  the  reduction  is  supposed  to  be  complete 
and  the  slag  become  fluid  and  homogeneous.  The  contents  of  the  crucible  are 
then  poured  out  into  a  mold,  or  the  crucible  is  cooled  and  broken  open,  and 
the  metallic  button  cleaned  from  slag  and  weighed. 

To  ores  of  iron  are  added  silica,  kaolin,  or  calcite  according  to  the  composi  - 
tion  and  acid  or  basic  nature  of  the  accompanying  minerals.  The  viscous 
slag  yielded  by  titaniferous  ores  is  best  thinned  by  fluorite.  The  charge  is 
melted  in  a  clay  crucible  thickly  lined  with  a  carbonaceous  material  (brasque) 
to  remove  the  oxygen  from  the  iron  oxides.  After  luting  on  the  crucible  cover, 
the  crucible  is  exposed  to  the  highest  heat  of  the  furnace.  The  resulting  but- 
ton has  approximately  the  composition  of  cast  iron,  and  its  hardness,  tenacity, 
and  the  color  and  appearance  of  its  surface  and  fracture,  influenced  by  the 
various  impurities  present,  are  believed  to  foretell  the  quality  of  pig  iron  that 
the  ore  would  yield  if  smelted  in  a  blast  furnace. 

Copper  ores  (mainly  sulfides,  sulfarsenides  and  carbonates)  are  first  roasted 
with  charcoal  to  burn  out  sulfur  and  arsenic,  assisted  by  an  admixture  with 
hematite,  then  with  ammonium  carbonate  to  decompose  cupric  sulfate.  The 
roasted  ore  is  mixed  with  argol,  flour  and  sodium  carbonate,  or  a  similar  flux, 
and  the  charge  melted  and  poured  into  a  mold.  The  copper  button  is  gen- 
erally  too  impure  for  direct  weighing  and  must  be  refined  by  melting  in  a  clay 


Fletcher,  Quantitative  Assaying  with  the  Blowpipe. 
School  of  Mines  Quart.  1892—368. 


THE   FIRE   ASSAY.  277 

dish  with  borax-glass.    The  results  of  the  assay  are  always  too  low  from  loss 
of  copper  in  the  slag. 

Nickel  ores  contain  sulfur,  arsenic,  cobalt,  copper,  iron,  etc.  Since  nickel 
is  infusible  at  furnace  temperatures,  it  is  combined  with  arsenic  and  weighed 
as  nickel  arsenide  (Ni4As2) ,  fusible  at  a  bright  red  heat.  The  process  is  to  roast 
the  ore,  first  alone,  then  with  charcoal,  and  finally  with  ammonium  carbonate. 
The  resulting  oxides  are  mixed  with  arsenic  and  a  reducing  flux  and  melted  down, 
yielding  a  button  of  the  arsenides  of  nickel,  copper,  cobalt  and  iron.  The  button  is 
scorified  with  borax-glass ;  first  the  iron  arsenide,  then  the  cobalt  arsenide  oxi- 
dizes and  the  products  pass  into  the  slag.  The  remaining  button  is  weighed;  if 
there  was  no  copper  in  the  ore  it  is  nickel  arsenide  only,  from  which  the  nickel 
can  be  calculated,  but  if  the  ore  contained  copper,  the  button  is  a  mixture  of 
copper  and  nickel  arsenides.  In  this  case  it  is  melted  with  a  weighed  amount  of 
pure  gold,  and  scorified  with  lead  and  ammonium  sodium  phosphate,  all  the 
arsenic  and  nickel  oxidizing  and  passing  into  the  slag.  The  remaining  button 
is  an  alloy  of  copper  and  gold ;  it  is  weighed  and  the  weight  of  the  gold  deducted, 
giving  the  weight  of  copper.  The  copper  is  calculated  to  copper  arsenide,  and 
knowing  the  weight  of  the  copper-nickel  arsenide,  the  nickel  arsenide  is  found 
by  subtraction 3  and  the  weight  of  the  nickel  calculated  therefrom. 

Native  platinum,  with  the  commonly  associated  minerals  and  metals — osmium, 
iridium,  etc.  —  is  pulverized  and  sifted.  The  sittings  are  mostly  the  sand  of  the 
ore  with  a  part  of  the  metals  and  are  assayed  in  about  the  same  way  as  an  ore  of 
silver,,  beginning  with  a  crucible  fusion.  On  the  sieve  remain  most  of  the  metals 
ready  to  be  scorified  with  lead  and  cupelled,  the  platinum  metals,  like  gold,  alloy- 
ing with  lead  and  resisting  oxidization.  The  buttons  from  the  cupellations  are 
unfused,  spongy  masses,  containing  considerable  lead,  and  must  be  purified  in 
the  wet  way  or  by  re-cupellation  with  a  weighed  amount  of  silver.  The 
buttons  are  then  dissolved  in  aqua  regia  and  the  platinum  metals  separated 
from  one  another  by  fractional  precipitation  or  otherwise.* 

All  the  ores  of  mercury  are  decomposed  on  ignition  with  a  carbonaceous 
compound,  lime,  or  iron,  with  the  liberation  of  mercury.  The  ore  is  mixed 
with  one  of  the  above  and  heated  in  a  glass  retort  and  the  volatilized  mercury 
received  in  cold  water.  When  the  distillation  is  finished,  the  water  is  decanted, 
the  mercury  dried  on  filter  paper  and  weighed.  For  the  poorer  ores  it  is 
advised  to  extract  the  mercury  with  aqua  regia,  evaporate  to  dryness,  and  distill 
the  residue.  Amalgams  and  fouled  mercury  are  distilled  without  any  addition. 

In  the  method  of  Eschkaf,  the  edge  of  a  deep  porcelain  crucible  is  ground  to 
fit  tightly  against  the  under  side  of  a  shallow  gold  dish  of  slightly  greater 
diameter.  The  ore  is  mixed  with  half  its  weight  of  clean  iron  filings, 
placed  in  the  crucible,  and  covered  with  a  layer  of  filings;  the  gold  dish 
is  weighed,  filled  with  cold  water  and  laid  on  the  crucible.  The  vapor  of 
the  mercury,  liberated  on  heating,  condenses  and  forms  an  amalgam  with  the 
gold.  The  dish  is  emptied,  rinsed  with  alcohol,  dried  at  a  low  heat  and  re- 
weighed —  the  increase  is  mercury. 

The  available  sulfur  in  impure  native  thion  or  in  pyrite  may  be  determined  by 
mixing  the  powdered  ore  with  sand  to  prevent  fusion,  and  subliming.  For  native 
sulfur  the  heat  need  only  be  moderate,  but  for  pyrite  should  be  considerably  higher. 

The  carbon  in  a  fuel  may  be  estimated  by  mixing  the  powdered  coal  with  a 
large  excess  of  lead  protoxide,  pouring  into  a  tube  of  refractory  glass,  and 
heating  to  above  the  melting  point  of  lead.  An  equivalent  of  lead  is  assumed 
to  be  formed  by  the  oxidation  of  the  carbon;  actually  the  method  is  only  a 
measure  of  all  the  reducing  matter  in  the  sample. 

*  Crookes,  Select  Methods,  446. 

t  Trans.  Amer.  Inst.  Mining  Engrs.  28—444. 


278  ELECTROLYSIS. 


ELECTROLYSIS, 

For  the  electrolytic  determination  of  a  metal,  a  current  of  electricity  is  passed 
through  an  aqueous  solution  of  a  combination  of  the  metal  with  a  suitable 
acid  radical,  by  means  of  electrodes,  usually  of  platinum.  When  the  condi- 
tions of  current  strength,  character  of  the  electrolyte,  superficial  area  of  the 
electrodes,  and  temperature  of  the  solution  are  within  certain  limits,  the  metal, 
slowly  but  completely,  deposits  on  the  cathode  as  a  film  closely  adherent  to 
the  electrode.  The  increase  in  weight  of  the  cathode  is  that  of  the  metal 
deposited  thereon. 

The  theory  of  electrolysis  propounded  by  Grotthus  in  1805  asserted  that  the 
radicals  of  the  metallic  compound  are  oppositely  electrified,  and  during  the 
passage  of  the  current  arrange  themselves  in  lines  with  their  similar  ends  in 
one  direction,  and  are  then  disrupt  by  the  electrical  attraction  of  the  elec- 
trodes. This  simple  hypothesis  assumed  the  integrity  9f  all  the  molecules 
and  their  symmetrical  arrangement;  it  is  no  longer  held. 

Of  modern  theories  and  their  variations  no  one  can  be  said  to  be  unassailable 
or  to  have  gained  entire  acquiescence.*  One  of  these  is  substantially  as 
follows. 

When  an  electrolyte  (a  compound  that  conducts  electricity)  is  dissolved  in 
water  it  separates  into  ions  more  or  less  completely  according  to  its  nature 
and  the  concentration  of  the  solution.  These  ions,  equal  in  number,  are 
charged  with  like  amounts  of  electricity,  the  cathions  (metals)  with  positive, 
and  the  anions  with  negative.  Under  the  influence  of  the  electric  current  equal 
quantities  of  positive  and  negative  electricity  leave  the  solution,  and  the  cathion 
ions,  bereft  of  their  electric  charges,  unite  to  form  molecules;  similarly  an 
equal  number  of  anion  ions  unite  to  form  molecules.  Moreover  there  is  an 
actual  migration  or  travel  of  the  ions  toward  their  respective  electrodes,  the 
cathions  toward  the  cathode  (negative  electrode)  and  the  anions  toward  the 
anode  (positive  electrode). 

Under  the  influence  of  the  current  the  electrolyte  may  undergo  one  of  three 
change  s.f 

1.  The  ions  may  be  transformed  into  others  charged  with  different  amounts 
of  electricity  to  those  formerly  held,  effecting  under  suitable  conditions,  the 
phenomena  commonly  known  an  oxidation  and  reduction.    Thus,  a  mercuric 
salt  is  changed  to  a  mercurous,  and  a  ferrous  to  a  ferric  salt. 

2.  The  anode  being  of  the  same  oxidizable  metal  as  that  of  the  electrolyte, 
the  cathions  integrate  at  the  cathode,  while  the  anions  do  not  leave  the  ionic 
condition  for   the  reason  that  an  equal  weight  of  the  anode  to  that  of  the 
molecularized  cathions  passes  into  solution  as  cathions,  in  this  manner  pre- 
serving the  electrical  balance  of  the   system.    As  au  example,    witness  the 
process  of  electro  typing,  the  electrolyte  a  saturated  acidified  solution  of  copper 
sulfate,  the  cathode  of  graphite  and  the  anode  of  sheet  copper;  while  the  cur- 
rent passes,  metallic  copper  is  deposited  on  the  cathode,  but  simultaneously 
an  exactly  equal  weight  of  copper  leaves  the  anode  becoming  copper  ions. 


*  Meyer,  Modern  Theories  of  Chemistry,  546  et  seq. 
t  Journ.  Franklin  Inst.  1901—201. 


ELECTHOLYSIS.  279 

3.  Where  the  anode  is  a  platinoid  metal,  graphite,  or  the  like,  it  does  not 
dissolve  as  in  (2),  but  both  cathions  and  anions  become  molecular,  the  former 
depositing  on  the  cathode,  and  the  latter  either  escaping  as  a  gas  at  the  anode 
or  reacting  with  the  solvent;  thus  cupric  sulfate  — 

CuS04  -f-  CuSO4  +  2H2O  =  Cu2  -f-  2H2SO4  -f-  O2. 

The  possibility  of  precipitating  a  metal  from  its  solution  is  a  matter  gov- 
erned by  the  difference  of  electric  potential  between  the  metal  and  its  possible 
electrolytes,  and  the  density  of  current  that  is  available.  To  a  certain  extent 
the  density  must  be  increased  in  proportion  to  the  place  of  the  metal  in  the 
electro- chemical  series,  the  most  positive,  potassium,  and  several  following 
requiring  a  current  far  beyond  that  ordinarily  at  the  command  of  the  chemist, 
lu  general,  when  the  current  density  reaches  a  limit  sufficient  under  the  con- 
ditions of  the  experiment  to  electrolyze  the  solvent,  hydrogen,  and  not  the 
metal,  appears  at  the  cathode. 


The  source  of  electricity  may  be  either  a  galvanic  battery,  a  thermopile,  a 
dynamo,  or  a  storage  battery.  Until  recent  years  the  first  named  was  invari- 
ably employed,  it  having  the  advantages  of  portability  and  low  first  cost. 

For  electrolytic  work  a  choice  of  several  forms  of  battery  is  allowed.  All 
are  constituted  of  two  plates  of  dissimilar  metals,  or  a  metal  and  carbon,  im- 
mersed in  a  suitable  electrolyte.  In  some  forms  both  plates  are  in  one  solu- 
tion, in  others  there  are  two  solutions,  one  plate  in  each,  separated  by  a 
porous  diaphragm,  usually  an  unglazed  clay  cup.  Elements  with  but  one 
liquid  are  liable  to  weakening  by  polarization  —  the  initiation  of  a  potential 
difference  in  the  opposite  direction  from  that  of  the  normal  current,  due  to  the 
collection  of  hydrogen  on  the  surface  of  the  negative  pole. 

In  trade  parlance  batteries  are  classified  as  "open-circuit"  and  "closed- 
circuit."  The  former  are  designed  for  such  purposes  as  exact  but  a  momen- 
tary or  intermittent  current,  and  if  kept  in  excitation  for  a  much  longer 
period,  rapidly  weaken  or  *  run  down.'  The  latter  furnish  a  less  intense 
though  more  constant  current  for  many  hours  or  days  without  recharging  with 
fresh  solutions,  and  are  more  suitable  for  electrolytic  work. 

The  exciting  fluid  (or  fluids)  of  every  battery  is  so  chosen  as  to  exert 
practically  no  chemical  action  on  the  plates  until  the  wires  leading  from 
them  are  brought  into  contact,  directly  or  through  a  solution  that  is  a  con- 
ductor. While  this  contact  is  maintained  the  circuit  is  said  to  be  f  closed.' 
The  chemical  action  that  generates  the  current  is  the  dissolution  of  the  elec- 
tro-positive plate  with  evolution  of  hydrogen,  the  gas  being  either  liberated 
at  the  surface  of  the  plate  or  reacting  with  the  solution.  The  varieties 
most  in  use  are  — 

1.  The  Daniell  cell.  A  bar  of  zinc  whose  surface  has  been  converted  to  a 
zinc  mercury  alloy  by  amalgamation,  is  held  in  a  porous  clay  cup;  outside  the 
cup  is  a  cylinder  of  sheet  copper,  the  whole  contained  in  a  glass  jar.  The 
porous  cup  is  filled  with  dilute  sulfuric  acid,  and  the  glass  jar  with  a  saturated 
solution  of  copper  sulfate.  As  the  zinc  plate  is  coated  with  zinc- amalgam  it  is 
not  acted  on  by  the  sulfuric  acid  until  the  circuit  is  closed.  Copper  wires, 
attached  by  soldering  or  screw-clamps  to  the  plates,  lead  to  the  electrodes, 
from  the  zinc  to  the  cathode  and  from  the  copper  to  the  anode.  A  cell  of 
one-gallon  capacity  produces  a  very  constant  current  of  about  1.079  volts. 

The  above  has  been  superseded  by  modifications  known  as  the  "  Gravity  ", 
"  Hill  ",  l(  Callaud  ",  etc.,  in  which  the  porous  cup  is  dispensed  with.  A  zinc 
disk  hangs  horizontally  near  the  top  of  the  glass  jar;  a  copper  plate  or  rosette 


280  QUANTITATIVE    CHEMICAL   ANALYSIS. 

rests  on  the  bottom  and  is  covered  with  crystals  of  copper  sulfate.  The 
jar  is  tilled  with  water,  and  after  standing  for  some  hours,  the  circuit  being 
closed,  the  liquid  separates  into  two  layers,  a  saturated  solution  of  copper 
sulfate  below,  and  a  (specifically  lighter)  dilute  solution  of  zinc  sulfate  above. 
When  in  operation  the  copper  of  the  copper  sulfate  is  deposited  on  the 
copper  plate,  while  the  sulfuryl  dissolves  an  equivalent  of  zinc  from  the 
zinc  plate.  The  copper  sulfate  is  removed  from  time  to  time  as  it  becomes 
exhausted. 

2.  Bunsen's  cell.  Inside  the  porous  cup  is  a  rod  made  of  dense  carbon  (gas- 
coke)  in  concentrated  nitric  acid;  outside  is  an  amalgamated  zinc  cylinder  in 
dilute  sulfuric  acid.     More  powerful  than  the  Daniell,  it  has  the  drawbacks  of 
giving  off  irritating  fumes  from  the  reduction  of  the  nitric  acid  by  hydrogen, 
and  of  early  polarization.    A  modification  called  the  "  electropion "  has  the 
same  elements  but  different  exciting  fluids,  namely,  chromic  and  sulfuric  acids 
in  the  porous  jar  and  water  in  the  outer  jar;  the  zinc  need  not  be  amalgamated. 

3.  Smee's  cell  is  a  single  fluid  battery  formed  of  two  plates  of  amalgamated 
zinc  and  between  them  a  plate  of  platinum  or  platinized  silver.    The  fluid  is 
dilute  sulfuric  acid.    It  furnishes  a  fairly  constant  current  of  .65  volts  for 
several  hours. 

4  The  Edison-Lelande  is  a  popular  type  for  electrolysis,  having  the  advan- 
tage of  a  solid  depolarizing  element.  The  elements  are  zinc  and  copper  oxide 
in  a  concentrated  solution  of  caustic  potash  covered  with  a  thin  layer  of 
paraffin  oil  to  prevent  entrance  of  carbon  dioxide  and  aqueous  vapor.  The 
electromotive  force  is  about  one  volt  and  the  internal  resistance  about  .3  ohm 
for  polar  surfaces  four  inches  square  at  a  distance  of  1.5  inch.  The  capacity 
is  from  300  to  600  ampere-hours. 

For  occasional  determinations  of  copper,  nickel,  etc.,  the  electropion  is  per- 
haps the  most  convenient,  and  for  routine  work,  two  or  three  Edison-Lelande 
cells  or  three,  to  six  gravity  cells,  connected  to  suit  the  work  in  hand. 

A  storage  battery  or  accumulator  has  two  plates  of  lead  in  dilute  sulfuric  acid ; 
when  a  current  from  a  battery  or  dynamo  is  passed  for  a  time  through  the  couple 
the  surface  of  one  plate  is  changed  from  metallic  lead  ultimately  to  the  bin- 
oxide.  Succeeding  this  transformation,  the  cell  is  at  such  a  potential  as  to 
generate  a  nearly  constant  current  for  many  hours,  the  binoxide  gradually  pass- 
ing to  the  state  of  sulfate.  Smith  states  that  he  is  able  to  secure  a  more  con- 
stant and  controllable  current  from  a  Julien  pile  than  from  any  other  source. 

Thermo-electric  piles  are  built  up  of  a  number  of  bars  of  an  alloy,  such  as 
zinc- antimony,  and  iron,  the  two  soldered  together  at  one  extremity.  A 
number  of  these  couples  are  grouped  radially  around  a  Bunsen  burner  whose 
heat  generates  a  weak  constant  current.  All  are  liable  to  derangement  and  are 
difficult  to  repair.  The  most  practical  form  is  said  to  be  that  designed  by  Gul- 
cher*  which  is  equivalent  to  two  large  Bunsen  cells ;  the  electromotive  force  is 
four  volts,  the  current  strength  three  amperes,  and  the  internal  resistance  .65 
ohm.  The  consumption  of  gas  is  about  six  cubic  feet  per  hour. 

Small  dynamos  suitably  wound  can  now  be  purchased,  and  generate  a  con- 
stant and  easily  regulated  current.  They  are  suitable  in  places  where  a  large 
number  of  determinations  are  made  periodically  and  motive  power  is  available. 

Through  the  extension  of  electric  lighting  during  recent  years  many  have  the 
opportunity  to  use  the  current  from  an  incandescent  light  socket.  Since  the 
voltage  is  far  too  great  for  the  purpose,  a  suitable  resistance  is  interposed, 
easiest  by  means  of  a  number  of  incandescent  bulbs. f 


*  StilJman,  Engineering  Chemistry,  7. 
t  Journ.  Anal.  Appl.  Ohem.  1892—129. 


ELECTROLYSIS. 

The  standards  of  measurement  of  the  electric  current,  omitting  those  in  less 
frequent  use,  are,  according  to  the  common  system: 

The  ampere,  the  unit  of  current  strength,  represented  by  the  unvarying  cur- 
rent that  will  dissociate  a  solution  of  silver  nitrate,  under  certain  specifica- 
tions, with  the  deposition  of  .06708  gram  of  silver  per  minute,  or  of  copper 
sulfate,  depositing  .01969  gram  of  copper  per  minute.  An  ampere  also  disso- 
ciates dilute  sulfuric  acid  with  the  evolution  of  10.436  Cc.  of  oxy-hydrogen  gas 
per  minute  measured  under  normal  conditions;  and  the  volume  of  these  gases- 
liberated  by  a  given  current  times  .0958  gives  the  strength  of  the  current  in 
amperes. 

The  volt  is  the  unit  of  potential  difference,  electromotive  force,  or  ten- 
sion. It  is  a  force  so  great  that  when  steadily  applied  to  a  conductor 
whose  total  resistance  is  one  ohm  will  cause  a  current  of  one  ampere 
to  flow.  It  is  represented  by  1000/U34:  of  the  electromotive  force  of  Clark's 
standard  voltaic  cell  at  a  temperature  of  15  °  . 

The  ohm  is  the  unit  of  resistance  to  the  passage  of  the  current  and  is  equiv- 
alent to  that  of  a  column  of  mercury  at  zero  Cent,  one  square  millimeter  in 
cross  section  and  1.063  meters  long.  The  Siemens'  unit,  frequently  used  to 
express  the  resistance  of  solutions  and  of  batteries,  is  the  resistance  of  a  column 
of  mercury  100  Cm.  long  and  one  square  Mm.  in  section,  at  zero  Cent.  The  recip  - 
rocal  of  the  ohm  is  the  unit  of  conductivity. 

The  unit  of  current  is  the  coulomb,  the  quantity  of  electricity  transferred  by 
a  current  of  one  ampere  in  one  second. 

The  density  of  a  current  signifies  the  quantity  of  electricity  per  square  deci- 
meter of  electrode,  and  is  found  by  dividing  the  current  strength  by  the  surface 
of  the  electrode  immersed  in  the  electrolyte. 

The  description  of  the  current  suitable  for  an  electrolytic  determination 
should  specify  the  ampere  units  per  square  centimeter  of  cathode  surface  and 
the  units  of  voltage.* 

Practically,  electrolytic  determinations  in  the  laboratory  are  restricted  to  the 
deposition  of  metals  from  aqueous  solutions.  To  secure  the  deposit  of  a 
compact,  tenacious  film  suitable  for  weighing,  and  in  a  reasonable  time,  the 
amperage  and  voltage  of  the  current  employed  for  each  electrolyte  must  be  ad  - 
justed  within  certain  specific  limits.  A  specific  minimum  voltage  is  required  for 
every  electrolyte,  while  the  amperage  determines  the  character  of  the  deposit, 
and  if  the  two  are  not  in  the  proper  relation,  the  deposition  may  be  incom- 
plete or  tardy,  or  the  film  brittle,  sandy,  or  spongy  from  occluded  gas.  The 
proper  current  for  each  metal  is  found  by  experiment;  in  general,  copper,  cad- 
mium, bismuth,  etc.,  need  only  currents  of  low  potential,  while  iron,  nickel, 
zinc,  etc.,  are  more  resisting. 

The  formula  for  calculating  the  current  pressure  required  for  the  decom- 

position of  an  electrolyte  is  Z  =  -     aonar    >  where  Z  is  the  decomposition 

tl 


pressure  in  volts;  w  the  thermal  modulus  expressed  as  the  gram-  calories  set 
free  in  the  formation  of  the  chemical  compound  referred  to  one  gram  of 
hydrogen  as  a  unit;  and  n,  the  number  of  valencies  dissolved  by  the  current. 
For  example,  cadmium  sulfate  is  decomposed  to  cadmium,  oxygen  and  sulfur 

trioxide,  CdSO4  =  Cd.O.SOs;  n  =  2,  and  w  =  89500,  hence  Z  = 

1.9  volts.  In  electrolytic  determinations  the  pressure  does  not  usually  exceed 
four  volts. 


Chem.  News,  1891— 2— i 


282  QUANTITATIVE    CHEMICAL   ANALYSIS. 

The  amperage  of  any  given  type  of  battery  is  increased  by  enlarging  the  sur--. 
face  of  the  elements,  which  may  be  done  either  by  increasing  the  size  of  the 
cell  or  by  connecting  two  or  more  cells  in  *  parallel '  or  *  multiple  arc,'  that  is, 
by  joining  the  connecting  wires  of  all  the  negative  poles  and  of  all  the  positive 
poles  together.  To  raise  the  voltage  (independent  of  the  size  of  any  given  bat- 
tery) the  cells  are  connected  *  in  series,'  that  is,  the  positive  pole  of  one  cell 
with  the  negative  pole  of  the  cell  adjoining;  or  a  more  powerful  type 
of  battery  is  substituted.  Other  combinations  may  be  arranged  by  con- 
necting the  cells  partly  in  parallel  and  partly  in  series,  this  arrangement 
being  designated  as  *  multiple  series.'  High  amperage  is  indicated  when  large 
weights  of  metal  are  to  be  deposited;  high  voltage  when  an  electrolyte  is  dif- 
ficult of  decomposition. 

Let  e  represent  the  electromotive  force  of  a  given  cell;  r,  the  internal  resist- 
ance to  the  current;  and  n,  the  number  of  cells  of  the  battery.  Then  for  a 
battery  of  n  cells  arranged  in  parallel,  the  electromotive  force  would  be  e  and 

the  internal  resistance  — .     On  the  other  hand,  if  the  cells  are  joined  in  series, 

the  electromotive  force  is  n.e,  and  the  internal  resistance  n.r.    If  arranged 
in  multiple  series,  each  group  of  a  elements  in  parallel  has  an  electromotive 

force  of  e  and  internal  resistance  of  --  ;  so  that  in  a  collection   of  n  such 


groups,  the  electromotive  force  is  n.e,   and  the  internal  resistance  is  — - — . 

The  adjustment  of  a  current  to  suit  a  particular  electrolyte  may  be  done  by 
reducing  one  more  powerful.  A  substitute  for  the  ordinary  resistance  board 
is  simply  a  long  iron  or  nickel  wire  of  small  diameter  stretched  zigzag  across  a 
board.  One  end  of  the  wire  is  connected  to  the  cathode,  and  a  brass  clip  to 
the  zinc  pole  of  the  battery.  The  clip  may  be  fixed  at  any  point  on  the  wire, 
thus  introducing  the  desired  resistance.  Another  plan  is  to  fill  a  large  glass 
tube  with  a  saturated  solution  of  zinc  sulfate;  each  end  of  the  tube  is  closed 
by  a  cork  through  which  passes  a  metallic  rod  having  a  metallic  plate  fixed 
transversely  to  its  inner  end.  In  proportion  to  the  distance  apart  of  the  disks 
is  the  resistance  increased. 

The  current  is  measured  in  the  usual  way  by  the  ammeter  and  voltmeter,  or 
by  measuring  the  volume  of  oxy-hydrogen  gas  liberated  when  the  current  is 
passed  through  dilute  sulfuric  acid.  In  all  cases  the  electrolytic  solution 
should  be  included  in  the  circuit. 


For  electrodes  a  material  is  used  that  is  a  good  conductor  of  electricity  and 
is  not  acted  on  by  the  electrolyte  or  associates  in  the  solution.  As  a  cathode 
mercury  allows  the  deposition  of  certain  metals  by  restraining  ionic  hydrogen 
from  becoming  molecular  as  it  would  do  on  the  surface  of  a  solid  electrode. 
Dense  carbon  and  graphite  have  been  advised  for  some  electrolytic  separa- 
tions. But  practically,  smooth  sheet  platinum  is  almost  without  exception 
made  the  medium  for  both  the  cathode  and  anode,  it  being  a  fairly  good  con- 
ductor and  can  be  cleaned  from  deposits  by  suitable  acids.  Previous  to  the 
deposition  of  gold  or  platinum,  a  thin  film  of  silver  is  deposited  on  the  cathode 
in  order  that  aqua  regia  may  be  used  for  dissolving  the  gold  or  platinum  de- 
posit and  not  attack  the  platinum  of  the  cathode. 

The  form  of  the  electrodes  depends  somewhat  on  the  concentration  of  the 
electrolytic  solution  and  the  nature  of  the  metal  to  be  deposited; 


ELECTROLYSIS.  283 

1.  The  cathode  may  be  a  platinum  dish  of  a  suitable  capacity  to  contain  the 
solution,  resting  on  a  coil  of  bright  copper  wire  connected  to  the  zinc  of  the 
battery.    The  anode  is  a  circular  platinum  plate  of  say  half  the  diameter  of 
the  dish,  to  whose  center  is  welded  a  thick  platinum  wire.    The  plate  is  sus- 
pended a  little  above  the  bottom  of  the  dish.    During  the  deposition  a  stream 
of  bubbles  arises  from  the  anode,  but  they  are  individually  so  small  that  there 
is  no  loss  by  projection  of  drops  if  the  dish  is  fairly  broad.    Objections  to  this 
arrangement  are  taat  any  solid  matter  separating  from  the  solution  during 
electrolysis,  or  dust  that  may  enter,  falls  to  the  bottom  and  is  inclosed  in  the 
deposit  and  weighed  with  it,  and  that  there  is  greater  evaporation  from  a  dish 
than  from  a  taller  and  narrower  vessel  and  consequently  a  wider  ring  of  the 
deposit  exposed  to  oxidation  by  the  air.    In  the  case  of  metals  whose  'perox- 
ides are  conductors  and  deposit  on  the  anode,  the  plate  is  made  the  cathode  and 
the  dish  the  anode. 

2.  Two  platinum  crucibles  or  dishes  may  form  the  electrodes,  the  smaller 
being  suspended  within  the  larger  and  at  such  a  distance  from  it  as  will 
accommodate  the  solution.    The  gas-bubbles  arising  from  the  anode,  which 
may  be  either  the. inner  or  outer  vessel  as  is  most  suitable,  tend  to  keep  the 
solution  homogeneous.    Electric  connections  are  made  to  the  outer  crucible 
by  coiling  copper  wire  around  it,  and  to  the  inner  by  a  tightly  fitting  cork 
through  which  is  passed  a  copper  wire  terminating  in  a  flat  coil  at  the  bottom. 

3.  The  anode  is  a  small  open  cylinder  of  platinum  foil,  the  cathode  a  larger 
cylinder,  both  welded  to  heavy  platinum  wires,  Fig.  141.    The  cylinders  may 
be  either  partly  or  wholly  immersed  in  the  solution.    A  narrow  longitudinal 
slit  in  each  allows  a  better  opportunity  for  the  continual  mixing  of  the  solution 
during  the  electrolysis. 

4.  The  cathode  is  a  truncated  cone  of  sheet  platinum,  with  or  without  perfo- 
rations in  the  sides,  the  anode  a  conical  coil  of  platinum  wire  hung  within 
the  cathode. 

Whichever  form  be  adopted,  it  is  essential  that  evaporation  be  prevented  as 
far  as  possible  (to  avoid  exposing  the  edge  of  the  deposit  to  the  air),  and  that 
the  surface  of  the  cathode  have  so  great  an  area  that  the  metal  is  deposited  in 
a  thin  film  only. 

.Supports  for  the  electrodes  or  their  suspensions  should  afford  an  amply 
large  surface  of  contact  with  the  leading  wires  from  the  battery  and  also  allow 
them  to  be  easily  and  quickly  disconnected.  Where  one  current  is  used  for  the 
simultaneous  precipitation  of  a  number  of  like  electrolytes,  the  cathode  of  one 
solution  is  connected  to  the  anode  of  the  one  next  adjoining,  and  the  terminal 
wires  to  the  battery. 


As  a  rule  an  electrolysis  proceeds  normally  when  the  solution  is  -at  the  tem- 
perature of  the  laboratory,  though  there  are  a  number  that  require  that  the  solu- 
tion be  maintained  near  the  boiling  point  — actual  boiling  is  apt  to  loosen  the 
deposit  from  the  cathode.  The  conductivity  of  a  solution  generally  increases 
with  a  rise  of  temperature;  thus,  in  electrolyzing  a  solution  of  gold  potassium 
cyanide,  the  cathode  a  platinum  dish  standing  near  a  window  in  cold  weather, 
the  gold  was  deposited  incompletely  and  only  on  the  side  of  the  dish  furthest 
from  the  window.  Yet  some  metals  separate  completely  only  in  the  cold.* 

The  concentration  of  the  metal  in  a  solution  can  usually  vary  within  wide 
limits  without  impairing  the  condition  of  the  deposit.  A  fair  concentration  is 


*  Journ.  Anal.  Chem.  1895—613. 


284  QUANTITATIVE    CHEMICAL   ANALYSIS. 

one  gram  of  metal  in  200  Cc.  of  liquid.  The  weight  of  metal  that  can  be  de- 
posited depends  on  the  area  of  the  cathode,  as  too  thick  a  deposit  is  apt  to  be 
rough  and  less  adherent  than  a  thin  film. 

To  the  rule  that  the  electrolyte  shall  be  in  a  clear  solution  for  deposition 
there  are  a  few  exceptions,  namely  where  the  electrolyte  may  be  in  the  form 
of  powder  gradually  dissolving  as  the  current  passes  and  the  metal  precipitates. 

The  presence  in  the  solution  of  the  electrolyte  of  other  inorganic  compounds 
that  are  not  decomposed  by  a  current  of  the  strength  employed,  has,  as  a  rule,  no 
interfering  effect.  Certain  organic  bodies,  however,  may  hinder  or  prevent  the 
deposition  of  a  metal  or  modify  the  usual  behavior;  for  example,  the  copper 
commonly  present  in  oil  of  cajupet  appears  on  the  positive  pole  as  copper  oxide 
when  the  oil  is  mixed  with  water  and  electrolyzed. 

Of  the  different  radicals  with  which  a  given  metal  may  be  combined,  the  most 
suitable  are  those  which  require  but  a  moderate  current  for  decomposition  at 
ordinary  temperatures.  With  some  exceptions  the  simple  salts  of  the  metals 
are  not  well  adapted  for  electrolysis,  as  a  moderate  current  precipitates  them 
but  slowly  or  not  at  all.  The  deposition  proceeds  more  rapidly  and  regularly 
with  double  salts,  the  consort  an  alkali  metal.  To  change  a  simple  to  a 
double  salt  it  is  usually  sufficient  to  add  an  excess  of  the  proper  alkali  salt  to 
the  solution. 

Following  is  a  synopsis  of  the  forms  of  combination  most  suitable  for  elec- 
trolytic deposition.  Some  metals  can  be  thrown  down  with  equal  success  from 
any  of  several  combinations,  others  from  but  one  or  two.  Special  conditions 
not  mentioned  here  must  be  observed  for  many  depositions. 

1.  Comparatively  few  metals  can  be  precipitated  satisfactorily  from  combina- 
tions with  an  inorganic  acid,  and  it  must  be  remembered  that  though  the  solu- 
tion be  neutral  at  the  beginning,  yet  as  the  electrolyte  is  decomposed  by  the 
current  the  free  acid  increases.    The  nitrates  of  copper,  silver  and  mercury  are 
decomposed  satisfactorily  provided  but  little  free  nitric  acid  be  in  the  solution; 
in  neutral  solutions  free  from  organic  matter,  the  peroxides  of  lead,  thallium, 
and  manganese  form  on  the  anode ;  bismuth,  silver  and  copper  can  be  precipi- 
tated from  sulfuric  solutions;  while  hydrochloric  acid  is  unfitted  for  metals 
other  than  tin,  platinum  and  palladium. 

2.  Most  of  the  metals  are  readily  deposited  when  combined  as  double  salts 
with  the  tartrates,  acetates,  oxalates,  or  formates  of  alkali  metals.    Those  that 
can  be  combined  with  ammonium  oxalate  are  the  members  of  the  zinc  and  cop- 
per groups,  and  some  of  the  platinum  group.    Metallic  oxalates  are  broken  up 
by  the  current  to  the  metal  and  carbon  dioxide,  or  in  the  case  of  the  more  elec- 
tropositive metals,  into  hydrogen  and  hydrocarbonate  of  the  metal.    For  man- 
ganese potassium  oxalate,  the  manganese  separates  as  peroxide  on  the  anode. 

Iron  with  ammonium  tartrate;  zinc,  cadmium,  and  uranium  (the  latter  sep- 
arating as  peroxide)  with  sodium  acetate  and  acetic  acid;  and  zinc  as  formate, 
yield  readily  to  the  current.  The  decomposition  products  of  these  radicals  are 
generally  quite  complex. 

Some  radicals  or  the  excess  of  an  organic  salt  have  a  specific  influence  toward 
protecting  the  metal  from  oxidation  during  deposition. 

3.  The  double  alkali  cyanides  of  the  copper,  zinc,  and  antimony  groups 
(except  arsenic)  readily  separate  under  comparatively  weak  currents.     (The 
cyanides  of  sodium  and  gold  or  silver  are  largely  employed  in  electro-plating.) 
The  double  sulfocyanides  of  the  iron  group  have  much  the  same  characteristics 
as  those  of  the  cyanides. 

4.  The  double  alkali  sulfldes  of  gold,  antimony  and  mercury  (soluble  in 
potassium  sulflde),  and  the  sulfo-salts  of  antimony  and  tin  are  decomposed  by 
a  current  of  adequate  strength. 


ELECTROLYSIS.  285 

5.  A  number  of  the  metals  may  be  combined  with  alkali  phosphates;  these 
are  cadmium,  bismuth,  tin,  the  manganese  group,  and  some  of  the  platinum 
group.     Brand*  advhes  the  combination  with  sodium  pyrophosphat.e  in  con- 
junction with  ammonium  carbonate  for  various  metals.     He  also  states  that 
such  double  salts  of  the  metals  as  form  peroxides  behave  electrolytically  differ- 
ent from  the  salts  hitherto  examined. 

6.  Members  of  the  silver  and  zinc  groups  are  precipitated  from  ammoniacal 
solutions  of  their  double  ammonium  sulfates. 

It  will  be  noticed  from  the  above  that  in  the  majority  of  cases  the  combina- 
tion of  the  metal  is  with  a  comparatively  weak  radical. 


The  time  required  for  a  complete  deposition  ranges  from  two  or  three  to 
twelve  hours  or  more,  according  to  the  concentration  and  character  of  the 
electrolyte,  the  power  of  the  battery,  temperature,  etc.  Usually  in  an  assay  no 
harm  results  from  a  longer  transmission  of  the  current  provided  that  it  be  un- 
interrupted, and  the  circuit  may  be  closed  in  the  evening  and  allowed  to  con- 
tinue over  night  if  convenient. 

The  precipitation,  as  a  rule,  is  more  complete  than  that  afforded  by  other 
gravimetric  methods,  yet  it  is  seldom  that  when  a  delicate  test  is  applied  to  the 
residual  liquid,  traces  of  the  metal  will  not  be  shown.  Where  the  most  accu- 
rate results  are  aimed  at,  the  liquid  should  be  concentrated  and  the  remaining 
metal  determined,  colorimetrically  where  possible. 

That  the  metal  has  separated  up  to  the  limit  required  may  be  assured  by  ap- 
plying any  one  of  the  delicate  qualitative  tests  for  the  metal  to  a  few  drops  of  the 
solution,  premising  that  the  solvent  is  not  of  a  nature  to  interfere  with  the  reac- 
tion. Of  a  few  metals  the  fading  of  the  color  of  the  solution  marks  the  abstraction 
of  the  metal.  Or  if  the  solution  is  diluted  somewhat  without  interrupting  the  cur- 
rent, so  that  an  uncoated  surface  of  platinum  be  brought  into  it,  the  non- 
appearance  of  a  deposit  within  the  space  of  an  hour  or  so  is  proof  that  only 
a  negligible  quantity  remains;  but  with  metals  of  nearly  the  color  of  platinum 
the  formation  of  a  slight  deposit  is  difficult  to  detect.  In  any  event  the  entire 
solution  should  be  tested  after  the  removal  of  the  electrodes. 

In  most  cases  when  the  electrolysis  is  finished,  the  cathode  may  be  withdrawn, 
rinsed  by  quick  immersion  in  a  beaker  of  water,  dried  and  weighed.  'But  if  the 
original  solvent,  or  that  formed  by  the  electrolysis,  is  of  a  nature  to  readily  act 
on  the  deposit,  a  slight  re -solution  might  take  place  however  expeditiously 
the  rinsing  was  done,  and  the  current  must  not  be  stopped  until  the  solvent  is 
displaced  by  water.  Here  the  liquid  is  drawn  off  by  a  small  glass  syphon  reach- 
ing to  the  bottom  of  the  vessel,  at  the  same  time  pouring  in  water,  carefully 
to  prevent  much  mixing.  The  cathode  is  then  disconnected  and  rinsed  as  before. 

Ordinarily  the  metal  may  be  dried  at  a  temperature  of  100  o  or  below,  though 
some  prefer  to  rinse  with  strong  alcohol  and  after  draining  for  a  moment,  to 
light  and  allow  to  burn  out,  leaving  the  cathode  ready  for  weighing.  Others 
would  follow  the  alcohol  with  ether  and  allow  spontaneous  drying. 

As  the  weight  of  the  deposit  is  learned  from  the  increase  in  weight  of  the 
cathode  it  is  best  to  heat  the  latter  to  redness  before  the  original  weighing  to 
burn  off  adhering  organic  matter,  and  care  must  be  taken  that  particles  of  the 
platinum  wire  are  not  detached  by  friction  of  the  binding -screw  connecting 
with  tbe  wire  from  the  battery.  A.  few  metals  readily  oxidize  with  increase  of 


*  Chem.  News,  1890-1—2. 


286  QUANTITATIVE   CHEMICAL    ANALYSIS. 

weight  on  exposure  to  the  air  and  must  be  protected  as  far  as  possible  — 
thallium,  for  example,  oxidizes  so  easily  that  a  method  has  been  devised  by 
Neumann  in  which  a  special  apparatus  is  provided  for  dissolving  the  deposit  in 
hydrochloric  acid  without  exposure  to  the  air,  calculating  the  weight  of  the 
thallium  from  the  volume  of  hydrogen  evolved. 

Wherever  the  deposit  is  to  be  weighed  directly  it  is  highly  important  that  it 
should  form  in  a  dense  and  closely  adhering  film.  In  this  shape  oxidation 
occurs  to  a  less  extent,  if  at  all,  than  if  it  were  loose  and  spongy.  Moreover  it 
is  more  easily  cleansed  from  the  adhering  solution,  and  there  is  not  the  danger 
from,  mechanical  loss  incident  to  one  granular  or  flaky.  This  condition  is  to  be 
secured  by  a  proper  regulation  of  the  current  and  the  concentration  and  tem- 
perature of  the  solution. 

Tbe  deposit  is  accounted  pure  metal,  and  in  the  majority  of  determinations 
this  assumption  is  warranted.  Yet  unquestionably  the  tendency  of  certain 
deposits  formed  in  complex  solutions  to  occlude  other  matter  has  not  been 
given  the  attention  it  merits.  Thus,  iron  when  precipitated  from  a  combination 
with  an  organic  radical  always  contains  some  carbon;  under  some  conditions 
copper  occludes  gases  and  small  amounts  of  other  metals  that  may  be  in  the 
solution* ;  peroxides  formed  on  the  anode  are  prone  to  inclose  compounds  not 
removed  by  washing.  It  is  in  technical  work  rather  than  scientific  that  com- 
plex solutions  are  dealt  with,  and  greater  precautions  as  to  current  strength, 
temperature  and  the  like  are  indicated. 

To  some  extent  the  appearance  of  the  deposit  is  an  indication  as  to  its  purity, 
denoted  by  a  uniform  lustre  and  the  color  of  the  unpolished  pure  metal. 
Local  discoloration  points  to  oxidation,  general,  to  the  presence  of  impurities. 

The  deposit  may  be  cleaned  from  the  electrode  by  immersion  in  a  suitable 
simple  acid;  if  of  gold,  chlorine  water  will  dissolve  it,  and  if  of  platinum  a 
digestion  in  hot  aqua  regia,  the  electrode  being  protected  by  a  film  of  silver 
deposited  previous  to  the  electrolysis. 

SEPARATION   BY  ELECTROLYSIS. 

Methods  for  the  separation  of  two  metals  in  one  solution  may  be  classified 
as  follows:  — 

1.  One  metal  may  be  so  electro-positive  that  only  a  current  of  extraordinary 
tension  will  effect  its  deposition ;  the  other  is  to  be  combined  with  a  suitable 
radical  and  deposited  by  a  current  of  ordinary  density. 

2.  Both  metals  being  precipitable,  they  are  combined  with  such  a  radical  that 
there  will  be  a  considerable  difference  in  potential,  and  the  current  is  adjusted 
to  a  tension  that  will  only  suffice  to  decompose  an  electrolyte  midway  between 
them  in  potential.     When  the  first  metal  has  completely  deposited,  the  cathode 
is  withdrawn  and  replaced  by  another  and  the  current  raised  to  the  tension 
required  for  the  separation  of  the  second. 

When  a  current  whose  voltage  is  gradually  increased  is  passed  through  a 
solution  holding  different  metals,  that  metal  having  the  smallest  potential  dif- 
ference in  relation  to  the  solution  separates  first,  and  alone  up  to  a  certain 
minimum  concentration  in  the  solution.  If  the  potential  difference  be  kept 
constant  for  a  time,  this  required  concentration  may  be  lowered  until  qualita- 
tive tests  show  practically  no  metal  remaining.  At  a  higher  tension  that  metal 
with  the  next  smaller  potential  difference  separates,  and  so  on  through  the 
series. 


*  Chcm.  News,  1889-2-24. 


ELECTROLYSIS.  287 

In  the  separation  of  two  metals  the  form  of  combination  best  suited  to  any 
given  pair  can  only  be  found  by  experiment,  as  there  seems  to  be  no  general 
rule  by  which  can  be  determined  what  combination  will  give  the  best  results. 
One  of  the  following  is  usually  selected :  an  acid  solution  of  the  nitrates  or 
phosphates;  doable  cyanides  orsulfldes;  or  acid  or  alkaline  double  sulfates, 
oxalates,  tartrates,  or  citrates.  In  one  of  these  combinations  can  one  group  of 
metals  be  separated  from  another,  though  each  combination  seems  best  fitted 
for  a  few  special  pairs. 

3.  Both  metals  being  precipitable,  they  are  thrown  down  successively  under 
different  conditions  of  temperature  and  strength  of  current.    Thus,  copper  is 
deposited  from  a  cold  solution  in  ammonium  oxalate  by  a  weak  current,  but 
not  iron;  while  iron  deposits  if  the  solution  be  hot  and  the  current  strong. 

4.  Both  metals  being  precipitable,  one  may  be  changed  in  valence  during  the 
passage  of  the  current  to  a  state  not  decomposed  by  an  ordinary  voltage. 

5.  One  metal  forms  a  stable  peroxide,  the  other  not.     Here  the  two  may  be 
deposited  simultaneously,  the  former  as  peroxide  on  the  anode,  the  latter  in 
the  metallic  state  on  the  cathode. 

The  part  played  by  the  decomposition  products  of  the  radical  of  the  metallic 
salts  or  the  excess  of  the  associated  compound  may  be  of  importance  to  a  per- 
fect separation,  as  the  electrolysis  of  one  of  the  metals  may  be  hastened  or 
retarded  ad  libitum,  while  that  of  the  other  metal  is  unchanged,  or  in  some  cases 
may  form  an  insoluble  compound  and  precipitate.  Thus,  free  nitric  acid  is 
decomposed  to  nitrogen  tetroxide  (N204),  ammonia,  water,  and  oxygen  (liber- 
ated at  the  anode);  hydrochloric  acid  plus  water,  to  hydrogen  and  oxides  of 
chlorine;  ammonium  oxalate  to  hydrogen  and  ammonium  bicarbonate;  potas- 
sium oxalate  to  hydrogen  and  potassium  hydrocarbonate ;  and  various  organic 
radicals  to  complex  dissociation  products. 

Thus,  if  during  the  electrolysis  of  iron  and  manganese  oxalates,  the  solution 
contain  a  .large  amount  of  ammonium  oxalate  and  is  kept  hot,  the  iron  deposits 
on  the  cathode,  but  the  manganese  remains  in  solution  until  the  ammonium 
oxalate  is  nearly  all  dissociated;  this  behavior  prevents  the  occlusion  of  some 
of  the  iron  in  the  manganese  dioxide  as  would  happen  were  both  metals  pre- 
cipitated concurrently.  The  nitrates  with  free  nitric  acid  act  similarly. 

6.  Both  metals  are  precipitated  on  one  cathode;  it  is  removed  and  made  the 
anode  in  another  solution  so  chosen  that  (1),  one  of  the  metals  only  will  be 
dissolved,  either  remaining  in  solution  or  again  plating  the  cathode;  (2),  both 
metals  dissolve,  one  remaining  in  solution,  the  other  depositing  on  the  cathode ; 
or  (3),  both  leave  the  anode,  one  depositing  on  the  cathode,  the  other  be- 
coming an  insoluble  compound. 

The  deposition  of  an  alloy  —  that  is,  of  two  metals  simultaneously  on  one 
electrode  —  is  not  difficult  when  they  are  not  far  apart  in  the  electro- chemical 
series,  but  in  proportion  as  they  diverge  the  current  must  be  more  carefully 
adjusted  that  it  may  not  be  so  weak  as  to  deposit  mainly  the  more  electro- 
negative metal,  nor  so  strong  as  to  favor  the  more  electro-positive.  By  this 
course  it  is  possible  to  separate  two  electrolytically  similar  metals  from  a 
dissimilar  third. 

For  example,  LeRoy*  separates  nickel  and  cobalt  from  iron  by  first  preparing 
an  ammouical  solution  of  their  ammonium  sulfates,  the  iron  being  held  in 
solution  by  ammonium  citrate.  The  three  are  precipitated  on  a  platinum 
cathode  by  a  suitable  current.  The  cathode  is  removed  to  a  concentrated 
ammoniacal  solution  of  ammonium  sulfate,  made  the  anode  of  the  circuit,  and 


*  Chem.  Newfl,  1891-1-1<>4. 


288  QUANTITATIVE    CHEMICAL    ANALYSIS. 

the  current  passed.  The  nickel  and  cobalt  dissolve  and  are  deposited  on  the 
cathode,  while  the  iron  dissolves  and  is  precipitated  by  the  ammonia  as  ferric 
hydrate. 

A  commercial  metal  containing  metallic  or  other  impurities  may  be  made  the 
anode  of  a  circuit  with  a  sheet  of  platinum  for  the  cathode.  With  a  suitable 
liquid  and  strength  of  current  the  metal  dissolves,  either  remaining  in  solution 
or  depositing  on  the  cathode  according  to  circumstances.  Most  or  all  of  the 
various  impurities  remain  insoluble,  in  the  form  of  a  powder  or  skeleton,  and 
may  be  filtered  from  the  liquid. 

7.  In  addition  to  the  above  there  are  various  methods  designated  by  some  as 
electrolytic  separations,  but  which  logically  belong  to  other  classes.  Of  such 
are  the  deposition  of  an  alloy  on  the  cathode  with  subsequent  separation  of 
the  members  by  other  than  electrolytic  means,  e.  g.t  by  volatilization  of  one 
metal  by  heating,  and  the  deposition  of  two  metals  from  individual  solutions 
after  a  separation  according  to  a  gravimetric  method. 

Electrolytic  separations  are  not  employed  to  the  extent  that  would  appear 
from  the  number  of  methods  that  have  been  published.  One  not  skilled  in 
electro-chemical  analysis  finds  considerable  difficulty  in  adjusting  the  current 
strength  to  harmonize  with  the  dissociation  factors  of  the  electrolytes,  or  in 
modifying  the  current  t>r  removing  the  electrode  directly  one  metal  has  been 
entirely  precipitated  and  before  the  dissociation  of  the  other  electrolyte 
begins.  This,  of  course,  does  not  refer  to  the  numerous  cases  where  the  sep- 
aration is  practically  identical  with  the  method  of  determination  of  one  metal 
and  allows  the  same  latitude  as  regards  the  general  conduct. 


Electrolytic  methods  as  compared  with  other  classes  of  gravimetric  analysis 
are  superior  in  that  various  inaccuracies  inherent  to  the  latter  are  not  incurred ; 
that  once  the  apparatus  is  arranged,  the  operation  proceeds  to  a  close  with 
but  little  attention;  that  the  product  is  obtained  ready  for  weighing;  and  that 
if  the  residual  solution  is  to  be  further  dealt  with,  there  are  introduced  no 
reagents  of  doubtful  purity,  difficult  of  removal,  or  interfering  in  any  way. 
Their  accuracy  in  many  cases  fits  them  for  refined  scientific  analyses  of  com- 
pounds of  the  heavy  metals,  and  even  for  the  determination  of  atomic  weights. 

In  technical  analyses  of  certain  alloys,  commercial  metals,  ores,  mattes  and 
slags,  simple  and  reliable  methods  are  provided  of  equal  or  greater  accuracy 
than  any  others  now  in  use.  For  copper  ores  and  products  electrolytic  pro- 
cesses have  largely  supplanted  all  others,  and  this  is  true  to  some  extent  of 
nickel  ores;  but  for  other  metals  volumetric  or  gravimetric  methods  still 
obtain  the  preference. 

The  disposition  of  some  writers  to  extend  the  field  of  electrolytic  analysis 
beyond  its  legitimate  boundaries  must  be  deprecated.  Many  schemes  for  the 
analysis  of  naturally  occurring  and  factored  commercial  articles  by  quasi-elec- 
trolytic methods  have  been  devised  and  considerable  ingenuity  shown  in  their 
adaptation,  but  one  at  all  versed  in  analysis  can  readily  see  their  inferiority  to 
other  methods  or  their  impracticability  for  the  average  laboratory.  For  a  large 
proportion  of  the  material  met  with  in  practical  analysis  electrolytic  schemes 
offer  no  advantage  whatever  and  are  inferior  as  a  whole  to  the  usual  methods. 


THE    METALS    AKD    COMMON    ACIDS.  289 


THE  METALS  AND  COMMON  ACIDS. 

The  literature  on  the  separation  and  determination  of  the  above  is  so  ex- 
tensive that  space  will  not  permit  of  more  than  a  mention  of  the  methods  best 
known  and  generally  available.  More  than  this  is  unnecessary  since  the 
student  has  easy  access  to  several  exhaustive  works  on  the  subject,  notably 
the  treatises  of  Fresenius,  Menchutkin,  Jagnaux,  Carnot,  and  Crookes. 

Solution. 

Water  dissolves  the  majority  of  the  salts  of  the  different  metals,  and  the 
free  acids  and  alkalies.  For  commercial  salts  a  trace  of  free  acid  may  be 
added  to  secure  a  clear  solution. 

Nitric  acid  dissolves  all  the  metals  except  gold  and  the  platinum  group,  and 
tin  and  antimony,  which  are  converted  to  oxides.  As  a  rule  this  acid  is  chosen 
as  the  solvent  for  commercial  metals  and  alloys.  With  a  few  exceptions,  sul- 
fldes  also  are  readily  decomposed  by  the  moderately  strong  acid  with  the  for- 
mation of  sulfate  and  nitrate  of  the  bases,  and  separation  of  free  sulfur,  but 
fuming  nitric  acid,  or  the  ordinary  acid  in  combination  with  some  strong  oxi- 
dizer,  is  preferable  for  the  reason  that  all  the  separated  sulfur  is  converted  to 
sulfuric  acid  on  protracted  heating  of  the  liquid. 

What  nitric  acid  is  toward  the  metals — an  almost  universal  solvent — is 
hydrochloric  acid  for  the  oxides,  all  but  a  few  readily  passing  into  solution. 
Where  a  choice  is  allowed  hydrochloric  is  preferred  to  either  nitric  or  sulfuric 
acid,  for  the  reasons  that  the  chlorides  are  generally  freely  soluble,  the  excess 
of  acid  is  readily  removed  by  evaporation,  and  that  most  precipitants  can  be 
applied  directly  to  a  hydrochloric  solution.  Most  peroxides  enter  into  solu- 
tion as  protochlorides  when  treated  with  hydrochloric  acid  and  a  reducer 
such  as  hydrogen  peroxide . 

Hot  concentrated  sulfuric  acid  converts,  with  evolution  of  sulfurous  acid,  a 
number  of  the  metals  to  sulfates  soluble  on  dilution  with  water,  and  is  some- 
times applied  in  the  form  of  melted  potassium  or  sodium  pyrosulfate.  Many 
native  oxides  and  silicates,  insoluble  in  other  acids,  yield  to  prolorfged  treat- 
ment with  sulfuric.  The  dilute  acid  dissolves  some  of  the  metals  (with  quan- 
titative evolution  of  hydrogen)  though  hydrochloric  can  nearly  always  be 
substituted  with  advantage. 

Sodium  hydrate  solution  dissolves  aluminum  and  zinc  and  is  sometimes  used 
for  the  separation  of  these  metals  from  other  elements.  Melted  potassium 
hydrate  will  resolve  some  refractory  oxides  and  other  insoluble  compounds, 
and  has  been  applied  in  connection  with  a  current  of  electricity  for  the  oxida- 
tion of  certain  native  sulfldes. 

Sodium  carbonate  fused  with  any  of  the  numerous  insoluble  native  and  arti- 
ficial silicates  forms  double  silicates  of  the  bases  soluble  in  water  or  a  dilute 
acid.  Other  fluxes  of  more  limited  application  are  sodium  fluoride  for 
silicates;  sodfum  carbonate  and  sulfur,  or  sodium  thiosulf ate  for  metals  and 
oxides  of  the  arsenic  group;  various  fusible  metallic  oxides,  borax,  boric 
acid,  etc.,  for  silicates;  soda-lime  for  chrome  iron  ore,  etc. 

Special  solvents  are  hydrofluoric  acid  for  native  silica  and  silicates,  nitric 

19 


290  QUANTITATIVE    CHKMICAL    ANALYSIS. 

or  hydrochloric  acid  in  conjunction  with  bromine,  potassium  chlorate  or  a 
hypochlorite  for  metallic  sulfides,  hydrochloric  or  sulf  uric  acid  with  a  reducing 
agent  for  peroxides,  etc. 

Determination. 

The  hydrates  of  the  iron  group,  manganese,  nickel,  cobalt  and  copper  and  the 
oxides  of  titanium,  zirconium  and  gallium  are  precipitated  by  the  alkalies.  In 
presence  of  a  strong  oxidizer,  managanese  and  nickel  come  down  as  perhy- 
drates,  and  with  a  reducing  agent  copper  as  cuprous  oxide.  Alumina  is  pre  - 
cipitated  from  a  solution  in  potassium  hydrate  by  heating  with  ammonium 
chloride. 

The  precipitates  are  usually  flocculent  and  as  a  rule  not  easy  to  filter. 
Where  a  choice  of  alkali  is  allowed  ammonia  is  preferable,  since  occluded 
ammonia  salts  are  volatile  on  ignition.  The  precipitates  of  aluminum  and 
chromium  hydrates  are  especially  difficult  to  wash,  while  the  perhydrates  and 
peroxides  are  more  granular.  Many  organic  and  a  few  inorganic  bodies  inter- 
fere with  precipitation  and  must  have  been  previously  removed. 

The  precipitates  pass  on  ignition  to  the  state  of  protoxide  or  sesquioxide ; 
manganese  becomes  the  tetroxide  and  titanium  the  dioxide.  Ignited  in  hydro- 
gen, some  oxides  are  reduced  to  the  metallic  state. 

During  electrolysis  a  few  metals  are  deposited  on  the  anode  in  the  form 
of  peroxides. 

Hydrogen  peroxide  readily  parts  with  an  atom  of  oxygen  leaving  a  residue 
(water)  neither  acid  nor  basic.  In  general,  in  acid  solutions  the  reagent 
reduces  peroxides,  while  in  alkaline  solutions  proto  •  salts  are  perduced ;  the 
products  remain  dissolved  or  precipitate,  or  if  insoluble  originally,  retain  tho 
solid  form  or  pass  into  solution  according  to  the  nature  of  the  acid  or  alkali. 

The  barium  group,  manganese,  lead  and  cadmium  are  precipitated  as 
carbonates  by  an  alkali  carbonate;  zinc  and  bismuth  as  basic  carbonates;  and 
copper  from  a  hot  solution  as  oxide.  The  precipitates  are  granular  and  easy 
to  wash,  and  with  the  exception  of  barium  and  strontium  carbonates,  pass  to 
the  oxides  on  strong  ignition. 

The  aluminum  group,  gallium  and  indium  are  precipitated  from  neutral  solu- 
tions by  barium  carbonate,  an  equivalent  of  barium  entering  the  solution. 
The  reaction  was  formerly  much  used  for  separations,  but  at  present  other 
methods  have  the  preference. 

The  silver,  copper  and  arsenic  groups  are  precipitated  as  suifides  from  acid 
solutions  by  hydrogen  sulflde;  the  iron  and  zinc  groups  from  alkaline  solutions 
by  ammonium  sulflde.  In  view  of  the  extensive  use  of  these  reagents  in  quali- 
tative analysis,  it  is  surprising  to  the  beginner  to  find  how  comparatively  seldom 
they  are  employed  in  a  quantitative  way.  The  reasons  are  (1),  the  tendency 
of  the  sulfldes  to  oxidize  during  filtration  and  washing  and  pass  through  the 
filter;  (2),  that  frequently  there  is  free  sulfur  admixed  with  the  precipitate,  and 
that  on  drying  or  ignition  there  are  left  indefinite  mixtures  of  the  sulfide  with 
sulfate  or  oxide,  so  that  the  precipitate  can  only  be  weighed  after  special 
preparation  to  insure  its  being  entirely  of  the  assumed  composition,  or  it  must 
be  redissolved  and  precipitated  in  some  other  combination;  (3),  the  transmis- 
sion of  a  gas  through  a  liquid  is  not  so  convenient  as  the  addition  of  a  liquid 
reagent,  and  the  separation  of  one  metal  from  another  can  usually  be  done  as 
completely  and  more  conveniently  by  some  other  reagent. 

The  sulfldes  of  gold,  silver  and  platinum  are  converted  to  the  metallic  state 
on  moderate  ignition,  but  those  of  arsenic,  antimony  and  mercury  are  volatile 
and  must  not  be  heated  above  100  ° .  Of  most  other  metals  there  is  left  on 
ignition  a  definite  crystalline  or  amorphous  sulflde  ready  for  weighing  if  the 


THE   METALS    AND    COMMON    ACIDS.  291 

precipitate  has  been  mixed  with  free  sulfur  and  ignited  in  an  atmosphere  of 
hydrogen.  Many  sulfides  pass  entirely  to  oxides  on  prolonged  roasting  in  air. 

Hydrogen  sulflde  is  an  energetic  reducing  agent,  often  used  for  the  reduc- 
tion of  solutions  of  per- salts,  though  the  separation  of  free  sulfur  necessitates 
an  extra  filtration. 

The  sulfates  of  lead  and  the  barium  group  are  pulverulent  precipitates,  all 
except  barium  sulfate  requiring  the  addition  of  alcohol  to  the  solution  and 
wash  water  to  lessen  the  loss  due  to  their  solubility  in  water.  All  can  be 
ignited  without  decomposition. 

A  volatile  acid  radical  combined  with  an  alkali  metal,  magnesium,  nickel, 
etc.,  is  displaced  by  the  sulfuric  radical  when  the  compound  is  evaporated 
with  a  slight  excess  of  dilute  sulfuric  acid  and  the  residue  gently  ignited,  the 
protosulfate  remaining. 

The  chlorides  of  silver,  mercurosum  and  lead  form  heavy  curdy  or  crystal- 
line precipitates  easy  to  wash,  that  of  lead  being  less  soluble  in  dilute 
alcohol  than  in  water.  Like  sulfuric  acid,  hydrochloric  may  be  used  to 
replace  a  volatile  acid  radical  combined  with  a  metal,  and  the  resulting  proto- 
chloride  weighed. 

Conversion  of  a  metallic  compound  to  the  metal.  Many  metals  can  be 
deposited  electrolytically  with  the  best  results. 

Gold,  platinum,  copper,  antimony,  tin,  lead  and  silver  are  precipitated  by 
metallic  zinc  from  slightly  acid  solutions  as  metallic  powders,  and  some 
insoluble  compounds  of  these  metals  are  completely  decomposed.  In  special 
cases  for  zinc  there  is  substituted  cadmium,  iron,  aluminum  or  magnesium, 
which  can  be  bought  nearly  pure  in  the  shape  of  foil  or  wire.  Although  this 
method  has  been  largely  supplanted  by  electrolysis,  yet  it  occasionally  finds 
use,  especially  in  separations,  from  its  quickness  and  convenience. 

Gold,  silver  and  platinum  compounds  are  decomposed  to  the  metals  by 
strong  reducers,  such  as  ferrous  sulfate,  oxalic  acid,  and  some  organic  com- 
pounds ;  and  mercury  by  the  powerful  reducer  stannous  chloride. 

Compounds  of  bismuth,  lead,  tin,  etc.,  are  reduced  to  the  metallic  state  by 
fusion  with  potassium  cyanide  or  sodium  formate,  or  ignition  in  hydrogen. 

The  oxalates  of  calcium,  zirconium,  cerium,  lanthanum,  and  didymium  are 
fine  powders,  generally  highly  insoluble.  On  ignition  they  pass  through  car- 
bonates to  oxides.  A  number  of  other  oxalates  are  fairly  insoluble  in  dilute 
alcohol  and  can  be  determined  volumetrically  by  potassium  permanganate. 

The  chromates  of  barium,  bismuth  and  lead  are  precipitated  by  potassium 
chromate  from  a  solution  containing  only  a  weak  acid  in  the  free  state,  prefera- 
bly chromic  acid.  The  precipitates  may  be  dried  and  weighed,  or  the  chromic 
radical  determined  volumetrically  and  the  equivalent  of  metal  found  by  calcula- 
tion. 

The  phosophates  of  aluminum  and  calcium,  and  the  ammonium  phosophates  of 
cadmium,  manganese  and  zinc  are  practically  insoluble  in  dilute  ammonia;  on 
ignition  the  latter  pass  to  the  pyrophosphates.  As  a  precipitant  the  substitution 
of  a  pyrophosphate  for  a  phosphate  has  been  advised  for  a  number  of  metals. 

The  platinchlorides  of  potassium,  ammonium,  thallium,  cerium,  rubidium 
and  ruthenium  are  crystalline  compounds  insoluble  in  dilute  alcohol  to  a  greater 
or  less  degree,  and  may  be  dried  without  decomposition.  An  alternate  method 
is  to  isolate  and  determine  the  platinum  contained. 

The  ferrocyanides  of  copper,  bismuth,  cadmium  and  gallium,  and  the  iodides 
of  bismuth,  thallium,  palladium  and  lead  are  sufficiently  insoluble  for  the  pur- 
pose of  separation,  though  other  methods  for  determination  of  these  metals  are 
preferred  as  a  rule. 


292  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Compounds  of  certain  metals  with  weak  acids  in  nearly  neutral  solutions 
are  decomposed  on  boiling,  with  the  separation  of  all  the  base  as  a  basic  com- 
pound ;  such  as  ferric  and  aluminic  succinates,  acetates,  etc.  The  method  ia 
one  for  separation  rather  than  determination. 

Nitroso-beta-napthol  precipitates  several  of  the  metals  from  an  acetic  solution 
and  gives  excellent  results  as  a  separant.  It  has  not  come  into  general  use, 
however. 

Various  specific  reagents  afford  accurate  and  convenient  means  for  the  sep- 
aration and  determination  of  certain  metals.  Such  are  a  sulfocyanide  for 
silver  and  copper,  tartaric  acid  for  potassium,  caesium  and  rubidium; 
ammonium  chloride  in  conjunction  with  alcohol  for  the  platinum  group; 
ammonia  for  uranium;  a  cyanide  for  silver;  tungstic  acid  for  barium  and  cal- 
cium; molybdic  acid  for  barium  and  lead;  gallic  acid  for  antimonic  com- 
pounds; etc. 


Argentic  nitrate  precipitates  the  halogens  and  hydrosulfuric,  phosphoric, 
arsenic,  chromic,  ferrocyanic,  ferricyanic,  sulfocyanic,  and  uric  acids  as  silver 
compounds,  and  as  a  rule  quite  completely.  Formic  and  a  few  other  organic 
acids  reduce  silver  nitrate  in  a  hot  aqueous  solution  to  metallic  silver. 

Barium  chloride  is  the  most  common  precipitant  for  sulfuric  acid  and  is 
sometimes  used  for  phosphoric,  chromic,  selenic  and  silicofluoric  acids. 
Barium  hydrate  is  the  usual  precipitant  for  carbonic  acid.  Calcium  chloride 
precipitates  phosphoric,  hydrofluoric,  oxalic,  tartaric  and  citric  acids,  and 
carbonic  acid  in  alkaline  solution. 

Lead  acetate  precipitates  arsenic,  phosphoric,  chromic,  vanadic,  hydrosul- 
furic,  molybdic,  citric  and  hydrofluoric  acids.  Mercurous  nitrate  precipitates 
molybdic,  phosphoric  and  chromic  acids. 

Magnesic-ammonic  chloride,  molybdic  acid,  ferric  hydrate,  and  uranium  nitrate 
each  react  with  phosphoric  and  arsenic  acids  to  form  insoluble  compounds. 
The  first  named  reagent  is  generally  employed  for  a  gravimetric  determination, 
the  others  for  separation  only. 

As  with  the  metals,  various  specific  reagents  are  available. 

On  the  evaporation  of  acid  solutions  of  silicic  and  tungstic  acids  to  dryness, 
these  bodies  pass  to  a  form  insoluble  in  water  and  dilute  mineral  acids,  and 
may  be  filtered,  ignited,  and  weighed  as  anhydrous  oxides. 

Certain  acid  radicals  can  be  determined  gravimetrically  only  after  conversion 
to  a  higher  or  lower  form;  such  are  a  chlorate,  reducible  by  zinc  and  sulfuric 
acid  to  a  chloride;  a  thiosulfate  or  sulfite  oxidized  by  bromine  to  a  sulfate;  a 
nitrite  converted  to  a  nitrate ;  etc.  In  mixtures  of  the  various  oxygen  com- 
pounds of  one  acidogen  element,  the  oxidation  or  reduction  may  be  done  by  a 
volumetric  solution. 

Solid  mixtures  of  a  neutral  salt  with  free  acid  can  often  be  separated  by 
washing  out  the  acid  by  alcohol,  alcohol -ether,  or  other  solvent  in  which  the 
salt  is  practically  insoluble.  Volatile  acids  are  separated  from  fixed  acids  by 
distillation,  usually  to  be  repeated  several  times. 

Volumetric  methods. 

Potassium  permanganate  has  perhaps  the  widest  range  of  application  of  any  of 
the  volumetric  solutions.  The  lower  compounds  of  iron,  tin,  antimony,  copper, 
uranium,  thallium,  molybdenum,  vanadium,  titanium,  and  tellurium  in  acid  solu- 
tions are  raised  to  higher  states  of  oxidation;  while  in  an  alkaline  solution  (or 
one  at  most  but  slightly  acid),  manganese  passes  to  insoluble  perhydrate. 


THE    METALS    AND    COMMON    ACJDS.  293 

Like  the  metals,  nitrous  acid  is  oxidized  to  nitric,  ferrocyanic  to  ferricyanic, 
sulfocyanic  to  hydrocyanic,  sulfurous  to  sulfuric,  etc. 

When  finely  divided,  some  metals,  such  as  arsenic,  tin  and  copper,  and  sub  - 
oxides  such  as  cuprous  oxide,  may  be  dissolved  in  ferric  chloride  and  the 
resulting  ferrous  chloride  titrated  by  permanganate.  Similarly,  certain  sulfides 
when  freshly  precipitated  are  oxidized  to  sulfates  on  digestion  with  a  ferric 
chloride  solution,  the  reagent  reduced  to  ferrous  chloride.  Conversely,  nitric 
acid  is  reduced  to  nitric  oxide  by  ferrous  chloride,  an  equivalent  of  ferric  chloride 
resulting. 

Oxalic  acid  is  oxidized  by  potassium  permanganate  to  carbon  dioxide  and 
water.  Since  many  of  the  metals  form  oxalates  insoluble  in  dilute  alcohol, 
they  may  be  precipitated  as  oxalates  in  a  dilute  alcoholic  solution,  the  precipi- 
tate filtered  and  decomposed  by  dilute  sulfuric  acid,  the  free  oxalic  radical 
titrated  by  permanganate,  and  the  weight  of  the  metal  calculated.  Metals  ad- 
mitting of  this  procedure  are  lead,  zinc,  calcium,  nickel,  cobalt,  cadmium, 
bismuth,  cerium,  lanthanum,  and  didymiurn.  A  shorter  method  is  to  precipi- 
tate the  metal  by  a  known  weight,  a  moderate  excess,  of  oxalic  acid  or  am- 
monium oxalate,  and  titrate  the  excess  in  an  aliquot  part  of  the  filtrate. 

A  similar  plan  may  be  followed  with  a  few  metals  using  a  ferrocyanide  as 
the  precipitant,  this  oxidized  to  ferricyanic  acid  on  titration  by  permanganate. 

Hydrogen  peroxide  in  the  form  of  a  weak  standard  solution  may  be  employed 
for  direct  titration,  but  is  more  often  used  as  an  adjunct  to  permanganate. 
Acting  on  the  one  hand  as  an  oxidizer  of  protoxides  or  suboxides  or  a  perducer 
of  lower  compounds  or  as  a  reducer  of  peroxides  or  per-salts,  and  on  the  other 
as  a  reducer  of  permanganate,  many  metals  can  be  determined  by  back  titra  • 
tion.  However,  methods  based  on  the  expulsion  of  the  excess  of  hydrogen 
peroxide  by  moderately  protracted  ebullition  of  a  solution  are  vitiated  by  the 
fact  that  a  dilute  aqueous  solution  of  the  peroxide  may  be  boiled  for  a  long 
time  yet  some  peroxide  remain  dissolved. 

Of  the  standard  acids,  hydrochloric  and  nitric  are  well  adapted  for  determina- 
tions of  the  caustic  alkalies  and  earths,  their  carbonates  and  bicarbonates ; 
sulfuric  and  oxalic  acids  are  not  so  suitable  for  the  "earths  and  earthy  carbon- 
ates on  account  of  the  insolubility  of  the  resulting  compounds.  As  bases,  the 
alkali  hydrates  are  suited  to  the  titration  of  the  inorganic  acids,  and  with 
proper  indicators,  to  the  organic  acids.  The  earthy  hydrates  have  a  more 
limited  use,  since  their  carbonates  are  insoluble,  as  are  their  phosphates,  sul- 
fates and  tartrates. 

A  number  of  the  metals  can  be  determined  by  an  indirect  volumetric  process, 
given  a  neutral  solution  of  a  normal  salt.  The  metal  is  precipitated  as 
hydrate  by  an  excess  of  standard  alkali  or  alkali  carbonate,  and  the  excess  of 
alkali  titrated  by  a  standard  acid.  Many  haloid  salts  are  decomposed  on 
digestion  with  silver  oxide,  the  reaction  yielding  an  insoluble  compound  of 
silver  and  free  base,  the  latter  determinable  by  standard  acid. 

Free  iodine  is  converted  to  hydriodic  acid  by  many  reducing  agents.  By 
standard  iodine  solution  may  be  titrated  thiosulf uric,  hyposulf urous,  sulfurous, 
arsenious,  antimonious  and  hydrocyanic  acids,  using  starch-paste  as  indicator,, 

Sodium  thiosulfate  is  oxidized  to  the  tetrathionate  by  free  iodine.  Any  chem- 
ical compound  that  on  distillation  with  hydrochloric  acid  yields  chlorine 
quantitatively  may  be  determined  by  passing  the  evolved  chlorine  into  a  solution, 
of  potassium  iodide,  then  determining  by  standard  solution  of  thiosulfate  the 
iodine  set  free.  Many  peroxides  and  per-saits,  the  chromates  and  bichromates* 
iodates,  chlorates,  etc.,  may  be  analyzed  in  this  way.  Finely  divided  metals 
that  unite  directly  with  iodine,  and  sulfldes  that  exchange  an  atom  of  sulfur 


294  QUANTITATIVE    CHEMICAL   ANALYSIS. 

for  one  of  iodine  can  be  determined  by  a  reverse  titration,  as  well  as  a  few 
compounds  that  decompose  potassium  iodide  with  absorption  or  liberation  of 
iodine. 

Potassium  bichromate  is  a  direct  oxidizer  like  permanganate,  though  having 
fewer  applications.  That  it  is  not  decomposed  by  most  varieties  of  organic  mat- 
ter in  cold  dilute  solution  is  an  advantage  at  times,  as  in  the  titration  of  inor- 
ganic bodies  in  presence  of  organic  matter.  The  standard  solution  reacts  with 
ferrous,  stannous,  and  cuprous  compounds,  and  arsenious,  antimonious  and 
nitrous  acids.  It  may  be  used  also  as  a  volumetric  precipitant  for  lead  and  a 
few  other  metals. 

The  haloid  compounds  of  silver  being  quite  insoluble,  all  soluble  inorganic 
bodies  containing  a  halogen  can  be  titrated  directly  by  silver  nitrate,  cessation 
of  precipitation  marking  the  end- point.  Many  carbonates,  nitrates,  and  like 
salts  can  be  converted  to  chlorides  by  repeated  evaporation  with  hydrochloric 
acid,  and  the  chlorates  and  perchlorates  by  nascent  hydrogen,  and  the  chloride 
titrated .  Silver  nitrate  is  also  used  for  the  titration  of  cyanides,  sulf ocyanides 
and  soluble  sulfides;  ammonium  silver  nitrate  for  sulfides  in  presence  of 
chlorine. 

Sodium  chloride  (or  bromide")  forms  with  silver  salts  insoluble  silver  chloride 
(or  bromide)  and  is  the  specific  volumetric  solution  for  this  metal,  though  others 
are  occasionally  used.  In  connection  with  silver  nitrate,  it  may  be  used  in  a 
reverse  titration  for  the  determination  of  the  halogens  combined  with  metals. 

Oxalic  acid  or  ammonium  oxalate  is  sometimes  used  as  a  precipitant  for  lead 
and  calcium,  the  end-point  found  by  filtering  a  portion  and  testing  by  the 
titrand.  Various  peroxides  and  per-salts  are  reduced  by  oxalic  acid,  the  deter- 
mination made  by  a  back  titration  with  permanganate. 

Stannous  chloride  reduces  most  peroxides  and  per-salts  to  the  normal,  and 
some  normal  salts  to  sub-compounds.  Examples  are  ferric,  chromic  and  man- 
ganic salts  reduced  to  the  normal,  cupric  salts  to  cuprous,  and  mercuric  salts 
to  mercurous  and  later  to  metallic  mercury.  The  indicator  for  these  titrations 
is  usually  ferric  sulfocyanide,  bleached  by  the  slightest  excess  of  the  titrand. 

Potassium  and  sodium  sulfides  precipitate  many  of  the  metals  as  normal  sul- 
fides and  are  chiefly  used  lor  copper  and  lead  determinations.  The  end-point 
is  found  by  spotting  drops  of  the  titrate  on  paper  impregnated  with  a  lead  com- 
pound which  shows  a  brown  stain  with  the  least  excess  of  sulfide. 

A  number  of  reagents  are  applied  for  special  determinations. 


Colorimetric  methods. 

Comparatively  few  of  the  metals  form  compounds  soluble  In  water  of  a 
color  sufficiently  deep  for  a  colorimetric  determination.  Direct  comparisons 
are  possible  with  aqueous  solutions  of  the  salts  of  gold,  platinum,  copper  and 
chromium,  and  the  ammonium  salts  of  copper,  cobalt  and  nickel.  Specific 
compounds  of  ferric  and  ferrous  iron,  chromium,  manganese,  lead,  gold,  plat- 
inum, vanadium,  etc.,  possess  a  color  suited  to  comparison.  In  all  cases,  the 
methods  are  best  adapted  to  small  proportions  of  the  metal  in  a  mixture,  and 
where  strict  accuracy  is  not  essential. 


ELEMENTARY   ORGANIC   ANALYSIS.  295 


ELEMENTARY  ORGANIC  ANALYSIS. 

A  determination  of  the  elements  composing  an  organic  or  partly  organic  body 
is  resorted  to  for  the  purpose  of  deducing  an  empirical  formula,  or  to  learn  the 
proportions  of  the  elements,  or  by  calculation,  the  compounds  of  a  mixture.  In 
most  cases  there  are  to  be  determined  carbon  and  hydrogen,  frequently  also 
oxygen  and  nitrogen,  and  sometimes  sulfur,  phosphorus,  a  halogen,  etc. 

The  purity  of  an  organic  compound  submitted  to  an  ultimate  analysis  to 
learn  its  formula  is  always  to  be  assured  by  appropriate  tests.  A  commercial 
article,  known  to  be  more  or  less  impure  is  usually  analyzed  without  previous 
preparation  beyond  drying  or  the  evaporation  of  an  aqueous  solution  to  dryness. 

All  the  methods  involve  the  destruction  of  the  compound.  Carbon  and 
hydrogen  are  converted  into  carbon  dioxide  and  water  which  are  collected  and 
weighed  and  the  weights  of  the  elements  calculated ;  nitrogen  is  either  isolated 
as  a  gas  whose  volume  is  measured,  or  converted,  by  assimilation  of  hydrogen, 
into  ammonia  to  be  determined  gravimetrically  or  volumetrically;  oxygen  is 
almost  invariably  determined  by  difference;  sulfur  and  phosphorus  by  oxida- 
tion to  their  respective  acids  and  a  gravimetric  determination;  and  other  ele- 
ments by  well  known  general  or  special  methods,  after  destruction  of  the 
organic  matter  by  oxidation. 

CARBON  AND  HYDROGEN. 

For  the  determination  of  these  elements  the  organic  substance  is  burned  by 
oxygen  furnished  by  some  easily  reduced  metallic  oxide  or  other  compound,  or 
in  a  current  of  oxygen  or  air,  or  both. 

The  simplest  form  of  apparatus  is  that  originated  many  years  ago  by  Liebig 
and  still  in  occasional  use.  The  combustion  is  done  in  a  tube  A,  Fig.  158, 
about  18  inches  long  and  1-2  inch  in  bore,  made  of  a  special  refractory  glass. 


Fig.  158. 

The  tube  is  drawn  out  at  one  end  to  a  narrow  tail  B  which  is  sealed,  the  other 
end  left  open.  It  is  charged  for  a  few  inches  with  granular  cupric  oxide,  then 
with  the  organic  body  previously  mixed  with  granular  copper  oxide,  then  to 
the  end  with  copper  oxide.  The  end  is  closed  by  a  cork  carrying  a  short  glass 
tube  C,  and  the  tube  laid  on  the  supporting  ridges  of  a  chauffer  D. 

A  light  glass  tube  E,  holding  dry  granular  calcium  chloride  kept  in  place  by 
plugs  of  cotton,  is  weighed  and  connected  by  a  short  piece  of  rubber  tubing 
with  C;  a  light  absorption  bulb  F,  partly  filled  with  a  strong  solution  of 
potassium  hydrate,  is  also  weighed  and  connected  to  E.  It  is  ascertained  that 
all  the  connections  are  air-tight,  and  the  apparatus  is  in  readiness  for  the 
combustion. 

The  charcoal  in  the  chauffer  is  kindled  a  few  inches  back  from  the  cork,  and 
as  the  copper  oxide  becomes  hot,  the  remainder  of  the  tube  is  heated,  grad- 


296 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


SliBSiKll^B^am 


•  «  —  -  mm-m.m;mmu. 


IlliliffiifllPW 


ually  approaching  the  tail.  The  organic  compound  first  chars,  then  burns  at 
the  expense  of  the  oxide  in  contact  with  it,  the  carbon  becoming  carbon 
dioxide,  the  hydrogen  water,  and  the  nitrogen  escaping  uncombined.  The 
formation  of  gases  in  the  tube  and  their  expansion  by  heat  drives  them  out 
through  E  and  F.  An  indication  of  the  presence  of  nitrogen  in  the  organic 
body  is  that  bubbles  pass  through  the  potash  during  the  entire  combustion, 
while,  if  absent,  all  the  gas  is  absorbed  toward  the  close  of  the  operation. 

The  water  from  the  combustion  is  retained  in  E  by  combination  with  the 
anhydrous  calcium  chloride,  forming  the  hydrated  compound  CaClg.Baq.  The 
carbon  dioxide  unites  with  the  potassium  hydrate  in  F  to  form  potassium 
carbonate. 

The  combustion  tube  remains  filled  with  gas  after  the  combustion  is  ended, 

and  to  sweep  this  also 
through  E  and  Fthe  open 
end  of  F  is  connected  by 
a  rubber  tube  to  an  aspi- 
rator bottle,  and  over  the 
tail  of  the  combustion 
tube  is  slipped  a  rubber 
tube  leading  from  a  sup- 
ply of  purified  and  dried 
air.  The  end  of  the  tail 
is  broken  off  by  nippers, 
Fig.  159.  and  air  drawn  slowly 

through  the  apparatus  until  the  gases  in  A  have  been  swept  out.  E  and  F 
are  weighed  after  cooling,  and  the  increase  over  their  previous  weights 
are  the  respective  weights  of  water  and  carbon  dioxide  they  have  taken  up. 
The  weights  of  carbon  and  hydrogen  in  the  organic  compound  are  calculated 
from  these  figures,  observing  that  if  any  moisture,  combined  water,  or  carbon- 
ates  decomposed  by  heat  were  in  the  compound,  a  deduction  from  the  weights 
of  water  and  carbon  dioxide  must  be  made  accordingly;  these  corrections  are 
deduced  from  separate  tests. 

Certain  defects  in  Liebig's  apparatus  are  remedied  in  the  modern  furnace  and 
train  which  differs  in  several  particulars,  though  arranged  on  the  same  general 
plan. 

1.  The  charcoal  furnace  has  been  superseded  by  one  burning  illuminating  gas, 
Fig.  159.  The  combustion  tube  is  supported  above  a  row  of  Bunsen  burners, 
each  with  a  stopcock.  Above,  before  and  behind  the  tube,  an  inch  or  two  dis- 
tant, are  movable  plates  of  fireclay  that  serve  to  radiate  the  heat  from  the 
burners  to  the  upper  part  of  the  tube  and  to  protect  it  from  draughts. 

Fletcher's  furnace,  Fig.  160.  is  entirely  of  fireclay  made  in  short  sections 
that  may  be  joined  to  accommodate  any  length  of  tube.  A  row  of  burners 

stands  at  the  front  of  the  furnace,  inclined  to 
throw  the  flames  under  the  tube,  entering  at 
perforations  in  the  front  of  the  furnace. 

Furnaces  burning  kerosene  or  gasoline  have 
been  designed  for  laboratories  not  provided 
with  illuminating  gas,  but  are  not  so  conveni- 
ent as  the  latter.    For  a  platinum  combustion 
tube  no  furnace  is  needed,  a  row  of  Bunsen 
burners  supply  ing  sufficient  heat  for  the  purpose. 
2.  A  glass  combustion  tube,  though  it  has  the 
Fig.  160.  merit  of  allowing  an  observation  of  the  prog- 


ELEMENTARY    ORGANIC   ANALYSIS.  297 

ress  of  the  combustion  throughout  the  operation,  can  be  heated  only  to  dull 
redness,  as  above  this  point  there  is  danger  of  the  tube  becoming  softened  and 
perforated  by  the  pressure  of  the  gases  within;  nor  is  it  always  easy  to  secure 
a  highly  refractory  glass  for  the  purpose.  So  that  for  bodies  leaving  a  dense 
coke  on  heating,  it  is  better  to  substitute  a  tube  of  another  material. 

A  porcelain  tube  will  bear  a  very  high  temperature,  and  with  proper  care  in 
heating  and  cooling  can  be  used  for  several  combustions.  The  tube  must  be 
well  glazed  interiorly,  and  of  a  sufficient  length  to  project  far  enough  beyond 
the  furnace  that  the  corks  or  rubber  stoppers  closing  the  ends  will  never 
become  heated  to  the  point  of  charring. 

A  platinum  tube  is  far  superior,  except  as  to  opacity,  to  one  of  any  other 
material;  the  cumbrous  and  expensive  furnace,  so  unpleasant  in  hot  weather, 
is  not  needed,  and  one  escapes  the  frequent  annoyance  of  a  determination 
spoiled  by  the  perforation  or  cracking  of  a  glass  or  porcelain  tube.  Though 
the  first  cost  is  high,  for  routine  work  it  is  more  than  compensated  by  the  dura- 
bility —  the  author  has  made  over  a  thousand  combustions  in  a  platinum  tube 
without  its  appreciable  injury.  That  platinum  is  permeable  to  gases  when  at  a  red 
heat  has  been  shown  to  be  of  no  significance  in  the  ordinary  combustion  process. 

The  best  form  of  tube  is  shown  in  Fig.  161 ;  one  end  is  contracted  to  a  narrow 
^  prolong,  and  in  the  other  end, 

7/  \  ^  reinforced  by  a  German  silver 

paJ '£ /  band,  is  inserted  a  hollow  metal 

plug  ground  to  fit  gas-tight, 
Fig.  161.  and  terminating  in  a  narrow 

metal  tube;  but  a  cork  and 
glass  tube  will  answer  the  purpose  quite  as  well  or  better. 

Schwartz  proposes  a  tube  of  seamless  copper,  the  corks  at  the  ends  being 
protected  from  conducted  heat  by  water-cooled  jackets,  and  Summers  *  arranges 
for  a  short  platinum  tube  on  the  same  plan.  Shimer  f  still  farther  reduces 
the  cost  of  platinum  apparatus  by  substituting  for  the  tube  a  large  platinum 
crucible  closed  by  a  water- jacketed  stopper. 

3.  Instead  of  oxidizing  the  organic  body  by  copper  oxide,  lead  chromate,  etc., 
alone,  it  is  more  prudent  to  transmit  during  the  operation  or  near  the  close,  a 
current  of  pure  oxygen,  finally  replacing  it  by  a  stream  of  air.    The  oxygen  and 
air  are  drawn  from  gasometers  or  gas-bags  or  from  metal  tanks,  and  before 
entering  the  combustion  tube  are  purified  by  being  passed  through  a  solution 
of  caustic  potash  to  remove  carbon  dioxide,  chlorine,  etc.,  then  through  con- 
centrated sulfuric  acid  to  absorb  moisture.    It  is  said  that  oxygen  compressed 
in  metal  cylinders  sometimes  contains  gaseous  hydrocarbons  taken  up  from  the 
oil  used  to  lubricate  the  compressor,  and  that  gases  stored  in  rubber  bags  may 
also  contain  carbonaceous   compounds ;  where   this    is  suspected  the  gas  is 
passed  first  through  a  short  porcelain  tube  containing  fibrous  asbestos  kept  at 
a  dull  red  heat,  or  through  a  capillary  platinum  tube  heated  to  redness  by  a 
Bunsen  burner,  thence  to  the  purifying  tubes  precedent  to  the  combustion  tube. 

4.  The  organic  body  may  be  burned  in  oxygen  alone,  omitting  any  oxidizing 
reagent.    A  solid  or  semi-solid  body  Is  weighed  in  a  porcelain  or  platinum 
boat,  Fig.  162,  and  after  the  combustion  the  boat  reweighed,  it  containing  any 
fixed  inorganic  constituents  of  the  substance.    If  the  organic  body  is  a  liquid 
of  high  boiling  point,  the  boat  is  filled  with  sand  or  powdered  copper  oxide  and 
the  liquid  imbibed  therein.    A  volatile  liquid  is  weighed  in  a  thin  glass  bulb 
with  a  capillary  stem  through  which  the  liquid  gradually  exudes  as  the  bulb 


*  Journ.  Amer.  Chem.  Socy.  1896—1087. 
t  Idem,  1899-557. 


298 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


becomes  heated;  or  the  bulb  may  be  sealed  up  after  filling,  and  eventually 

broken  by  the  expansion  of  the 
liquid  on  heating.  An  extra  long 
combustion  tube  is  needed  for 
volatile  liquids,  that  the  surface  of 
hot  copper  oxide  traversed  by  the 
gases  may  be  ample  to  oxidize  any 
organic  vapor  or  carbon  mon- 
oxide produced,  and  the  tube  must 
Fig  162  V  -V  be  neated  stowly  and  cautiously. 

5.  During  the  combustion  there 

may  be  formed  small  amounts  of  oxides  of  nitrogen,  and  these  passing  to  the 
potash  bulb  will  be  absorbed  and  count  as  carbon  dioxide.  The  oxides  are 
decomposed  on  contact  with  hot  metallic  copper,  their  oxygen  combining  with 
the  metal.  For  this  purpose  a  roll  of  copper  foil  is  placed  between  the  copper 
oxide  and  the  asbestos  plug  next  the  cork  at  the  front  of  the  tube ;  the  copper 
should  be  free  from  sulfur,  and  previously  superficially  oxidized  and  reduced  to 
remove  occluded  hydrogen. 

Behind  the  copper  foil  is  another  plug  of  asbestos,  then  a  few  inches  of 
copper  oxide  whose  purpose  is  to  oxidize  any  volatile  matter  or  carbon  mon- 
oxide resulting  from  incomplete  combustion.  It  is  held  to  place  by  another 
asbestos  plug.  As  substitutes  for  the  copper  oxide  there  have  been  proposed 
manganic  oxide,  or  where  the  combustion  is  done  in  a  stream  of  oxygen,  platin- 
ized asbestos,  or  simply  fibrous  asbestos,  the  latter  acting  mechanically  to  pro- 
mote the  reaction  between  the  carbon  monoxide  and  oxygen. 

Organic  bodies  containing  sulfur  are  mixed  with  lead  chromate,  as  this 
reagent  both  oxidizes  sulfur  and  sulfurous  acid  and  combines  with  sulfuric 
acid.  Semi -organic  bodies  containing  alkalies  leave  a  residue  of  alkali  carbon- 

ate  on  burning,  and  the  results 
for  carbon  are  consequently  too 
low;  such  bodies  are  mixed 
with  potassium  bichromate  and 
Fig.  163.  lea(*  chromate,  the  excess  of 

chromic  acid  in  the  former  rea- 
gent expelling  the  carbon  dioxide  from  the  alkali  carbonate. 

6.  The  drying  tube,  Fig.  163,  is  provided  with  an  extra  empty  bulb  at  A  in 
order  that  the  larger  part  of  the  water  produced  may  condense  therein,  and 
the  calcium  chloride  remain  drier  and  more  efficient  from  having  less  moisture 
to  absorb.  For  the  straight  tube  may  be  substituted  a  U-tube  containing 
anhydrous  phosphoric  acid  or  glass  splinters  saturated  with  concentrated  sul- 
furic acid,  or  a  specially  arranged  tube  holding  both  these  absorbents  which 
are  more  energetic  desiccators  than  calcium  chloride. 

As  an  absorbent 
of  carbon  dioxide  a 
strong  solution  of 
caustic  potash  or 
soda  is  the  usual 
reagent.  Numer- 
ous forms  of  con- 
tainers have  been 
invented,  all  de- 
signed to  secure  as 
intimate  and  pro- 
longed contact  be- 


Fig.  164. 


ELEMENTARY    ORGANIC    ANALYSIS. 


299 


tween  the  lye  and  gas  as  possible,  though  the  important  feature  of  presenting 
a  small  surface  of  glass  to  the  atmosphere  has  generally  been  neglected. 
The  forms  shown  in,  Fig.  164  need  no  description;  that  of  Bowen  is  one  of 
the  best  in  point  of  strength  and  compactness. 

Since  the  current  of  unabsorbable  gases  passing  through  the  bulb  carries 
away  a  little  moisture  from  the  potash  solution  (unless  it  is  unusually  highly 
concentrated),  a  small  calcium  chloride  tube  is  attached  and  weighed  with 
it;  in  Fig.  164,  the  horizontal  tube  is  fitted  to  the  bulb  by  a  ground  joint. 

In  cases  where  a  large  weight  of  carbon  dioxide  is  expected  from  a  combus- 
tion two  potash-bulbs  should  be  joined  tandem.  Many  believe  that  a  U-tube 
filled  with  soda-lime  (a  mixture  of  granular  sodium 
and  calcium  hydrates)  is  a  safer  absorbent  for  carbon 
dioxide  than  potash  solution  ;  the  tube  is  guarded  from 
loss  of  moisture  by  an  attached  tube  of  chloride  of 
calcium.  For  the  soda-lime  tube  some  would  sub- 
stitute one  filled  with  pumice  fragments  saturated 
with  the  strongest  potash  lye. 

Before  beginning  a  combustion  the  perfect  gas- 
tightness  of  the  corks  and  rubber  connections  must  be 
assured.  The  tube  A  of  a  small  mercury  manometer 
Fig.  165,  is  connected  to  the  exit  tube  of  the  potash 
bulb,  and  the  entrance  to  the  combustion  tube  is 
closed.  Air  is  blown  in  at  B  until  the  mercury  in  C 
is  depressed  into  the  bend,  and  the  stopcock  is  closed. 
If  the  inequality  in  height  of  the  columns  of  mercury 
remains  unchanged  or  nearly  so  for  ten  minutes,  the 
connections  may  be  considered  tight,  but  it  must  be 
remembered  that  during  this  time  any  variation  in  the 
temperature  of  the  air  inclosed  in  the  train  will  alter 


s' 


the  pressure  on  the  mercury.  The  usual  directions  are  to  produce  a  partial 
vacuum  in  the  train  instead  of  compression,  but  except  where  the  tension  dur- 
ing the  combustion  is  to  be  less  than  atmospheric,  the  latter  is  the  better  plan,  as 
an  imperfect  connection  may  yet  be  secure  against  external  atmospheric  pressure. 

The  details  to  be  observed  in  making  a  combustion  differ  somewhat  accord- 
ing to  the  character  of  the  organic  body  and  individual  practice,  but  in 
any  case  require  the  undivided  attention  of  the  operator.  The  usual  routine 
is  about  as  follows,  the  directions  applying  to  a  porcelain  or  platinum  tube  and 
the  use  of  a  current  of  oxygen  for  oxidation. 

The  combustion  tube  having  been  cleaned  and  the  copper  oxide  near  the 
front  placed  in  position,  a  current  of  pure  dry  air  is  passed  through  it  to 
remove  adhering  moisture.  A  boat  containing  a  known  weight  of  some  pure 
organic  compound  such  as  cane-sugar,  is  pushed  to  the  middle  of  the  tube 
followed  by  a  roll  of  platinum  gauze.  The  potash  bulb  and  prolong  and  the 
chloride  of  calcium  tube  are  weighed  and  connected  to  the  combustion  tube. 
Then  the  tightness  of  the  connections  is  tested  by  the  manometer. 

The  section  of  the  combustion  tube  containing  the  copper  oxide  is  heated  to 
redness  while  a  slow  current  of  pure  oxygen  is  passed  through  the  apparatus. 
Then  the  burners  beneath  the  platinum  gauze  are  lighted,  aud  finally  those 
beneath  the  boat.  The  flames  are  raised  until  the  tube  is  at  a  dull  red  heat. 
The  sugar  will  be  burned  in  a  few  minutes  time,  whereupon  the  stream  of 
oxygen  is  replaced  by  one  of  pure  air,  and  the  flames  gradually  lowered  and 
finally  extinguished,  those  under  the  copper  oxide  last.  When  the  air  has 
displaced  the  (heavier)  oxygen,  the  drying  tube  and  potash  bulb  and  prolong 


300  QUANTITATIVE    CHEMICAL    ANALYSIS. 

are  detached,  their  orifices  closed  by  glass  plugs  or  other  means,  allowed 
to  cool  in  the  balance  case,  and  weighed. 

The  apparatus  is  now  in  readiness  for  the  analysis  of  any  organic  substance, 
the  combustion  of  sugar  serving  to  indicate  by  the  close  agreement  of  the 
results  with  those  calculated  from  the  formula,  that  no  defects  exist  in  the 
apparatus  or  connections. 

A  roll  of  metallic  copper  in  the  combustion  tube  is  unnecessary  for  non- 
nitrogenous  bodies.  la  burning  those  containing  nitrogen  it  is  essential  that 
the  copper  be  not  superficially  oxidized,  as  it  would  then  be  powerless  to 
decompose  the  oxides  of  nitrogen.  For  organic  bodies  containing  a  halogen, 
there  may  be  interposed  between  the  chloride  of  calcium  tube  and  the  potash 
bulb,  a  U-tube  containing  in  one  limb  dry  granular  cuprous  chloride  to  absorb 
halogen  acids,  and  in  the  other  limb  silver  foil  to  retain  free  halogens}  and 
calcium  chloride  to  absorb  any  water  from  the  cuprous  chloride. 

For  small  amounts  of  organic  bodies  that  are  readily  combustible,  the  cur- 
renp  of  oxygen  may  be  dispensed  with  and  air  alone  relied  on  for  their  oxida- 
tion. The  process  here  is  to  be  carried  on  more  slowly  than  where  oxygen  gas 
is  used.  In  either  case,  a  blank  combustion  should  not  cause  an  increase  in 
weight  of  the  drying  tube  or  potash  bulb  of  more  than  a  milligram. 

Since  the  mixture  of  oxygen  or  air  with  a  highly  volatile  liquid  is  explosive, 
the  combustion  of  the  latter  is  begun  by  passing  a  stream  of  pure,  dry  nitrogen 
until  it  is  gasified,  relying  on  the  hot  copper  oxide  to  oxidize  what  vapor 
reaches  it. 

Levoir*  proposes  an  apparatus  for  the  combustion  of  organic  compounds 
that  dispenses  with  the  ordinary  combustion  furnace.  The  compound  is  held 
in  a  small  platinum  tube  and  surrounds  a  spiral  of  platinum  wire.  The  ar- 
rangement is  placed  in  a  glass  combustion  tube,  the  ends  of  the  platinum  wire 
projecting  and  the  combustion  tube  filled  with  oxygen.  An  electric  current 
is  passed  through  the  wire  sufficient  to  heat  it  to  redness,  and  the  organic 
body  burns  rapidly  in  the  oxygen. 

Berthollet  effects  the  combustion  in  a  '  calorimetric  bomb.'f  This  is  a  thick 
steel  cup  lined  with  platinum  and  having  a  tube  projecting  through  the  cover. 
A  platinum  basket  containing  the  organic  body  is  hung  about  the  center  of  the 
cup  and  in  contact  with  it  is  a  thin  platinum  wire  connected  to  platinum  rods 
passing  to  the  exterior  of  the  apparatus.  The  cover  is  fastened  on  tightly  and 
oxygen  forced  in  until  the  pressure  is  about  25  atmospheres,  the  tube  closed, 
and  a  current  of  electricity  passed  through  the  wire  heating  it  to  redness  and 
igniting  the  organic  body.  The  combustion  is  instantaneous  and  total  <  The 
resulting  gases  are  withdrawn  through  the  tube  in  the  cover  and  passed  through 
absorbing  tubes  as  usual. 

The  ultimate  analysis  of  a  compound  gas  or  a  mixture  of  gases  can  be  done 
by  passing  a  measured  volume  through  a  combustion  tube  arranged  as  for  vola- 
tile organic  bodies.  Or  with  the  gas  may  be  united  a  suitable  proportion  of 
oxygen,  hydrogen,  or  oxyhydrogen,  as  needed,  and  the  mixture  exploded  in  a 
eudiometer  or  burned  by  platinized  asbestos. 


Moist  combustion.}:  The  elements  of  many  organic  bodies  are  oxidized  par- 
tially or  completely  by  the  action  of  an  aqueous  solution  of  some  strong  oxidi- 
zer.  Of  the  several  reagents  proposed,  potassium  permanganate  and  chromic 


*  Chem.  News,  1890-1-37. 

t  Idem,  1888    2—284 ;  Stlllman  Engineering  Chem.  126. 

I 


ELEMENTARY    ORGANIC    ANALYSIS.  301 

acid  have  a  general  application,  while  potassium  manganate,  chloric  acid,  hydro- 
gen peroxide,  lead  peroxide,  and  others,  are  available  only  in  special  cases. 
According  to  the  nature  of  the  organic  body  and  the  oxidizer,  acidity,  temper- 
ature and  time  of  digestion,  and  other  conditions,  there  is  determined:  (1)  the 
weight  of  oxygen  required  for  oxidation,  in  which  case  the  analysis  is  rather 
of  the  nature  of  a  proximate  than  an  ultimate  one;  or,  (2)  the  weight  of  car- 
bon dioxide  produced,  when  the  process  is  but  a  variation  of  the  ordinary  fur- 
nace combustion. 

A.  Oxidation  by  permanganate.  Usually  this  reagent  is  applied  in  a  solution 
made  strongly  alkaline,  though  sometimes  in  conjunction  with  sulfuric  acid. 
The  organic  body,  that  may  be  soluble  or  insoluble  in  water,  is  digested  with  a 
measured  excess  of  a  standard  aqueous  solution  of  potassium  permanganate 
under  fixed  conditions  of  time,  temperature  and  concentration.  In  most 
cases  the  reaction  ends  with  the  removal  of  three  atoms  of  the  available  oxygen 
from  the  reagent,  leaving  insoluble  manganese  dioxide,  though  under  some 
circumstances  all  five  atoms  react.  The  temperature  during  the  operation 
should  be  above  100®  for  complete  oxidation  (Wanklyn).  Smith  states  that 
in  an  acid  solution  there  occurs  a  secondary  reaction  between  the  precipitated 
manganic  oxide  and  the  excess  of  permanganate  with  liberation  of  oxygen,  but 
that  it  may  be  prevented  in  great  part  by  the  presence  of  a  suflMent  quantity 
of  ferric  phosphate.  If  a  chloride  is  contained  in  the  organic  body  it  will  react 
with  sulfuric  acid  to  form  hydrochloric  acid,  this  decomposing  perman- 
ganate. 

After  digestion  for  the  specified  time,  the  solution  is  either  (1)  filtered 
through  asbestos  and  the  excess  of  permanganate  in  the  filtrate  determined, 
either  by  direct  titration  by  oxalic  acid  or  ferrous  sulfate,  or  better  by  a 
back  titration  by  one  of  these  and  permanganate;  or  (2),  the  manganese  in  the 
precipitate  is  determined  by  a  gravimetric  or  volumetric  process;  or  (3), 
without  filtration  both  the  precipitate  and  excess  of  permanganate  are  reduced 
by  excess  of  oxalic  acid  and  the  excess  titrated  back  by  permanganate. 
Prom  the  weight  of  oxygen  consumed  in  the  decomposition  of  the  organic 
body  sometimes  a  calculation  may  show  the  percentage  of  the  oxidizable  ele- 
ments, but  usually  this  is  not  to  be  relied  on,  for  comparatively  few  bodies  are 
completely  oxidized  by  permanganate,  and  a  partial  oxidation  is  not  so  regular 
and  definite  that  any  positive  conclusions  can  be  drawn  from  the  oxygen  con- 
sumed —  analogous  bodies  may  react  quite  differently  when  treated  under  the 
same  conditions.* 

In  the  determination  of  carbon  by  oxidizing  the  organic  body  by  permanga- 
nate and  weighing  the  carbon  dioxide  produced,  the  evolved  gas,  as  in  a  furnace 
combustion,  Is  passed  first  over  dry  calcium  chloride,  then  into  potash  solu- 
tion. But  for  the  permanganate  it  is  better  to  substitute  the  (usually)  more 
energetic  oxidizer  chromic  acid  in  sulfuric  acid  solution. 

For  the  determination  of  the  humus  of  soils  Warrington  and  Peake  digest 
with  permanganate  first  made  alkaline  by  potassium  hydrate,  then  after  acidifi- 
cation by  sulfuric  acid.  In  the  first  digestion  potassium  carbonate  and  probably 
some  potassium  oxalate  are  formed;  in  the  second  the  oxalic  acid  is  decom- 
posed by  the  permanganate,  and  the  carbon  dioxide  formed,  plus  that  liberated 
by  the  sulfuric  acid  from  the  potassium  carbonate,  is  passed  into  a  tube  of 
soda-lime  and  the  increase  in  weight  found.  Determinations  by  permanganate 


*  Journ.  Anal.  Chem.  3—387. 


302  QUANTITATIVE    CHEMICAL   ANALYSIS. 

of  carbon  in  a  soil  averaged  92  per  cent  of  that  given  by  tube-combustion, 
while  the  chromic  acid  process  yielded  only  80  per  cent. 

For  the  determination  of  humus  in  aqueous  solution,  Baulin  *  would  apply 
the  oxidizing  power  of  recently  precipitated  manganic  hydrate.  He  directs  to 
prepare  a  liquid  containing  manganic  hydrate  in  suspension  by  mixing  solutions 
of  potassium  permanganate  and  manganous  sulfate  in  the  ratio  of  three  mole- 
cules of  the  former  to  one  of  the  latter,  boiling  until  the  mutual  decomposition 
is  complete.  The  solution  of  humus,  in  quantity  not  greater  than  will  reduce 
one- half  of  the  manganic  hydrate,  is  added,  and  the  mixture  acidified  by  sul- 
furic  acid  and  boiled  for  eight  hours.  The  humus  acts  to  reduce  the  manganic 
to  manganous  hydrate  which  then  dissolves  in  the  sulfuric  acid.  What  man- 
ganic hydrate  remains  unacted  on  is  determined  by  dissolving  in  standard 
oxalic  acid,  and  titrating  back  the  excess  of  the  latter  by  permanganate.  A 
blank  determination  is  made  with  the  same  weights  of  reagents  as  were  used 
in  the  analysis,  and  the  difference  in  the  volumes  of  standard  permanganate  is 
the  basis  for  calculation. 

B.  Oxidation  by  chromic  acid.  Chromic  acid  in  conjunction  with  sulfuric 
acid  attacks  a  greater  number  of  organic  bodies  than  does  permanganate. 
The  reactions  are  3C-f-4CrO3  +  6H2SO4=3CO2-f  2Cr2(S04)3  +  6H2O;  and 
3H2  +  2CrO3  +  3H2SO4  =  6H2O4-Cr2(SO4)3.  It  is  said  that  the  celluloses  and 
carbohydrates  are  entirely  converted  to  gaseous  products,  as  are  some  typical 
benzenoid  compounds,  and  urea  in  presence  of  a  little  nitric  acid,  but  the  mono- 
basic acids  of  the  fatty  series  are  not  fully  oxidized ;  amid  and  imid  nitrogen 
prevent  complete  oxidation. f 

Up  to  a  certain  concentration  the  aqueous  solution  of  the  two  acids  may  be 
heated  to  boiling  without  an  inter-reaction  taking  place;  at  a  higher  concen- 
tration the  chromic  acid  is  decomposed  —  2CrO3+  3H2SO4  =  Cr2(SO4)8  +  30  -f- 
3H2O  —  the  decomposition  occurring  at  a  lower  temperature  the  greater  the 
concentration.  Hence  the  concentration  of  the  reagent  must  be  adjusted  to 
the  method  employed,  which  may  be  (1),  a  determination  of  the  oxygen  con- 
sumed by  the  organic  compound,  where  the  secondary  reaction  would  increase 
the  result,  and  (2),  a  determination  of  the  carbon  dioxide  evolved,  where  the 
event  of  the  secondary  reaction  is  immaterial. 

(1).  The  determination  of  oxygen  consumed  has  been  applied  by  several 
chemists  to  various  organic  compounds.  Heidenhain  J  remarks  that  quantitative 
oxidation  is  possible  only  with  a  small  number  of  substances,  but  almost 
quantitative  results  are  obtained  with  very  many  substances.  By  experiment 
he  found  that  23  Cc.  of  fifth-normal  potassium  bichromate  mixed  with  15  Cc. 
of  concentrated  sulfuric  acid  could  be  boiled  for  15  minutes  without  appre- 
ciable reduction  of  any  chromic  acid,  and  arranges  that  this  concentration  shall 
not  be  exceeded  in  the  united  volumes  of  the  reagent  and  solution  of  the  organic 
body.  The  mixture  is  heated  to  boiling  for  ten  minutes  in  a  flask  provided 
with  a  simple  form  of  reflux  condenser  to  prevent  evaporation  of  any  great 
amount  of  water.  The  excess  of  chromic  acid  is  determined  by  reduction  by 
standard  ferrous  sulfate  and  back  titration  of  the  excess  of  the  latter.  The  cal- 
culation is  based  on  a  factor  derived  from  experiments  on  the  chemically  pure 
organic  body;  — usually  but  two  determinations,  namely  of  100  per  cent  and 
40  per  cehit,  are  needed  to  construct  a  table  by  interpolation. 


*  Chem.  News,  1890-1— 155. 

t  Idem,  1888—2—21. 

\  Journ.  Anal.  Chem.  1893—71. 


ELEMENTARY   ORGANIC   ANALYSIS. 


303 


Fig.  166. 


2.  The  more  concentrated  solution  of  chromic  acid  in  stronger  sulfuric  acid 
has  a  broader  application  for  the  reason  that  most non- volatile  organic  bodies, 

either  soluble  in  water  or  insoluble, 
are  eventually  completely  oxidized. 
The  advantages  over  the  furnace- 
combustion  process  for  the  deter- 
mination of  carbon  are  the  compara- 
tive simplicity  of  the  apparatus,  the 

possibility  of  analyzing  moist  or  pasty  matter  without  previous 
drying,  and  that  fewer  precautions  are  required  for  certain 
compounds,  such  as  the  organic  salts  of  the  alkalies.  Against 
the  process  is  the  uncertainty  of  a  complete  oxidation  of 
refractory  bodies,  necessitating  an  extra  attachment  for  insur- 
ing the  conversion  of  any  carbon  monoxide  to  dioxide,  the  long 
boiling  needed  for  some  compounds  or  forms  of  elementary 
carbon,  the  inapplicability  to  volatile  bodies,  and  its  lim- 
itation to  the  determination  of  carbon  only. 

The  usual  train  of  apparatus  is  shown  in  Fig.  166.  The  weighed  sample  is 
dropped  into  the  flask  A  and  covered  with  a  strong  solution  of  chromic  acid  or 
potassium  bichromate.  The  funnel-tube  B  is  filled  with  concentrated  sulfuric 
acid.  Through  a  second  hole  in  the  cork  passes  the  end  of  a  reflux  condenser 
C  whose  object  is  to  condense  the  steam  from  A  and  return  the  water,  in  this 
way  avoiding  the  rapid  liquefaction  of  the  chloride  of  calcium  in  the  drying 
tube  D.  This  tube  is  of  a  larger  size  than  that  of  the  usual  combustion  train 
since  it  is  not  to  be  weighed.  The  potash  bulb  E  is  one  of  the  ordinary  forms. 
The  apparatus  is  connected  gas-tight,  water  started  through  the  condenser, 
and  the  sulfuric  acid  run  into  A.  When  the  evolution  of  gas  slackens  the  solu- 
tion is  boiled  and  a  current  of  pure  air  passed  through  the  apparatus  entering 
at  the  funnel  tube.  Finally  the  potash  bulb  is  detached  and  weighed,  observing 
the  usual  precautions. 

During  the  boiling  of  the  mixture  in  A  some  of  the  reagent  may  reach  the 
cork  or  rubber  stopper  and  act  upon  it  with  generation  of  carbon  dioxide.  To 
provide  against  this  a  flask  can  be  had  with  a  ground  glass  stopper  in  which  is 
fused  the  funnel  tube,  the  outlet  tube  projecting  from  the  upper  part  of  the 
neck  of  the  flask. 

Should  the  substance  contain  a  halogen,  the  potash  bulb  is  guarded  by  a  tube 
containing  a  solution  of  silver  sulfate.  To  insure  the  combustion  of  any  carbon 
monoxide  or  hydrocarbons  escaping  from  the  flask,  between  the  condenser  and 
train  there  may  be  interposed  a  porcelain  tube  filled  with  copper  oxide  kept  at 
a  red  heat.  It  should  be  remembered  that  any  carbonate  in  the  sample  will 
also  furnish  carbon  dioxide. 

The  process  is  largely  used  for  the  determination  of  the  combined  carbon  of 
iron  and  steel,  operating  on  the  carbonaceous  residue  left  after  solution  of 
the  metal  in  certain  reagents  (vide  Iron).  To  determine  the  carbon  dioxide 
more  rapidly  than  by  absorption  in  potash  and  weighing,  Phelps  proposes  to 
pass  the  gas  into  a  known  volume  of  baryta  water,  and  titrate  what  baryta 
remains  in  solution.  Handy*  prevents  absorption  of  carbonic  acid  from  the  air 
during  the  filtration  by  causing  a  current  of  purified  air  to  play  over  the  surface 
of  the  liquid  in  the  funnel,  then  determines  the  baryta  by  standard  acid. 

Or  the  carbon  dioxide  may  be  passed  into  a  gas- measuring  tube,  the  vol- 
ume observed  and  reduced  to  normal  conditions,  then  into  an  absorption  tube 


Journ.  Amer.  Chem.  Socy.  1895—247. 


304 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


Fig.  167. 


containing  potash  lye,  and  the  residual  air,  plus  any  oxygen  evolved  from  the 
chromic  acid,  measured. 

NITROGEN. 

Nitrogen  may  be  determined  either  (1;,  by  the  'absolute  method  '  of  isolat- 
ing it  in  the  elementary  form  and  measuring  the  gas;  or  (2),  by  converting  it 

into  ammonia,  this  to  be  deter- 
mined gravimetrically,  volumet- 
rically,  or  by  colorimetry. 

1.  A.  In  the  absolute  method, 
originated  by  Dumas,  the  or- 
ganic substance  is  burned  by 
the  oxygen  from  copper  oxide, 
and  the  liberated  nitrogen  is  sep- 
arated from  the  water  and  car- 
bon dioxide  also  produced,  then 
measured  in  a  gas  tube  and  its 
weight  calculated  from  its  nor- 
mal volume. 

A  hard  glass  tube  A,  Fig.  167,  about  30  inches  long,  is  closed  at  one  end  by 
fusion,  and  in  it  are  charged  in  succession  layers  of  potassium  bicarbonate, 
copper  oxide,  a  mixture  of  the  organic  body  with  copper  oxide,  a  close  roll  of 
metallic  copper,  and  alternate  layers  of  copper  oxide  and  metallic  copper  until 
nearly  full.  The  front  end  is  closed  by  a  plug  of  asbestos  behind  a  well-fitting 
cork.  Through  the  cork  projects  a  long  glass  tube,  the  outer  part  bent  down- 
wards and  the  end  upwards  to  convey  the  gas  into  the  measuring  tube. 

Preceding  the  combustion  the  closed  end  of  the  tube  is  heated  until  carbon 
dioxide  from  the  bicarbonate  has  driven  out  all  the  air  from  the  tube,  this 
shown  by  passing  a  little  of  the  escaping  gas  into  a  tube  of  potash  lye,  when  all 
should  be  absorbed.  Then  there  is  placed  over  the  end  of  B  a  graduated  gas 
tube  C  filled  with  strong  potash  solution.  The  combustion  tube  is  now  heated, 

beginning  at  the  front  and  pro- 
ceeding until  the  bicarbonate  is 
reached,  which  is  again  heated 
until  the  evolved  carbon  di- 
oxide has  swept  all  the  gases 
into  the  gas  tube .  In  the  latter 
the  steam  condenses  and  the 
carbon  dioxide  and  any  traces 
of  chlorine  are  absorbed,  leav- 
ing only  nitrogen .  The  tube  is 
transferred  to  a  jar  of  water, 
and  the  volume  of  moist  nitro- 
gen measured  with  the  usual 
precautions,  and  reduced  to 
normal  conditions  wherein  one 
cubic  centimeter  of  nitrogen 
weighs  .00125616  gram.  If 
through  inadvertence  a  little 
nitrogen  dioxide  should  have 
escaped  decomposition  by  the 
copper  and  accompany  the  ni- 
trogen, it  may  be  absorbed  by 


Fig.  168. 


ferrous  sulfate,  and  one-half  (N202 
ducted. 


N2-{-O2)  Hie  diminution  in  volume  de- 


ELEMENTARY   ORGANIC   ANALYSIS.  305 

Of  the  many  modifications  of  the  details  of  the  process  there  may  be  men- 
tioned that  of  Simpson  who  substitutes  a  mixture  of  copper  oxide  and  mercuric 
oxide  for  the  copper  oxide,  and  a  mixture  of  magnesium  carbonate  and  mer- 
curic oxide  for  the  sodium  bicarbonate ;  of  Johnson  and  Jenkins  who  aid  the 
combustion  by  a  little  potassium  chlorate  in  the  extreme  posterior  of  the  tube, 
relying  on  the  metallic  copper  at  the  anterior  to  fix  any  excess  of  oxygen  generated 
from  the  chlorate ;  and  of  Meyer,  who  substitutes  lead  chromate  for  copper  oxide 
to  retain  any  sulfur  gases  coming  from  sulfur  in  the  organic  body. 

Or  the  combustion  may  be  conducted  in  vacuo,  dispensing  with  the 
bicarbonate.  The  combustion  tube,  Fig.  168,  is  connected  by  rubber  tubing 
to  a  Sprengel  mercury  vacuum-pump.  To  insure  the  air  tightness  of  the 
connection  it  is  surrounded  at  a  and  c  by  larger  tubes  containing  glycerine 
or  mercury.  On  opening  the  stopcock  g  a  stream  of  mercury  flows  down 
the  tube  from  the  reservoir  e,  drawing  air  from  the  combustion  tube  until 
it  becomes  practically  vacuous,  the  air  escaping  from  d.  A  gas-measuring  tube 
filled  with  mercury  is  then  inverted  over  d,  and  the  combustion  tube  (charged 
as  above  except  the  omission  of  the  bicarbonate)  is  heated  as  before.  The 
pump  transfers  the  product  of  combustion  to  the  gas  tube.  When  the  combus- 
tion is  ended  the  gas  tube  is  transferred  to  a  trough  of  potash  solution  to 
absorb  the  carbon  dioxide,  and  the  volume  of  the  residual  nitrogen  is  read.  A 
small  error  comes  from  the  presence  of  occluded  air  or  other  gases  in  the  tube, 
liberated  on  heating, 

B,  Anumber  of  organic  bodies  are  decomposed  by  an  alkaline  solutionof  sodium 
hypobromite  or  hypochlorite,  one  of  the  products  being  gaseous  nitrogen.  The 
complete  isolation  of  the  nitrogen  is  seldom  obtained  however,  though  a  nearly 
complete  evolution  may  be  had  in  some  cases  by  the  addition  of  a  catalytic 
agent,  The  process  has  an  extensive  application  for  the  determination  of  urea 
in  urine,  q.  v. 

2,  A.  In  the  method  of  Will  and  Varrentrapp  the  nitrogen  of  the  organic 
body  is  converted  into  ammonia  by  heating  with  soda-lime ;  it  is  said  that  the 
conversion  is  effected  through  the  dissociation  of  aqueous  vapor  —  3C  -f-  2N  -+- 
3H2O  =  SCO  -f  2NHa.  The  ammonia  is  led  through  a  mineral  acid  by  which  it 
is  absorbed,  and  is  determined  by  the  usual  methods  for  ammonia.* 
The  apparatus  is  shown  in  Fig.  169.  The  combustion  tube,  A  B,  that  may  be 

of  glass  or  iron,  con- 
tains at  the  sealed  end  a 
layer  of  oxalic  acid  or 
calcium  oxalate  which 
on  heating  decomposes 
with  evolution  of  carbon 

monoxide  and  dioxide ;  next  to  this  is  the  sample  mixed  with  soda-lime  or  quick 
lime,  and  if  rich  in  nitrogen,  with  some  sugar  or  other  non-nitrogenous  sub- 
stance whose  gaseous  products  dilute  the  ammonia  and  lessen  the  danger  of 
any  passing  the  acid  unabsorbed.  The  third  layer  is  soda-lime  only,  held 
in  place  by  a  plug  of  asbestos.  Joined  to  the  combustion  tube  is  a  container 
for  the  acid,  such  as  the  "  nitrogen  bulb  "  C,  partly  filled  with  dilute  hydro- 
chloric acid.  The  combustion  is  carried  on  in  the  usual  way,  proceeding  to 
heat  the  tube  gradually  back  to  the  oxalic  acid.  When  the  gases  from  the 
decomposition  of  the  latter  have  driven  out  the  ammonia,  the  solution  of 
ammonic  chloride  is  poured  into  a  beaker  and  precipitated  by  chloroplat- 
inic  acid  with  the  usual  precautions ;  the  ammonium  platinchloride  may  be 


*  Journ.  Anal.  Chem.  1888—335. 

20 


306  QUANTITATIVE    CHEMICAL   ANALYSIS. 

weighed  as  such,  or  the  platinum  separated  by  simple  ignition  or  otherwise, 
and  weighed. 

It  is  more  usual,  however,  to  determine  the  ammonia  in  a  volumetric  way . 
Here  the  bulb  contains  a  fixed  volume  of  standard  sulfuric  acid,  and  after  the 
combustion  the  unneutralized  excess  is  titrated  by  standard  alkali  and  litmus. 

The  method  has  the  confidence  of  most  technical  chemists  of  affording 
accurate  results  when  carefully  worked;  an  especial  precaution  is  that  of 
limiting  the  heat  to  bare  redness  lest  ammonia  be  dissociated.  But  some 
organic  bodies  yield  volatile  compounds  when  ignited  with  soda-lime,  and 
cannot  be  analyzed  by  this  method  except  with  several  modifications. 

B.  Ruffle's  method  *  is  similiar  to  the  above,  differing  in  that  sodium  thio- 
sulfate,  sulfur,  and  carbon  or  an  organic  compound  are  added  to  the  soda- 
lime  mixed    with  the  organic  body.    These    additions    make  it  possible  to 
convert   the  nitrogen  of    some  compounds  into    ammonia,   where  soda-lime 
alone  would   fail.    The  reaction  is  said  to  be  that  the  thiosulfate  {urnishes 
sulfurous  acid  which  reacts  with  oxides  of  nitrogen  to  form  ammonia  and  sul- 
furic acid,  e.  g.,  N2O  +  4SO2  -f  3H2O  =  2NH3  -f  4SO3. 

Several  other  mixtures  of  a  similar  character  to  the  above  have  been  pro- 
posed. The  Ruffle  method  is  highly  regarded  by  many  chemists,  especially  for 
the  determination  of  nitric  nitrogen. 

C.  The  method  due  to  Kjeldahl,  although  of  comparatively  recent  origin,  has 
largely  supplanted  other  methods.    Its  advantages  are  that  it  can  be  applied  to 
heterogeneous  mixtures  difficult  to  powder,  and  to  liquids,  pastes  and  hygro- 
scopic   matter    in    general    without   previous    drying;    that    the    apparatus 
required   is    less    complicated  and    more    compact;    and  that   a    number  of 
determinations  can  be  carried  on  simultaneously  by  one  operator. 

A  small  weight  of  the  organic  body  is  heated  with  concentrated  sulfuric 
acid  until  a  clear  solution  results,  when  powdered  potassium  permanganate 
is  sifted  in.  The  liquid  is  diluted  with  water,  made  alkaline  by  sodium 
hydrate,  and  distilled  until  the  freed  ammonia  has  gone  over  with  water. 
The  receiver  contains  an  acid  to  fix  the  ammonia,  which  is  then  determined  as 
in  the  soda-lime  process,  supra. 

The  reactions  occurring  during  the  decomposition  of  the  organic  body 
have  been  described  as  follows:  First,  the  sulfuric  acid  absorbs  any  water 
that  the  sample  may  contain;  second,  the  acid  reacts  with  the  carbon  and 
hydrogen  to  form  carbon  monoxide  and  water  and  is  reduced  to  sulfurous 
acid;  third,  sulfurous  acid  and  nitrogenous  compounds  dissociate  water,  the 
nitrogen  takes  up  hydrogen  forming  certain  intermediate  products  but  ulti- 
mately ammonia,  while  the  sul- 
furous acid  is  oxidized  to  sul- 
furic; fourth,  the  potassium 
permanganate  completes  the  oxi- 
dation of  any  part  of  the  sub- 
stance resisting  the  sulfuric  acid 
alone. 

About  one-half  gram  of  the  or- 
ganic body  is  weighed;  if  a  liquid 

Fi     17Q  <—:— '   it   is   placed   in   a  bulb   with  a 

capillary  neck.  The  substance  is 
covered  with  ten  to  twenty  Cc.  of  concentrated  sulfuric  acid,  and  the  mixture 
boiled  until  all  the  organic  matter  has  been  destroyed  and  the  acid  become 


*  Chem.  News,  1890-1—231 ;  Journ.  Chem.  Socy.  40—451  and  21—161. 


ELEMENTARY    ORGANIC   ANALYSIS.  307 

clear  and  colorless  or  but  slightly  tinted.  The  permanganate  is  added  and 
the  solution  heated;  then  cooled,  diluted  with  water,  and  transferred  to  a 
larger  flask  arranged  for  distillation  as  shown  in  Fig.  170.  The  condenser  is 
of  block-tin  or  platinum  (since  the  ammonia  would  dissolve  a  little  glass 
were  the  condenser  of  this  material),  or  simply  a  long  air-cooled  tube,  the 
exit  dipping  into  the  acid  in  the  receiver, 

Into  the  flask  is  poured  an  excess  of  sodium  hydrate  solution.  The  alka- 
line liquid  is  then  distilled  (best  in  a  current  of  steam)  until  the  ammonia 
has  passed  over  into  the  acid.  To  prevent  the  carrying  over  of  any  of  the 
alkaline  liquid  as  spray,  some  form  of  trap  is  interposed  between  the  flask 
and  condenser;  one  is  shown  at  A  in  Fig.  170. 

The  determination  of  the  ammonium  in  the  distillate  can  be  done  gravimetri- 
cally  by  conversion  to  ammonium  platinchloride,  but  a  volumetric  method  is 
more  usual.  Of  the  latter  the  plan  of  back  titration  by  a  standard  alkali  is 
most  common ;  a  variation  is  the  use  of  baryta  water  with  rosolic  acid  as  indi- 
cator. Or  the  distillate  may  be  treated  with  potassium  iodide  and  iodate,  when 
the  free  sulf  uric  acid  liberates  an  equivalent  of  iodine  —  5KI  -|-  KIOs  -f  3H2SO4  = 
3K2SO4  +  3Ig  -|-  3H2O  —  and  the  iodine  titrated  by  thiosulfate  and  starch  paste. 
A  rapid  approximate  method  is  that  of  liberating  nitrogen  through  the  agency 
of  sodium  hydrate  and  hypobromite  and  measuring  the  gas.  For  small  amounts 
of  ammonia,  Nessler's  test  is  the  most  accurate. 

Many  modifications  of  the  original  method  of  decomposition  by  sulf  uric  acid 
have  been  proposed,  but  mostly  of  dubious  worth.*  Various  oxidants  or  car- 
riers of  oxygen  have  been  advised  as  additions  to  the  sulfuric  acid,  such  as 
cupric  oxide,  mercury,  etc.,  the  sulfates  of  these  bases  being  supposed  to 
materially  assist  in  the  dissolution  of  the  organic  body.  An  objection  to  the 
use  of  mercury,  the  most  common  addition,  is  the  need  of  subsequent  removal 
by  a  sulfide,  and  the  inaccuracy  following  the  introduction  of  an  excess. f 

Gunning  modifies  the  method  by  the  substitution  of  solid  sodium  pyrosulfate 
for  sulfuric  acid  thus  securing  a  more  powerful  oxidizing  action.  The  pro- 
cess has  received  the  favor  of  many  chemists.  Riviere  and  BailhatcheJ  prefer 
sodium  pyrophosphite  to  the  pyrosulfate. 

As  described,  the  original  method  is  not  suited  for  the  determination  of  nitric 
nitrogen  especially  in  presence  of  much  chlorine,  It  is  said  that  nitro,  nitroso, 
azo,  diazo,  hydrazo,  and  amidoazo  bodies  and  compounds  of  nitrous  acids,  the 
hydrazines,  and  probably  cyanogen  compounds  cannot  be  satisfactorily  deter- 
mined. To  adapt  the  method  for  nitrates,  to  the  sulfuric  acid  Jodlbauer  adds 
phenolsulfuric  acid,  zinc  dust,  and  a  few  drops  of  platinic  chloride  solution ; 
the  nitric  acid  becomes  nitro-phenol,  this  by  the  reducing  action  of  zinc  be- 
comes amido-phenol,  and  this  by  the  action  of  sulfuric  acid  becomes  ammonia. 
Salicylic  and  benzoic  acids  are  in  use  for  the  purpose,  finally  oxidizing  the 
excess  by  permanganate,  and  it  has  been  found  of  advantage  to  mix  a  nitrate 
with  gypsum  before  the  digestion. 

THE  HALOGENS. 

In  the  method  of  Carius,  the  compound  is  oxidized  by  nitric  acid  in  presence 
of  silver  nitrate,  and  the  compound  of  halogen  and  silver  weighed.  A  tube  of 
stout  glass  is  sealed  up  at  one  end,  and  the  sample  with  some  powdered  silver 
nitrate  introduced;  if  the  organic  body  is  a  volatile  liquid  it  is  held  in  a  weigh- 


*  Journ.  Anal.  Chem.  2—299. 

t  Journ.  Socy.  Dyers  &  Col.  1897—81. 

t  Analyst,  1896-267. 


308  QUANTITATIVE    CHEMICAL   ANALYSIS. 

ing-tube  closed  by  a  glass  stopper,  or  sealed  up  in  a  light  glass  bulb  after- 
ward broken  by  shaking  the  tube.  Concentrated  nitric  acid  is  then  poured  in 
to  partly  fill  the  tube,  and  the  open  end  is  drawn  out  to  a  small  bore  and  the 
orifice  sealed  by  a  blowpipe  flame. 

The  tube  is  then  heated  in  an  oven  for  several  hours  at  a  temperature  of 
from  150°  to  300°,  allowed  to  cool,  held  upright,  and  the  tip  of  the  prolong 
softened  by  a  blowpipe  flame,  when  the  gases  liberated  by  the  reaction  will 
force  their  way  through  the  plastic  glass;  should  inflammable  gases  have  been 
formed,  a  safer  plan  is  to  cut  off  the  end  at  once.  The  prolong  is  now  broken 
off,  the  solution  poured  out,  diluted,  and  the  silver  compound  filtered  and 
weighed.  A  simpler  plan  is  to  weigh  the  silver  nitrate  introduced  with  the 
sample,  and  determine  what  is  left  uncombined,  easiest  by  titration  with 
standard  sodium  chloride. 

While  the  results  of  the  method  are  unexceptionable,  the  length  of  tim^  required 
for  decomposition,  the  danger  of  explosion  of  the  tube  during  the  heating  or 
on  opening  it,  and  various  manipulative  difficulties  seriously  detract  from  the 
usefulness  of  the  method. 

A  solid  substance  may  be  calcined  in  contact  with  a  base  that  will  fix  the 
halogens  or  their  acids.  Quicklime  forms  compounds  of  calcium  that  may 
be  readily  converted  into  corresponding  silver  compounds  and  weighed. 
Kopp's  method  is  that  of  heating  with  ferric  oxide  in  a  tube  open  at  one  end, 
the  reaction  yielding  soluble  ferric  haloids  ready  for  precipitation  by  argentic 
nitrate.  Kekule  digests  the  substance  with  water  and  sodium  amalgam  whose 
action  is  to  form  the  sodium  salts;  but  the  method  is  of  limited  application. 

Meilliere  evaporates  a  solution  or  extract  with  calcium  nitrate  in  a  platinum 
crucible,  after  which  a  slight  ignition  destroys  all  the  organic  matter.  The 
aqueous  extract  of  the  residue  is  free  from  phosphates,  and  after  acidulation 
"With  sulfuric  acid  and  the  addition  of  calcium  carbonate  to  decolorize  and 
neutralize  the  liquid,  the  halogens  in  the  filtrate  may  be  titrated  by  standard 
silver  nitrate  and  potassium  chromate. 

Combustion  in  the  calorimetric  bomb  of  Berthollet  may  be  applied  to  the 
determination  of  the  halogens ;  for  chlorine  a  little  arsenious  acid  should  be 
added  to  fix  this  element. 

Plympton  and  Groves  propose  to  burn  the  organic  body  in  a  Bunsen  flame 
under  an  inverted  funnel,  drawing  the  products  of  combustion  through  the 
stem  into  a  solution  of  sodium  hydrate  by  which  the  halogens  are  retained. 

SULFUR. 

In  the  combustion  of  organic  bodies  containing  sulfur  there  is  formed  sulfur- 
ous  acid  mainly,  this  converted  to  sulfuric  by  an  oxidizer.  In  the  furnace  com- 
bustion for  carbon  and  hydrogen  the  substance  is  mixed  with  lead  chromate, 
or  a  mixture  of  lead  chromate  and  potassium  chromate,  instead  of  copper 
oxide,  since  this  compound  both  oxidizes  the  sulfurous  acid  and  reacts  with 
the  sulfuric  to  form  lead  sulfate.  %. 

Sulfur  is  always  determined  by  first  converting  it  into  sulfuric  acid,  then 
precipitating'by  barium  chloride  and  weighing  the  precipitate  of  barium  sulfate. 
Many  schemes  have  been  proposed  for  the  oxidation. 

One  process  is  that  of  burning  the  organic  body  in  a  combustion  tube  in  a 
current  of  mixed  oxygen  and  nitric  oxide.*  The  sulfur  is  thus  brought  to  the 
highest  state  of  oxidation  and  the  acid  may  be  caught  in  any  convenient  reagent. 
Prunier  mixes  the  substance  with  80  to  100  parts  of  powdered  potassium  per- 


*  Berichte,  19-1910;  Chem.  News,  1888—2-96. 


ELEMENTARY  ORGANIC  ANALYSIS.  309 

raanganate  and  heats  the  mixture  in  an  ordinary  combustion  tube.  Oxygen  is 
given  off  from  the  permanganate  at  about  240  °  .  The  products  of  combustion 
are  passed  through  a  solution  of  potassium  permanganate  which  absorbs  the 
sulfuric  acid  and  oxidizes  any  sulfurous  acid.  After  filtering  from  deposited 
manganese  oxide,  the  filtrate  is  acidified  and  the  sulfuric  acid  precipitated  as 
usual.  Carbon  may  be  determined  in  the  same  operation  by  annexing  a  tube 
of  baryta  water ;  after  the  combustion  is  finished  the  carbon  dioxide  is  set  free 
by  a  mineral  acid  and  measured  or  absorbed  and  weighed. 

The  combustion  may  be  made  in  oxygen  under  high  pressure  in  a  Berthollet 
calorimetric  bomb  containing  a  little  water  for  the  absorptipn  of  the  sulfur 
oxides;  a  large  percentage  of  sulfur  is  more  readily  oxidized  if  the  organic 
substance  has  been  mixed  with  an  equal  weight  of  a  pure  carbohydrate. 
Burton  *  modifies  Sauer's  method  for  liquids  and  gases  by  burning  in  a  lamp  or 
gas-burner,  passing  the  gases  formed  in  the  combustion  into  a  standard 
solution  of  potassium  hydrate,  and  titrating  back  by  a  standard  acid  and  methyl 
orange,  this  indicator  indifferent  to  carbonic  acid.  Or  the  combustion  gases 
may  be  passed  through  bromine  water  or  a  solution  of  bromine  in  hydrochloric 
acid  for  the  oxidation  of  the  sulfurous  acid  and  retention  of  the  sulfuric  acid. 

Oxidation  in  the  wet  way  may  be  done  according  to  the  method  of  Carius  for 
halogens,  with  the  modification  of  omitting  the  silver  nitrate.  The  solution 
after  heating  in  the  closed  tube  is  evaporated  to  dryness,  previously  adding  a 
little  sodium  nitrate;  the  residue  is  taken  up  by  strong  hydrochloric  acid, 
diluted  and  filtered,  and  the  sulfuric  acid  (now  as  sodium  sulfate)  precipitated 
by  barium  chloride,  and  the  barium  sulfate  weighed  as  usual.  For  refractory 
bodies,  such  as  asphalt,  a  preliminary  digestion  with  fuming  nitric  acid  is 
advised,  and  any  residue  left  after  heating  in  the  tube  is  subjected  to  an  exam- 
ination for  sulfur  by  one  of  the  fusion  methods  mentioned  below. 

Some  organic  compounds  are  destroyed  and  the  sulfur  oxidized  to  sulfuric 
acid  by  heating  with  a  liquid  oxidizer.  For  this  purpose  there  have  been  pro- 
posed nitric  acid  with  potassium  chlorate,  potassium  hydrate  solution  and  the 
passage  of  a  current  of  chlorine,  alkaline  potassium  permanganate,  chromic 
and  nitric  acids,  etc.  But  many  of  these  are  likely  to  fail  with  bodies  strongly 
resisting  oxidation,  however  prolonged  the  digestion  or  boiling. 

For  decomposition  in  the  dry  way,  Liebig's  directions  are  to  mix  the  sub- 
stance with  potassium  nitrate  or  chlorate  and  project  the  mixture  by  small 
portions  into  potassium  hydrate  kept  melted  in  a  platinum  crucible,  finally 
heating  over  a  blast-lamp.  A  later  method  is  to  melt  the  substance  with  a 
mixture  of  sodium  carbonate  and  potassium  hydrate  and  slowly  add  sodium 
peroxide  until  the  carbon  is  burned. f  It  would  appear  that  Eschka's  scheme 
for  sulfur  in  coals  (g.  •#.)  and  cast  iron  could  be  extended  to  many  other  organic 
bodies. 

Debus  would  mix  the  organic  substance  with  potassium  bichromate  and 
sodium  carbonate  and  fuse  the  mixture  in  a  combustion  tube;  as  a  guard  some 
of  the  reagent  is  placed  before  and  behind  the  mixture.  When  the  tempera- 
ture has  risen  to  redness  a  current  of  oxygen  is  passed  into  the  tube  until  all 
the  carbon  has  burned. 

PHOSPHORUS. 

Phosphorus  is  oxidized  in  much  the  same  way  as  sulfur,  Carius'  method 
of  decomposition  and  that  of  fusion  with  alkali  carbonate  and  nitrate  being 
most  in  use.  After  bringing  the  -phosphorus  to  the  state  of  phosphoric  acid 


*  Amer.  Journ.  Science,  1889—472. 
t  Chem.  Centralb.  1896-66. 


310  QUANTITATIVE    CHEMICAL   ANALYSIS. 

the  clear  solution  may  be  at  once  precipitated  by  magnesic  solution,  or  in  pres- 
ence of  metals  forming  permanent  precipitates  with  ammonia,  preceded  by  a 
separation  as  ammonium  phosphomolybdate. 

METALS. 

The  metals  of  semi-organic  compounds  may  be  converted  into  oxides  or  car- 
bonates as  the  case  may  be,  by  ignition  in  air  or  oxygen  until  the  carbon  is 
burned.  This  is  the  simplest  plan  and  is  applicable  for  all  metals  that  are  not 
volatile  at  the  heat  of  the  calcination.  For  the  volatile  metals  the  organic 
matter  may  be  oxidized  by  the  method  of  Carius  or  by  any  of  the  liquid  oxi- 
dizers  mentioned  under  the  determination  of  sulfur.  After  destroying  the 
organic  matter  the  determination  follows  the  usual  course  of  inorganic 
analysis. 

COMBINATION   METHODS. 

Various  schemes  have  been  proposed  for  the  simultaneous  determination  of 
two  or  more  elements.  That  for  carbon  and  hydrogen  is  entirely  successful, 
but  none  of  the  others  have  come  into  general  use,  principally  for  the  reason 
that  the  determination  of  any  one  element  requires  the  careful  adjustment  of 
reagents  and  a  certain  peculiar  routine  of  manipulation  that  must  be  more  or 
less  modified  when  another  element  has  also  to  be  determined.* 


*  Analyst,  1897—277 ;  Amer.  Journ.  Sci.  41—40. 


PROXIMATE   ORGANIC  ANALYSIS.  311 


PROXIMATE  ORGANIC  ANALYSIS. 

The  processes  for  the  separation  and  determination  of  organic  bodies  fol- 
low to  some  extent  those  for  inorganic;  but  while  to  the  latter  we  can  apply, 
as  a  rule,  a  number  of  reactions  that  yield  products  suitable  for  weighing  or 
measuring,  the  majority  of  organic  compounds  are  indifferent  to  the  common 
precipitants ;  again,  for  many  inorganic  compounds  there  are  specific  reagents 
allowing  a  perfect  separation,  while  the  number  of  specific  reagents  for  or- 
ganic bodies  are  comparatively  few. 

Another  distinction  is  that  with  inorganic  bodies  the  reagents  have  a  broader 
scope,  a  given  reagent  being  applicable  in  general  to  all  or  nearly  all  the  com- 
binations in  which  the  reacting  element  or  radical  may  enter,  though  for  ana- 
lytical purposes  it  may  be  restricted  to  a  few  or  but  one  for  practical  reasons. 
But  with  organic  compounds  there  enter  into  consideration  the  configuration 
of  the  molecule  of  which  the  group  to  be  determined  is  a  part,  stereoisomerism, 
and  the  modification  in  behavior  that  substituted  groups  may  undergo,  these 
narrowing  the  scope  of  the  various  processes  so  far  that  there  are  but  few 
that  can  be  termed  general  for  any  one  group. 

A  few  characteristics  not  usually  met  with  in  inorganic  bodies,  such  as  vola- 
tility at  moderate  temperatures,  solubility  in  organic  liquids,  an  easily  meas- 
urable boiling  and  congealing  point,  etc.,  are  applied  analytically. 

In  the  practice  of  proximate  organic  analysis  we  may  be  called  on  to  inves- 
tigate— 

1.  A  single  organic  compound,  presumably  pure,  which  is  to  be  identified  or 
classified,  elementary    analysis  failing  to    disclose  the  constitution  further 
than   can  be  deduced  from  an  empirical  formula.    Here  a  determination  of 
some  one    group  of  the  molecule  — hydroxyl,  ethoxyl,  diazo,  etc.  — will  be  a 
basis  for  the  calculation  of  the  rational  formula  and  structural  constitution. 

2.  A  similar  case  is  where  the  compound    is   semi -organic.    Usually   the 
determination  of  the  inorganic  part  and  an  elementary  analysis  of  the  organic 
part  can  be  done  by  simple  and  accurate  processes  and  often  suffice  to  de- 
scribe the  compound ;  if  not  they  are  supplemented  by  a  determination  of  one 
or  more  of  the  radicals. 

3.  A   mixture  of  several  organic   compounds.    The  constituents  may   be 
either  analogous  bodies,  as  mixtures  of  different  oils,  waxes,  vegetable  alka- 
loids, artificial  dye-stuffs,  fruit  essences,  etc.,  or  heterogeneous,  as  are  many 
natural  and  manufactured    articles.    In   the  analysis   of  the  former  class, 
attempts  at  a  separation  of  the  constituents    by  methods  designed  for   the 
determination  of  one  of  them,  will  often  fail  on  account  of  the  similarity  of 
behavior  of  the  others  toward  the  reagent.    Fractional  solution,  fractional 
precipitation,  fractional  distillation,  etc.,  may  be  made  to  yield  a  fair  separa- 
tion, but  as  a  rule,  attributive  methods  are  the  main  resource.    Heterogeneous 
mixtures   present    fewer     difficulties,    and  often  admit   of    quite    accurate 
separations. 

4.  Lastly,  we  may  have  to  consider  heterogeneous  natural  or  artificial  products 
that  are  partly  inorganic.    Such  are  animal  and  vegetable  matter  generally, 
animal  secretions  and  excretions,  medicinal  preparations,  the  waste  products 


312  QUANTITATIVE    CHEMICAL    ANALYSIS. 

of  manufactures,  etc.,  etc.    With  material  so  diverse,  no  general  statement  can- 
be  laid  down  as  to  the  methods  available,  since  each  example  is  a  special 
problem  and  often  one  of  no  inconsiderable  difficulty. 
We  may  here  glance  at  the  methods  commonly  used. 

SOLUTION. 

Of  organic  bodies  in  general,  water  or  one  of  the  common  organic  liquids  will 
serve  as  a  solvent  for  the  majority;  the  employment  of  the  mineral  acids,  so 
frequent  in  inorganic  analysis,  is  here  comparatively  rare.  Lyes  of  the  caustic 
and  carbonated  alkalies  will  dissolve  a  few  bodies  that  are  insoluble  in  other 
menstrua,  and  a  few  are  only  dissolved  by  concentrated  sulf uric  acid.  Special 
solvents  have  a  considerable  use  for  separations. 

DETERMINATION. 

1.  By  evaporating  the  aqueous  or  other  solution  of  the  compound  and  weigh- 
ing the  residue.    This,  the  simplest  plan,  is  not  practicable  in  many  cases ; 
volatility  of  the  organic  body,  increased  by  the  vaporization  of  the  solvent, 
may  cause  a  serious  loss ;  a  reaction  between  the  solvent  and  solute  may  be  set 
up  at  temperatures  above  ordinary;  and  the  heat  applied  to  evaporate  the 
solvent  may  cause  partial  decomposition,  and  contact  with  the  air  allow  oxida- 
tion, especially  about  the  period  of  solidification. 

The  usual  plan  is  to  pour  the  solution  into  a  tared  flat -bottomed  dish  and 
evaporate  at  as  low  a  temperature  as  practicable,  finally  drying  the  residue 
at  a  gentle  heat  or  in  the  desiccator.  Spontaneous  evaporation  is  safer  for 
slightly  volatile  bodies,  and  evaporation  in  vacuo  is  often  a  necessary 
precaution. 

2.  By  precipitation  of  the  organic  body.    The  solvent  may  be  changed  to  one 
in  which  the  compound  is  much  less  soluble,  e.  g.,  by  the  large  dilution  of  an 
alcoholic  solution  with  water.    The  result  of  the  determination  is  generally  too 
low  from  the  incomplete  insolubility  of  the  compound  in  the  mixed  liquids. 
The  accuracy  is  increased  by  removing  by  evaporation  at  a  low  heat  as  much  of 
the  original  solvent  as  can  be  done  without  separation  of  the  compound ;  though 
if  the  application  of  heat  is  allowable,  the  process  of  (1)  is  to  be  preferred. 

Another  plan  for  reducing  the  solvent  power  of  the  liquid  is  that  of  saturat- 
ing the  solvent  with  some  inorganic  salt  — *  salting  out.'  Aqueous  solutions  of 
some  dyes,  proteids,  etc.,  are  precipitated  nearly  completely  by  stirring  in  solid 
sodium  chloride,  magnesium  sulfate,  or  similar  salt,  until  the  liquid  is  satu- 
rated therewith. 

A  few  bodies  in  aqueous  solution  coagulate  when  the  liquid  is  boiled  or  upon 
the  addition  of  an  acid,  alcohol,  or  a  ferment. 

Rarely  do  organic  compounds  unite  as  a  whole  with  a  reagent  to  form  an 
insoluble  precipitate.  An  instance  is  the  combination  of  anilin  with  chloro- 
platinic  acid  —  2C8H6NH2  +  H2PtCl6  =  (C6H6NH2HCl)2PtCl4. 

3.  By  volumetric  analysis.    Some  organic  bodies  admit  of  direct  titration  by 
the  common  volumetric  solutions  or  those  of  special  reagents.    It  must  be  re- 
membered that  the  ionic  combination  in  which  a  group  exists  in  the  solution  of 
an  organic  compound  determines  whether  the  reaction  on  which  a  titration  is 
based  will  or  will  not  take  place. 

Free  bases,  such  as  phenylhydrazin,  antipyrine,  etc.,  are  titratable  directly 
by  a  standard  acid  and  suitable  indicator,  and  this  is  also  possible  of  a  combi- 
nation of  a  strong  base  with  an  acid  weaker  than  the  indicator  selected. 

Similarly,  the  free  organic  acids  are  titrated  by  standard  alkali.    If,  for  lack 


PROXIMATE    ORGANIC   ANALYSIS.  313- 

of  a  suitable  indicator,  or  on  account  of  a  highly  colored  titrate,  a  direct  titra- 
tion  is  difficult,  the  acid  is  treated  with  an  excess  of  standard  sodium  hydrate,* 
an  excess  of  ammonium  chloride  added,  and  the  ammonia  set  free  by  the  excess 
of  sodium  hydrate  is  distilled  into  water  and  titrated  by  an  acid.  Or  sodium 
carbonate  may  be  mixed  with  the  acid  solution  and  the  freed  carbon  dioxide 
dried  and  passed  into  potash  bulbs  and  weighed .  Compounds  of  a  weak  base 
and  a  strong  acid,  e.  g.,  the  sulfates  of  the  aromatic  amines,  some  alkaloidal 
chlorides,  etc.,  can  be  titrated  by  alkali  as  though  the  acid  radical  were  com- 
bined with  hydrogen. 

Of  occasional  use  are  strong  reducing  solutions  for  oxidizable  compounds, 
such  as  stannous  chloride  for  reducing  nitro-groups  to  ammo-derivatives,, 
and  oxidizing  solutions  for  reducible  compounds.  Arsenic  acid  oxidize* 
phenylhydrazin  to  phenol;  beta-napthol  in  a  carbon  tetrachloride 
solution  is  converted  to  monobromnapthol  by  titration  with  bromine; 
hydrazin  is  converted  by  potassium  permanganate  to  nitrogen,  water  and 
ammonium  sulfate ;  hydroxylamin  to  nitrogen  and  water  by  vanadic  acid,  etc. 

4.  By  determination  of  some  element  or  radical  of  the  organic  compound, 
and  calculation  to  the  original  body.  When  nitrogen,  sulfur,  or  phosphorus- 
is  a  constituent,  the  determination  of  the  element  is  comparatively  easy  and 
accurate;  however,  the  percentage  of  the  element  in  the  compound  is  usually 
quite  small,  and  in  the  calculation  of  the  latter  from  the  former  the  errors 
of  determination  are  correspondingly  magnified. 

Nitrogen  is  determined  by  the  methods  of  ultimate  organic  analysis.  A 
number  of  organic  bodies  are  usually  calculated  from  the  nitrogen  contained 
in  preference  to  direct  weighing,  notably  the  proteids,  the  multiplier  in  this- 
case  being  6.25  or  6.33,  since  the  average  nitrogen  content  of  proteids  is  about 
16  per  cent.  Another  example  is  the  analysis  of  commercial  Prussian  blue 
(principally  ferric  ferrocyanide  3Fe(CN)2.4Fe(CN)8);  multiplication  of  the 
found  nitrogen  by  the  factor  4.4  gives  approximately  the  weight  of  true  pigment 
iu  the  sample,  the  other  constituents  being  nitrogen -free. 

A  large  number  of  nitrogenous  organic  bodies  are  decomposed  by  certain. 
well-known  reagents,  the  nitrogen  being  evolved  quantitatively  or  nearly  so. 
Thus,  diazo -compounds  by  boiling  with  dilute  sulfuric  acid,  phenylhydrazin. 
with  iodine,  hydrazides  with  Fehling's  solutiona  and  hydrazine  salts  with 
platinic  chloride  or  potassium  permanganate.  Some  reagents  containing 
nitrogen  are  themselves  decomposed  and  the  total  nitrogen  obtained  in  the 
determination  is  the  sum  of  that  coming  from  the  organic  compound  and  the 
reagent ;  thus  aspartic  acid  with  nitrous  acid  yields  malic  acid  and  nitrogen  — 
C4H7NO4  +  HN02  =  C4H605  -f  N2  +  H2O ;  similarly,  the  aliphatic  amines. 

For  a  determination,  the  organic  body  is  treated  in  a  small  flask  with  the 
reagent,  and  the  evolved  nitrogen  conducted  into  a  gas-measuring  tube.  The 
weight  of  nitrogen  is  calculated  from  the  corrected  volume.  Another  form  of 
apparatus  that  may  be  used  is  that  shown  in  Fig.  120. 

Certain  nitrogenous  compounds,  such  as  the  nitrophenols,  on  heating  with 
zinc- dust  and  mercury  are  converted  first  to  amidophenols,  then  to  ammonium 
sulfate.  The  ammonia  in  the  product  is  determined. 

From  a  few  bodies,  as  the  thioureas,  sulfur  is  precipitated  directly  by 
ammoniacal  silver  nitrate  and  the  silver  sulfide  formed  converted  to  metallic 
silver  by  ignition.  Other  bodies  are  decomposed  with  the  formation  of  an  in- 
soluble metallic  sulfide  when  an  aqueous  solution  is  digested  with  a  metallic 
oxide.  But  in  general,  sulfur  is  determined  by  oxidation  to  sulfuric  acid,  as 
by  the  plan  of  Carius,  by  fusion  with  an  alkali  carbonate  and  nitrate  or  other 


314  QUANTITATIVE    CHEMICAL    ANALYSIS. 

oxidizing  mixture,  or  by  suspension  in  potash  lye  through  which  chlorine  is 
passed.     The  sulf uric  acid  is  determined  in  the  usual  way. 

Phosphorus  is  a  normal  constituent  of  some  animal  and  some  vegetable 
matters.  It  is  oxidized  to  phosphoric  acid  by  proceeding  as  for  sulfur, 
and  the  phosphoric  acid  determined  as  the  magnesium  compound. 

5.  By  the  weight  of  a  reagent  required  to  change  the  molecular  structure. 
Many  of  these  reactions  have  been  applied  ia  technical  analysis,  and  for 
uniformity  and  comparison,  a  certain  fixed  weight  of  the  material  is  taken 
for  each  test,  and  the  result  of  the  determination  called  the  '  value  '  or  *  number  ' 
of  the  material.  For  example  the  •  saponification  value '  of  a  wax  is  the 
number  of  milligrams  of  potassium  hydroxide  combining  with  the  acids  from 
one  gram  of  the  wax;  the  *  iodine  number7  is  the  weight  of  iodine  entering 
100  grams  of  an  organic  compound;  similarly  the  '  acetyl number  ',  *  carbonyl 
number',  *  methoxyl  number  '  ,etc. 

A.  By  precipitation.  With  some  classes  of  organic  bodies  certain  inorganic 
and  organic  reagents  produce  compounds  more  or  less  insoluble.  In  a  few 
instances  the  insolubility  is  sufficient  to  allow  an  accurate  determination,  but 
the  majority  are  too  freely  soluble  to  admit  of  more  than  approximate  results. 
Instead  of  directly  weighing  the  precipitates,  it  is  often  better  to  determine 
some  element  or  radical  contained.  Some  of  the  principal  reactions  follow. 

Phenylhydrazin  (a  colorless  oily  compound  CeHsN^Hs),  phenylhydrazin  hy- 
drochloride,  and  various  substitution  products,  all  yield  phenylhydrazons 
with  bodies  containing  the  carbonyl  group.  The  products  separate  in  the  form 
of  crystalline,  flocculent,  or  oily  compounds,  most  readily  and  completely  from 
warm  acetic  acid  solutions.  The  reaction  has  received  considerable  practical 
application  in  the  determination  and  differentiation  of  the  sugars.  The  precipi- 
tates may  be  dried  and  weighed;  or,  since  phenylhydrazons  are  not  decomposed 
by  Fehlings  solution,  a  known  weight  of  phenylhydrazin  hydrochloride  is  used 
for  the  precipitation  and  the  excess  determined  by  boiling  with  Fehlings 
solution;  the  nitrogen  of  the  reagent  is  liberated,  and  is  measured  after  passing 
Into  a  eudiometer. 

Carbamyl  chloride  reacts  with  hydroxyl  derivatives  with  the  formation  of 
carbamates  and  hydrochloric  acid.  The  reaction  takes  place  at  ordinary  tem- 
peratures in  ethereal  solution.  In  the  crystalline  precipitate  is  determined 
nitrogen,  this  equaling  the  number  of  hydroxyl  groups  in  the  original. 

Phenyl  isocyanate  gives  ethereal  phenyl-carbamates  with  hydroxyl  com- 
pounds —  R.OH  +  C6H5N.CO  =  R.C6H5.NH.CO.O.  The  reaction  takes  place  at 
a  boiling  temperature,  The  product  is  washed  by  ether  and  cold  water,  and 
recrystallized  from  alcohol. 

Sodium  sulfite  throws  down  crystalline  precipitates  from  concentrated  solu- 
tions of  the  aldehyds;  for  example,  CH3.COH  -f  NaHSOg  =  CH3.CHOH.NaS03 
(sodium-oxyethyl  sulfonate). 

Iodine  forms  additive  or  substitutive  compounds  with  many  organic  bodies, 
e.  g.t  with  diazo  compounds  —  CHN2.COO.R  -f-  I2  =  CHI2. COO. R  -f-  N2.  For 
pure  organic  bodies  a  simple  plan  of  determination  is  to  incorporate  an  excess 
of  iodine  with  the  alcoholic  solution  of  the  body,  evaporate  to  dryness,  expel 
the  excess  of  iodine  by  a  gentle  heat,  and  weigh  the  residue;  bodies  insoluble 
in  alcohol  can  be  triturated  with  iodine  and  the  excess  volatilized.  The 
method  pursued  for  glycerides  is  described  under  Oils,,  A  reaction  of  practical 
importance  is  that  between  aceton  and  iodine,  iodoform  being  produced  — 
C8H6O  +  2I2  -f-  KOH  =  CHI3  +  CH3COH  +  Kl  +  H2O. 

lodoso  compounds  react  with  hydriodic  acid,  being  reduced  to  iodides  and 


PROXIMATE    ORGANIC    ANALYSIS.  315 


liberating  iodine  from  the  reagent;  thus,  CellslO  (iodosobenzene)  -(-  2HI  = 
C6H5I  +  h  +  H20. 

B.  By  the  introduction  of  a  determinable  element  or  group.  An  organic 
radical  is  introduced  into  the  molecule  of  the  organic  compound  and  afterwards 
determined  ;  for  example,  the  replacement  of  the  hydrogen  of  a  hydroxyl 
group  of  a  compound  by  an  acetic,  benzole,  or  phenylsulf  uric  group. 

Replacement  by  the  acetyl  group  is  a  common  analytical  scheme  for  the  de- 
termination of  organic  acids,  certain  oils,  phenols,  amines,  alcohols,  etc. 
The  acetylation  is  done  by  different  reagents  according  to  the  character  of 
the  molecule  to  be  acted  upon  ;  namely  by  treating  the  compound,  or  its  solu- 
tion in  an  organic  solvent,  with  acetic  anhydride,  acetyl  chloride,  or  anhydrous 
acetic  acid  and  phosphorus  oxychloride,  then  heating  to  a  specified  tempera- 
ture under  atmospheric  or  a  higher  pressure.  For  example  — 

(OR)2.C3H6.OH   (a  diglyceride)  -f  (C2H8O)20  (acetic  anhydride)  = 
(OR)2.C3H5.O.C2H3O-1-C2H3O.OH  (acetic  acid;. 

CsHu.OH  (amyl  alcohol)  +  CHs-COCl  (acetyl  chloride)  4-  K2COs  = 
CsHnO.  CO.CHs  (amyl  acetate)  -f  KC1  -f  KHCOs. 

3  (C6H6)OH  (phenol)  +  3CH3COOH  (acetic  acid)  +POCJ8  (phosphorus  oxy- 
chloride) =  3(C«H5)(C2HSO2)  (phenyl  acetate)  +  3HC1  -f-  H3PO4. 

The  determination  of  the  introduced  acetyl  group  can  be  done  in  some  cases 
by  noting  the  increase  in  weight  of  the  compound  due  to  the  assimilation  of 
C2H20  (that  is,  C2H3O  minus  H).  Another  plan  is  to  weigh  the  acetic  anhy- 
dride taken  for  the  acetylation,  and  after  the  reaction  is  complete,  to  convert 
the  excess  into  acetic  acid  by  treatment  with  hot  water  —  (CH3)2C2O3  +  H2O  = 
2CH3COOH  —  and  determine  the  acetic  acid  by  titration  with  standard  alkali, 
calculating  the  result  to  acetic  anhydride.  The  weight  reacting  with  the  or- 
ganic body  is  found  by  difference. 

The  most  common  method  is  that  of  separating  the  acetylated  compound 
from  the  excess  of  the  reagent,  then  hydrolyzing  it  by  the  action  of  water,  an 
acid,  or  a  solution  of  a  caustic  alkali  or  earth,  and  determining  the  acetic  acid 
produced  from  the  acetyl  radical.  Thus,  the  product  in  the  first  equation, 
supra  — 

(OR)2.C3H5  OC2H3O  +  H20  =  (OR)2.C3H5.OH  -f  CH3.COOH  (acetic  acid). 
The  freed  acetic  acid  is  titrated  by  standard  alkali.  Another  plan  is  to  hy- 
drolize  the  derivative  by  potassium  hydrate  solution,  acidify  by  phosphoric 
acid,  and  distill  the  free  acetic  acid.  In  the  distillate  the  acid  is  neutralized  by 
baryta,  the  excess  of  baryta  removed  by  passing  carbon  dioxide,  and  the  baryta, 
in  solution  as  barium  acetate,  determined  gravimetrically  and  the  acetic  acid 
calculated. 

Along  similar  lines  the  methyl  group  may  be  introduced  into  a  compound  by 
the  agency  of  methyl  iodide,  the  propionic  radical  by  propionic  anhydride,  etc. 

Benzoyl  chloride  and  its  various  derivatives  are  employed  to  introduce  the 
benzoyl  radical  into  bodies  containing  the  hydroxyl,  amino  or  imide  groups, 
reacting,  in  presence  of  sodium  hydrate,  with  alcohols,  phenols,  amines,  and 
amido-phenols  to  form  crystalline  precipitates,  e.  g. 

C6H5.OH  (phenol)  +  C6H5.CO.C1  (benzoyl  chloride)  +  NaOH  =  C6H5  O.CO.C6H5 
(benzoicphenyl  ester)  +NaCl  -f  H2O. 

C6H5.NH2  (anilin)  -f  Cells.CO.Cl  -f  NaOH=  CeHs.NH.CO.Ce^  (benzanilid)  -f 
NaCl  +  H2O. 

The  benzoylation  is  usually  carried  on  iu  dilute  aqueous  solution  made 
strongly  alkaline,  and  at  ordinary  temperatures.  The  benzoyl  derivatives  are 


316  QUANTITATIVE    CHEMICAL    ANALYSIS. 

usually  white  crystalline  precipitates  that  retain  traces  of  the  reagent  with 
some  tenacity.  The  determination  of  the  benzoyl  radical  is  done  by  hydro- 
liz'ng  the  derivative,  it  becoming  benzoic  acid.  The  hydrolysis  is  effected  by 
hydrochloric  acid  saturated  with  benzoic  acid,  heating  the  mixture  in  a  closed 
flask.  The  benzoic  acid  remains  as  a  powder  (solubility  in  water  1  in  400), 
and  is  filtered  and  washed  with  water  saturated  with  benzoic  acid.  The  filter 
and  contents  are  thrown  into  water  and  the  acid  titrated  by  standard  alkali 
and  phenolphthalein.  Or  the  benzoyl  derivative  may  be  hydrolyzed  by  alco- 
holic sodium  hydrate,  the  liquid  acidified  by  phosphoric  acid,  and  the  benzoic 
acid  distilled  over  in  a  current  of  steam;  the  acid  in  the  distillate  is  titrated 
as  before. 

Compounds  containing  the  carboxyl  group  are  etherified  on  boiling  with 
absolute  alcohol  containing  gaseous  hydrochloric  acid.  The  resulting  esters 
are  then  determined  by  saponification  with  alkali  in  the  usual  way. 

Aldehyds  and  ketons  react  with  hydroxylamin  to  form  oxims,  respectively 
aldoxims  and  ketoxims;  for  example  — 

CeHs  COH  (benzoic  aldehyd)  -f-NH2OH  (hydroxylamin)  =C6H5,CHN.OH  (ben~ 
zaldoxim)  +  H2O. 

CeHsCO.CeHs  (diphenyl  keton)  -f-  NH2OH  =  C6H5.CNOH.C6H5  (benzophenon 
oxim)  +  H20. 

The  oxim  is  produced  by  acting  on  the  organic  body  by  hydroxylamin 
hydrochloride  and  sodium  carbonate.  The  oxims  as  a  class  are  crystalline 
compounds  soluble  in  water  from  whence  they  may  be  extracted  by  agita- 
tion with  ether. 

C.  A  reaction  yielding  a  determinable  extrinsic  product.  The  free  carboxyl 
group  is  usually  determined  by  titration  of  the  compound  by  standard  alkali, 
but  if  circumstances  render  this  proceeding  difficult,  an  indirect  process  may 
be  substituted.  One  of  these  is  based  on  the  liberation  of  an  equivalent  of 
hydrogen  sulflde  from  a  soluble  sulflde  by  a  free  organic  acid  —  thus,  CH3.COOH 
(acetic  acid)  -fNaHS  (sodium  hydrosulflde)  =  CH3.COONa  (sodium  acetate)  -f 
H2S.  The  volume  of  the  gas  evolved  is  determined  by  conducting  the  opera- 
tion in  a  modified  form  of  the  familiar  Victor  Meyer  vapor  density  apparatus. 
Or  the  acid  is  dissolved  in  alcoholic  potash  in  excess,  the  liquid  compounded 
with  strong  alcohol,  and  carbon  dioxide  passed  through  it  to  convert  the 
excess  of  potassium  hydrate  into  carbonate  and  bicarbonate  which  precipitate. 
After  filtering,  the  potassium  in  the  solution  (combined  with  the  organic  acid 
radical)  is  determined  by  the  usual  method. 

Organic  acids  liberate  iodine  from  a  mixture  of  potassium  iodide  and  iodate; 
e.g.,  6R.COOH-j-5KI  +  KIO3  =  6R.COOK  +  3I2-h3H2O.  The  iodine  is  de- 
termined by  treating  the  liquid  with  potassium  hydrate  and  an  excess  of 
hydrogen  peroxide,  when  a  molecule  of  oxygen  is  liberated  for  every  molecule 
of  iodine  — 12  -f-  2KOH  -f  H202  =  O2  -f  2KI  -|-  2H20.  The  volume  of  oxygen  ia 
found  by  having  the  reaction  take  place  in  a  nitrometer  or  similar  instrument. 

The  iodoso  (IO)  and  the  iodoxy  (I02)  groups  liberate  iodine  from  an  acid 
solution  of  potassium  iodide,  the  former  freeing  two  atoms,  and  the  latter  four 
atoms,  corresponding  to  their  oxygen  content. 

The  amids  and  amido -compounds  are  decomposed  by  contact  with  nitrous 
acid  evolving  nitrogen  — 

C2H3O.NH2  (acetamid)  -fHNO2  =  C2H3O.OH  (acetic  acid)  +  N2  +  H2O. 
one-half  of  the  total  nitrogen  coming  from  the  amid. 

Zeisel's  method  *  for  the  determination  of  methoxyl  is  one  very  generally 


*  Chem.  News,  1891—1—37  and  Analyst,  1898—297. 


PROXIMATE    ORGANIC    ANALYSIS.  317 

applicable.  The  principle  is  the  conversion  of  the  methyl  group  into  methyl 
iodide  by  acting  on  the  organic  body  with  hydriodic  acid  —  R.CH3O  +  HI  = 
CH3I  +R. OH  —  then  converting  the  methyl  iodide  into  silver  iodide  by  means 
of  an  alcoholic  solution  of  silver  nitrate  —  CH3I  -f  AgN03  =  Agl  -j-  CH3NO3. 

The  apparatus  for  the  determination  is  in  three  parts  joined  by  rubber  tub- 
ing; (1),  a  flask  closed  by  a  cork  carrying  a  tube  through  which  carbon  dioxide 
enters  and  passes  through  the  entire  apparatus,  and  a  reversed  condenser  to 
return  water- vapor  to  the  flask;  (2),  a  guard-tube,  arranged  to  be  heated  to 
about  56  ° ,  holding  water  in  which  red  phosphorus  is  suspended,  or  a  solution 
of  potassium  arsenite;  and  (3),  a  train  of  absorption  bulbs  containing  an 
alcoholic  solution  of  silver  nitrate. 

The  apparatus  is  connected  air-tight,  the  organic  body  and  an  excess  of 
hydriodic  acid  placed  in  the  flask,  and  all  the  air  in  the  train  displaced  by  car- 
bon dioxide.  The  mixture  in  the  flask  is  boiled,  and  the  gaseous  iodmethyl 
passes  over,  through  the  guard  tube  in  which  any  hydriodic  acid  or  iodine 
that  might  accompany  it  is  retained,  into  the  absorption  bulbs  where  silver 
iodide  is  precipitated.  The  precipitate  is  purified  and  weighed.  Volatile 
organic  compounds  may  be  analyzed  if  precautions  are  taken  against  their 
distillation.  The  method  cannot  be  used  where  sulfur  is  a  constituent  of  the 
organic  body. 

The  withdrawal  of  the  methyl  group  from  a  compound  is  known  as  demethy- 
lation,  and  has  some  applications  in  analysis.  Thus,  guaiacol  is  reduced  to 
pyrocatechin  and  homopyrocatechin  by  the  action  of  hydrobromic  acid  — 
C6H4.OH.O.CH3  +  HBr  =  C6H4(OH)2  -f  CH3Br.  —  these  products  extracted  by 
ether,  and  subsequently  separated  by  benzene. 

Easily  reducible  metallic  salts  are  acted  on  by  some  organic  bodies  with  sep- 
aration of  the  metal  or  a  lower  oxide.  Silver  nitrate  is  reduced  to  metallic  silver 
by  many  organic  compounds,  in  some  instances  in  molecular  ratio.  The  reduc- 
tion of  a  cupric  salt  to  cuprous  oxide  is  familiar  through  the  many  applica- 
tions of  Fehlings  solution  to  organic  analysis.  Other  reagents  that  have  a 
more  limited  use  are  mercuric,  stannic,  and  arsenic  compounds. 

The  reduction  of  potassium  permanganate  and  potassium  bichromate  have 
been  utilized  for  determinations  of  sugars,  organic  acids  and  their  salts,  and 
the  like,  as  has  the  reduction  of  ferric  chloride  to  ferrous  chloride  by  hydra- 
zin,  hydroxylamin,  and  similar  bodies. 

6.  Colorimetrically.  Many  organic  bodies  are  intrinsically  or  after  alteration 
by  the  action    of  reagents,  of    a  color   sufficiently  pronounced  for  a  colori- 
metric  comparison. 

Various  compounds,  notably  the  vegetable  alkaloids,  develop  intense  colors 
with  certain  reagents,  but  the  colors  are  evanescent  and  greatly  modified  by 
associated  bodies. 

The  pentosans  and  pentoses  produce  with  phlorglucin  and  hydrochloric 
acid  a  cherry-red  color  that  allows  a  fair  comparison.  The  aldehyds  repro- 
duce the  red  color  of  fuchsin  solution  that  has  been  previously  decolorized 
by  sulfurous  acid. 

7.  Attributive  methods.    In  inorganic  analysis  one  elects  attributive  methods 
in  preference  to  direct  determinations  for  considerations  of  rapidity  or  con- 
venience, but  in  organic  analysis  their  employment  is  frequently  a  matter  of 
necessity  from  the  want  of  other  methods. 

Inequality  in  specific  gravity  is  often  applied  in  technical  work  to  mixed  liquids 
or  solutions  and  occasionally  to  solid  mixtures.  Tables  are  drawn  up  from 
direct  experiments  showing  the  gravity  corresponding  to  each  percentage  of 


318  QUANTITATIVE    CHEMICAL,   ANALYSIS. 

one  constituent  of  the  mixture,  usually,  however,  extending  only  through  the 
range-of  the  proportions  commonly  found  in  commercial  articles.  Of  the  other 
physical  data,  the  rotation  of  polarized  light  is  largely  applied  in  the  determin- 
ation of  sugar,  less  often  to  the  oils,  urine,  the  proteids,  etc. 

The  conducting  power  of  solutions  for  electricity  has  been  employed  for  the 
determination  of  the  basicity  of  organic  radicals  combined  with  sodium.  The 
salt  is  made  up  in  a  dilute  aqueous  solution,  usually  one  gram-molecule  in  32 
and  322  liters,  and  an  electric  current  passed  through  it  by  electrodes  of  plat- 
inum covered  with  platinum  black.  The  resistance  of  the  solution  is  measured 
by  a  modified  Wheatstone's  bridge,  the  adjustment  indicated  by  a  telephone 
and  commutator  in  the  circuit. 

Of  the  chemical  methods,  the  reacting  quantity  to  a  given  reagent  is  often 
great  enough  between  members  of  a  mixture  to  be  a  reliable  basis  for  a  cal- 
culation. 

The  saponification  value  is  of  service  in  the  analysis  of  mixtures  of  oils, 
waxes,  etc.,  but  even  though  the  values  of  the  members  may  differ  consider- 
ably, in  many  instances  they  are  too  uncertain  to  be  relied  on,  varying  to  an 
excessive  degree  by  reason  of  origin,  age,  mode  of  preparation  and  other 
factors;  e.  g.t  the  saponiflcation  value  of  oil  of  rose  geranium  may  be  anywhere 
from  32  to  75  according  to  the  locality  of  the  growth  of  the  plant. 

The  combining  equivalent  of  the  members  of  a  chemical  series  will  often 
show  a  scale  of  numbers  increasing  or  decreasing  regularly  with  the  rank  of 
the  members.  Thus,  the  formic  acid  series,  each  acid  combining  with  a 
specific  proportion  of  barium  to  form  a  neutral  salt,  ranging  from  28. 60  per 
cent  for  capric  acid  to  70.25  per  cent  for  formic.  In  a  mixture  of  any  two  of 
the  acids  the  weight  of  barium  needed  for  neutralization  is  d  in  the  usual 
formula. 

In  the  ash  of  commercially  pure  organic  substances  the  ratio  of  one  base 
to  another  is  practically  constant  and  may  be  used  as  a  basis  for  the  deter- 
mination of  an  impurity  or  associate. 

The  ratio  between  the  oxygen  consumed  from  potassium  permanganate  and 
potassium  bichromate  in  the  modst  oxidation  process  is  said  to  be  specific  for 
some  organic  bodies  —  thus  starch  and  sugar  withdraw  much  more  oxygen 
from  bichromate,  while  the  reverse  is  true  of  tannin.* 

SEPARATION. 

By  solvents.  The  following  scheme  f  aims  to  separate  mixtures  of  the  com- 
mon organic  bodies  into  four  groups  by  the  application  of  various  solvents  in 
succession.  It  is  understood  that  in  most  cases  the  separation  is  but  approxi- 
mate. 

On  agitating  the  solid  or  liquid  mixture  with  water  acidulated  by  sulfuric  acid  and  a 
suitable  solvent  immiscible  therewith  (ether,  chloroform,  amyl  alcohol,  benzene,  or 
petroleum  ether),  the  following  distribution  will  occur: 

A. 

The  Acidulated  Aqueous  Liquid  will  contain  carbohydrates,  soluble  alkaloids  and  acids, 
organic  bases,  proteids,  etc.,  which  may  be  further  separated  by  adding  a  moderate  excess 
of  soda,  and  again  shaking  with  a  suitable  immiscible  solvent,  when  there  will  be 
obtained :  — 


*  Journ.  Amer.  Chem.  Socy.  1898—498. 
t  Allen,  Coml.  Org.  Anal.  1—60. 


PROXIMATE    ORGANIC   ANALYSIS. 


319 


IN  THE  IMMISCIBLE  LAYER  — 

Most  Vegetable  Alkaloids;  as  quinine,  strych- 
nine, aconltine,  atropine,  nicotine  cin- 
chonlne,  morphine,  (the  last  two  with 
difficulty). 

Coal  Tar  Bases;  as  anallne  and  its  homo- 
logues  (rosanlline),  chrysotoluldlne 
(pyridlne),  homolognes  of  pyrldine. 


IN  THE  ALKALINE  AQUEOUS   LIQUID  — 

Carbohydrates;  as  sugars,  gums,  dextrin. 
Soluble  Alcohols;  as   methyl  alcohol,  ethyl 

alcohol,  glycerine. 
Soluble  Acids;   as    acetic,    oxalic,    lactic, 

malic,  tartarlc,  sulphophenlc. 
Certain  Alkaloids  or  Organic  Bases;  as  cura- 

nine,    urea,  glycocine,   solanlne,    and 

possibly  clnchonine,  morphine  and  py- 

ridine. 

Certain  coloring  matters  ;  as  Indigo  products. 
Proteids  and  their  allies ;  as  albumin,  casein, 

gelatin. 


B. 

The  Immiscible  Layer  will  contain-  hydrocarbons,  oils,  various  acids,  resins,  coloring  mat- 
ters, phenols,  glucosides,  etc.,  which  may  be  further  separated  by  agitating  the  liquid  with 
water  containing  caustic  soda,  when  there  will  be  obtained: 


IN  THE  ALKALINE  AQUEOUS  LIQUID  — 

Fatty  Acids;  as  stearlc,  oleic,  valeric. 
Various  other  Acids;  as   benzole,  salicylic, 

phthalic,  meconic. 
Acid  Dyes  and  Coloring  Matters;   as   picric 

and  chrysophanic  acids,  alizarin,  aurin, 

bllirubln. 

Acid  Resins;  as  colophony. 
Phenols;  as   carbolic    and    cresyllc  acids, 

thymol,  creasote. 
Certain  Glucosides,  etc. ;  as  santonin,  can- 

tharidin,  picrotoxln. 


IN  THE  IMMISCIBLE  LAYEK  — 

Solid  Hydrocarbons;  as  paraffin,  naphtha- 
lene, anthracene. 

Liquid  Hydrocarbons;  as  petroleum  prod- 
ucts, rosin-oil,  benzene. 

Essential  Oils;  as  turpentine. 

Nitro- compounds;  as  nitro-benzene. 

Ethers  and  their  Allies;  as  ether,  chloro- 
form, compound  ethers,  nitre-glycerin. 

Fixed  Oils,  Fats,  and  Waxes. 

Neutral  Resins  and  Colouring  Matters. 

Camphors;  as  laurel -camphor,  borneol, 
menthol. 

Alcohols  Insoluble  or  nearly  Insoluble  in 
water;  as  amyl  and  cetyl  alcohols, 
chloresterin. 

Certain  Glucosides,  etc. ;  as  saponin,  digita- 
lin,  santonin. 

Certain  Weak  Alkaloids;  as  caffeine,  col- 
chicine,  narcotlne,  pipeline,  theobro- 
mine. 

After  this  partial  separation,  the  four  liquids  are  further  treated,  either  in 
the  same  manner  with  other  solvents,  or  otherwise,  as  the  nature  of  the  sub- 
stance seems  to  call  for. 

Separation  by  precipitation,  crystallization,  distillation.  As  in  inorganic 
analysis,  in  some  instances  the  members  of  a  group  of  allied  bodies  can  be 
separated  by  successive  precipitations.  Thus,  a  separation  of  the  proteids  by 
the  method  of  Schjerning  *  — 

A.  Albumin  I  is  precipitated  by  tin  chloride. 

B.  Albumin  I,  albumin    II,    and    denuclein  by  lead  acetate  or  mercuric 
chloride. 

C.  Albumin  I,  albumin  II,  denuclein  and  propeptone  by  ferric  acetate. 

D.  Albumin  I,  albumin  II,  denuclein,  propeptone  and  peptone  by  uranium 
acetate. 

E.  Albumin  I,    albumin  II  and  propeptone    by  magnesium  sulfate.    The 
weights  of  the  several  precipitates  are  sufficient  data  for  calculating  the  pro- 
portion of  each  proteid. 

Fractional  precipitation  is  applied  to  certain  groups  all  of  whose  members 


*  Zelts.  anal.  1898—413. 


320 


QUANTITATIVE    CHEMICAL.   ANALYSIS. 


form  precipitates  of  varying  solubility  with  a  given  reagent;  e.  g.t  the  toluidins 
with  oxalic  acid,  the  acid  oxalate  of  paratoluidin  requiring  6600  parts  of  water 
for  solution,  and  the  corresponding  ortho  compound  only  200  parts. 

Distillation  as  a  means  of  separating  a  volatile  from  a  non-volatile  body  has 
many  applications  in  technical  analysis,  as  has  fractional  distillation  for  the  ap- 
proximate separation  of  complex  liquids  all  of  whose  members  are  volatile  at  a 
moderate  heat,  e.  g.,  petroleum,  liquid  fatty  acids,  the  isomers  of  the  oxycar- 
bonic  acids,  etc.  As  circumstances  dictate,  the  distillation  is  conducted  in  a 
current  of  some  gas  or  steam,  or  in  vacuo. 

Duclaux  has  described  a  method  lor  the  determination  of  the  proportions 
of  the  volatile  members  of  the  fatty  acid  series  in  a  mixture  of  two  or 
more  —  namely  formic  to  caproic.  On  distilling  a  dilute  aqueous  solution  of 

any  one  of  these  acids  the  acid  comes 
over  with  the  water  at  a  specific  rate. 
Shown  graphically  in  Fig.  171  are  the 
results  obtained  by  him  for  the  first 
six  acids  of  the  series.  The  experi- 
ments were  made  by  fractionally  dis- 
tilling 110  Cc.  until  100  Cc.  had  come 
over,  receiving  each  ten  Cc.  sepa- 
rately and  determining  the  amount  of 
acid  it  contained.  The  ordinates 
represent  the  percentages  of  acid  in 
the  distillate  and  the  abscissae  the 
percentages  of  the  volumes  of  the  dis- 
tillate, taking  the  acid  in  the  original 
110  Cc.  as  100  per  cent.  It  will  be  seen 
that  of  formic  and  acetic  acids  the 
acidity  of  the  vapor  is  less  than  that  of 


Fig.  171. 


the  liquid  throughout  the  distillation,  though  Increasing  in  strength  as  the 
operation  proceeds,  while  the  reverse  is  true  of  the  other  four  acids;  and 
that  in  the  residue  of  ten  Cc.  in  the  retort  there  were  retained  of  formic 
acid  36.5  per  cent,  of  acetic  20  per  cent,  etc.,  but  none  of  caproic. 

With  a  mixture  of  two  acids  the  curve  of  the  rate  of  distillation  lies  be- 
tween those  of  the  constituents,  approaching  the  one  whose  proportion  in 
the  mixture  is  the  greater.  The  dotted  line  represents  a  mixture  in  equiva- 
lent proportions  of  acetic  and  propionic  acids. 

In  practice  the  total  acidity  of  a  mixture  of  the  free  acids  is  determined 
volumetrically  by  neutralizing  them  by  a  standard  alkali;  they  are  again  set 
free  by  an  excess  of  (non-volatile)  sulfuric  acid  and  fractionally  distilled.  But 
practically,  the  curves  are  so  altered  by  the  effect  of  condensation  in  the  retort 
(page  225)  that  the  method  is  restricted  to  the  identification  of  one  or  possibly 
two  acids  in  a  solution. 

Separation  by  ferments.  Of  the  ferments  that  convert  organic  matter  into 
less  complex  products,  the  most  familiar  is  brewers  yeast,  well  known  for  its 
action  on  the  starches  and  sugars.  Diastase  and  taka-diastase  are  convenient 
and  powerful  agents  for  the  sacchariflcation  of  starch,  and  inulase  for  the  con- 
version of  inulin  to  levulose.  Brewers  yeast  allows  the  aldoses  to  be  differen- 
tiated, only  the  tri-  hex-  and  nonoses  fermenting,  and  it  is  said  that  in  a 
solution  of  ammonium  paramandalate  the  micro  -organism  pennicilium  glaucum 
destroys  only  the  laevo-modification,  while  sacchr.  ellipsodeus  destroys  only  the 
dextro-modification. 

An  oxydase  known  as  laccase  obtained  from  the  sap  of  the  lac-tree  of  Japan, 


PROXIMATE   ORGANIC   ANALYSIS.  321 

is  a  soluble  oxidizing  ferment;  added  to  a  phenol,  the  mixture  absorbs  oxygen 
from  the  air  —  thus,  in  four  hours,  with  hydroquinon  there  was  absorbed  32 
Cc.  of  oxygen  per  gram  of  hydroquinon;  with  pyrocatechin,  17.4  Cc.  per  gram; 
and  with  resorcin,  only  .5  Cc.* 

Saponification  —  the  conversion  of  an  acid-ester  into  an  acid  and  an  alcohol  — 
is  of  importance  in  many  branches  of  technical  analysis,  notably  as  a  means  of 
parting  certain  oils,  waxes,  etc.  from  non-saponifiable  matter  in  general.  The 
reagent  most  used  for  the  purpose  is  a  caustic  alkali  in  alcoholic  or  concen- 
trated aqueous  solution. 

Other  bodies  that  may  be  termed  saponiflable  in  a  broader  use  of  the  term 
are  the  amids  —  e.  g.,  C2H3O.NH.C2H5  (ethyl  acetamid)  +  NaOH  =  NII.C2H5.H 
(ethylamin)  4- NaC2HaO2  (sodium  acetate);  amido  and  nitrile  groups,  hydro- 
lyzed  to  ammonia;  the  higher  alcohols  of  the  ethane  series;  etc. 

By  polymerization,  etc.  Various  compounds  are  converted  to  polymers  or 
other  products  soluble  in  water  or  the  reagent,  leaving  associates  insoluble. 
An  application  of  this  principle  is  in  the  assay  of  the  oil  of  cassia,  the  most  im- 
portant constituent  of  which  is  an  aldehyd  called  cinnamaldehyd.  On  treating 
the  oil  with  a  strong  solution  of  sodium  bisulfite  the  cinnamaldehyd  yields  a 
product  soluble  in  the  reagent,  while  the  other  constituents  (non-aldehyds)  re- 
main undissolved.  The  assay  is  conducted  in  a  special  flask  of  about  50  Cc. 
capacity,  having  a  long  neck  holding  about  6  Cc.  and  graduated  into  cubic 
centimeters  and  tenths.  The  oil  and  reagent  are  mixed  in  the  flask,  then  water 
poured  in  until  the  non-aldehyds  have  risen  into  the  neck  where  their  volume 
is  measured. 

Other  applications  are  for  the  determination  of  turpentine  in  the  commercial 
article  (g.  0.),  aQd  nitrobenzene  in  commercial  benzene  by  nitrification. f 


*  Amer.  Annual  of  Photography,  1898—72. 
t  Journ.  Socy.  Chem.  Ind.  1884—74. 

21 


322  QUANTITATIVE   CHEMICAL   ANALYSIS. 


CHLOKIMETRY. 

In  the  older  processes  for  the  manufacture  of  bleaching  powder,  chlorine  is 
generated  by  the  action  of  hydrochloric  acid  (from  sodium  chloride  and  sulf uric 
acid)  on  a  superoxide  of  manganese,  and  conducted  over  calcium  hydrate.  The 
manganese  superoxide  is  either  the  native  ore,  consisting  of  the  minerals  wad, 
pyrolusite,  etc.,  with  more  or  less  gangue,  or  the  manganic  hydrate  regenerated 
from  the  spent  manganese  chloride  ( Weldon's  mud)  by  precipitation  by  lime  in 
contact  with  air. 

The  percentage  of  metallic  manganese  in  an  ore  is,  in  this  connection,  a  matter 
of  indifference  to  the  bleach-maker,  he  is  simply  wishing  information  as  to  what 
volume  of  chlorine  will  be  furnished  by  a  ton  of  a  given  ore.  The  weight  of 
chlorine  evolved  is  in  the  ratio  of  two  atoms  of  chlorine  to  one  atom  of  availa- 
ble oxygen  in  the  ore, 

MnO2    +  4HCl  =  Mn012    +  2H2O  -f  2C1.     1.227  Ibs.  yields  1  Ib.  01. 
Mn203  +  6HC1  =  2Mn012  +  3H2O  +  2C1.     2.228    "        «        "      " 
Mn8O4  -f  8HC1  =  3Mn012  +  4H2O  +  2C1.     2.327    "        "        "      « 

Since  the  ores  are  generally  mixtures  of  the  different  oxides  and  contain 
sand,  clay,  etc.,  every  purchase  must  be  tested  to  ascertain  its  chlorine  value. 
Moreover  there  are  occasionally  associated  with  the  manganese  superoxide 
certain  minerals  of  a  reducing  nature  soluble  in  hydrochloric  acid  or  the  acid 
plus  chlorine,  and  that  proportionally  reduce  the  yield  of  chlorine.  Thus, 
taking  as  an  extreme  case  a  mixture  of  magnetite  (Fe2O3.FeO)  and  hausman- 
nite  (Mn3O4)  in  the  molecular  proportion  of  two  to  one  —  that  is  containing 
66.97  percent  of  magnetite  and  33.03  percent  of  hausmannite  —  there  is  con- 
tained 2.31  per  cent  of  available  oxygen;  but  the  manufacturer  on  heating  the 
mixture  with  hydrochloric  acid  will  obtain  no  chlorine  whatever  (theoretically), 
since 

Mn3O4  -f  2Fe304  +  24HC1  =  3MnCl2  -f  3Fe2Cl6  +  12H2O. 

All  the  analytical  methods  hitherto  proposed  are  based  on  the  principle  of 
measuring  the  oxidizing  power  of  the  available  oxygen,  by  its  direct  or  indi- 
rect reaction  with  a  known  weight  of  a  reducer  and  determining  the  excess  of 
the  latter.  The  following  are  in  common  use: 

1.  Pattinson's.  The  finely  powdered  ore,  dried  at  110® ,  is  placed  in  a  small 
flask  and  dissolved  in  a  warm  mixture  of  a  specified  volume  of  standard  fer- 
rous sulfate  with  sulfuric  acid,  when  the  reactions 

MnO2  -f  2FeS04  +  2H2S04  =    MnS04  -f  Fe2(SO4)3  +  2HsO. 
Mn2O3  4-  2FeSO4  +  3H2SO4  =  2MnSO4  -f  Fe2(S04)3  +  3H2O. 
Mn3O4  4-  2FeSO4  +  4H2S04  =  3MnSO4  +  Fe2(SO4)3  -f  4H20. 
take  place,  each  atom  of  available  oxygen  in  the  ore  changing  two  atoms  of 
ferrosum  to  ferricum.    When  the  reaction  is  over  the  excess  of  ferrous  sulfate 
is  determined  by  titration  by  standard  permanganate,  and  the  available  oxygen 
and  corresponding  chlorine  calculated. 

Throughout  the  experiment  the  ferrous  sulfate  is  protected  from  oxidation 
by  the  air  by  transmission  of  a  current  of  carbon  dioxide  through  the  flask. 
Or  the  flask  may  be  closed  by  a  cork  from  which  passes  an  open  glass  tube 


CHLOKIMETKY.  323 

doubly  bent,  the  exit  dipping  into  water;  the  air  originally  in  the  flask  is  dis- 
placed by  carbon  dioxide  by  the  admixture  of  a  little  sodium  bicarbonate  with 
the  ore,  this  reacting  with  the  acid  before  the  ore  is  attacked. 

A  grave  objection  to  the  above  method  is  the  use  of  dilute  sulfuric  acid  as 
the  solvent  in  which  magnetite  and  other  reducing  mineral  compounds  are  not 
readily  soluble;  and  though  the  process  be  modified  by  the  substitution  of  hy- 
drochloric acid  for  sulfuric  (and  potassium  bichromate  for  permanganate), 
still  there  is  lacking  the  aid  of  the  powerful  oxidizer  free  chlorine.  Taking  the 
above  example  of  the  mixture  of  magnetite  and  hausmannite,  if  all  the  magne- 
tite dissolves  in  the  sulfuric  acid  the  result  will  correctly  indicate  no  available 
oxygen  in  the  mixture  and  no  chlorine  would  be  evolved  in  practice,  since 

MD304  +  2Fe304  +  12H2SO4  +  (FeSO4)  =  3MnSO4  +  3Fe2(SO4)3  +  12H2O  -f 
(FeSO4). 

the  ferrous  sulphate  remaining  entirely  unoxidized.  But  if  none  of  the  mag- 
netite dissolved,  the  result  would  show  the  whole  of  the  available  oxygen  of  the 
hausmancite.  As  only  a  part,  and  perhaps  only  a  small  part,  of  the  magnetite 
would  dissolve,  the  result  would  be  in  so  far  misleading. 

2.  Bunsen's  method.    The  error  referred  to  in   (1)  is  not  incurred  in  the 
method  of  Bunsen  which  imitates  the  process  of  the  manufacture  of  chlorine  on 
a  large  scale,  and  furnishes  corresponding  results. 

The  powdered  ore  in  a  small  flask  is  boiled  with  concentrated  hydrochloric 
acid  and  the  evolved  chlorine  led  through  a  solution  of  potassium  iodide.  The 
chlorine  displaces  an  equivalent  of  iodine  according  to  the  equation  2 KI  -{-  Clg 
=  2KCt  -f  I2.  The  freed  iodine,  held  in  solution  by  the  excess  of  potassium 
iodide,  is  titrated  by  a  standard  solution  of  sodium  thiosulfate  with  starch- 
paste  as  indicator  —  Ig  +  2Na2S203  =  Nal  +  Na2S406.  The  excess  of  potassium 
Iodide  does  not  interfere  with  the  titration.  The  end-point  is  shown  by  the 
breaking  up  of  the  intensely  blue  iodide  of  starch  ((C^H^O^I^HI  ?)  and  the 
consequent  decolorization  of  the  solution.  From  the  weight  of  the  liberated 
iodine  is  calculated  that  of  the  chlorine. 

DeKoninck  and  Lecruer*  assert  that  in  the  process  as  carried  out  on  these 
lines,  much  hydrochloric  acid  vapor  passes  into  the  solution  of  potassium 
iodide  and  forms  therein  bydriodic  acid  which  is  readily  decomposed  by  air 
with  the  liberation  of  iodine.  They  prefer  to  mix  the  ore  with  two  or  three 
parts  of  water  and  pass  into  the  mixture  and  through  the  absorbent,  in  suc- 
cession, carbon  dioxide,  gaseous  hydrochloric  acid,  and  carbon  dioxide. 

3.  Fresenius  and  Will's  method.    Oxalic  acid  is  decomposed  to  carbon  diox- 
ide and  water  by  a  higher  oxide  of  manganese:  e.  g.,  the  binoxide  — 

MnO2  +  H2C2O4  =MnO      +H2O   -f2CO2;  or 

Mn02  +  H2C204  +  H2S04  =  MnSO4  +  2H2O  +2CO2  — 
the  other  superoxides  reacting  in  a  similar  way. 

The  ore  is  placed  in  the  flask  A,  Fig.  2,  and  covered  with  dilute  sulfuric 
acid.  An  excess  of  solution  of  oxalic  acid  is  poured  into  B,  and  D  is  half 
filled  with  concentrated  sulfuric  acid.  The  apparatus  is  weighed  and  the  ox- 
alic solution  run  into  the  flask;  the  liberated  carbonic  acid  passes  out  through 
D,  dried  meanwhile  by  the  sulfuric  acid.  When  the  reaction  is  over  the  liquid 
in  the  flask  is  boiled  and  a  current  of  dry  air  drawn  through,  entering  at  B  and 
leaving  at  D,  to  remove  the  last  traces  of  carbon  dioxide  from  the  solution. 
The  apparatus  is  cooled  and  reweighed,  the  loss  from  the  preceding  weight 
being  carbon  dioxide,  from  which  the  weight  of  available  oxygen  can  be  calcu- 
lated by  the  above  equations. 


Chem.  News,  1891—1—280. 


324  QUANTITATIVE    CHEMICAL    ANALYSIS. 

A  modification,  somewhat  more  accurate,  is  to  determine  the  volume  of 
carbon  dioxide  by  measuring  it  in  a  nitrometer,  Fig.  118. 

4.  Baumann  applies  the  reaction  of  hydrogen  peroxide  for  the  determination, 
one  atom  of  available  oxygen  from  the  manganese  superoxide  uniting  with  one 
atom  of  oxygen  from  hydrogen  peroxide  to  form  a  molecule;  thus  with 
manganese  sesquioxide  — 

Mn2O3  +  H2O.O  =  2MnO  -f-  O2  +  H2O. 

The  oxygen  formed  maybe  determined  in  two  ways;  gravimetrically,  by  the  loss 
in  weight  when  the  operation  is  done  in  a  weighed  flask  as  in  Fresenius  and 
Will's  method  supra;  or  volumetrically,  by  acting  on  the  superoxide  by  a 
known  volume  (an  excess)  of  standard  solution  of  hydrogen  peroxide,  then 
titrating  back  the  excess  by  standard  permanganate. 


In  Weldons  mud  there  are  contained  besides  manganese  superoxides,  the 
oxides  of  calcium  and  other  bases  which  generate  no  chlorine  when  the  mud  is 
dissolved  in  hydrochloric  acid,  though  using  up  their  equivalents  of  acid.  For 
their  determination  a  weighed  amount  of  the  mud  is  heated  with  a  known 
volume,  an  excess,  of  normal  oxalic  acid.  The  acid  reacts  with  the  manganese 
binoxide  to  form  manganous  oxalate  —  Mn02  -f-  2H2C204  =  MnC204  -f  2II2C03  — 
this  requiring  two  equivalents  of  oxalic  acid  for  one  of  manganese  binoxide. 
The  other  bases  also  form  oxalates.  An  aliquot  part  of  the  filtered  solution  is 
titrated  by  normal  sodium  hydrate  and  phenol-phthalein.  Deducting  two 
equivalents  of  acid  for  the  manganic  oxide  (whose  amount  has  been  previously 
found)  the  remainder  is  the  oxalic  acid  required  for  the  other  bases,  from 
which  may  be  calculated  the  corresponding  weight  of  hydrochloric  acid  required 
for  their  solution.  * 


For  the  determination  of  the  chlorine  and  hydrochloric  acid  in  the  gases  from 
the  chlorine  generator,  a  simple  method  is  to  pass  five  liters  through  a  solution 
of  pure  caustic  soda  by  which  both  are  absorbed  —  2C12  -f  4NaOEI  =  2NaOCl  -f 
2NaCl  -f  2H2O ;  and  HC1  -f  NaOH  =  NaCl  -f  H2O.  On  titrating  the  solution  by 
standard  arsenious  acid  —  2NaOCl  +  As2O3  =  2NaCl  -f-  As20s  —  each  atom  of  the 
oxygen  of  the  hypochlorite  corresponds  to  two  atoms  of  the  chlorine  in  the  gas. 
The  total  chlorine  is  determined  by  titration  by  standard  silver  nitrate.  A 
simple  calculation  gives  their  respective  proportions. 

Another  method  is  due  to  Younger.  The  gas  is  drawn  into  an  aspirator 
bottle  having  a  gauge  showing  the  volume  of  gas  entering  it  expressed  in  both 
cubic  centimeters  and  in  cubic  feet  and  decimals.  Before  entering  the  aspirator 
the  gas  is  passed  through  a  wash  bottle  in  the  form  of  a  long  (20  inches)  tube, 
containing  100  Cc.  of  a  solution  of  arsenious  acid,  the  reagent  in  the  solution 
being  an  amount  to  exactly  fix  one  gram  of  chlorine  —  (As20s  -|-  2C12  +  2Ef2O  = 
As205  +  *HCl)  ;  the  solution  also  contains  a  little  sulfate  of  indigo  whose  blue 
color  is  bleached  by  free  chlorine. 

The  gas  is  drawn  slowly  through  the  wash-bottle  until  the  indigo  Is  just  de- 
colorized, showing  that  all  the  arsenious  acid  is  oxidized.  The  aspirator  is 
closed  and  the  volume  of  gas  drawn  in  is  read  in  cubic  feet.  If  exactly  one 
cubic  foot,  for  example,  then  the  gas  contains  15.432  grains  (one  gram)  of 
chlorine  per  cubic  foot,  and  other  volumes  proportionally. 


*  Lunge,  Chemieche-technische  Untersuchung,  430. 


CHLORIMETRY.  325 

To  determine  the  hydrochloric  acid  in  the  gas,  ten  Cc.  of  the  solution  in  the 
wash-bottle  is  drawn  out  by  a  pipette  and  titrated  by  decinormal  silver  nitrate; 
this  is  tantamount  to  titrating  the  entire  100  Cc.  with  normal  silver  nitrate. 
The  excess  of  silver  solution  required  for  precipitation  over  28.2  Cc.  is  due  to 
the  hydrochloric  acid  in  the  gas,  and  the  proportion  may  be  readily  calculated. 
The  figure  28.2  is  the  volume  of  silver  solution  required  to  combine  with  the 
4HC1  liberated  in  the  oxidation  of  the  arsenious  acid  as  shown  in  the  above 
equation;  i.  e.t  as  one  Cc.  of  normal  silver  nitrate  is  equivalent  to  .036458 
gram  of  HC1,  then  28.2  Cc.  are  equivalent  to  1.028  grams  of  HC1,  the  amount 
formed  by  the  hydrogenation  of  one  gram  of  chlorine. 

Between  the  wash-bottle  and  aspirator  is  interposed  a  tube  containing  a  solu- 
tion of  potassium  iodide.  As  soon  as  any  chlorine  passes  the  wash-bottle  it 
liberates  iodine  which  colors  the  solution  yellow,  thereby  furnishing  an  indi- 
cation in  addition  to  that  of  bleaching  the  indigo. 

BLEACHING  POWDER. 

The  formula  of  this  material  is  variously  stated  by  different  authorities;  it 
is  essentially  a  mixture  of  calcium  hypochlorite  with  calcium  chloride  and  a 
little  calcium  chlorate.  Its  commercial  value  as  a  bleach  depends  on  the  pro- 
portion of  loosely  combined  chlorine  of  the  hypochlorite.*  Theoretically,  as- 
suming the  formula  CasHeOeCU,  it  contains  about  39  per  cent  of  the  element  in 
this  state,  a  good  commercial  quality,  when  fresh,  from  32  to  37  per  cent.f  It 
slowly  decomposes  on  keeping,  with  loss  of  available  chlorine.  As  a  bleaching 
agent  calcium  chloride  is  of  no  value;  nor  is  the  chlorate,  and  the  method 
adopted  for  the  determination  of  available  chlorine  should  differentiate  be- 
tween that  from  the  hypochlorite  and  that  formed  by  the  action  of  the  reagents 
on  the  chlorate. 

To  prepare  the  powder  for  analysis  it  is  ground  in  a  mortar  with  cold  water, 
allowed  to  settle  for  a  moment,  and  the  turbid  liquid  poured  into  a  liter  flask. 
The  coarser  grains  remaining  in  the  mortar  are  again  triturated  and  lixiviated 
as  before,  and  the  operation  continued  until  all  has  been  transferred,  when  the 
liquid  is  made  up  to  the  mark  with  water.  Before  taking  out  any  part  of  the 
contents  of  the  flask  for  analysis,  a  thorough  shaking  up  is  necessary,  since  the 
insoluble  matter  retains  a  little  chlorine. 

As  in  the  analysis  of  manganese  ores,  all  the  methods  measure  the  oxidizing 
power  of  the  chlorine  by  means  of  a  reducing  agent,  but  here  the  measure- 
ment may  be  done  (1),  directly,  or  (2),  by  a  determination  of  the  excess  of  the 
reducer,  J 

In  the  former  class  is  an  old,  now  discredited  method  directing  to  mix  with 
the  solution  of  bleaching  powder  a  solution  of  ferrous  chloride  in  hydrochloric 
acid,  and  throw  in  a  weighed  piece  of  copper.  The  mixture  is  boiled  in  a  hask 
for  some  time  without  access  of  air.  The  reaction  CaOCl2  +  Cu2  +  2HCl  = 
CaCl2  +  CU2C12  4-  H2O  ensues,  the  ferrous  chloride  acting  only  as  a  carrier  of 
oxygen.  The  loss  in  weight  of  copper  represents  an  equivalent  of  available 
chlorine  in  the  powder. 

Penot's  method  is  based  on  the  principle  that  an  arsenite  is  oxidized  to  an 
arseniate  by  chlorine,  and  also  that  if  a  mixture  of  starch  and  potassium  iodide 
is  touched  with  a  solution  containing  free  chlorine  the  mixture  becomes  blue 
from  the  combination  of  starch  with  the  iodine  liberated  from  the  iodide.  Ta 


*  Biederman,  Chem.  Kal.  308. 

t  Journ.  Anal.  App.  Chem.  1892—231. 

t  Chem.  News,  1892-2—114. 


326  QUANTITATIVE    CHEMICAL    ANALYSIS. 

the  bleaching  powder  suspended  in  water,  is  added  from  a  burette  a  standard 
solution  of  sodium  arsenite,  testing  a  drop  of  the  fluid  after  each  addition  by 
allowing  it  to  fall  on  paper  coated  with  a  mixture  of  starch  and  potassium 
iodide.  When  no  color  is  produced  it  is  known  that  all  the  available  chlorine 
has  reacted  with  the  arsenite. 

Another  method  is  almost  identical  with  that  of  Pattinson  for  the  analysis  of 
a  manganese  superoxide.  The  powder  is  treated  with  sulfuric  acid  and  an  ex- 
cess of  a  standard  solution  of  ferrous  sulfate,  and  the  unoxidized  iron  titrated 
by  permanganate  or  bichromate.  Here  the  chlorate  also  oxidizes  an  equivalent 
of  iron. 

Lunge  takes  advantage  of  the  reaction  between  calcium  hypochlorite  and  hy- 
drogen peroxide  —  CaOC]2  +  H202  =  CaC)2  +  H2O  +  O2  or  CaO2C2  +  2H2O2  = 
CaCIs  +  2H2O  -f  2O2.  The  bleaching  powder  is  mixed  with  a  solution  of  hydro- 
gen peroxide  in  water  in  a  nitrometer  (page  144)  and  the  evolved  oxygen  meas- 
ured, its  weight  calculated,  and  the  result  converted  to  chlorine  —  one  atom 
of  oxygen  corresponding  to  one  atom  of  chlorine. 

When  potassium  iodide  and  hydrochloric  acid  are  added  to  bleaching  powder, 
the  chlorine  liberates  an  equivalent  of  iodine  which  may  be  immediately 
titrated  by  sodium  hyposulflte  and  starch  paste,  or  better,  by  an  excess  of  hy- 
posulfite  and  back  with  iodine. 

Of  all  the  methods  proposed,  that  of  Penot  is  probably  the  most  used,  since 
arsenious  acid  (the  solution  of  bleaching  powder  being  normally  alkaline)  is 
not  acted  on  by  calcium  chlorate,  unlike  many  of  the  other  reducing  agents. 
For  a  separate  determination  of  the  calcium  chlorate  the  following  may  be 
outlined:  — 

1.  The  chlorate  is  calculated  from  the  difference  between  the  result  given  by 
the  method  of  Penot  (i.  e.t  the  available  chlorine  in  the  hypochlorite),  and  the 
result  from  some  other  method  that  measures  the  oxidizing  power  of  both  the 
hypochlorite  and  chlorate.    Another  plan  is  to  titrate  the  solution  of  the  pow- 
der by  arsenious  acid  in  both  alkaline  and  acid  solutions ;  in  the  former  the 
arsenious  acid  is  acted  on  but  slowly  by  the  chlorate,   practically  not  at  all 
during  a  titration,  while  an  acid  solution  of  a  chlorate  readily  oxidizes  it  in  a 
warm  liquid. 

2.  Drey  fuss  first  treats  the  solution  of  the  bleaching  powder  with  a  slight 
excess    of  ammonia.    The  hypochlorite  is  decomposed    to  calcium  chloride, 
while  the  chlorate  is  only  transformed  to  ammonium  chlorate  — 

3CaOCl2  +  2NH3=  3CaCl2  +  3H2O  -f  N2;  and 
Ca(ClO3)2  +  2NH4OH  =  2NH4C1O3  -f  Ca(OH)2. 

the  solution  is  then  evaporated  until  the  excess  of  ammonia  is  dissipated,  and 
made  up  to  a  definite  volume. 

Of  a  standard  solution  of  cupric  sulfate,  a  measured  volume  is  titrated  by  a 
solution  of  stannous  chloride  in  hydrochloric  acid,  — 

2CuSO4  +  4HC1  +  SnCl2  =  Cu2012  -f  SnCl4  +  2H2SO4, 

the  end  point  shown  by  the  discharge  of  the  blue  color.  The  liquid  is  now 
essentially  a  solution  of  pure  cuprous  chloride. 

An  aliquot  part  of  the  bleaching  powder  solution  prepared  as  above  is  added 
to  the  cuprous  chloride  solution ;  the  ammonium  chlorate  reacts  to  form  cupric 
chloride  — 

3Cu2Cl2  -f  NH4C1O3  +  6HC1  =  3Cu2Cl4  -f  NH4C1  +  3H2O. 

The  solution  is  once  more  titrated  by  the  stannous  chloride  solution  till  color- 
less to  determine  the  amount  of  cupric  chloride  formed,  from  which  the  amount 
of  chlorate  is  calculated.  To  standardize  the  stannous  chloride  solution ,  a 
Mmilar  experiment  is  made  on  pure  potassium  chlorate. 


CHLORIMETRY.  327 

3.  Fresenius*  directs  to  mix  the  aqueous  extract  of  the  powder  with  neu- 
tral lead  acetate  solution  in  excess.  A  precipitate  falls,  a  mixture  of  lead 
chloride  and  lead  hydroxide.  The  hypochlorite  slowly  reacts  with  the  lead 
chloride  — 2CaOCl2  +  PbCl2  =  PbO2  +  2CaCJ2  +  Cl2;  later  the  chlorine  reacts 
with  the  excess  of  lead  acetate  —  Ck  +  2Pb(C2H302)2  -f  2H2O  =  PbCl2  -f-  PbO2 
-j-  4HC2H3O2.  The  liquid  is  filtered  and  the  filtrate  concentrated.  The  calcium 
chlorate,  not  decomposed  in  the  preceding  reactions,  is  determined  by  acidulat- 
ing the  filtrate  by  hydrochloric  acid,  distilling  the  liberated  chlorine  (CaC103)2  + 
12HC1  =  CaCl2  +  6C12  +  6H2O)  into  potassium  iodide,  and  titrating  by  thiosul- 
fate  the  iodine  set  free. 


Eau  de  Javelle  is  essentially  a  solution  of  sodium  hypochlorite  in  water.  It  has 
normally  an  alkaline  reaction  due  to  sodium  hydrate  and  carbonate,  and  it  is 
sometimes  desirable  to  determine  these  constituents.  The  sodium  hydrate  may 
be  titrated  directly  by  a  standard  acid  and  phenol-phthalein;  the  red  color  per- 
sists as  long  as  free  alkali  remains,  but  is  then  immediately  bleached  by  the 
free  chlorine,  (NaOCl-f  2HC1  =  NaCl  +  C12  -f-  HzO).  Since  chlorine  is  tem- 
porarily liberated  throughout  the  titration,  several  additions  of  minute  amounts 
of  the  indicator  are  made  while  the  titration  is  in  progress. 

Or  the  oxygen  of  the  hypochlorite  may  be  eliminated  by  heating  the  eau  with 
a  slight  excess  of  ammonia  —  3NaOCl  -f  2NH3  =  3NaCl  -f  N2  +  3H2O.  After 
evaporating  until  the  excess  of  ammonia  has  been  expelled  the  liquid  is  titrated 
by  normal  acid  and  methyl  orange,  or  first  by  phenol-phthalein,  then  by  methyl 
orange  (page  121).  The  reaction  with  ammonia  has  also  been  applied  for  the 
determination  of  available  chlorine  in  bleaching  powder,  measuring  the  nitrogen 
evolved. 

Another  way  to  decompose  the  hypochlorite  is  by  treatment  with  precipitated 
cobalt  sesquioxide  or  nickel  sesquioxide  suspended  in  water.  Sodium  chloride 
and  the  protoxide  of  the  metal  are  formed,  the  free  alkali  and  alkali  carbon- 
ate remaining  unchanged  —  NaOCl  +  Co2O3  =  NaCl  -f  2CoO  +02. 


*  Zeits.  angew.  1895—501. 


328 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


IRON  AND  STEEL. 

The  chemical  reactions  involved  in  the  processes  of  smelting  and  refining 
these  metals  are  so  well  comprehended  that  quite  an  effective  control  of  the 
character  of  the  products  is  permitted,  and  there  is  maintained  at  every  large 
works  a  laboratory  for  the  systematic  examination  of  the  raw  materials  and 
products  and  by-products.  From  the  analytical  data  in  conjunction  with  me- 
chanical tests,  the  details  of  the  processes  of  manufacture  are  modified,  aiming 
at  tKe  production  of  the  largest  output  of  the  best  quality  for  the  specific  pur- 
pose intended,  and  at  the  lowest  cost. 

Pig  iron  and  steel  are  essentially  iron  containing  small  proportions  of  carbon, 
sulfur,  silicon,  and  phosphorus,  and  sometimes  copper,  titanium,  arsenic, 
nickel,  etc.,  and  alloyed  with  a  little  manganese.  The  state  in  which  the  non- 
metallic  elements  exist  —  whether  chemically  combined  with  iron  or  manganese 
or  with  each  other,  or  simply  dissolved  in  the  matrix  of  iron  —  is  as  yet  a  mat- 
ter of  controversy;  and  while  the  influence  of  carbon  on  the  physical  properties 
of  iron  is  fairly  well  known,  the  effect  of  the  other  elements  usually  contained, 
though  quite  as  pronounced,  is  more  obscure  and  can  only  be  traced  in  a  gen- 
eral way. 

Pig  iron  and  cast  iron  contain  much  larger  proportions  of  carbon  and  silicon 
than  the  refined  metals,  part  of  the  carbon  being  in  the  free  state  (graphitic 
carbon,  graphite,  kish).  The  difference  in  composition  between  the  various 
grades  of  raw  and  refined  metals  can  be  seen  in  the  following  analyses  of  typi- 
cal specimens ;  however,  the  percentages  often  vary  largely  from  the  figures 
here  given. 

It  will  be  noticed  that  the  percentages  of  combined  carbon  and  manganese 
are  greater  in  the  white  pig  iron  than  in  the  gray,  while  the  graphite  and  sili- 
con are  less;  the  mottled  iron  is  intermediate.  .Also  that  in  the  milder  grades 
of  steel  (those  comparatively  low  in  carbon)  the  percentages  of  the  constitu- 
ents closely  approach  those  in  wrought  iron.  From  the  composition  alone  it 
is  often  impossible  to  state  positively  that  a  given  sample  is  mild  steel  or 
wrought  iron,  though  as  a  rule  the  percentage  of  manganese  is  higher  and  the 
phosphorus  and  sulfur  lower  in  the  former.  The  percentage  of  slag  and  oxides, 
character  of  the  fracture  shown  on  breaking  by  tension  or  transversely,  and 
the  microscopical  appearance  of  a  slightly  etched  plane  surface  afford  evidence 
as  to  the  method  of  manufacture. 


A. 

B. 

C. 

D. 

E. 

F. 

G. 

H. 

I. 

J. 


Combined 
carbon. 
2.45 

Graphite. 
1.46 

Silicon 
2.23 

2.92 

1.20 

1.12 

3.37 

.35 

.42 

4.22 

.18 

1.06 

6.78 

trace. 

2.28 

.10 

.06 

.16 

.11 

— 

.01 

.18 

— 

.03 

.55 

— 

.12 

1.05 

— 

.15 

Iron. 

Silicon. 

Sulfur.  Phosphorus.  Manganese,  etc. 

2.23 

.09 

.21 

.35 

93.21 

1.12 

.08 

.16 

,40 

94.12 

.42 

.11 

.17 

.61 

94.97 

1.06 

.01 

.13 

22.15 

72.25 

2.28 

— 

.28 

79.76 

10.90 

.16 

.15 

.17 

.22 

99.14 

.01 

.08 

.07 

.43 

99.30 

.03 

.02 

.03 

.25 

99.49 

.12 

.07 

.09 

1.00 

98.17 

.15 

.05 

.06 

.42 

98.27 

IKON    AND    STEEL. 


329 


A.  Gray  pig  iron  for  founding. 

B.  Mottled  pig  iron  for  founding. 

C.  White  pig  iron  for  making  wrought  iron. 

D.  Spiegel-eisen  (manganiferous  pig  iron),  "22  per  cent." 

E.  Ferro- manganese  (ferriferous  manganese), "  80  per  cent." 

F.  Wrought  iron,  of  fair  quality. 

G.  Bessemer  steel,  for  wire. 

H.  Open  hearth  steel,  for  boiler  plate. 
I.  Bessemer  steel,  for  railroad  rails. 
J.  Open  hearth  steel,  for  tools,  moderately  hard. 


An  outline  of  the  processes  of  smelting  iron  ore  and  the  manufacture  of  steel  and 
wrought  iron  may  be  of  interest. 

1.  For  the  manufacture  of  pig  iron,  calculated  weights  of  iron  ore,  coke  and  limestone 
are  in  turn  charged  into   a  tall  cylindrical  furnace  supplied  with  heated  air  blown  in 
through  tuyeres  near  the  bottom.    At  the  region  of   the  tuyeres  coke  burns  to  carbon 
dioxide  (C  +  O2=  CO2) ;  the  gases  passing  upward  meet  incandescent  coke,  and  the  car- 
bon dioxide  is   reduced  to  carbon  monoxide  (CO2+0=2CO).    The  carbon  monoxide 
reacts  with  the  iron  ore  to  produce  metallic  iron  (Fe2O3  +  SCO  =  Fe2  +  3CO2),  and  also  to 
a  limited  extent  with  the  silica  of  the  gangue  of  the  ore  to  form  silicon  (SiO2  +  2CO  =  Si  + 
2CO2) ;  similarly  the  phosphoric  acid  and  sulfur  trioxide  of  the  ore  and  coke  are  reduced 
to  phosphorus  and  sulfur. 

The  metallic  iron,  in  intimate  contact  with  finely  divided  carbon,  alloys  with  about  four 
per  cent  of  this  element  and  the  silicon,  phosphorus,  and  part  of  the  sulfur,  and  melts, 
finally  dropping  into  the  reservoir  (crucible)  at  the  bottom  of  the  furnace,  where  it  col- 
lects, protected  by  a  layer  of  specifically  lighter  slag  from  oxidation  by  the  blast,  and  is 
tapped  out  periodically. 

The  object  of  the  limestone  is  to  flux  the  gangue  of  the  ore  (sand,  clay,  etc.)  and  the 
ash  of  the  coke,  these  being  infusible  at  furnace  temperatures.  In  contact  with  lime  there 
is  formed  a  comparatively  easily  fusible  slag,  a  complex  silicate  of  lime,  alumina,  magne- 
sia and  manganese  protoxide,  containing  a  small  amount  of  calcium  sulfide. 

The  gas  arising  to  the  top  of  the  furnace  is  a  mixture  of  carbon  monoxide,  carbon  diox- 
ide (partly  from  the  limestone)  and  nitrogen  (from  the  air).  It  is  combustible  and  there- 
fore carried  down  to  be  burned  in  the  regenerative  stoves  used  to  heat  the  air-blast 
entering  the  tuyeres. 

2.  Wrought  iron  is  made  from  pig  iron,  preferably  of  the  white  or  mottled  grades.    A 
charge  of  iron  is  melted  in  a  horizontal  furnace  whose  bed  is  made  of  a  pure  variety  of 
iron  ore.    Through  the  action  of  the  iron  oxide  and  the  air  the  silicon  and  manganese  of 
the  pig  iron  are  gradually  oxidized,  and  to  a  great  extent  the  phosphorus  and  sulfur,  the 
carbon  burning  to  carbon  monoxide.    The  ferric  oxide  of  the  bed  is  partially  reduced  to 
ferrous  oxide  which  unites  with  the  silica,  manganese  oxide,  and  phosphoric  acid  to  form 
»n  easily  fusible  slag.    As  the  pig  iron  gradually  loses  its  carbon  and  silicon  it  becomes 
proportionately  less  fusible,  and  finally  comes  to  the  form  of  a  pasty  mass  inclosing  con- 
siderable of  the  slag.    The  mass  is  divided  into  several  parts  which  are  gathered  into  balls 
and  each  ball  subjected  to  extensive  mechanical  treatment  (squeezing,  hammering  and 
rolling)  at  a  high  heat  to  expel  the  slag.    The  product  is 

wrought  iron,  graded  according  to  the  quality  of  the 
original  pig  iron  and  the  extent  of  mechanical  working 
the  product  has  undergone. 

3.  Acid  Bessemer  steel  is  made  in  a  converter  shown  in 
section  in  Fig.  172,  lined  with  a  mixture  that  is  essentially 
silica.  In  the  bottom  are  numerous  small  holes  through 
which  enters  air  under  high  pressure.  The  converter 
being  partly  filled  with  melted  pig  iron,  the  air  forces 
its  way  upward  through  it,  the  oxygen  converting  an 
equivalent  of  iron  into  protoxide.  The  protoxide  im- 
mediately reacts,  first  with  silicon  to  form  silica  (Si  + 
2FeO  =  SiO2  +  Fe) ;  later  with  manganese  (Mn  + 
FeO  =  MnO  +  Fe) ;  and  finally  with  carbon  (0  +  FeO  = 
CO2  +  Fe).  During  the  oxidation  of  these  elements  a 
brilliant  flame  emerges  from  the  converter,  suddenly 
ceasing  when  oxidation  is  complete  ^.at  or  just  before 
this  moment  the  converter  is  turned  to  a  horizontal  position  and  the  blast  shut  off. 

The  metal  in  the  converter  is  now  nearly  pure  iron  containing  considerable  ferrous 


330  QUANTITATIVE    CHEMICAL   ANALYSIS. 

oxide,  and  covered  by  a  thin  layer  of  slag,  a  silicate  of  ferrous  oxide,  manganese  oxide, 
and  the  earths  (from  the  lining  principally).  To  it  is  added  a  suitable  proportion  of 
melted  spiegel-eisen  (an  alloy  of  manganese,  iron  and  carbon),  for  three  purposes;  (1),  a 
part  of  the  manganese  reacts  with  the  ferrous  oxide  converting  it  to  iron  (FeO  -f  Mn  = 
Fe  -f  MnO),  the  manganese  oxide  passing  into  the  slag;  (2),  the  remainder  of  the  manga- 
nese alloys  with  the  metal ;  it  has  a  beneficial  influence  on  the  quality  of  the  steel;  and 
(3),  the  carbon  enters  the  metal  conferring  the  desired  hardness.  The  steel  is  now  poured 
into  ingot  molds. 

4.  The  manufacture  of  acid  open-hearth  steel  is  based  on  three  propositions.  (1)  When 
pig  Iron  and  wrought  iron  of  suitable  qualities  and  in  the  proper  relative  proportions 
found  by  calculation,  are  melted  together,  the  product  conforms  chemically  to  the  compo- 
sition of  steel.  (2)  As  in  the  Bessemer  process,  when  melted  pig  iron  is  exposed  for 
some  time  to  a  current  of  air  the  carbon,  silicon,  and  manganese  are  oxidized  and  elimi- 
nated, until  at  a  certain  stage  the  composition  is  that  of  steel.  (3)  The  same  effect  as  in 
(2)  transpires  when  pig  iron  and  iron  oxide  (iron  ore)  are  melted  together,  the  oxygen 
being  supplied  by  the  ore.  In  current  open-hearth  practice  the  three  are  combined. 

Pig  iron,  scrap  wrought  iron,  and  iron  ore  in  suitable  proportions  are  together  charged 
into  the  open-hearth  furnace,  of  which  one  form  is  that  of  a  horizontal  cylinder  lined  with  a 
mixture  of  silica  and  fire -clay.  A  gas  flame  enters,  alternately  at  one  end  and  the  other, 
heating  the  upper  part  of  the  lining.  The  charge  is  melted  by  radiated  heat.  The  silicon, 
manganese,  and  carbon  of  the  pig  iron  are  lessened  in  percentage  In  the  total  amount  of 
metal  (pig  iron  plus  wrought  iron)  as  in  (1),  and  almost  entirely  eliminated  by  oxidation  as 
in  (2)  and  (3).  The  metal  has  now  become  practically  pure  iron  containing  a  small  propor- 
tion of  ferrous  oxide,  and  is  converted  to  steel  by  the  addition  of  high  grade  spiegel-eisen 
(ferromanganese)  as  in  the  Bessemer  process. 

The  distinction  between  the  *  acid '  and'  basic '  Bessemer  and  open-hearth  processes  lies 
in  the  composition  of  the  slag  formed  therein.  In  the  acid  processes  the  silica  of  the  slag 
is  largely  in  excess  of  the  bases,  and  conversely.  In  the  acid  process  the  phosphorus  of 
the  pig  iron  is  continually  oxidized  to  phosphoric  acid  and  unites  with  ferrous  oxide,  but 
is  as  continually  displaced  by  silica  and  reduced  back  to  phosphorus  again  to  return  to  the 
metal;  so  that  the  resulting  steel  contains  practically  all  the  phosphorus  of  the  pig  iron. 
In  the  basic  processes  the  phosphorus  is  oxidized,  and  the  phosphoric  acid  unites  with 
lime  and  enters  and  remains  permanently  in  the  slag,  and  hence  a  pig  iron  high  in  phos- 
phorus and  correspondingly  low  in  price  can  be  used  to  make  low-phosphoric  steel.  The 
linings  of  the  converter  and  open-hearth  furnaces  are  mainly  silica  for  the  acid  process; 
and  for  the  basic,  magnesia,  lime  and  magnesia,  or  a  neutral  material  such  as  carbon  or 
chrome  Iron  ore,  with  addition  of  limestone  to  the  charge. 


Steel  and  the  softer  irons  are  prepared  for  analysis  by  drilling  the  metal  with 
a  clean  twist-drill,  using  no  water  or  oil;  filing  is  not  to  be  recommended  as 
more  or  less  foreign  matter  is  apt  to  be  mixed  with  the  filings..  Since  the 
drillings  of  soft  pig  iron  are  made  up  of  light  scales  of  graphite  with  much 
heavier  fragments  of  iron,  a  perfect  mixture  for  the  determination  of  carbon, 
etc.,  can  only  be  secured  by  moistening  the  whole  of  the  drillings  with  pure 
alcohol,  dividing  down  the  paste  to  the  weights  desired  for  the  determinations, 
and  drying. 

The  harder  grades  of  pig  iron,  chilled  iron,  spiegel-eisen,  ferro-manganese 
and  quenched  steel  are  broken  to  a  coarse  powder  in  a  steel  mortar. 

Before  considering  the  methods  of  analysis  in  detail,  let  us  examine  the  action 
of  the  various  common  solvents  on  the  metals  and  their  impurities.  With  the 
exception  of  a  few  of  the  rarer  alloys  all  the  commercial  grades  of  iron  and 
steel  are  soluble  in  dilute  mineral  acids  and  in  concentrated  hydrochloric  acid. 
For  the  determination  of  manganese,  silicon,  copper  and  other  impurities  that 
form  no  volatile  compounds  with  hydrogen,  any  acid  may  be  the  solvent,  while 
phosphorus  and  sulfur  are  retained  in  solution  only  by  an  oxidizer  such  as 
nitric  acid  or  bromine  in  hydrochloric  acid.  Special  solvents  effect  the  sepa- 
ration of  iron  and  manganese  from  combined  or  dissolved  carbon. 


IRON    AND    STEEL.  331 

1.  Hydrochloric  acid,  both  concentrated  and  dilute,  dissolves  manganese  to 
manganous  chloride,  and  iron  to  ferrous  chloride;  but  unless  precautions  are 
taken  against  contact  of  the  air  some  ferric  chloride  also  is  formed.    The  com- 
bined (or  dissolved)  carbon  for  the  most  part  unites  with  nascent  hydrogen 
and  passes  off  as  odorous  gases  said  to  be  of  the  propyl  series;  a  small  pro- 
portion, less  if  the  solvent  be  heated,  separates  in  the  solid  form.    The  gra- 
phitic carbon  remains  undissolved.    The  combined  silicon  is  oxidized,  and  more 
or  less  passes  into  solution  according  to  the  strength  of  acid,  temperature, 
etc.,   while  crystallized  and  graphitoid  silicon  remain.    A  part,  usually  the 
greater  part,  rarely  the  whole,   of    the  sulfur  combines  with    nascent  hy- 
drogen   and    passes  off    as    hydrogen    sulflde:    the    remainder   is   left    in 
the  residue    of   graphite,    etc.,    possibly   as    an    organic    compound,  possi- 
bly as  iron  disulflde.    Of  the  phosphorus,  part  passes  off  as  gaseous  hydro- 
gen   phosphide,    the   proportion    varying    with    the    percentage    of    carbon 
in  the  metal;    part,  enters    the    solution    as    phosphoric    acid;    and    part 
remains  in  the  insoluble  residue.    When  iron  containing  a  large  percentage  of 
phosphorus  is  dissolved  in  dilute  hydrochloric  acid  there  is  left  a  black  residue 
said  to  be  composed  of  iron,  phosphorus,  hydrogen,  oxygen,  and  water. 

Of  the  other  elements,  copper  remains  in  the  metallic  state;  titanium  is  also 
left,  probably  as  titanium  carbide;  while  manganese  oxide,  ferrous  oxide  and 
slag  are  dissolved.  The  above  statements  are  to  be  accepted  as  true  only  in  a 
general  way,  since  the  nature  and  proportions  of  the  associates  influence  the 
solubility. 

2.  Dilute  sulfuric  acid  is  similar  in  action  to  hydrochloric  but  is  somewhat 
inferior  in  solvent  power  and  the  reactions  are  less  sharp  and  complete.    There 
is  less  ferric  compound  formed  with  an  equal  exposure  to  the  air  than  where 
the  solvent  is  hydrochloric  acid. 

3.  Hot  concentrated  sulfuric  acid  and  melted  sodium  pyrosulfate  attack  the 
finely  divided  metals,  and  on  dilution  with  water  the  sulfates  dissolve,  leaving 
silica  and  graphite.    These  reagents  are  seldom  used  however. 

4.  A  mixture  of  hydrofluoric  with  one  of  the  mineral  acids  has  the  effect  of 
taking  silica  into  solution  and  the  liquid  does  not  gelatinize  on  standing  or 
evaporation  (hindering  filtration),  and  for  some  determinations  on  siliconifer- 
ous  pig  irons  saves  an  evaporation  to  dryness  to  render  silica  insoluble. 

5.  Cold  concentrated  nitric  acid  causes  iron  to  become  passive  and  no  fur- 
ther action  ensues.    On  boiling  the  acid  the  metal  is  slowly  dissolved,  the  sulfur 
being  converted  entirely  to  sulfuric  acid ;  this  is  about  the  only  occasion  for 
the  use  of  nitric  acid  of  this  strength. 

6.  Nitric  acid  of  moderate  dilution  dissolves  iron  to  ferric  nitrate  and  man- 
ganese to  manganous  nitrate;  some  samples  of   pig  iron  become  passive  in 
the  cold  acid  of  a  gravity  of  1.2.    The  combined  carbon  dissolves  on  heating 
to  a  clear  brown  solution,  the  graphite  remaining  unacted  on.    Silicon  partly 
passes  into  solution  as  hydrated  silica,  the  more  the  weaker  the  acid.    Sulfur 
is  converted  to  sulfuric  acid  provided  the  acid  is  in  large  excess  and  hot 
throughout  the  operation,  otherwise  some  sulfur  may  remain  as  a  fine  powder 
suspended  in  the  liquid  or  escape  as  sulfurous  acid  or  hydrogen  sulflde.    The 
phosphorus  is  oxidized,  mainly  to  phosphoric  acid,  and  passes  into  solution. 
Copper  dissolves  as  cupric  nitrate,  and  arsenic  as  arsenic  acid. 

7.  Aqua  regia  combines  the  solvent  power  of   hydrochloric  acid  with  the 
oxidizing  effect  of  nitric,  and  has  a  few  applications  on  this  account. 

8.  A  cold  aqueous  solution  of  bromine,  chlorine,  or  iodine  dissolves  iron 
and  manganese  as  halogen  compounds,  leaving  the  carbon  and,  more  or  less 
completely,  the  other  impurities,  including  the  slag  and  oxides.    Similarly, 


332  QUANTITATIVE    CHEMICAL   ANALYSIS. 

solutions  of  easily  reducible  per-salts  such  as  ferric  chloride,  mercuric  chloride, 
etc.,  are  reduced,  the  iron  and  manganese  assimilating  the  freed  rest  and 
passing  into  solution.  Salts  of  copper  and  silver  are  decomposed,  their  metals 
being  deposited  while  Iron  and  manganese  are  dissolved ;  the  double  chloride 
of  copper  and  an  alkali  metal  first  takes  up  the  iron  and  manganese,  at  the 
same  time  depositing  their  equivalents  of  metallic  copper;  then  the  excess  of 
the  reagent  dissolves  the  copper  as  cuprous  chloride. 

9.  Cold  dilute  hydrochloric  or  salfuric  acid  containing  an  oxidizing  agent, 
such  as  chromic  acid,  dissolves  the  metal  without  the  evolution  of  hydrogen, 
this  being  immediately  converted  into  water  by  reacting  with  the  chromic  acid. 
The  combined  carbon  is  mainly  left  in  the  insoluble  residue  of  graphite. 
Chromic  acid  in  hot  concentrated  sulfuric  acid  acts  energetically  to  oxidize  all 
the  carbon  to  carbon  dioxide. 


Silicon  may  be  determined  by  dissolving  the  metal  in  hydrochloric  acid,  evap- 
orating to  thorough  dryness,  and  redissolving  the  bases  in  nitric  or  hydrochloric 
acid.  All  the  silicon  is  converted  into  insoluble  silica  which  is  filtered,  ignited 
and  weighed.  It  is  seldom  obtained  pure,  however,  containing  iron  oxide,  scale, 
etc.  For  purification  it  may  be  fused  with  a  little  sodium  carbonate,  the  melt 
dissolved  in  hydrochloric  acid,  evaporated  to  dryness,  the  residue  treated  with 
dilute  hydrochloric  acid,  and  the  silica,  now  pure,  ignited  and  weighed.  Another 
plan  is  to  volatilize  the  silica  by  hydrofluoric  acid,  evaporate  and  weigh  the  resi- 
due ;  the  loss  in  weight  is  silica. 

In  the  method  of  Drown  the  pig  iron  or  steel  is  dissolved  in  a  mixture  of 
dilute  sulfuric  and  nitric  acids  and  the  solution  evaporated  until  the  excess  of 
sulfuric  becomes  concentrated ;  at  this  point  the  silicon  hydrate  is  dehydrated 
and  soluble  carbonaceous  compounds  oxidized.  The  residue  is  treated  with 
hot  dilute  hydrochloric  acid  to  dissolve  the  ferric  and  manganous  sulfates, 
filtered,  and  the  residue  of  graphite  and  silica  ignited  until  the  former  has 
burned  away.  Pure  silica  is  left.  It  is  immaterial  as  regards  accuracy  as  to 
whether  the  metal  is  originally  dissolved  in  sulfuric  or  in  nitric  acid  or  as  to 
the  excess  of  either.  But  the  plan  of  dissolving  in  dilute  sulfuric  acid  and  then 
oxidizing  by  nitric  requires  only  about  one-fourth  of  the  volume  of  the  latter 
as  compared  with  the  use  of  nitric  as  the  solvent  —  a  consideration  in  labora- 
tories where  many  determinations  are  made  daily. 

For  rapid  determinations  of  silicon  in  iron  from  a  blast  furnace,  the  melted 
metal  is  run  into  cold  water,  solidifying  in  the  form  of  chilled  globules  easy  to 
pulverize  in  a  steel  mortar.  The  powder  is  dissolved  in  hot  hydrochloric  acid 
in  a  platinum  dish,  boiled  to  dryness,  redissolved  in  acid,  and  filtered  by  suc- 
tion. The  paper  and  carbon  are  burned  off  in  a  current  of  oxygen.  The 
results  are  only  approximations,  yet  are  close  enough  for  the  purpose  —  that 
of  ascertaining  the  grade  of  iron  and  the  state  of  the  furnace  —  and  can  be  re- 
ported in  less  than  fifteen  minutes  from  the  time  of  catching  the  sample. 

In  the  result  of  a  determination  of  silicon  in  iron  is  included  the  silicon  of 
any  slag  contained  in  the  metal. 

Slag  and  oxides  of  manganese  and  iron.  These  are  general  constituents  of 
refined  iron,  but  are  in  less  amount  in  pig  iron  and  steel.  None  of  the  methods 
for  their  determination  can  be  considered  satisfactory. 

Various  solvents  have  been  proposed  for  the  solution  of  the  iron  and  man- 
ganese without  the  slag  and  oxides  being  attacked ;  such  are  aqueous  solutions 
of  iodine,  bromine,  ferric  chloride,  and  simple  and  double  salts  of  copper ; 


IRON   AND    STEEL.  333 

iodine  dissolved  in  solution  of  potassium  iodide  or  ferric  chloride;  certain 
metallic  oxides  suspended  in  water,  etc.  The  residue  left  after  treating  the 
metal  by  one  of  these  solvents  is  boiled  with  potash  solution  to  dissolve  free 
silica  and  carbon  hydrates,  the  residue  ignited  to  burn  the  graphite,  and  weighed 
as  a  mixture  of  slag  and  oxides.  If  considerable  in  amount,  the  silica  con- 
tained may  be  determined  by  volatilizing  it  by  hydrofluoric  acid,  and  the  pro- 
portion of  slag  calculated,  knowing  its  approximate  composition. 

In  Pourcel's  method  the  metal  is  placed  in  a  porcelain  boat  at  the  middle  of 
a  porcelain  tube  through  which  is  passed  a  current  of  oxygen-free  dry  chlo- 
rine. The  volatile  ferric  chloride  formed  is  carried  to  the  cooler  parts  of  the 
tube,  depositing  as  anhydrous  golden-yellow  flakes,  the  graphite,  slag,  oxides, 
etc,  remaining  in  the  boat. 

Tucker*  heats  the  powdered  metal  to  fusion  in  a  brasque  clay  crucible  with 
exclusion  of  air;  the  button  of  metal  is  weighed,  and  the  loss  in  weight  is  as- 
sumed to  be  the  oxygen  of  the  oxides  which  has  combined  with  the  carbon  of 
the  metal  or  the  brasque,  and  escaped  as  carbon  monoxide  or  dioxide.  A  cor- 
rection is  applied  for  carbon  taken  up  by  the  metal  from  the  brasque,  found  by 
determining  the  carbon  in  the  metal  before  and  after  the  operation. 


Manganese.  Here  the  difficult  problem  is  to  separate  a  large  amount  of  iron 
from  a  small  proportion  of  manganese  —  difficult,  because  in  nearly  all  the  ac- 
cepted methods  for  their  separation  either  the  iron  is  precipitated  as  a  compound 
of  a  voluminous  flocculent  form  that  can  scarcely  be  washed  completely,  or  the 
manganese  as  manganic  oxide  or  hydrate,  remarkable  for  their  power  of  occlud- 
ing the  compounds  of  other  metals. 

1.  Ammonia  precipitates  ferric  hydrate  from  a  ferric  and  manganous  solution ; 
theoretically  all  the  manganese  should  remain  in  solution  as  a  manganous  am- 
monium salt,  but  this  is  true  only  when  it  is  present  in  but  a  very  small  pro- 
portion and  the  solution  very  dilute,  otherwise  much  accompanies  the  ferric 
hydrate. 

2.  Barium  carbonate  precipitates  the  iron  from  a  neutral  ferric  solution  as 
basic  ferric  hydrate  mixed  with  the  excess  of  the  precipitant,  most  or  all  of  the 
manganese  remaining  in  solution.    The  filtrate  is  acidified,  the  barium  that  has 
gone  into  solution  removed  by  sulfuric  acid,  and  the  manganese  determined  in 
the  filtrate. 

An  excellent  method  where  the  proportion  of  manganese  to  iron  is  large 
is  to  neutralize  the  solution  of  ferric  and  manganous  chlorides  with  sodium 
carbonate,  then  stir  in  a  slight  excess  of  pure  lead  carbonate,  lead  replacing 
the  iron.  'The  filtrate  is  acidified  and  treated  with  hydrogen  sulflde  to  remove 
the  lead,  leaving  only  manganous  chloride  in  solution. f 

3.  An  acetic  acid  solution  of  nitroso-beta-napthol  was  proposed  as  a  pre- 
cipitant for  ferric  salts  by  von  Knorre.J    A  neutral  or  faintly  acid  solution  of 
a  ferric  salt  yields  a  bulky  brown  compound,  ferri-nitroso-napthol,  Fe(CioH6O. 
NO>;   manganese  is  not  precipitated.    In  the  filtrate  the  manganese  is  sepa- 
rated as  binoxide  by  making  the  solution  ammoniacal  and  passing  a  current  of 
air  loaded  with  bromine  vapor. 

4.  On  boiling  a  dilute  neutral    solution  containing  iron  as    a    ferric    salt, 
all  the    iron  comes    down  as  ferric  hydrate,    a  manganous  salt  remaining 


*  Journ.  Iron  &  Steel  Inst.  1881—205. 
t  Journ.  Anal.    Chem.  2-291. 
1  Chem.  News,  1889—1—232. 


334  QUANTITATIVE     CHEMICAL    ANALYSIS. 

undecomposed.  The  method  is  not  much  in  use,  for  the  reason  that  as  the 
precipitation  takes  place  in  a  neutral  solution  the  manganese  is  held  up  less 
completely  than  in  an  acid  solution,  as  in  (5) . 

5.  Of  all  the  methods  for  material  wherein  the  iron  largely  preponderates 
over  the  manganese  and  where  the  iron  is  precipitated  for  the  separation t 
probably  the  best  separation  is  afforded  by  the  '  basic  acetate  '  method.    To 
the  slightly  acid  solution  of  ferric  and  manganous  chlorides  or  sulfates  is 
added  an  excess  of  an  alkali  acetate   (preferably  the  ammonium  salt).    On 
boiling  the  dilute  solution  all  the  iron  precipitates  as  basic  ferric  acetate. 
Unfortunately  the  precipitate  is  so  bulky  and  slimy  that  it  is  difficult  to  filter 
and  impossible  to  thoroughly  wash,  and  must  be  dissolved  and  reprecipitated 
one  or  more  times  for  a  complete  separation.    Succinnic  acid  is  said  by  Frese- 
nius  to  afford  a  precipitate  denser  and  easier  to  wash.     . 

A  combination  of  the  methods  (4)  and  (5)  is  the  addition  of  an  insufficient 
proportion  of  acetate  to  transpose  with  all  the  iron  and  manganese  chlorides. 
It  is  claimed  that  manganese  acetate  is  readily  decomposed  on  boiling,  with 
formation  of  manganese  hydrate ;  and  that  if  less  of  the  acetate  be  used  than 
will  convert  all  the  iron  to  acetate,  all  the  manganese  will  remain  as  chloride  or 
sulfate.  In  this  case  the  precipitation  of  the  iron  is  due  partly  to  the  decom- 
position of  the  acetate  and  partly  to  the  decomposition  of  a  neutral  solution. 
However,  the  distinction  is  not  of  importance  if  viewed  by  the  dissociation 
theory.* 

The  basic  acetate  method  is  well  suited  to  material  containing  much  manga- 
nese with  little  iron,  as  in  spiegels,  f  erro- manganese,  and  manganese  ores.  In 
such  cases  the  separation  may  with  advantage  be  preceded  by  a  careful  separa- 
tion by  ammonia  to  withdraw  most  of  the  manganese  from  the  iron. 

6.  Of  the  few  methods  wherein  the  manganese  is  separated  by  precipitation 
leaving  the  iron  in  solution  is  the  deservedly  favored  one  due  to  Ford.f    The 
metals  are  obtained  as  nitrates  dissolved  in  strong  nitric  acid;  on  dropping 
crystals  of  potassium  chlorate  into  the  boiling  solution,  first  the  carbonaceous 
matter  is  oxidized,  then  the  manganese  precipitates  as  binoxide.     The  reaction 
is  undoubtedly  complex;  perhaps  the  leading  one  is 

5Mn(NO3)2  +  2KC1O3  -f-  4H2O  =  5MnO2  +  2KNO3  -f-  8HNO8  +  C12. 
Nearly  all  the  iron  is  left  in  solution.  For  several  reasons  it  is  better  to  filter 
without  dilution,  so  the  liquid  is  passed  through  an  asbestos  felt  or  sand  filter. 
After  washing  with  colorless  concentrated  nitric  acid,  then  with  water,  the 
precipitate  is  dissolved  in  sulfurous  or  hydrochloric  acid.  As  a  small  amount 
of  ferric  nitrate  is  invariably  carried  down  mechanically,  it  is  removed  by  a  basic 
acetate  separation. 

Ford's  method  is  well  adapted  to  material  in  which  the  proportion  of  iron 
exceeds  that  of  manganese,  but  where  manganese  preponderates,  the  basic 
acetate  method  is  preferable  for  some  reasons. 

7.  Rothe  J  proposes  a  separation  based  on  the  solubility  of  ferric  chloride  in 
ether  and  the  insolubility  of  manganous  chloride.    The  solution  of  the  chlo- 
rides of  the  two  metals  is  made  slightly  acid  and  extracted  by  ether  in  a 
special  form  of  apparatus;  the  same  procedure  is  followed  as  for  the  extraction 
of  an  organic  body  from  an  aqueous  solution. 

8.  Blum  §  describes  a  method  based  on  the  precipitation  of  manganese  from 


*  Chem.  News,  1873—1—14  and  Journ.  Anal.  Appl.  Chem.  1892—94. 
t  Trans.  Amer.  Inst.  Mining  Engrs.  9—397. 
t  Mitth.  d.  Vers-Anst.  10-123. 
§  Chem.  News,  1887-1-236. 


IKON    AND    STEEL.  335 

an  ammoniacal  solution  of  ferric  and  manganous  tartrates  by  potassium  ferro- 
cyanide. 

The  separation  having  been  accomplished  by  one  of  the  above  methods,  the 
manganese  may  be  determined  by  precipitation  either  as  the  binoxide  or  as  the 
ammonium  phosphate.  For  the  former  the  solution  is  nearly  neutralized  by 
ammonia,  ammonium  acetate  and  bromine  added,  then  an  excess  of  ammonia, 
and  the  solution  boiled  for  a  time.  The  manganese  precipitates  as  manganic 
hydrate  — 
Mil  (C2H302)2  +  5Br  +  8NH3  +  H2O  =  MnOjjxHgO  +  5  NH4Br  +  2NH4C2H3O2  +  N. 

After  filtering  and  washing  with  very  slightly  acidulated  water,  the  precipi- 
tate is  strongly  ignited  and  weighed  as  Mn3O4,  though  its  composition  may 
vary  somewhat  in  the  ratio  of  manganese  to  oxygen.  • 

The  second  form  is  the  better ;  the  solution  is  treated  hot  with  ammonium 
phosphate  and  ammonia  (page  243),  and  after  ignition  the  precipitate  is  weighed 
as  manganese  pyrophosphate. 

Volumetric  methods.  All  these  depend  on  the  formation  of  a  definite  oxide 
of  manganese  higher  than  the  protoxide,  the  determination  of  the  active  oxygen 
therein  by  means  of  a  standard  solution  of  some  reducing  agent,  and  calcula- 
tion of  the  manganese  from  the  atomic  ratio. 

1.  Pattinson's.    The  hot  solution,  containing  the  iron  as  ferric  chloride,  is 
neutralized    and    treated    with  calcium  carbonate    and  bromine  or  calcium 
hypochlorite,    giving  a  precipitate  of    ferric  and  manganic  hydrates.    After 
filtration  and  washing,  the  precipitate  is  treated  with  dilute  sulfuric  acid  and  a 
measured  volume  of  ferrous  sulfate  solution.    The  sulfuric  acid  dissolves  the 
ferric  hydrate  to  ferric  sulfate,  and  the  acid  and  ferrous  sulfate   dissolve  the 
manganic  hydrate  to  manganous  sulfate,  an  equivalent  of  ferrous  iron  being 
converted  to  ferric.    The  excess  of  ferrous  sulfate  is  found  by  titration  by 
standard  permanganate,  and  from  the  iron  oxidized,  the  weight  of  the  man- 
ganese.   Assuming  the  manganic  hydrate  to  have  the  formula  MnO2.xH2O,  the 
reactions  are 

MnCl2  +  2CaCO3  -f  2Br  +  2H2O  =  MnO2  +  CaCJ2  -f  CaBr2  +  2H2C03. 

MnO2  -f  2FeS04  +  2H2SO4  =  MnSO4  +  Fe2(SO4)3  -f-  2H2O. 

10FeSO4  +  K2Mn2O8  +  8H2SO4  =  MnSO4  +  Fe2(SO4)3  +  2MnSO4  +  K2S04 

+  8H2O. 

Hydrogen  peroxide  has  been  recommended  for  the  oxidizing  agent,  but  it  is 
doubtful  if  the  precipitate  always  has  a  constant  composition. 

2.  Vortmann    treats  the  solution  with  potassium  hydrate  and  a  measured 
volume,  an  excess,  of  decinormal  iodine  solution;  the  iron  precipitates  as  ferric 
hydrate,  and  the  manganese  as  peroxide  — 

MnCl2  -f  I2  +  4KOH  =  MnO2  +  2KC1  -f  2KI  +  2H20. 

After  diluting  to  a  certain  volume  and  filtering,  an  aliquot  part  of  the  filtrate  is 
acidified  and  the  iodine  in  excess  titrated  back  by  decinormal  sodium  thiosul- 
fate  and  starch-paste. 

3.  In  Troilius'  method  the   manganese  is  precipitated  alone  by  potassium 
chlorate  as  in  Ford's  method.    After  filtering  through  asbestos  and  washing, 
the  precipitate  is  dissolved  in  excess  of  standard  solution  of  ferrous  sulfate  and 
sulfuric  acid,  and  the  excess  of  ferrous  iron  titrated  back  by  potassium  bichro- 
mate.   Modifications   of    the  method  are   due  to  Williams,*  who  substitutes 
oxalic    acid   for   the  ferrous  sulfate  and  potassium    permanganate  for   the 
bichromate 

Mn02  +  H2C204  +  H2S04  =  MnS04  +  2H2CO3; 


Trans.  Amer.  Inst.  Mining1  En^rs.   10—100. 


336  QUANTITATIVE    CHEMICAL    ANALYSIS. 

by  Julian,*  who  omits  the  filtration  and  dissolves  the  binoxide  in  the  diluted 
nitric  solution  with  a  standard  solution  of  hydrogen  peroxide,  titrating  its 
excess  with  permanganate;  and  by  Smith,  who  treats  the  residue  with  an 
excess  of  hydrogen  peroxide  and  measures  the  evolved  oxygen. 

4.  Volhard'sf  method  is  based  on  the  principle  that  zinc  is  intermediate  in 
chemical  potential  between  iron  and  manganese.     The  sample  is  dissolved  in 
nitric  acid,  evaporated  with  sulfuric  acid,  and  the  sulfates  dissolved  in  water 
and  filtered  if  necessary.    In  the  cold  solution  the  iron  is  precipitated  by  zinc 
oxide, 

Fe2(SO4)3  +  3ZnO  -f  3H20  =  Fe2(OH)6  -f  3ZnSC>4, 

leaving  the  manganese  in  solution.  After  making  up  to  a  definite  volume  and 
filtering,  an  aliquot  part  of  the  filtrate  is  treated  with  a  drop  of  nitric  acid  (to 
destroy  traces  of  organic  matter) ,  heated  to  near  boiling,  and  titrated  by  stan- 
dard permanganate ;  both  the  manganese  of  the  manganous  sulfate  and  that  of 
the  permanganate  are  converted  into  insoluble  manganic  oxide, 

6MnSO4  -f  2K2Mn2O8  +  4H2O  =  10MnO2  -f  4KHSO4  +  2H2SO4. 
The  end  of  the  titration  is  shown  by  a  persistent  faint  pink  color  of  the  solution 
from  a  slight  excess  of  permanganate. 

The  process  is  simplified  by  Sarnstroem,  who  adds  a  slight  excess  of  sodium 
bicarbonate  to  the  acid  solution  of  ferric  and  manganous  chlorides:  the  iron 
precipitates  as  ferric  hydrate  and  the  manganese  stays  in  solution .  The  titra- 
tion is  made  without  filtering  off  the  ferric  hydrate,  as  its  presence  is  an  advan- 
tage in  clarifying  the  solution  during  titration. 

All  these  volumetric  methods  give  slightly  low  results.  The  cause  has  been 
ascribed  to  (1)  that  not  the  binoxide  as  assumed,  but  some  lower  oxide  or 
mixture  of  oxides  (e.  g.,  MnioOig)  forms  the  precipitate ;J  (2)  the  manganese 
binoxide  as  it  forms  occludes  some  unprecipitated  mangauous  nitrate  which,  of 
course,  has  no  oxidizing  action  on  the  ferrous  solution;  or  (3)  manganese 
binoxide  reacts  with  manganous  nitrate  to  form  manganese  sesquioxide,  inferior 
in  oxidizing  power  to  the  binoxide.  In  steel  and  pig  iron  the  error  is  practi- 
cally inconsiderable,  but  with  material  like  spiegel-eisen  and  ferro-manganese 
it  may  reach  to  one  per  cent  or  more  of  the  manganese  and  cannot  be  neglected. 
It  is  always  best  to  run  a  parallel  determination  on  a  sample  of  similar  material 
in  which  the  manganese  has  been  determined  gravimetrically,  and  make  a 
correction  therefrom. 

5.  Chatard's  method.  When  a  weak  solution  of  manganous  nitrate  is  acidi- 
fied by  nitric  acid  and  boiled  with  lead  peroxide  or  bismuth  tetroxide,  all  the 
manganese  is  perduced  to  permanganic  acid  — 

2Mn(NO3)2  -f-  5Pb02  -f  6HNO3  =  H2Mn2O8  +  5Pb(N03)2  +  2H2O. 
The  excess  of  lead  binoxide  is  removed  by  filtration  through  asbestos  (paper 
would  decompose  the  permanganic  acid),  and  the  solution  titrated  until  color- 
less by  a  weak  standard  solution  of  any  suitable  reducing  agent,  such  as  oxalic 
acid,  hydrogen  peroxide,  ferrous  sulfate,  mercurous  nitrate,  etc.    Babbitt  § 
prefers  sodium  arsenite  — 
5Na8AsO3  +  H2Mn208  +  19HNO8=  5H3As04  +  2Mn(N03)2+  15NaNO3+  3H2O. 

6.  In  the  method  of  Moore  the  manganese  is  converted  to  sesquichloride 
and  determined  by  titrating  to  protochloride  by  a  ferrous  solution,  or  from  a 
reverse  titration  by  ferrous  sulfate  and  permanganate.    The  solution  of  iron 


*  Journ.  Amer.  Chem.  Socy.  1893—113. 

t  Trans.  A.  I.  M.  E.  10—201. 

j  Trans.  Amer.  Inst.  Mining  Engrs.  11-323  etc. 

§  Amer.  Chem.  Journ.  9—58. 


IRON    AND    STEEL.  337 

and  manganese  is  evaporated  nearly  to  dryness  and  from  ten  to  twenty  Cc.  of 
syrupy  phosphoric  acid  added,  then  a  few  crystals  of  potassium  chlorate,  and 
the  liquid  warmed  until  the  smell  of  chlorine  has  gone.  The  sesquioxide  re- 
mains in  solution  coloring  it  a  deep  violet. 

Colorimetric  methods.*  One  of  these,  formerly  much  in  use  in  steel  works 
laboratories,  is  based  on  the  measurement  of  the  intense  red  color  of  perman- 
ganic acid.  The  reaction  producing  the  acid  is  given  in  (5)  supra.  A  decigram 
of  steel  is  dissolved  in  a  large  test-tube  in  20  Cc.  of  dilute  nitric  acid;  the  solu- 
tion is  heated  and  a  few  grams  of  lead  peroxide  added,  and  after  boiling  for  a 
few  minutes  the  excess  of  reagent  is  allowed  to  settle,  leaving  a  clear 
supernatant  solution  of  permanganic  acid.  An  equal  weight  of  another  steel 
whose  content  of  manganese  is  known  is  treated  in  the  same  way;  the  two 
liquids  are  decanted  into  graduated  tubes  of  the  same  diameter,  and  the  darker 
of  the  solutions  is  diluted  with  water  until  the  tints  are  identical.  Then  the 
percentages  of  manganese  are  in  proportion  to  the  volumes  of  the  two  solu- 
tions. 


Phosphorus.  This  element  is  believed  to  exist  in  steel  and  pig  iron  wholly 
or  at  least  mainly,  as  an  iron  or  manganese  phosphide,  possibly  as  iron 
titanum  phosphide,  while  in  wrought  iron  there  is  reason  to  suppose  that 
part  is  in  the  intermingled  slag  as  ferrous  or  manganous  phosphate.  Recent 
researches  indicate  that  phosphorus,  like  carbon,  may  be  in  two  forms  or 
combinations,  the  proportions  varying  to  some  extent  with  the  temperature  of 
quenching  the  steel ;  they  are  differentiated  by  their  unequal  solubility  when 
the  steel  is  treated  with  a  slightly  acid  solution  of  mercuric  chloride. 

Phosphorus,  unlike  sulfur,  cannot  well  be  determined  by  evolution  as  a  hy- 
drogen compound,  so  that  invariably  the  analysis  is  begun  by  dissolving  the 
metal  in  an  oxidizing  reagent,  usually  nitric  acid  of  the  specific  gravity  of  1.2 
or  1.13;  less  often  in  aqua  regia.  With  nitric  acid  of  1.2  the  silicon  of  the 
metal  is  oxidized  to  silicic  acid  and  dissolves,  but  on  standing  or  when  the 
phosphoric  acid  is  precipitated,  the  silica  partially  separates ;  while  with  acid  of 
1.13  the  silica  remains  in  solution. 

But  whatever  the  strength  of  acid  a  phenomenon  is  observed,  namely  that 
instead  of  the  whole,  only  a  part  (about  two-thirds)  of  the  phosphorus  is  ox- 
idized to  a  compound  precipitable  by  the  usual  precipitants  for  phosphoric 
acid;  nor  is  the  remainder  converted  thereto  by  boiling  or  evapo- 
ration of  the  solution  to  dryness,  but  only  by  heating  the  residue 
from  evaporation  up  to  about  120 °,f  or  by  boiling  the  original  solution 
with  some  strong  oxidizer  such  as  potassium  permanganate  or  chromic 
acid.  This  peculiarity  was  formerly  attributed  to  the  presence  of 
silicone,  later  to  carbon  compounds;  it  occurs,  however,  to  about  an 
equal  extent  in  pig  iron  and  comparatively  pure  soft  steel.  Possibly  some 
lower  oxide  of  phosphorus  is  formed  and  restrained  from  passing  to  ortho- 
phosphoric  acid  by  the  associates;  possibly  me ta- or  pyro- phosphoric  modifi- 
cations may  be  the  explanation.  This  characteristic  has  led  some  writers  to 
erroneously  depreciate  the  delicacy  of  the  reaction  between  molybdic  acid, 
ammonia  and  phosphoric  acid  in  presence  of  much  ferric  nitrate. 

Having  the  metal  in  solution  and  the  phosphorus  completely  oxidized  by 
heating  the  residue  from  evaporation  or  otherwise,  first  the  phosphoric  acid  is 


*  Trans.  Amer.  Inst.  Min.  Engrs.  51—102  and  Jonrn.  Anal.  Chem.  1887—88  and  176.    Age 
of  Steel,  1901—23. 

t  Trans.  Amer.  Inst.  Min.  Engrs.  12—518. 

22 


338  QUANTITATIVE   CHEMICAL   ANALYSIS. 

to  be  separated  from  the  iron  and  manganese,  then  precipitated  in  a  combina- 
tion suitable  for  weighing. 

A.  The  "acetate -magnesia"  method.    The  principles  are  these.     (1)  On 
boiling  a  nearly  neutral  solution  of  ferric  acetate  the  compound  is  decomposed, 
all  the  iron  precipitating  as  basic  ferric  acetate,  and  the  precipitate  carries 
down  with  it  as  ferric  phosphate  all  the  phosphoric  acid  that  may  be  in  solution 
if  not  too  great  in  amount.    Ferrous  and  manganous  acetates  are  not  decom- 
posed by  boiling  their  solutions.     (2)  From  a  solution  of  ferric  chloride  and 
phosphoric  acid  ammonia  precipitates  ferric  hydrate  and  phosphate,  but  if  cer- 
tain organic  bodies  are  also  present,  no  precipitation  takes  place.     (3)  "  Mag- 
nesia mixture  "  (an  ammoniacal  solution  of  magnesium  ammonium  chloride) 
precipitates  phosphoric  acid  as  magnesium  ammonium  phosphate,  which  on 
ignition  loses  ammonia,  becoming  magnesium  pyrophosphate. 

The  nitric  acid  solution  of  the  metal  is  evaporated  to  dryness,  heated  to  120  ° , 
dissolved  in  hydrochloric  acid,  and  filtered  from  the  silica  and  graphite.  It  is 
said  that  pyrophosphoric  acid  is  formed  on  heating,  but  however  this  may  be, 
it  is  reverted  to  the  ortho-modification  on  dissolving  the  residue  in  hydro- 
chloric acid.  Nearly  all  the  ferric  chloride  is  reduced  to  ferrous  chloride  by 
sulfurous  acid,  sodium  acetate  added,  and  the  solution  nearly  neutralized, 
boiled,  and  filtered.  The  small  precipitate  consisting  of  basic  ferric  acetate 
and  ferric  phosphate  is  dissolved  in  hydrochloric  acid  and  a  crystal  of  citric  or 
tartaric  acid.  An  excess  of  ammonia  is  added  and  the  clear  solution  precipi- 
tated by  magnesia  mixture.  The  precipitate  of  ammonium  magnesium  phos- 
phate is  filtered  off,  washed  by  dilute  ammonia,  ignited,  and  weighed. 

Instead  of  igniting  the  precipitate  it  may  be  determined  volumetrically  by 
finally  washing  with  water,  dissolving  in  excess  of  weak  standard  hydrochloric 
acid,  and  titrating  back  by  standard  ammonia.  Malot  *  would  dissolve  it  in 
nitric  acid  of  1.2  gravity  and  titrate  by  standard  uranium  nitrate ;  instead  of  the 
usual  mode  of  determining  the  end-point  by  spotting  with  ferrocyanide, 
he  prefers  to  mix  with  the  titrate  some  extract  of  eochineal  which  forms  a 
green  precipitate  (a  lake)  with  uranium  nitrate. 

B.  The  "  molybdate-magnesia  "  method.    For  general  work  this  is  probably 
the  most  reliable,  and  for  any  given  class  of  material,  quite  as  accurate  as  any 
method.    The  separation  of  the  phosphoric  acid  from  the  iron  and  manganese 
is  by  the  characteristic  reaction  of  the  acid  with  a  colloidal  solution  of  molyb- 
denum trioxide  in  presence  of  ammonium  nitrate,  the  formation  of  a  canary 
granular  precipitate  of    ammonium  phospho-molybdate.    The    precipitate    is 
almost,  practically  quite,  insoluble  in  nitric  acid  in  presence  of   molybdic 
acid,  and  in  very  dilute  acids  and  solutions  of  their  salts;  but  is  readily  de- 
composed by  alkalies,  the  products  passing  into  solution.    Authorities  differ 
as  to  the  exact  formula  of  the  precipitate;  it  approximates  (NH4)3PO4.11MoO5 
,6H2O. 

The  metal  is  dissolved  in  dilute  nitric  acid,  the  solution  evaporated  to  dryness 
and  the  residue  heated  to  120  °  or  higher.  The  residue  is  then  heated  with 
concentrated  hydrochloric  acid  and  diluted  with  water,  when  all  should  go  into 
solution  except  silica  and  graphite.  After  filtering,  the  solution  is  evaporated 
to  dryness  and  the  residue  heated  with  concentrated  nitric  acid  and  diluted 
with  water.  There  is  now  in  solution  ferric  and  manganous  nitrates  and  phos- 
phoric acid;  the  free  nitric  acid  is  nearly  neutralized  by  ammonia,  and  the 
phosphoric  acid  precipitated  by  a  large  excess  of  a  solution  of  molybdic  acid 
in  ammonium  nitrate  and  nitric  acid,  and  the  mixture  digested  for  several 


*  Chem.  News,  1892-1—62. 


IRON   AND   STEEL.  339 

hours,  or  what  amounts  to  the  same,  vigorously  stirred  for  ten  minutes  or 
more.  The  precipitate  is  filtered,  washed  with  dilute  molybdic  solution  or 
acidulated  water,  and  dissolved  in  dilute  ammonia. 

A  little  alumina,  ferric  oxide  or  silica  is  usually  carried  down  with  the  yellow 
precipitate.  To  remove  them,  the  solution  is  neutralized  by  hydrochloric  acid, 
heated  and  filtered.  The  small  precipitate  retains  traces  of  phosphoric  acid 
which  can  be  recovered  by  dissolving  the  precipitate  in  dilute  nitric  acid  and 
precipitating  by  molybdic  solution.  A  simpler  plan  is  to  add  a  small  crystal  of 
citric  acid  to  the  ammoniacal  solution,  the  ammonium  citrate  preventing  their 
precipitating  with  the  magnesium  ammonium  phosphate. 

Modifications  of  the  above  are  in  the  use  of  nitric  acid  of  a  gravity  of  about 
1.13,  and  the  oxidation  of  all  the  phosphorus  by  potassium  permanganate  or 
other  oxidizer.  This  avoids  the  necessity  of  evaporation,  as  the  silica  remains 
in  solution  in  the  acid  and  does  not  interfere  with  the  precipitation  of  the  phos- 
phoric acid. 

Molybdic  acid  is  freely  soluble  in  dilute  ammonia  and  so  the  precipitate  of 
ammonium  magnesium  phosphate  should  retain  none  of  it.  But  ammonium 
compounds  of  other  oxides  of  molybdenum  than  the  trioxide  are  prone  to  be 
occluded,  and  to  prevent  or  remove  this  contamination  it  has  been  recom- 
mended: (1)  before  precipitation  to  oxidize  by  bromine  water  the  lower 
molybdenum  oxides  to  the  trioxide;  (2)  before  precipitation  to  saturate  the 
ammoniacal  solution  by  hydrogen  sulfide,  then  acidulate  and  filter  off  the 
precipitate  of  molybdenum  sulflde;  or  (3)  to  strongly  ignite  the  magnesium 
pyrophosphate  to  volatilize  the  molybdenum  oxides. 

C.  Riley's  method*  proposes  to  unite  the  advantages  of  the  acetate-mag- 
nesia and  the  molybdate -magnesia  methods.    He  proceeds  according  to  the 
former  until  there  is  obtained  the  small  precipitate  of  basic  ferric  acetate  con- 
taining the  ferric  phosphate,  dissolves  it  in  nitric  acid,  precipitates  by  molybdic 
solution,  and  continues  according  to  the  latter  method.    In  reality  the  scheme 
includes  the  weakest  points  of  both  —  the  liability  of  incomplete  precipitation 
of  the  phosphoric  acid  in  the  former,  and  in  the  latter  the  contamination  of  the 
magnesium  precipitate  by  molybdenum  oxides  and  the  impurities  of  commercial 
molybdic  acid. 

D.  The  direct  molybdate  method.    A  scheme,  formerly  much  in  use,  is  that 
of  obtaining  the  precipitate  of  ammonium  phospho-molybdate  of  as  definite  a 
composition  and  as  free  from  extraneous  molybdic  acid  as  possible,  filtering 
on  a  tared  paper  or  through  two  counterpoised  filters  or  a  Gooch  crucible, 
washing  with  water  or  dilute  alcohol,  drying  the  precipitate  at   100  ° ,  and 
weighing  it.    Under  these  conditions  the  precipitate  is  assumed  to  contain 
1.63  per  cent  of  its  weight  of  phosphorus.    To  favor  the  formation  of  a  pre- 
cipitate of  constant  composition,  Carnot  would  dissolve  the  washed  precipitate 
in  dilute  ammonia,  and  reprecipitate  under  fixed  conditions. 

Modifications  of  the  above  are:  (1)  instead  of  taring  the  filter,  the  dried  pre- 
cipitate may  be  brushed  out  upon  a  tared  watch-glass,  the  slight  mechanical 
loss  in  this  operation  having  less  effect  on  the  accuracy  of  the  determination 
than  would  usually  follow,  on  account  of  the  small  percentage  of  phosphorus 
in  the  precipitate ;  C2)  the  undried  washed  precipitate  is  dissolved  through  the 
filter  in  dilute  ammonia,  catching  the  solution  in  a  tared  basin,  and  evaporating: 
to  dryness  on  the  water  bath;  (3)  the  precipitate  may  be  moderately  ignited 
to  expel  the  ammonia  and  water  and  the  residue  weighed  as  24MoO3.P20s;  (4) 
the  precipitate  may  be  suspended  in  water  or  other  liquid  and  its  weight 


*  Journ.  Chem.  Socy.  1878—104. 


340  QUANTITATIVE   CHEMICAL   ANALYSIS. 

determined  by  the  increased  specific  gravity  of  the  latter;  (5)  the  ammonia  in 
the  precipitate  may  be  determined  by  distilling  the  precipitate  with  an  excess  of 
sodium  hydrate  and  Nesslerizing  the  distillate ;  the  phosphorus  and  ammonia 
are  said  to  bear  a  more  constant  correllation  in  the  yellow  precipitate  than  does 
the  phosphorus  to  the  molybdic  acid. 

For  rapid  approximate  determinations  in  laboratories  of  steel  works  the 
Goetz  modification  *  of  Eggertz'  method  is  in  common  use.  A  small  standard 
weight  of  steel  is  dissolved  in  dilute  nitric  acid,  the  phosphorus  oxidized  by 
permanganate,  and  the  manganic  oxide  dissolved  as  in  Hundeshagen's  method 
(post)-,  the  solution  is  transferred  to  a  pear-shaped  bulb,  Fig.  3,  with 
a  graduated  prolong  of  standard  internal  diameter.  Molybdic  solution  is  added 
and  the  bulb  whirled  in  a  centrifugal  machine  for  a  few  minutes.  The  apparent 
volume  of  the  precipitate,  packed  in  the  prolong,  is  read.  As  the  prolong 
is  of  a  uniform  diameter  the  height  of  the  column  is  proportional  to  the  weight 
of  the  precipitate,  and  is  graduated  by  the  maker  directly  in  tenths  and  hun- 
dredths  of  one  per  cent  of  phosphorus. 

E.  A  number  of  other  schemes  for  separating  all  or  most  of  the  iron  and 
manganese  from  the  phosphorus  have  been  proposed,  such  as  by  dissolving  the 
metal  in  a  solution  of  ferric  chloride  or  of  copper  ammonium  chloride,  the 
phosphorus  remaining  in  the  residue ;  precipitating  the  iron  electrolytically,  by 
a  ferrocyanide,  etc.,  but  none  have  come  into  general  use. 

F.  Volumetric  methods.    All  these  measure  either  the  molybdic  acid  or  the 
molybdic  plus  phosphoric  acids  in  the  yellow  precipitate,  and  therefore  depend 
tor  their  accuracy  upon  the  constancy  of  the  composition  of  the  precipitate. 
As  this  is  conceded  to  be  somewhat  variable,  either  per  se  or  by  the  co-precipi- 
tation or  subsequent  precipitation  of  molybdic  acid,  even  when  the  conditions 
of  the  operation  comply  with  fixed  rules,  volumetric  methods  should  be  re- 
stricted to  material  in  which  the  percentage  of  phosphorus  is  approximately 
known,  and  where  any  considerable  variation  from  the  expected  percentage 
can  be  checked  by  a  gravimetric  method.     In  their  favor   is  the  magnitude 
<of  the   ratio    between   the    molybdenum    trioxide    and   the    phosphorus    of 
the  precipitate  —  about  91.4  to  1.63,  and  the  small  percentage  of  phosphorus 
contained  in  merchantable  iron  and  steel. 

Emmerton's  method. f  A  clear  nitric  solution  of  the  metal  is  prepared  as  in 
the  molybdate-magnesia  method;  all  the  iron  is  precipitated  by  ammonia,  then, 
without  filtering,  redissolved  in  a  limited  excess  of  nitric  acid.  The  solution  is 
heated  and  precipitated  by  molybdic  solution,  with  vigorous  shaking.  It  is  then 
filtered,  and  the  yellow  precipitate  washed  with  a  weak  solution  of  ammonium 
nitrate  or  sulfate,  and  dissolved  in  dilute  ammonia.  The  ammoniacal  solution 
is  strongly  acidified  by  sulf  uric  acid  and  metallic  zinc  introduced ;  the  nascent 
hydrogen  evolved  reduces  the  molybdic  trioxide  to  a  lower  oxide  or  mixture  of 
oxides,  said  to  be  Mo^Oig  (Emmerton) ,  or  Mo2O3  (Cheever) 

12MoO3  +  17Zn  -f-  17H2SO4  =  Moi2Oi9  +  17ZnSO4  +  17H20;  or 
2MoO3  +  3Zn  +  3H2SO4  =  Mo2O3  +  3ZnSO4  +  3H2O. 

"When  the  reduction  is  complete,  the  solution  becoming  of  a  dark  green  color, 
the  excess  of  zinc  is  withdrawn  by  filtration.  In  the  filtrate  the  molybdenum, 
suboxide  is  re-oxidized  to  the  trioxide  by  titration  by  an  empirical  standard 
solution  of  potassium  permanganate  — 

5Moi2Oi9+  17K2Mn2O8  +68H2SO4=60MoO3  +  34KHSO4  +  34MnS04-f-51H2O;  or 
€Mo2O3  +  3K2Mn2O8  +  12H2S04  =  10MoO3  +  6KHSO4  +  6MnS04  +  9H20. 


*  Zelts.  angew.  1889—638. 

t  Trans.  Amer.  Inst.  Mining  Engrs.  15—93  and   Oompt.  Bend.  59—301. 


IRON    AND    STEEL.  341 

As  the  oxidization  proceeds,  the  dark  color  of  the  liquid  gradually  lightens 
until  colorless,  and  another  drop  of  permanganate  reddens  it.  All  the  operations 
are  to  be  performed  according  to  specified  directions. 

The  molybdic  acid  may  also  be  reduced  by  passing  the  solution  through  a  Jones* 
'*  reductor,"  *  Fig.  173.  The  tube  A  is  filled  with  powdered  zinc  and 
the  flask  F  connected  to  a  filter  pump.  The  solution  is  poured 
into  the  funnel  B  and  is  forced  into  F  by  the  pressure  of  the  air  on 
the  surface.  After  washing  the  zinc  by  dilute  sulfuric  acid  and 
water  (preventing  the  passage  of  air  by  keeping  the  tube  full  of 
liquid)  the  solution  is  ready  for  titration.  The  formula  of  the 
molybdenum  suboxide  formed  is  said  to  differ  somewhat  from 
that  given  above. 

Hundeshagen's  method,  modified!.  The  steel  is  dissolved  in  di- 
lute nitric  acid  and  the  phosphorus  completely  oxidized  by  boiling 
with  potassium  permanganate.  The  precipitated  manganic  oxide, 
coming  from  decomposition  of  permanganic  acid,  is  reduced  by 
heating  with  a  few  grains  of  sugar  and  passes  into  solution  as 
nitrate,  e.  g., 
24MnO2  +  C^HzjOii  -f  48HNO3  =  24Mn(NO3)2  +  12CO2  •+-  35H2O. 

The  yellow  precipitate  is  thrown  down  by  molybdic  solution,  the 
deposition  hastened  by  brisk  stirring  or  shaking,  filtered,  and 
washed  with  a  neutral  solution  of  potassium  nitrate.  It  is  then 
dissolved  in  an  excess  of  weak  standard  sodium  hydrate —  Fig.  173.  V 

(NH4)3.llMo03.PO4  +  25NaOH  =  3NH4OH  +  llNa2MoO4  -f  Na3PO4  +  11H2O;  or 
(NH4)6(PO4)2.  (MoO3)24  +  46NaOH  =  (2NH4)2HPO4  -f  (NH4)2MoO4  +  22H2O  -f- 
23Na2MoO4. 

The  excess  of  alkali  is  then  found  by  titration  by  weak  standard  nitric  acid 
and  phenol-phthalein.  The  alkali  and  acid  are  standardized  against  ammo- 
nium phosphomolybdate  precipitated  under  the  same  conditions  as  obtain  in 
the  analysis. 

A  colorimetric  method  depends  on  the  brown  color  developed  when  molybdic 
acid  is  reduced  by  stannous  chloride.  The  yellow  precipitate  is  dissolved  in 
potassium  hydrate  solution,  this  boiled  to  expel  ammonia,  cooled  and  acidified 
by  hydrochloric  acid,  and  stannous  chloride  added.  The  color  is  compared 
with  that  of  a  standard  solution  of  molybdic  acid  in  hydrochloric  acid.  The 
same  reaction  is  the  basis  of  a  volumetric  method ;  the  slight  excess  of  stannous. 
chloride  is  converted  to  stannic  chloride  by  mercuric  chloride,  and  the  molyb- 
dous  oxide  titrated  by  bichromate  of  potassium, 


Sulfur.  With  respect  to  the  nature  and  concentration  of  the  reagent  chosen 
to  dissolve  the  iron  or  steel,  the  sulfur  may:  (1)  pass  into  solution  as  sulfuric 
acid  or  a  soluble  sulfate;  (2)  remain  in  the  insoluble  residue  as  iron  or  man- 
ganese sulfide  or  as  free  sulfur;  or  (3)  be  converted  to  gaseous  hydrogen, 
sulfide. 

1.  The c  aqua  regia'  method.  The  metal  in  thin  shavings  or  fine  powder  is  treated 
with  concentrated  nitric  acid;  prolonged  boiling  is  required  for  solution  since 


*  Trans.  Amer.  Inst.  Mining  Engrs.  15—625. 

t  Journ.  Anal.  Appl.  Chem.    1892—82  and  204 ;  Idem,  6—242. 


342  QUANTITATIVE    CHEMICAL   ANALYSIS. 

the  metal  becomes  passive  when  immersed  in  the  cold  concentrated  acid.  To 
the  solution  is  added  a  little  potassium  nitrate  to  form  potassium  sulfate  (fer- 
ric sulfate  is  decomposed  on  heating),  evaporated  to  dryness,  and  heated  for 
some  time  to  110°  to  render  silica  insoluble.  The  residue  is  dissolved  in  hy- 
drochloric acid,  filtered  from  silica  and  graphite,  largely  diluted,  and  the  sul- 
furic  acid  precipitated  by  barium  chloride.  The  method  is  fairly  accurate,  the 
plus  error  from  contamination  of  the  barium  sulfate  by  iron  oxides,  silica,  etc., 
being  compensated  to  a  degree  by  the  solubility  of  the  precipitate  in  acid  ferric 
chloride.  The  weighed  precipitate  may  be  purified  by  boiling  with  concentrated 
hydrochloric  acid,  diluting  with  water,  adding  a  little  barium  chloride,  then 
filtering  and  weighing  as  before;  the  more  usual  method,  inaugurated  by  a 
fusion  with  sodium  carbonate,  may  easily  introduce  more  impurities  than  are 
extracted. 

Other  solvents  for  the  metal  are  aqua  regia  which  dissolves  the  iron  promptly, 
but  there  is  said  to  be  some  danger  of  loss  of  sulfur  by  volatilization  of  sulfur 
chloride;  bromine  in  hydrochloric  acid;  potassium  bromide  or  potassium  chlo- 
rate with  nitric  acid;  etc.*  •-.  , 

The  injurious  effect  of  the  ferric  chloride  to  dissolve  barium  sulfate  or  con- 
taminate it  is  avoided  by  precipitating  the  iron  by  ammonia,  and  determining 
the  sulfuric  acid  in  the  concentrated  filtrate. 

2.  The  metal  is  treated  with  some  solvent  which  has  neither  an  oxidizing  action 
nor  generates  hydrogen  with  the  iron  or  manganese.    In  this  class  are  nearly 
neutral  ferric  chloride  — 

Fe  -f  FeaCle  =  SFeCk,  and  Mn  -f  Fe2Cle  =  MnCb  +  2FeCl2  — 
copper  ammonium  chloride,  etc.    All  these  leave  the  sulfur  in  the  residue,  from 
which  it  may  be  dissolved  by  concentrated  nitric  acid  and  the  solution  treated 
as  in  (1).      The   precipitation  taking  place   in    a  solution  almost   free  from 
ferric  chloride,  there  is  no  loss  from  solubility  and  the  precipitate  is  pure. 

3.  In  Devolution  methods"  the  sample  is  dissolved  in  dilute  hydrochloric 
acid,  and  the  hydrogen  evolved,  carrying  the  hydrogen  sulfide,  is  passed  through 
an  absorbing  medium.    For  a  gravimetric  determination  the  absorbent  is  (1), 
a  solution  of  an  oxidizer,  such  as  potassium  permanganate,  bromine  in  hydro- 
chloric acid,  or  hydrogen  peroxide  in  ammonia,  these  converting  the  hydrogen 
sulflde  into  sulfuric  acid  to  be  precipitated  by  barium  chloride  and  weighed 
as  barium  sulfate;  or  (2),  a  solution  of  some  metallic  salt,  as  an  ammoniacal 
solution  of  silver  nitrate,  lead  acetate,  or  cadmium  chloride;  in  these  a  sulfide 
of   the  metal  is  precipitated,  the  precipitate  filtered  off,  washed,  dried  and 
weighed  as  such,  or  oxidized  by  nitric  acid  to  the  sulfate,  and  the  sulfuric  acid 
determined  in  the  usual  way. 

One  of  the  many  forms  of  apparatus  is  shown  in  Fig.  73.  The  weighed 
drillings  are  placed  in  the  flask  A,  and  the  funnel  tube  B  is  filled 
with  dilute  acid.  The  absorption  bulb  C  holds  the  oxidizing  or  ab- 
sorbing reagent.  The  acid  in  B  is  run  into  A,  and  when  the  metal 
has  dissolved,  the  solution  is  heated  to  boiling  to  expel  what  hydrogen 
sulfide  remains  dissolved  and  in  the  gas  in  the  flask.  At  the  same  time  a  stream 
of  air  may  be  forced  through  the  apparatus  entering  at  B,  by  pressure  or  by 
suction  at  D.  In  more  accurate  analyses,  the  air  in  the  flask  and  bulbs,  that 
might  oxidize  some  of  the  hydrogen  sulflde  to  sulfurous  acid,  is  displaced  by  a 
stream  of  hydrogen  or  carbon  dioxide  before  running  in  the  hydrochloric  acid. 
As  a  further  precaution,  Campredon  f  would  pass  the  gases  through  a  red-hot 


*  Journ.  Amer.  Chem.  Socy.  1901—675. 
t  Chem.  News,  1895-2—15. 


IRON   AND   STEEL.  343 

porcelain  tube  preceding  the  absorption  bulb,  the  hydrogen  reducing  any  sul- 
furous  compounds  to  hydrogen  sulflde. 

A  quick  volumetric  method,  much  in  use  at  steel  works,  is  that  of  passing  the 
evolved  gases  through  a  strong  solution  of  potassium  hydrate  or  sodium 
hydrate,  the  hydrogen  sulflde  reacting  to  form  alkali  sulflde.  The  caustic 
solution  is  largely  diluted,  then  acidified  by  hydrochloric  acid,  this  setting  free 
hydrogen  sulflde  which  remains  dissolved  in  the  bulky  cold  liquid.  The  solu- 
tion is  immediately  titrated  by  a  standard  solution  of  iodine,  using  starch- 
paste  for  an  indicator. 

Owing  to  the  absorption  of  hydrocarbons  by  the  alkali  solution,  the  standard- 
izing of  the  iodine  solution  is  best  done  by  a  parallel  determination  on  a  similar 
grade  of  metal  whose  content  of  sulfur  has  been  ascertained  by  a  gravimetric 
determination. 

Boucher*  modifies  the  above  by  absorbing  the  hydrogen  sulfide  in  dilute 
caustic  soda  solution,  pouring  this  into  a  strongly  acidified  solution  of  ferric 
chloride,  and  titrating  the  ferrous  chloride  (reduced  from  ferric  by  the  hydrogen 
sulflde),  by  weak  standard  potassium  bichromate. 

For  the  caustic  alkali  solution  may  be  substituted  an  ammoniacal  solution  of 
cadmium  sulfate  or  zinc  sulfate.  After  filtering,  the  precipitated  sulflde  is 
dissolved  in  dilute  acid  and  titrated.  An  advantage  of  this  plan  is  that  the 
interfering  effect  of  the  hydrocarbons  absorbed  by  the  alkali  is  to  a  great  extent 
eliminated. 

An  approximate  technical  method  is  described  by  Arnold  and  Hardy.  The 
gases  from  the  evolution  flask  are  passed  through  a  series  of  some  fifteen  small 
absorption  tubes,  each  containing  exactly  two  cubic  centimeters  of  a  solution 
of  lead  acetate  of  a  fixed  concentration.  The  strength  of  the  lead  solution  is 
such  that  each  absorption  tube  contains  the  exact  amount  of  lead  precipitable 
by  the  hydrogen  sulfide  corresponding  to  .01  per  cent  of  sulfur  in  the  standard 
weight  of  metal  dissolved.  When  the  gas  has  passed,  the  number  of  bulbs 
showing  a  precipitate  or  coloration  represents  the  number  of  hundredths  of  one 
per  cent  of  sulfur  in  the  steel.  No  hydrogen  sulfide  is  retained  in  solution  by 
the  liquid  in  any  bulb  on  account  of  the  large  volume  of  hydrogen  accompany- 
ing it. 

The  results  of  all  determinations  are  likely  to  be  somewhat  too  low,  especially 
for  pig  iron  or  cast  iron,  as  some  of  the  sulfur  remains  with  the  insoluble 
matter  in  the  flask,  perhaps  as  an  organic  compound  or  as  ferrous  disulfide. 
Blair  f  observed  in  a  sample  of  pig  iron  containing  titanium  and  vanadium,  that 
the  major  part  of  the  sulfur  was  in  a  form  of  combination  insoluble  in  either 
hydrochloric,  nitric,  or  nitro-hydrochloric  acids,  and  was  only  to  be  resolved  by 
fluxing.  According  to  Moore  the  condition  of  the  carbon  in  cast  iron  exerts  an 
influence  on  the  amount  of  sulfur  retained  in  the  insoluble  residue,  a  greater 
proportion  remaining  in  the  case  of  chilled  iron  than  in  the  same  iron  that  has 
slowly  cooled  from  a  fused  condition. 

In  all  accurate  analyses,  therefore,  the  insoluble  residue  is  filtered  from  the 
solution  of  ferrous  chloride,  treated  with  nitric  acid,  evaporated  to  dryness, 
taken  up  by  hydrochloric  acid,  filtered,  and  the  filtrate  tested  by  barium  chlo- 
ride; if  a  weighable  amount  of  barium  sulfate  forms  it  is  determined  as  usual. 
The  solution  of  the  ferrous  chloride  may  also  be  tested  by  barium  chloride, 
though  it  seldom  if  ever  contains  sulfur. 

4.  Phillips  J  calls  attention  to  the  readiness  with  which  finely  powdered  iron 


*  Chem.  News,  1897—121. 

t  Journ.  Amer.  Chem.  Socy.  1897—114. 

1  Idem,  1897-1079. 


344  QUANTITATIVE    CHEMICAL    ANALYSIS. 

is  oxidized  by  a  melted  mixture  of  sodium  carbonate  and  nitrate,  and  the 
simultaneous  complete  oxidation  of  all  the  sulfur  to  sulfuric  acid.  Bamberg 
recommends  a  modification  of  the  aqua  regia  method,  wherein  the  solution  of 
the  metal  in  concentrated  nitric  acid  is  evaporated  with  sodium  nitrate  in  a 
platinum  dish  and  the  residue  ignited.  During  the  ignition  the  sulfur  is  ex- 
posed to  the  intense  oxidizing  action  of  the  melted  sodium  nitrate  and  the 
decomposing  ferric  nitrate.  The  melt  is  lixiviated  by  a  solution  of  sodium 
carbonate,  acidulated,  evaporated  to  separate  silica,  the  residue  taken  up  by 
dilute  hydrochloric  acid,  filtered,  and  the  sulfuric  acid  precipitated  by  barium 
chloride. 

5.  Eggertz  many  years  ago  proposed  a  colorimetric  scheme  for  wrought  iron 
and  steel,  in  which  a  decigram  of  the  metal  is  dissolved  in  dilute  sulfuric  acid 
in  a  small  flask,  the  hydrogen  sulfide  evolved  impinging  on  a  polished  silver 
plate.  A  tarnish  of  silver  sulfide  ensues,  the  depth  of  the  tint  being  in  propor- 
tion to  the  percentage  of  sulfur  in  the  metal,  ranging  from  light  yellow  (.01 
per  cent)  to  brown  (.04  per  cent).  For  comparison,  irons  containing  known 
amounts  of  sulfur  are  treated  in  the  same  way.  The  results  are  only  approxi- 
mate at  best,  and  the  method  is  but  little  used.  Later  the  principle  was  re- 
vived by  Wiborgh  *  who  passes  the  gas  through  a  disk  of  white  cotton  cloth 
mordanted  with  a  standard  solution  of  cadmium  acetate;  a  yellow  coating  of 
cadmium  sulflde  is  produced,  deep  in  proportion  to  the  hydrogen  sulflde  trans- 
piring. . 


Carbon.  Since  no  practical  methods  for  the  proximate  analysis  of  iron  and 
steel  have  as  yet  been  devised,  a  discussion  of  the  question  as  to  the  condition 
or  form  of  combination  in  which  carbon  exists  in  the  metals  need  not  be  entered 
into.  Two  conditions  are  readily  differentiated,  however :  (A),  combined  (or 
dissolved)  carbon,  by  its  behavior  when  the  metal  is  dissolved  (1)  in  a  non- 
oxidizing  acid,  the  carbon  passing  off  as  a  mixture  of  gaseous  hydrocarbons; 
(2),  in  hot  dilute  nitric  acid,  the  carbon  slowly  entering  into  solution  as  a  hy- 
drated,  highly  tinctorial  compound ;  and  (3) ,  in  a  solvent  of  a  reducible  nature, 
leaving  the  carbon  as  a  soft,  black,  pulverulent  residue  insoluble  in  dilute  acids, 
probably  a  carbohydrate  (CsH^O  ?),  that  dries  to  a  fine  powder  burning  like  tin- 
der when  heated  in  oxygen  or  air.  (B),  Carbon  in  the  graphitoidal  form 
(shortly  graphite),  practically  unaffected  by  acids  and  exhibiting  the  physical 
characteristics  of  the  mineral  —  slow  combustibility,  an  unctuous  feel,  etc. 
There  is  also  a  third  form,  a  psuedo-graphite,  whose  properties  are  intermedi- 
ate between  the  other  two.  The  carbon  in  steel  is  assumed  to  be  entirely  in 
the  combined  state  except  in  metals  of  some  unusual  chemical  composition  or 
that  have  been  subjected  to  abnormal  physical  treatment. 

Outside  of  comparative  methods,  the  general  procedure  in  a  determination  is 
to  convert  the  carbon  into  carbon  dioxide  and,  by  weighing  or  otherwise,  find 
the  proportion  of  carbon  contained.  A  great  number  of  methods  have  been 
devised,  nearly  all  for  the  estimation  of  the  total  carbon  of  the  metal. 

1.  The  carbon  is  oxidized  to  carbon  dioxide  simultaneously  with  the  oxida- 
tion of  the  iron  and  manganese. 

A.  The  metal  is  heated  in  a  current  of  oxygen,  either  alone  or  mixed  with  a 
reagent  readily  parting  with  oxygen.  It  is  essential  that  it  be  in  the  form  of 
the  thinnest  shavings  or  as  a  fine  powder,  that  the  coating  of  sintered  magnetic 
oxide  or  sesquioxide  of  iron  may  not  protect  the  interior  from  oxidization. 


*  Journ.  Socy.  Ohem.  Ind.  9—16. 


IRON   AND   STEEL.  345 

The  metal  is  spread  over  the  bottom  of  a  capacious  porcelain  or  platinum  boat,, 
then  heated  to  bright  redness  in  a  porcelain  or  platinum  combustion  tube 
arranged  as  for  an  ultimate  organic  analysis.  The  success  of  the  operation  is 
more  certain  if  a  greater  weight  of  some  carbon-free  oxidizer  like  copper  oxide, 
lead  dichromate  or  potassium  chlorate  is  intermixed  with  the  metal. 

B.  The  metal  is  dissolved  directly  in  a  hot  concentrated  solution  of  chromic 
acid  in  moderately  dilute  sulfuric  acid.  The  iron  and  manganese  dissolve  as 
ferric  and  manganous  sulfates,  while  the  carbon  is  oxidized  to  carbon  dioxide 
by  the  combined  action  of  chromic  acid  and  ferric  sulfate;  above  a  certain 
concentration  of  the  reagent,  oxygen  also  is  produced  — 2Cr03  +  3H2SO4  = 
Cr2 (864)3  ~f  SO  +  3H2O.  For  graphitic  irons  it  is  a  safer  plan  to  pass  the  gases 
through  a  porcelain  tube  filled  with  copper  oxide  kept  at  a  red  heat,  in  order 
to  oxidize  any  carbon  monoxide  or  hydrocarbons  of  the  gas. 

2.  The  iron  and  manganese  are  separated  from  the  carbon,  and  the  carbon- 
aceous residue  submitted  to  analysis. 

A.  The  metals  are  volatilized  by  heating  in  a  current  of  dry,  oxygen-free 
chlorine.    The  metal  is  weighed  in  a  porcelain  boat  and  the  boat  pushed  to  the 
middle  of  a  combustion  tube,  whose  anterior  end  is  sealed  by  a  water- trap. 
The  ferric  chloride  formed  on  heating  sublimes  to  anhydrous  yellow  flakes. 
When  all  the  metal  has  been  carried  over,  the  tube  is  cooled  and  the  boat  trans- 
ferred to  an  apparatus  for  ultimate  organic  analysis. 

B.  When  iron  or  steel  is  dissolved  in  dilute  hydrochloric  acid  under  the 
influence  of  an  electric  current  of  suitable  density,  the  chlorine  liberated  by 
the  electrolysis  of  the  acid  unites  with  the  iron  to  form  ferrous  chloride,  and 
the  hydrogen  escapes  at  the  negative  pole.    As  no  hydrogen  is  evolved  at  the 
surface  of  the  iron,  the  carbon  does  not  pass  off  as  hydrocarbons,  but  remains 
in  the  solid  form,  retaining  the  shape  of  the  original  fragments  of  the  metal. 

The  metal  drillings  are  held  in  a  basket  of  platinum  gauze  which  hangs  in 
the  acid  and  is  connected  to  the  copper  pole  of  a  battery;  to  the  zinc  pole  is 
connected  a  piece  of  platinum  foil  hung  some  distance  from  the  basket.  The 
current  is  known  to  be  properly  adjusted  when  the  stream  of  the  heavy  solu- 
tion of  ferrous  chloride  falling  from  the  basket  is  colorless. 

C.  Bromine  in  aqueous  solution,  and  iodine  in  a  solution  of  potassium  iodide 
unite  directly  with  iron  and  manganese  to  form  their  bromides  or  iodides,  and 
since  no  hydrogen  is  evolved,  leave  the  carbon  as  a  residue.    It  is  essential 
that  the  solvent  be  at  or  below  the  ordinary  temperature  while  the  metal  is 
undergoing  solution  since  the  carbon  has  a  rather  high  chemical  potential  at  the 
instant  of  liberation,  and  a  soluble  compound  with  the  halogen  may  occur 
to  some  extent  at  higher  temperatures. 

D.  Solutions  of  various  metallic  salts  that  are  reduced  to  a  lower  state  of 
oxidation,  or  from  which  the  whole  or  part  of  the  base  may  be  displaced  by 
iron  or  manganese,  have  been  proposed ;  such  are  cupric,  mercuric,  and  ferric 
chlorides  in  aqueous  solution,  and  silver  chloride  in  the  solid  form.    Thus,  a 
slightly  acid  solution  of  ferric  chloride  dissolves  an  atom  of  iron  by  parting 
with  two  atoms  of  chlorine;  and  a  cold  solution  of  potassium  dichromate  in 
dilute  sulfuric  acid  dissolves  the  metal  without  evolution  of  hydrogen  — 

K2Cr2O7  +  8H2SO4  +  Fe2  =  Fe2(SO4)3  -f  Cr2(SO4)3  +  2KHSO4  +  7H2O. 
Both  cupric  sulfate  and  chloride  deposit  an  equivalent  of  copper  for  the  iron, 
taken  up  if  desired,  by  heating  with  dilute  hydrochloric  acid  or  ferric  chloride 
in  which  the  carbon  residue  is  insoluble.  A  mixture  of  cupric  and  ferric 
chlorides  has  been  advised,  the  latter  compound  reacting  with  the  cuprous 
chloride  as  soon  as  produced,  the  products  being  cupric  and  ferrous  chlorides. 
Lunge  asserts  that  there  is  a  loss  of  carbon  during  solution  in  cupric  salts 


346 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


from  the  formation  of  gaseous  carbon  compounds,  amounting  to  .027  per  cent 
with  cupric  sulfate  as  a  solvent,  and  to  .015  per  cent  with  copper  ammonium 
chloride.  To  recover  the  carbon  in  the  gases  he  would  dissolve  the  metal 
under  a  slow  current  of  pure  air;  from  the  dissolving  vessel  the  air  passes  over 
hot  copper  oxide  to  burn  the  carbon  gases  to  carbon  dioxide,  thence  through 
potash  bulbs  to  absorb  the  latter  and  allow  its  being  weighed. 

The  double  chloride  of  an  alkali  metal  and  copper  dissolves  iron  without  the 
deposition  of  copper  —  Fe  -f  2CuCl2(KCl)2  =  FeCl2  -f  Cu2Cl2(KCl)4;  or  taking 
the  formula  Fe3C  to  represent  the  normal  combination  of  carbon 
and  iron,  3  Fe3C  +  18CuCl2(KCl)2  +  H20  =9FeCl2  +  9Cu2Cl2(KCl)4  +C3H20(?). 
A  strongly  acidulated  solution  of  a  double  chloride  has  been  found  to 
yield  higher  results  for  carbon  than  a  neutral  solution.  It  may  be  that  the  lat- 
ter fails  to  decompose  all  the  iron  carbide  or  some  one  variety  of  this  com- 
pound, afterward  dissolved,  with  evolution  of  hydrogen  and  loss  of  carbon,  on 
washing  the  residue  with  dilute  hydrochloric  acid. 

The  carbon  left  after  extraction  of  the  iron  and  manganese  by  one  of  the 
methods  given  above  cannot  well  be  dried  and  weighed  directly  since  it  is  com- 
bined with  hydrogen  and  oxygen,  perhaps  in  indefinite  proportions,  or  con- 
tains graphite,  and  is  often  mixed  with  silica,  scale  or  sand  coming  from  the 
metal  drillings  and  insoluble  in  dilute  acid.  Therefore  the  carbon  contained  is 
best  determined  by  the  usual  process  for  the  determination  of  carbon  in  an 
organic  compound.  The  liquid  is  filtered  on  purified  asbestos  and  the  residue 
washed  with  dilute  hydrochloric  acid  and  water. 

For  the  oxidation  of  the  carbon  and  the  determination  of  the  carbon  dioxide 
produced,  one  has  a  choice  of  several  methods.  The  dried  residue  may  be 
mixed  with  powdered  cupric  oxide  and  burned  in  a  current  of  air;  burned  alone 
in  air  or  oxygen;  or,  without  drying,  transferred  to  a  flask  and  oxidized  by 
chromic  and  sulfuric  acids.  The  carbon  dioxide  formed  is  passed  through  a 
drying  tube,  thence  through  a  potash  bulb. 

Shimer  *  has  arranged  a  special  apparatus  for  the  combustion,  a  substitute 
for  the  platinum  combustion  tube.  As  shown  in 
section  in  Fig.  174,  a  platinum  crucible  is  closed 
by  a  hollow  metal  stopper  A,  through  which  cir- 
culates cold  water,  and  the  junction  made  gas- 
tight  by  a  rubber  band.  Around  the  upper  part 
of  the  crucible  is  a  water-cooled  ring  B,  to  pre- 
vent the  rubber  gasket  from  being  heated.  Pass- 
ing through  the  stopper  are  inlet  and  outlet 
tubes  C  and  D,  for  the  gases. 

Combined  carbon.  This  is  usually  found  by 
difference,  subtracting  the  result  for  the  graphite 
from  that  for  the  total  carbon.  Two  methods 

have  been  proposed  for  a  direct  determination. 

A.  The  metal  is  dissolved  in  dilute  sulfuric  acid 
^'  and    the  hydrogen,  hydrocarbons,  and    hydrogen 

sulfide  passed  through  a  porcelain  tube  containing  red-hot  copper  oxide;  the 
hydrogen  burns  to  water,  the  carbon  to  carbon  dioxide,  and  the  sulfur  to  sul- 
furic acid.  The  carbon  dioxide  is  collected  and  weighed  as  in  an  elementary 
analysis. 

B.  The  metal  is  dissolved  in  melted  potassium  pyrosulfate,  sulfurous  and 
carbonic  acid  gases  being  evolved.  The  mixed  gases  are  passed  through 
chromic  acid  to  oxidize  and  absorb  the  sulfurous  acid,  then  over  hot  copper  oxide 


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/ 

V 

s-  
/  — 

1 

\ 

1 

Journ.  Amer.  Chem.  Socy.  1901—227. 


IRON    AND    STEEL.  347 

to  burn  any  carbon  monoxide  or  hydrocarbons  in  the  gases,  finally  into  a  potash- 
bulb.  The  graphite  of  the  metal  is  said  to  be  unattacked  and  may  subsequently 
be  determined  alter  lixiviating  the  flux  with  water. 

Graphite.  It  is  generally  assumed  that  in  steel,  wrought  iron,  spiegel-eisen 
and  ferro-manganese  all  the  carbon  is  combined  or  dissolved  in  the  metal,  but 
graphitic  carbon  is  also  present  in  pig  iron,  cast  iron,  malleable  castings,  and 
some  spiegels.  The  graphite  is  determined  by  dissolving  the  sample  in  a  dilute 
acid,  boiling,  filtering  and  washing  with  dilute  potassium  or  ammonium  hydrate 
and  water  —  some  recommend  also  alcohol  and  ether  —  to  remove  any  adhering 
hydrocarbons,  and  making  a  combustion  of  the  residue  as  above  described. 
Or  the  residue  may  be  dried  and  weighed,  then  burned  in  a  platinum  crucible 
and  the  weight  of  the  remaining  silica,  etc.,  deducted.  In  any  case,  the  results 
are  open  to  criticism,  as  with  hydrochloric  or  sulfuric  acid  for  the  solvent, 
part  of  the  combined  carbon  may  separate  in  a  form  insoluble  in  the  washing 
fluids,  and  with  nitric  acid,  the  finely  divided  carbon  may  not  entirely  escape 
oxidation.  Titanium  carbide,  a  common  constituent  of  pig  iron,  is  insoluble 
in  hydrochloric  acid,  but  readily  decomposed  by  nitric. 

Colorimetric  method  for  combined  carbon.  Eggertzf  in  the  year  1862,  ob- 
served that  when  pure  iron  is  dissolved  in  hot  moderately  dilute  nitric  acid  the 
solution  is  practically  colorless  after  moderate  dilution  with  water,  while  in 
the  case  of  a  steel,  the  carbon,  at  first  separating  in  flocks,  slowly  dissolves  and 
colors  the  acid  brown  or  greenish-brown,  the  intensity  of  the  tint  varying  directly 
with  the  proportion  of  carbon  in  the  steel.  On  this  principle  he  contrived 
the  method  now  in  general  use.  A  standard  weight  (.1  to  1  gram)  of  drillings 
is  dissolved  in  a  test-tube  in  a  standard  volume  (3  to  15  Cc.)  of  nitric  acid  of 
about  1.2  specific  gravity.  The  solution  is  heated  in  the  water-bath  for  ten  to 
thirty  minutes,  then  diluted,  cooled,  and  filtered  if  necessary,  and  poured  into 
a  comparison  tube  graduated  in  cubic  centimeters.  A  '  standard '  steel  (one 
whose  carbon  has  been  determined  gravimetrically)  is  treated  at  the  same 
time  in  exactly  the  same  manner.  The  darker  of  the  solutions  is  then  diluted 
until  the  tints  are  the  same,  when  the  percentages  of  carbon  in  the  standard 
and  sample  are  directly  as  the  volumes  of  the  respective  solutions. 

A  rack  of  hermetically-sealed  tubes  (Fig.  144)  containing  solutions  of  steels 
of  different  percentages  of  carbon  may  be  prepared,  but  their  colors  are  not 
permanent,  fading  on  exposure  to  light.  This  objection  applies,  though  in  a 
less  degree,  to  such  substitutes  as  tinctures  of  roasted  coffee,  caramel,  etc., 
and  solutions  of  ferric  salts  tinged  with  copper  and  cobalt. 

The  condition  or  form  of  combination  of  carbon,  consequent  on  the  mode  of 
manufacture  or  subsequent  mechanical  or  heat  treatment  of  the  steel,  deter- 
mines to  a  certain  extent  the  depth  of  tint,  so  that  the  steel  selected  for  a  stand- 
ard must  have  been  manufactured  by  the  same  process  and  have  received  approx  - 
imately  the  same  after-treatment  as  the  sample  to  be  compared  with  it.f  Fol- 
lowing the  general  rules  for  colorimetric  determinations,  the  solutions  to  be 
compared  must  be  clear  (free  from  suspended  sulfur,  scale,  etc.)  and  of  about 
the  same  concentration,  that  is,  the  standard  is  to  be  of  approximately  the  same 
percentage  of  carbon  as  the  sample  to  be  tested.  Observing  these  precautions, 
the  method  in  practiced  hands  is  remarkably  accurate  for  one  of  its  class. 

The  nitric  acid  solutions  of  the  softer  steels  are  apt  to  differ  in  color  as  well 
as  tint,  making  a  comparison  diflScult.  Stead  directs  to  precipitate  the  iron  as 
ierric  hydrate  by  solution  of  sodium  hydrate  in  excess;  the  colorific  carbon  com- 


*  Trans.  Amer.  Inst.  Mining  Engrs.  12—303. 
t  Chem.  News,  1894—1—251. 


348  QUANTITATIVE    CHEMICAL    ANALYSIS. 

pounds  are  left  in  solution,  their  tints  intensified  but  their  colors  identical. 
After  diluting  the  solutions  to  a  definite  volume  and  filtering,  aliquot  portions 
of  the  filtrate  are  compared  in  the  usual  way.  He  has  devised  a  special  color- 
meter  for  matching  the  faint  shades. 


Copper.  All  or  nearly  all  of  this  element  remains  in  the  insoluble  residue 
when  the  metal  is  dissolved  in  dilute  sulfuric  acid.  After  saturating  the  solu- 
tion with  hydrogen  sulfide,  or  boiling  with  a  little  sodium  thiosulfate  to  pre- 
cipitate any  traces  of  copper  that  may  have  gone  into  solution,  the  liquid  is 
filtered ;  the  residue  is  treated  with  nitric  acid,  evaporated  to  dryness,  taken  up 
with  hydrochloric  acid,  and  filtered  from  silica  and  graphite.  From  the  filtrate 
the  copper  is  precipitated  as  sulfide,  roasted  to  cupric  oxide  and  weighed;  or 
the  filtrate  is  evaporated  with  sulfuric  acid  and  the  copper  deposited  electro- 
lytically. 


Arsenic.  On  dissolving  steel  in  dilute  hydrochloric  acid  the  arsenic  present 
does  not  escape  as  arsine  but  remains  in  solution  as  arsenic  chloride,  and  on 
dilution  and  treatment  with  hydrogen  sulflde,  precipitates  as  arsenious  sulflde. 
The  steel  is  dissolved  in  dilute  hydrochloric  acid,  and  treated  with  a  little 
sulfurous  acid,  then  warmed  until  the  sulfurous  acid  is  dissipated.  A  stream 
of  hydrogen  sulflde  precipitates  the  sulflde ;  the  solution  is  filtered  and  the  pre- 
cipitate digested  with  potassium  sulfide  which  dissolves  the  arsenic  but  leaves 
the  copper  and  iron  sulfldes.  The  solution  is  filtered  and  the  filtrate  acidified, 
reprecipitating  the  arsenic  as  sulfide.  Again  filtering,  the  precipitate  is  oxi- 
dized to  arsenic  acid  by  hydrochloric  acid  and  potassium  chlorate.  The  solu- 
tion is  made  ammoniacal  and  precipitated  by  magnesia  mixture,  the  arsenic 
coming  down  as  ammonium  magnesium  arseniate.  This  compound,  analogous 
to  the  corresponding  phosphate,  leaves  magnesium  pyroarsenate  on  ignition. 

In  another  method,  the  arsenic  is  converted  into  arsenic  acid  by  nitric  acid; 
on  heating  with  hydrochloric  acid  and  a  strong  reducer,  the  arsenic  passes  to 
arsenic  chloride  that  may  be  distilled  at  the  boiling  point  of  strong  hydro- 
chloric acid.  The  steel  is  dissolved  in  a  mixture  of  nitric  and  sulfuric  acids 
and  evaporated  until  all  nitric  acid  is  expelled  and  the  residue  forms  a  dry 
cake.  This  is  transferred  to  a  flask,  powdered  ferrous  sulfate  and  strong 
hydrochloric  acid  added,  and  the  mixture  distilled  into  cold  water.  In  the  dis- 
tillate the  arsenic  is  determined  by  precipitating  by  hydrogen  sulfide  and  pur- 
suing the  usual  course  for  a  determination. 


Iron.  In  analyses  of  commercial  iron  and  steel  the  iron  is  usually  deter- 
mined by  difference,  and  frequently  also  in  spiegels  and  ferromanganese  in 
which  the  percentage  of  iron  is  secondarily  important  to  that  of  manganese.  A 
fair  approximation  to  the  percentage  of  iron  in  the  two  manganiferous  metals 
can  be  had  by  determining  the  manganese,  deducting  a  certain  percentage  for 
carbon,  silicon,  etc.,  and  calling  the  remainder  iron.  The  percentage  deducted 
for  the  carbon,  silicon,  etc.,  varies  with  the  manganese  content  of  the  metal 
but  is  quite  constant  for  a  given  percentage. 

A  direct  determination  is  made  by  a  process  similar  to  that  used  for  the  deter- 


IRON   AND    STEEL.  349 

mination  of  iron  and  iron  ores.  The  metal  is  dissolved  in  dilute  sulf  uric  acid  and 
the  ferrous  sulf  ate  titrated  by  potassium  permanganate,  or  in  hydrochloric  acid 
and  the  ferrous  chloride  titrated  by  potassium  bichromate.  Usually  the  carbon 
must  be  eliminated,  since  dissolved  hydrocarbons  reduce  permanganate  and  to  a 
less  extent,  bichromate ;  this  can  be  accomplished  by  heating  the  solution  to 
boiling,  filtering,  and  treating  the  filtrate  with  potassium  chlorate.  The  liquid 
is  then  reduced  by  zinc  or  other  reagent  and  titrated  as  usual. 


Of  commercial  iron  and  steel,  carbon,  silicon,  sulfur,  phosphorus,  and 
manganese  are  invariable  constituents,  and  as  their  effect  on  the  quality  of  these 
metals  has  been  studied  and  approximate  limits  established  for  the  different  va- 
rieties, they  are  always  included  in  an  analysis.  Copper  and  arsenic  are  not  infre- 
quently found,  and  titanium,  tin,  zinc,  nickel  and  a  few  other  elements  more 
rarely,  although  minute  traces  of  many  unsuspected  elementary  constituents 
would  probably  be  found  by  a  careful  search.  For  the  methods  of  determin- 
ing the  elements  not  described  here,  reference  may  be  had  to  several  practical 
works  on  iron  and  steel  analysis  that  have  been  published  by  Troilius,  Blair, 
von  Jonsdorff,  and  Arnold. 

Besides  the  familiar  carbon -hardened  steel  there  are  on  the  market  a  number 
of  special  alloys  of  remarkable  physical  properties,  known  from  the  influential 
adjective  as  tungsten-  manganese-  chrome-  nickel-  silicon-  titanium-  and 
aluminum-steel,  some  of  these  having  attained  considerable  practical  im- 
portance. For  the  production  of  such  steels  there  are  manufactured  alloys  of 
iron  with  a  high  percentage  of  these  elements,  designated  as  ferro-chrome, 
ferro- tungsten,  silico-spiegel,  etc.  The  methods  for  the  determination  of 
these  elements  follow  in  the  main  those  well  known  and  of  general  application. 


350 


QUANTITATIVE    CHEMICAL   ANALYSIS, 


IRON  AND  MANGANESE  ORES. 

The  minerals  constituting  the  ores  of  iron  are  either  hematite  (FetOs)r 
limonite  (Fe2O3.xH2O),  magnetite  (Fe8O4),  or  siderite  (FeCO3).  The  common 
associates  are  oxides  of  manganese,  quartz,  various  hydrous  and  anhydrous  sili- 
cates of  the  earths  and  alkalies,  etc.,  and  small  percentages  of  phosphoric  acid 
compounds,  and  sulfur  in  the  form  of  pyrite  or  gypsum.  Frequently  also  are 
found  bituminous  matter  and  titanic  acid,  more  rarely  the  oxides  or  other  com- 
pounds of  arsenic,  copper,  zinc,  nickel,  chromium,  etc.  All  ores  contain  hy- 
groscopic moisture  in  variable  quantity. 

The  commercial  value  of  an  ore  is  based  primarily  on  the  content  of  iron  and 
manganese,  and  freedom  from  titanic  acid,  phosphoric  acid  and  sulfur.  It  Is 
also  a  desideratum  that  the  gangue  contain  a  considerable  proportion  of  lime  or 
magnesia  rather  than  be  wholly  or  mainly  silica  or  aluminum  silicates.  A 
•  Bessemer  ore '  is  one  suitable  for  the  manufacture  of  rail-steel  by  the  acid 
Bessemer  process ;  that  is,  that  the  resulting  steel  shall  not  contain  over  about 
one-tenth  of  one  per  cent  of  either  phosphorus  or  sulfur.  Where  charcoal  is 
the  fuel  used  in  the  blastfurnace  the  limits  of  phosphorus  and  sulfur  in  a  Bes- 
semer ore  may  be  taken  as  not  to  exceed  a  ratio  of  .07  percent  to  65  per  cent 
of  iron  in  the  ore;  with  coke  fuel,  the  sulfur  in  the  ore  must  be  lower. 

Manganese  ores,  from  which  are  smelted  spiegel-eisen  and  ferro- manganese, 
are  essentially  one  or  a  mixture  of  the  hydrated  or  anhydrous  higher  oxides  of 
manganese,  and  usually  contain  associated  ferric  oxide  or  hydrated  oxide. 
They  are  valued,  next  to  the  content  of  manganese  and  iron,  by  their  freedom 
from  silica.  Phosphorus  is  nearly  always  comparatively  high  in  these  ores, 
and  baryta  is  a  frequent  associate. 

Below  are  a  few  analyses  of  different  varieties  of  ores,  all  dried  at  100  ° 
previous  to  analysis. 


A. 

B. 

C. 

D. 

E. 

Ferrous  oxide  

1.22 

24.49 

45.27 

.36 



Ferric  oxide  

87.67 

63.22 

.64 

72.28 

4.37 

Silica  

5.21 

2.99 

11.24 

12.76 

10.96 

Alumina  

2.07 

1.07 

8.14 

.31 

1.73 

.93 

.65 

1.72 

.42 

6.77 

Magnesia  

.56 

.20 

1.51 

.17 

.66 

Manganese  oxide.  .  .  . 

.30 

1.21 

2.83 

67.60  (Mn3O4) 

Phosphoric  acid  

.14 

.07 

.17 

.54 

.21 

Sulfur.  

.03 

2.11 

trace 

.10 

.08 

Carbon  dioxide  

30.32 

Titanic  acid  

3.20 

Organic  matter  —  \ 
Combined  water.  .  .  / 

1.48 

.28 

.98 

10.40 

6.33 

Baryta  

.37 

99.61 

99.49 

99.99 

100  17 

99.08 

Metallic  iron  

63.32 

63.51 

35.66 

50.91 

3.06 

Phosphorus  

.061 

.035 

.074 

,237 

.092 

Manganese  

.19 

.76 



2.19 

48.71 

A,  Lake  Superior  hematite.    B,  New  York  magnetite.    C,  Pennsylvania  aider* 
ite.    D,  Tennessee  limonite.    E,  Chilian  pyrolusite. 


IRON   AND   MANGANESE   ORES.  351 

Most  iron  ores  dissolve  but  slowly  and  with  difficulty  in  sulfuric  acid  an- 
are  practically  insoluble  in  nitric  acid,  so  that  hydrochloric  is  the  invariable 
solvent.  If  it  is  desirable  for  any  given  determination  that  the  bases  be 
combined  as  sulfates  or  nitrates,  the  hydrochloric  solution  is  evaporated  with 
an  excess  of  sulfuric,  or  boiled  down  two  or  three  times  with  nitric.  A  few 
ores  are  so  dense  in  structure  as  to  be  but  slowly  acted  on  by  concentrated 
hydrochloric  acid ;  here  a  previous  ignition  in  hydrogen  will  reduce  the  oxide 
to  metal,  or  the  addition  of  a  reducing  agent  to  the  hydrochloric  acid,  if 
allowable,  will  hasten  the  operation. 

Iron.  This,  the  most  important  constituent  of  an  iron  ore,  is  always  deter- 
mined volumetrically  since  gravimetric  methods  are  far  more  laborious  and 
not  more  accurate.  The  volumetric  methods  depend  on  the  conversion  of  a 
ferrous  compound  to  a  ferric  compound  by  a  perducer,  usually  potassium 
permanganate  or  bichromate ;  less  frequently  the  conversion  of  a  ferric  to  a 
ferrous  compound  by  a  reducing  agent. 

A.  By  standard   permanganate.    All   the    iron  is  brought    to  the  state   of 
ferrous  sulfate  in  a  dilute  sulfuric  acid  solution,  and  titrated  by  standard  potas- 
sium permanganate.    A  gram  of  the  dried  ore  is  dissolved  in  hydrochloric 
acid,  diluted  and  filtered;  if  any  iron  is  contained  in  the  insoluble  residue,  it 
is  brought  into  solution,  easiest  by  heating  the  residue  contained  in  a  platinum 
dish,  with  hydrofluoric  and  sulfuric  acids,  evaporating  to  remove  the  hydro- 
fluosilicic  and  excess  of  hydrofluoric  acids,  dissolving  the  residue  in  hydro- 
chloric acid,  and  adding  the  solution  to  the  main  solution.    An  excess  of  sul- 
furic acid  is  added  and  the  liquid  evaporated  until  all  hydrochloric  acid  is  ex- 
pelled, then  diluted  with  water.    The  ferric  sulfate  in  solution  is  reduced  to 
ferrous  sulfate  by  metallic  zinc,  then  decanted  from  the  excess  of  zinc,  and  at 
once  titrated.    The  accuracy  of  the  result  is  vitiated  only  by  carbonaceous  mat- 
ter in  quantity  too  great  to  be  destroyed  by  the  evaporation  with  sulfuric  acid, 
and  which  should  have  been  eliminated  by  roasting  the  ore  previous  to  dissolving 
it;  and  by  titanic  acid  q.  v.    The  reaction  of  the  titration  is 

10FeSO4  +  K2Mn2O8  +  9H2SO4  =  5Fe2(SO4)3  -f  2KHSO4  +  2MnSO4  +  8H2O. 
Although  proceeding  in  this  manner  gives  satisfactory  results,  the  time  re- 
quired for  evaporation  and  the  slow  reduction  by  zinc  are  serious  drawbacks  in 
technical  work.  A  hydrochloric  solution  of  iron  cannot  be  directly  titrated  by 
permanganate  owing  to  a  secondary  reaction  between  the  permanganate  and  acid, 
but  if  one  of  a  class  of  certain  metallic  compounds  (manganous  sulfate,  lead 
chloride,  mercuric  sulfate,  etc.)  is  present  inconsiderable  amount,  the  extent 
of  this  secondary  reaction  is  minimized,  though  the  red  coloration  showing  the 
end-point  is  far  more  fugitive  than  in  their  absence ;  the  deep  yellow  color  of 
ferric  chloride,  tending  to  obscure  the  end-point,  may  be  lightened  by  the 
addition  of  phosphoric  acid.  The  time  of  reduction  may  be  cut  down  to  a 
few  minutes  by  passing  the  ferric  solution  through  a  Jones'  reductor. 

B.  By   standard  potassium   bichromate.    In  titrating  by  this  reagent  the 
equation  is 

6FeCl2  -f  K2Cr2O7  -f  14HC1  =  3Fe2Cl6  +  2KC1  +  Cr2Cl6  -f  7H2O. 
no  reaction  taking  place  between  dilute  hydrochloric  acid  and  bichromate  in 
the  cold. 

Since  the  green  and  yellow  colors  of  chromic  and  ferric  chlorides  will  not 
allow  the  faint  yellow  of  the  excess  of  bichromate  to  be  seen,  the  point  of  entire 
conversion  of  ferrous  into  ferric  is  denoted  by  the  absence  of  a  blue  precipi- 
tate or  blue  coloration  when  a  drop  of  the  titrate  is  mixed  with  a  drop  of  a 


352  QUANTITATIVE   CHEMICAL   ANALYSIS. 

weak  solution  of  potassium  ferricyanide.  The  latter  is  spread  in  drops  over  a 
slightly  greased  porcelain  plate,  and  after  each  addition  of  a  small  volume  of 
the  titrand,  a  drop  of  the  titrate  is  transferred  to  one  of  the  ferricyanide  and 
the  effect  noted.  The  end-point  is  a  clear  brown  color. 

In  titrations  by  permanganate,  at  first  the  red  color  vanishes  immediately 
below  the  surface  of  the  titrate,  growing  more  diffusive  as  the  oxidation  pro- 
ceeds, and  one  can  correspondingly  diminish  the  rate  of  flow,  finally  to  drops 
only.  No  such  direct  indication  is  afforded  by  bichromate,  yet  one  can  observe 
in  the  sequence'  of  tests  a  gradual  diminution  in  the  density  of  the  blue  precipi- 
tate (ferrous  ferricyanide),  later  the  lessening  intensity  of  the  blue  coloration, 
and  so  follow  the  progress  of  the  oxidation. 

As  zinc  chloride  interferes  with  the  reaction  with  ferricyanide,  the  reduction 
is  effected  by  some  other  reagent.  A  stream  of  sulfurous  acid  or  hydrogen 
sulfide  may  be  passed  through  the  liquid ;  as  any  excess  of  either  would  reduce 
bichromate,  the  liquid  is  boiled  for  a  sufficient  time  in  a  flask  provided  with  a 
-cork  and  Bunsen's  valve,  which  allows  steam  to  escape  but  prevents  the 
entrance  of  air.  After  cooling,  the  flask  is  opened  and  the  titration  proceeded 
with. 

A  far  more  rapid  process  is  the  reduction  of  the  hot  solution  by  a  slight  ex- 
cess of  stannous  chloride;  the  excess  is  then  converted  into  stannic  chloride 
by  the  addition  of  mercuric  chloride.  The  reactions  are 

Pe2Cl6  (yellow  solution)  -\-  SnCl2  =  2FeCl2  (colorless  solution)  -f-  SnCl4;  and 
SnCJ2  +  2HgCl2  =  SnCl4  +  Hg2012. 

The  liquid,  now  ready  for  titration,  contains  stannic,  mercuric,  and  mercurous 
chlorides,  neither  of  which,  in  a  cold  solution,  affects  bichromate  or 
ferricyanide. 

With  manganese  ores  containing  but  relatively  little  iron  it  is  better  to 
dissolve  a  quantity  of  several  grams,  precipitate  the  iron  by  ammonia,  filter  and 
dissolve  the  ferric  hydrate  (and  whatever  manganic  hydrate  that  may  have 
co-precipitated)  in  hydrochloric  acid,  and  proceed  with  the  solution  as  usual. 

All  things  considered,  the  best  means  of  finding  the  strength  of  the  perman- 
ganate or  bichromate  solutions  is  by  iron  wire  (page  209).  Some  would  pre- 
pare a  stock  solution  of  ferric  chloride,  determine  the  concentration  by  a 
gravimetric  process,  and  for  standardization,  weigh  a  suitable  volume  and 
continue  exactly  as  with  the  hydrochloric  solution  of  an  ore.  By  this  pro- 
cedure it  is  claimed  that  certain  sources  of  error  affecting  the  assay  (e.  gr.,  loss 
through  volatility  of  ferric  chloride  on  boiling  the  solution)  are  practically 
counteracted  by  corresponding  errors  during  the  standardization.  But  it  is 
highly  probable  that  the  errors  incurred  in  gravimetrically  determining  the  iron 
in  the  stock  solution  will  be  at  least  as  great  as  those  the  scheme  is  intended 
to  rectify;  moreover,  the  losses  or  grains  that  would  be  experienced  by  a  nearly 
pure  ferric  solution  may  be  considerably  increased  or  diminished  by  the  soluble 
constituents  of  an  ore  or  its  gangue.  With  the  latter  point  in  view,  others 
would  prepare  a  large  quantity  of  a  finely  powdered  ore,  and  assume  that  the 
average  of  a  number  of  determinations  made  by  one  or  several  operators  is  the 
true  iron-content.  This  "  standard  ore  "  is  then  used  to  set  the  strength  of 
the  permanganate  or  bichromate  solutions,  observing  the  precaution  of  treating 
it  in  exactly  the  same  manner  as  the  ores  to  be  assayed.  But  since  all  the 
determinations  of  iron  in  the  standard  ore  were  presumably  made  with  iron 
wire  or  its  equivalent  as  a  basis,  not  only  is  its  intercalation  of  no  avail  for 
correcting  the  losses  or  gains  referred  to,  but  may  per  se  introduce  other  errors 
(from  imperfect  mixing,  segregation,  absorption  of  moisture,  etc.)  of  greater 
consequence. 


IRON  AND  MANGANESE   ORES.  353 

Volumetric  determination  by  reduction.  A  number  of  reducing  agents  have 
been  proposed  for  the  titration  of  ferric  solutions,  but  owing  to  the  instability 
of  the  solutions  from  their  ready  absorption  of  oxygen  from  the  air,  the  methods 
are  not  regarded  with  much  favor.  Here  all  the  iron  in  the  titrate  must  be 
in  the  form  of  ferric  chloride  with  no  other  oxidizer  present;  any  ferrous 
choride  may  be  oxidized  by  addition  of  a  slight  excess  of  potassium  perman- 
ganate, chlorine  water,  or  potassium  chlorate  to  the  strongly  acid  solution, 
and  boiling  off  the  excess  of  chlorine. 

A.  Stannous  chloride  abstracts  two  atoms  of  chlorine  from  ferric  chloride  — 

Fe2Cl6  (yellow)  +  SnCJ2  =  2FeCl2  (colorless)  +  SnCl4. 
and  reduces  ferric  sulfocyanide  to  ferrous  sulfocyanide  — 

Fe2(CNS)6  (red)  -f  SnCl2  +  2HC1  =  2Fe(CNS)2  (colorless)  +2HCNS  -f  SnCl4. 

When  standard  solution  of  stannous  chloride  is  added  to  a  hot  acidified  solu- 
tion of  ferric  chloride  the  yellow  color  fades  as  the  reaction  proceeds.  If  when 
the  titrate  has  become  but  faintly  yellow  potassium  sulfocyanide  be  added  and 
the  titration  resumed,  a  sudden  blanching  of  the  red  solution  indicates  that  all 
the  iron  has  passed  to  the  ferrous  state.  Some  prefer  to  add  at  once  an  excess 
of  stannous  chloride  and  titrate  back  by  standard  iodine  solution  and 
starch-paste. 

Another  indicator  is  due  to  Campbell ;  the  green  hue  produced  by  the  addi- 
tion of  a  little  cobaltous  chloride  to  the  ferric  chloride  solution  changes  to  a 
clear  blue  as  soon  as  all  the  ferric  chloride  is  reduced  to  ferrous. 

B.  Cuprous  chloride,  according  to  Winkler,  has  an  advantage  over  stannous 
chloride  in  that  reduction  takes  place  in  the  cold  as  promptly  as  at  higher  tem- 
peratures, and  that  after  bleaching  the  sulfocyanide,  the  next  drop  of  the  titrand 
clouds  the  titrate  with  insoluble  cuprous  sulfocyanide,  a  confirmation  of  the 
former  indication.    The  reagent  rapidly  oxidizes  on  exposure,  however. 

C.  Sodium  thiosulf ate  reduces  ferric  chloride  — 

Fe2Cl6  +  2Na2S2O3  =  2FeCl2  +  2NaCl  +  Na2S4O6. 

while  thiosulf  uric  acid  is  not  decomposed  by  cold,  very  dilute  free  acetic  acid. 
The  free  hydrochloric  acid  in  the  solution  of  the  ore  is  replaced  by  acetic,  by  the 
addition  of  sodium  acetate  until  the  red  color  of  ferric  acetate  appears,  then 
hydrochloric  acid  until  again  light  yellow.  An  excess  of  standard  sodium  thio- 
sulfate  is  run  in,  and  the  excess  titrated  back  by  standard  iodine  and  starch-paste. 
Better  results  are  had  by  the  interposition  of  potassium  iodide  which  reacts 
with  ferric  chloride  with  liberation  of  iodine.  The  iodine  is  then  determined 
by  standard  thiosulfate  and  starch-paste. 

D.  Moralit  bases  a  method  on  the  precipitation  of  ferric  chloride  and  ferric 
sulfocyanide  by  potassium  f errocyanide  — 

2Fe2Cl6  +  3K4Fe(CN)6=  Fe7(CN)18  +  12KC1. 

2Fe2(CNS)6  +  3K4Fe(CN)8  =  Fe7(CN)18  +  12KCNS. 

For  an  indicator  he  adds  ether  and  a  little  potassium  sulfocyanide  to  the 
titrate.  The  ferric  sulfocyanide  dissolves  in  the  floating  layer  of  ether,  com- 
municating its  characteristic  red  color,  while  Prussian  blue  is  insoluble.  The 
end-point  is  the  decolorization  of  the  ether.  The  process  is  conducted  with 
several  precautions. 

To  standardize  reducing  volumetric  solutions,  crystallized  ferric  ammonium 
sulfate  is  suitable;  or  if  ferric  chloride  is  preferred,  iron  wire  may  be  dis- 
solved in  hydrochloric  acid,  the  ferrous  chloride  peroxidized  by  potassium 
permanganate  or  potassium  chlorate,  and  the  solution  boiled  until  all  free 
chlorine  has  disappeared. 


354  QUANTITATIVE    CHEMICAL   ANALYSIS. 

E.  A  nearly  obsolete  method  is  that  of  Fuchs,  who  digests  a  weighed  sheet 
of  copper  in  the  ferric  chloride  solution  for  several  days  with  exclusion  of  air. 
The  copper  loses  in  weight  according  to  the  reaction  Fe2CJe  +  Cua  =  2FeCl2  + 
Ci^CIs,  63.6  parts  in  weight  lost  by  the  copper  corresponding  to  56  parts  of  iron. 

Ferrous  oxide.  The  protoxide  of  iron  may  be  in  an  ore  in  the  form  of 
magnetite  or  allied  minerals,  or  as  siderite,  or  the  gangue  may  contain 
pyrite,  a  ferrous  silicate,  etc.  For  a  determination  the  ore  is  dissolved  in 
hydrochloric  acid  in  a  flask  through  which  flows  a  current  of  carbonic 
acid  gas;  after  cooling,  the  solution  is  immediately  titrated  by  weak 
standard  bichromate.  The  insoluble  matter  is  filtered  off  and  dissolved 
in  hot  hydrofluoric  and  hydrochloric  acids,  also  with  exclusion  of  air, 
most  conveniently  accomplished  by  placing  the  residue  and  acids  in  a  large 
platinum  crucible  and  heating  on  a  water-bath,  the  crucible  covered  by  a  large 
funnel  into  which  is  passed  a  current  of  carbon  dioxide.  The  solution  is 
titrated  by  bichromate.  After  titration,  any  insoluble  residue  should  be  in- 
spected for  coarse  particles  of  pyrite ;  if  any  are  found  the  residue  is  decom- 
posed by  nitric  acid  and  the  iron  determined. 

The  results  for  ferrous  oxide  in  the  hydrochloric  solution  of  the  ore  are  in 
many  cases  somewhat  too  low  from  the  fact  that  most  ores,  especially  the 
hematites  and  limonites,  contain  small  amounts  of  the  higher  oxides  of  man- 
ganese, such  as  isomorphous  manganese  sesquioxide.  The  author  has 
obtained  fair  results  by  washing  the  ore  before  dissolving  it,  with  a  hot,  very 
dilute  solution  of  oxalic  acid,  and  has  detected  magnetite  in  manganese  ores  by 
this  scheme.  When  an  ore  contains  certain  organic  matters,  a  part  of  the  fer- 
ric chloride  is  reduced  to  ferrous  and  the  results  are  correspondingly  high ; 
here  it  is  well  to  endeavor  to  wash  out  the  organic  matter  by  some  organic 
solvent,  or  to  remove  it  by  elutriation  with  water  or  a  heavier  neutral  liquid. 

Silica  and  bases.  The  proximate  constituents  of  an  ordinary  iron  ore  may 
be  divided  as  regards  solubility  into  two  classes. 

A.  Soluble    in    hydrochloric  acid.    Ferrous  and  ferric  oxides  and  ferrous 
carbonate,    manganese  oxides,  calcium  and  magnesium  carbonates,    calcium 
sulfate,  calcium  phosphate,  and  the  silica  and  bases  of  certain  silicates. 

B.  Insoluble   in    hydrochloric    acid.     Crystallized   and    amorphous  silica, 
crystallized  titanic  acid,  and  many  silicates  that  may  contain  as  bases  iron, 
manganese,  aluminum,  calcium,  and  magnesium  oxides  and  the  alkalies. 

No  sharp  line  can  be  drawn  as  to  the  deportment  of  certain  compounds 
such  as  pyrite,  aluminum  phosphates,  titanates,  organic  matter  and  the  like, 
as  according  to  the  fineness  of  the  powder,  strength  of  acid,  time  of  digestion, 
and  other  considerations,  they  may  pass  entirely  or  partly  into  solution,  or 
remain  insoluble. 

Hence  in  the  analysis  of  an  ore  the  only  safe  course  is  to  first  treat  the 
powder  with  hydrochloric  acid,  and  after  filtering,  decompose  the  residue  by 
fusion  with  sodium  carbonate.  The  melt  is  disintegrated  by  water,  dissolved 
in  hydrochloric  acid,  and  united  with  the  original  solution.  After  removing 
silica,  the  bases  are  separated  seniatim  in  the  following  order:  iron  and 
aluminum,  manganese,  calcium,  magnesium.  Determinations  of  the  alkalies, 
titanic  acid,  sulfur  and  sulfuric  acid,  combined  water,  organic  matter,  etc., 
are  made  on  separate  portions  of  the  ore. 

Silica  is  determined  by  evaporating  to  dryness  the  solution  obtained  as  above ; 
on  taking  up  by  dilute  hydrochloric  acid,  usually  only  silica  remains,  but  in 
some  ores  it  may  be  contaminated  by  titanic  acid  or  (rarely)  tungstic  acid.  In 
this  case  the  residue  may  be  evaporated  with  hydrofluoric  and  sulfuric  acids, 
ignited,  and  the  weight  of  the  residue  deducted  from  the  previous  weight. 

A  lean  (highly  silicious)  ore  within  certain  limits  of  composition  is  decom- 


IRON   AND    MANGANESE   ORES.  355 

posed  on  heating  to  bright  redness,  with  the  formation  of  silicates  easily  de- 
composed by  hydrochloric  acid,  the  silica  gelatinizing;  on  evaporation  and  reso- 
lution the  silica  is  left  yi  a  nearly  pure  condition. 

In  presence  of  much  ferric  or  aluminic  chloride  silica  can  hardly  be  made 
entirely  insoluble  by  one  evaporation  and  heating  of  the  residue,  and  the  silica 
remaining  on  resolution  in  acid  is  apt  to  be  very  impure. 

Alumina.  From  the  similarity  of  their  reactions,  ferric  and  aluminic  com- 
pounds generally  accompany  each  other  in  the  process  of  separating  other 
bases.  The  separation  of  aluminum  from  iron  is  here  the  more  difficult  from 
the  relatively  large  amount  of  the  latter.  The  simple  means  of  precipitating 
the  iron  by  potassium  or  sodium  hydrate  leaving  the  alumina  in  solution,  is  im- 
perfect, even  when  the  iron  has  been  previously  reduced  to  the  ferrous  state, 
and  the  precipitate  becomes  granular  tetroxide ;  when  the  two  are  precipitated 
by  ammonia  as  mixed  hydrates,  ignited  to  oxides,  fused  with  sodium  carbonate 
and  the  melt  lixiviated  by  water,  much  alumina  is  retained  in  the  ferric  oxide ; 
and  the  indirect  method  of  weighing  the  mixed  sesquioxides,  redissolving 
and  determining  the  iron  volumetrically  and  the  alumina  by  difference  is  never 
satisfactory.  The  following  methods  give  fair  to  good  separations. 

1.  Sodium  thiosulfate  added  to  a  boiling  slightly  acid  solution  containing 
much  phosphoric  acid,  precipitates  aluminum  phosphate  mixed  with  sulfur, 
while  iron  is  only  reduced  to  a  ferrous  salt.    Carnot  believes  that  the  reaction 
is  favored  when  the  only  free  acid  in  the  solution  is  acetic.    The  precipitated 
aluminic  phosphate    is  dissolved  in  hydrochloric  acid,  filtered  from  sulfur, 
reprecipitated,  and  finally  weighed  as  the  phosphate  Al2OsP2O5.    The  reaction 
with  thiosulfate  is  A12(SO4)3  -+-  3Na2S2O3  =  A]2O3  -f  3S  -f  3SO2  -f  3Na2SO4. 

2.  Iron  in  not  too  large  an  amount  may  be  precipitated  electrolytically  on  a 
cathode    of  platinum,*  or  in    any  amount  on  one  of  mercury,  forming  an 
amalgam. f 

3.  If  the  concentrated  solution  of  the  chlorides  be  saturated  with  gaseous 
hydrochloric  acid  and  mixed  with  an  equal  volume  of  ether,  the  aluminum  is 
precipitated  as  granular  Al2Cle.l2H2O4 

4.  Borntrager  §  takes  advantage  of  the  solubility  of  ferric  oleate  in  petroleum. 
After  neutralizing  the  solution  of  ferric  and  aluminic  chlorides  with  potassium 
hydrate,  the  oleates  are  precipitated  by  neutral  potassium  oleate,  filtered  and 
dried.    The  ferric  oleate  is  then  lixiviated  by  hot  kerosene  and  the  residual 
aluminum  oleate  calcined  to  alumina  and  weighed. 

5.  Other  methods  have  been  proposed,  based  on  the  precipitation  of  the  iron 
by  an  excess  of  trimethylamin ;  the  relative  insolubility  in  water  of  basic  ferric 
nitrate  as  compared  to  that  of  basic  aluminic  nitrate;  precipitation  of  the   iron 
by  nitroso-beta-napthol ;  etc. 

Manganese.  After  the  separation  of  the  ferric  and  aluminum  oxides  from  the 
other  bases  by  ammonia  or  an  acetate,  the  manganese  in  the  filtrate  is  thrown 
down  by  ammonium  acetate  and  bromine  followed  by  ammonia ;  on  boiling,  all  the 
manganese  separates  as  flocculent  hydrated  binoxide.  The  lime  and  magnesia 
usually  found  in  iron  and  manganese  ores  complicate  matters,  since  the  pre- 
cipitate is  prone  to  carry  down  these  bases  in  considerable  amount.  ||  Probably 
the  best  plan  for  their  removal  from  the  precipitate  is  to  dissolve  it  in  hydro- 
chloric acid,  neutralize  the  solution  by  ammonia,  and  precipitate  the  manganese 


*  Journ.  Anal.  Chem.  3—91  and  4-488. 
t  Idem,  5-627 . 

I  Am.  Journ.  Sci.  52—416. 
§  Zeits.  anal.  32—187. 

II  Chem.  News,  1889—2—262. 


356  QUANTITATIVE    CHEMICAL    ANALYSIS. 

as  sulflde;  a  less  tedious  process  is  that  of  neutralizing  the  hydrochloric  solu- 
tion of  the  precipitate  by  ammonia,  adding  a  sufficiency  of  ammonium  acetate, 
and  again  precipitating  the  manganese  by  bromine  and  Ammonia. 

If  small  in  quantity  the  manganese  binoxide  may  be  ignited  until  it  has  passed 
to  the  tetroxide  (MnsC^)  and  weighed;  but  if  considerable  in  amount  it  is  better 
to  redissolve  and  determine  as  pyrophosphate. 

Where  the  manganese  of  an  ore  is  not  too  small  in  amount  the  state  of  oxida- 
tion can  be  found  by  one  of  the  methods  of  chlorimetry  that  recognizes  the 
presence  of  ferrous  oxide. 

Lime  and  magnesia  are  determined  in  the  filtrate  from  the  manganese  binoxide 
by  the  usual  methods  for  these  bases,  precipitating  the  calcium  as  oxalate,  and 
in  the  filtrate  the  magnesium  as  ammonium  magnesium  phosphate.  Where 
many  analyses  are  in  hand  the  oxalate  of  calcium  may  be  dissolved  in  hydro- 
chloric acid  and  the  oxalic  radical  titrated  by  standard  permanganate;  and  the 
ammonium  magnesium  phosphate  dissolved  in  standard  acid  and  the  excess 
titrated  back  by  standard  alkali. 

Phosphorus  is  determined  by  dissolving  the  ore  in  hydrochloric  acid  and  pro- 
ceeding substantially  as  in  the  determination  of  this  element  in  steel,  but  as  all 
the  phosphorus  of  an  ore  is  already  combined  with  oxygen  as  phosphoric  acid, 
the  precaution  of  evaporating  the  solution  and  heating  the  residue  is  here  un- 
necessary. Usually  combined  with  calcium  (as  apatite),  there  are  often 
found  other  combinations  that  are  insoluble  in  acids,  so  that  the  residue 
from  the  solution  of  the  ore  in  hydrochloric  acid  should  be  examined 
for  phosphorus,  though  this  precaution  may  be  omitted  for  ores  from  certain 
localities  that  are  known  to  contain  all  the  phosphoric  acid  in  soluble  combina- 
tions. Simply  igniting  the  insoluble  residue  of  some  ores  or  grinding  it  to  a 
fine  powder  will  so  attenuate  the  particles  or  alter  the  combination  of  the 
phosphoric  acid  by  inter-reactions,  that  all  the  acid  may  be  dissolved  out  by 
heating  the  residue  with  strong  hydrochloric  acid.  In  ores  where  all  the  phos- 
phoric acid  is  as  apatite,  nearly  or  quite  all  may  be  extracted  by  simply  boiling 
the  ore  with  dilute  nitric  acid. 

Sulfur  may  be  in  the  gangue  as  iron  disulfide,  sulfate  of  calcium,  etc.,  and 
for  its  determination  the  ore  is  dissolved  in  hydrochloric  acid  containing  nitric 
acid  or  other  oxidizer.  Calcium  sulfate  is  soluble  in  dilute  hydrochloric  acid, 
but  if  barium  sulfate  is  suspected,  the  insoluble  residue  should  be  fused  with 
sodium  carbonate,  the  alkali  sulfate  lixiviated  by  water,  the  filtered  solution 
acidified,  and  the  silica  separated  by  evaporation  The  sulfuric  acid  in  the 
united  filtrates  is  precipitated  by  barium  chloride  and  weighed  as  barium  sul- 
fate in  the  usual  way. 

The  sulfur  existing  in  the  gangue  as  pyrite  or  arsenopyrite  and  that  as  calcium 
or  barium  sulfate  may  be  approximately  distinguished  as  follows :  The  finely 
powdered  ore  is  digested  with  a  strong  solution  of  ammonium  carbonate  and 
filtered;  in  the  filtrate  is  ammonium  sulfate  due  to  the  reaction  between  the 
ammonium  carbonate  and  calcium  sulfate.  From  the  residue  the  cal- 
cium carbonate  and  any  barium  carbonate  are  dissolved  out  by  cold 
dilute  hydrochloric  acid.  Now  containing  pyrite  and  barium  sulfate,  the 
residue  is  fluxed  by  sodium  carbonate  and  nitrate,  lixiviated  by  water,  and  the 
sulfuric  acid  in  the  filtrate  and  the  barium  in  the  residue  determined.  A  simple 
calculation  will  distribute  the  sulfur  among  these  compounds. 

Arsenic  is  sometimes  found  in  ores,  and  precipitates  more  or  less  completely 
along  with  phosphoric  acid  when  the  latter  is  thrown  down  by  molybdic 
acid;  magnesic  solution  also  precipitates  arsenic  acid  as  ammonium  magnesium 
arsenate.  Arsenic  may  be  removed  from  the  hydrochloric  acid  solution  of 


IRON  AND  MANGANESE  ORES.  357 

an  ore  by  evaporating  two  or  three    times    with  oxalic  and    hydrochloric 
acids,  it  volatilizing  as  arsenic  chloride. 

In  Pattinson's  *  method  the  hydrochloric  solution  of  the  ore  is  treated  with 
sodium  thiosulfate  to  reduce  ferric  chloride,  the  sulfurous  acid  boiled  off, 
and  the  cooled  solution  strongly  acidified  by  hydrochloric  acid.  Powdered 
zinc  sulflde  is  stirred  in  to  precipitate  the  arsenic  as  sulfide,  which  is  then 
filtered  off  and  determined  by  direct  weight  or  otherwise.  The  filtrate  is 
mixed  with  a  little  ferric  chloride  and  an  excess  of  calcium  carbonate;  ferric 
hydrate  and  phosphate  are  precipitated,  and  after  filtering  may  be  dissolved 
and  the  phosphorus  determined  as  in  the  acetate  process. 

Another  method  is  based  on  the  volatility  of  arsenic  chloride.  The  ore  is 
brushed  into  a  flask,  moistened  with  nitric  acid  and  heated  to  decompose 
arsenopyrite,  and  distilled  with  concentrated  hydrochloric  acid  and  ferrous 
sulfate.  The  distillate  is  received  in  water  and  the  arsenic  precipitated  by 
hydrogen  sulfide,  the  arsenic  sulfide  washed  with  carbon  disulflde  to  extract 
free  sulfur,  dried  and  weighed. 

Titanic  acid.  The  minerals  menaccanite,  rutile,  sphene,  and  anatase  are  fre- 
quent associates  of  magnetic  ores,  but  are  seldom  met  with  in  other  varieties. 
From  the  negative  character  of  titanium  compounds  the  determination  is  not 
an  easy  matter.  The  native  oxides  are  insoluble  in  hydrochloric  or  nitric  acid, 
but  dissolve  in  melted  potassium  hydrofluoride,  and  in  hot  concentrated  sul- 
furic  acid  best  applied  in  the  form  of  melted  sodium  or  potassium  pyrosulfate. 

The  methods  for  separation  from  bases  apply  the  insolubility  of  sodium 
titanate  in  water,  the  precipitation  of  titanic  acid  from  its  solution  by  ammonia, 
or  the  precipitation  of  beta-titanic  acid  on  boiling  a  dilute  solution  containing- 
but  a  very  small  proportion  of  free  acid,  especially  a  mineral  acid.  The  most 
common  method  is  to  get  all  the  bases  of  the  ore  as  sulfates,  either  by  direct 
fusion  with  sodium  pyrosulfate  or  otherwise,  and  the  titanium  as  sodium  tita- 
nate. After  dissolving  in  a  large  volume  of  cold  water  and  filtering,  the  ferric 
sulfate  is  reduced  to  ferrous  sulfate  (to  prevent  the  decomposition  of  the  former 
compound  and  precipitation  of  ferric  hydrate  on  boiling) ;  sulfurous  acid  is  a 
convenient  reagent  for  the  reduction.  The  solution  is  nearly  neutralized,  then 
boiled  for  some  time.  The  precipitate  of  titanic  acid,  containing  some  iron 
oxide  and  a  (usually  inconsiderable)  amount  of  phosphoric  acid,  is  fused  with  a 
pyrosulfate  and  the  above  treatment  repeated ;  now  nearly  pure,  it  is  ignited 
and  weighed  as  titanium  dioxide. 

In  Classen's  method,  the  hydrochloric  solution  is  mixed  with  hydrogen  per- 
oxide and  the  iron  precipitated  by  potassium  hydrate ;  the  titanium  remains  in 
solution  as  the  trioxide  (TiO3).  An  aliquot  part  is  filtered  off  and  boiled  until 
the  hydrogen  peroxide  is  decomposed,  part  of  the  easily  reducible  titanic 
trioxide  separating  at  the  same  time.  After  acidifying  and  heating  until  the 
trioxide  has  been  reduced  to  the  binoxide,  the  latter  is  precipitated  by  a  very 
slight  excess  of  ammonia.  After  weighing,  the  precipitate  should  be  examined 
for  alumina  and  silica. 

A  volumetric  method  is  founded  on  the  reduction  of  titanium  dioxide  to 
titanium  sesquioxide,  which  may  then  be  reoxidized  by  standard  potassium  per- 
manganate. The  reducing  agent  is  metallic  zinc,  and  the  process  is  the  same 
as  for  the  determination  of  iron,  except  that  the  titrate  must  be  carefully 
shielded  from  the  air  on  account  of  the  far  greater  susceptibility  of  titanium 
sesquioxide  to  oxidation.  The  volume  of  the  titrand  used  in  the  titration  is 
that  required  for  the  oxidation  of  both  the  titanium  sulfate  and  the  ferrous  sul- 
fate that  may  be  present,  and  to  determine  the  latter,  the  titrate  is  treated  with. 


*  Journ.  Socy.  Chem.  Ind.  12—119. 


358  QUANTITATIVE    CHEMICAL    ANALYSIS. 

hydrogen  sulfide,  this  reagent  reducing  only  the  ferric  sulfate.  A  second  titra- 
tion  is  made,  and  the  difference  is  the  volume  of  permanganate  oxidizing  the 
titanium  sesquisulf ate  — 

5Ti2(SO4)3  -f  K2Mn208  +  9H2SO4  =  10Ti(SO4)2  +  2KHSO4  +  2MnSO4  +  8H20. 

A  colorimetric  method  suitable  for  the  small  proportions  found  in  market- 
able ores,  applies  the  intense  yellow  color  produced  when  (fluorine -free) 
hydrogen  peroxide  solution  is  added  to  a  solution  of  titanium  sulfate.  The 
Standard  is  a  sulfuric  solution  of  pure  titanic  acid. 

The  determination  of  phosphorus  is  interfered  with  by  titanic  acid ;  if  the 
hydrochloric  solution  be  evaporated  to  dryness  a  compound  of  ferric  oxide, 
titanic  acid  and  phosphoric  acid  remains  insoluble  with  the  silica;  in  precip- 
itating titanic  acid  by  boiling  the  dilute  solution,  part  or  all  of  the  phosphoric 
acid  is  also  carried  down ;  and  it  is  said  that  the  presence  of  titanic  acid  in  a 
nitric  solution  prevents  the  complete  precipitation  of  phosphoric  acid  by 
molybdic  solution. 

After  fusion  of  a  mixture  of  the  two  acids  with  sodium  carbonate  and  lixivia- 
tion  of  the  fusion  by  hot  water,  sodium  phosphate  passes  into  solution,  while 
all  the  titanium  remains  in  the  residue  as  acid  sodium  titanate.  In  the  method 
of  Jennings,  the  ore  is  treated  with  hot  concentrated  hydrochloric  acid,  and, 
without  filtering,  the  liquid  neutralized  and  the  ferric  chloride  nearly  reduced 
to  ferrous  chloride  by  sulfurous  acid;  then  acetic  acid  is  added  and  the  liquid 
boiled.  The  residue  and  precipitate,  containing  all  the  phosphoric  acid  as 
ferric  phosphate,  is  fused  with  sodium  carbonate  and  the  melt  lixiviated  by 
hot  water  —  sodium  silicate,  aluminate,  and  phosphate  dissolve.  The  phos- 
phorus is  then  determined  in  the  nitrate  by  one  of  the  usual  methods.  A 
simpler  plan  is  to  flux  the  ore  at  once  with  sodium  carbonate,  plus  a  little  nitre 
to  oxidize  ferrous  compounds  and  organic  matter,  then  lixiviate  with  water. 

Volatile  at  a  red  heat.  On  ignition  to  bright  redness  an  ore  loses  combined 
water,  carbon  dioxide  and  organic  matter,  and  about  one -half  of  the  sulfur 
of  pyrite.  Unless  it  is  deemed  advisable  to  examine  further,  the  result  is 
put  down  as  "  volatile  at  redness  "  or  shortly  "volatile  matter."  Reduction 
of  ferric  to  ferrous  oxide  by  organic  matter,  and  loss  of  oxygen  by  a  higher 
oxide  of  manganese  may  increase  the  result,  and  oxidation  of  magnetite  or 
siderite  may  decrease  It. 

The  separate  determination  of  combined  water,  organic  matter  and  carbon 
dioxide  can  be  done  as  follows.  A  portion  of  the  ore  is  decomposed  by  hot 
dilute  sulfuric  acid  in  a  closed  flask  and  the  carbon  dioxide  led  first  through 
a  condenser  to  remove  most  of  the  aqueous  vapor,  then  through  a  calcium 
chloride  drying  tube,  and  finally  into  a  weighed  potash  bulb  to  absorb  the 
carbon  dioxide,  determined  by  the  increase  -in  weight.  The  water  of  com- 
position of  the  mineral  and  gangue  is  determined  by  igniting  the  ore  (pre- 
viously dried  at  100°),  in  a  current  of  dry  air,  catching  the  moisture  in  a 
weighed  calcium  chloride  tube.  The  carbon  of  organic  matter  by  igniting  the 
ore  in  a  current  of  oxygen  and  absorbing  the  carbon  dioxide  in  a  weighed  pot- 
ash bulb  following  a  drying  tube;  from  the  increase  in  weight  of  the  bulb  is 
deducted  the  weight  of  the  carbon  dioxide  found  by  treatment  with  an  acid. 
An  approximation  to  the  amount  of  organic  matter  may  be  had  by  assuming  it 
to  be  bituminous  in  character  and  to  contain  about  75  per  cent  of  carbon. 

Hygroscopic  water  is  found  from  the  loss  sustained  in  drying  the  ore  at  100  o . 
Since  the  percentage  of  moisture  is  usually  decreased,  sometimes  increased, 
by  pulverizing  and  exposure  to  the  air,  the  determination  is  the  more  reliable 
the  less  manipulation  the  sample  has  undergone  previous  to  the  drying.  For 
large  shipments  of  ore  a  weight  of  from  100  to  2,000  Ibs.  is  a  suitable  quantity. 


COAL.  359 


COAL. 

The  coals  are  amorphous  minerals,  black  or  brown  in  color,  of  a  specific 
gravity  ranging  from  1.2  to  1.8,  and  hardness  from  .5  to  2.5.  According  to  Bal- 
zer,  coals  are  mixtures  of  complex  carbon  compounds  of  a  genetic  and  pos- 
sibly homologous  series.  Carbon  is  the  predominating  constituent,  while  all 
varieties  contain  more  or  less  hydrogen,  oxygen,  and  small  amounts  of  com- 
bined or  occluded  nitrogen,  and  hygroscopic  moisture.  As  associates  are  com- 
monly found  slates,  calcite,  fire-clay,  pyrite  or  marcasite,  gypsum,  etc. 

The  different  varieties  may  be  enumerated  as  follows,  though  no  sharp  lines 
divide  them,  one  variety  merging  into  another. 

1.  Brown  coal,  lignite,  and  peat  coal  are  incompletely  carbonized  woody  fiber 
and  therefore  contain  an  unusually  large  proportion  of  oxygen  ("up  to  30  per 
cent)  and  moisture.    Their  general  appearance  and  lightness  are  characteristic, 
and  also  their  comparative  susceptibility  to  the  action  of  chemicals. 

2.  Cannel  (candle)  coal  by  its  close  grain  and  dull  fracture  suggests  an  in- 
durated wax.    The  proportion  of  moisture  is  low,  as  is  that  of  fixed  carbon, 
while  the  volatile  hydrocarbons  may  be  as  high  as  70  per  cent.    The  sulfur  and 
ash  are  usually  very  moderate. 

3.  Bituminous  coal  is  black,  with  a  fracture  that  is  flat,  somewhat  resinous  in 
luster,  and  often  iridescent.    The  layers  are  frequently  interstratified  by  films 
of  pyrite  or  calcite.    In  composition  the  moisture  ranges  from  1  to  5  per  cent, 
volatile  hydrocarbons  from  30   to  45,  fixed  carbon  from  40  to  60,  and  ash  from 
2  to  20.    The  sulfur  is  seldom  below  1  per  cent  and  may  reach  5  or  more. 

A  coking  or  caking  coal  is  one  that  on  slow  destructive  distillation  leaves  a 
hard,  dense,  lustrous,  strongly  coherent  cellular  residue,  coke. 

4.  Semi-bituminous  coal  is  intermediate  between  bituminous  and  anthracite 
and  therefore  contains  more  fixed  carbon  than  the  former  and  less  volatile 
matter  and  moisture. 

5.  Anthracite  is  distinguished  by  its  superior  hardness  and  conchoidal  or 
sub-conchoidal  fracture,  a  high  resistance  to  chemicals,  and  slow  smokeless 
combustion.    The  purest  varieties,  including  the  so-called  graphitic  anthra- 
cite, are  little  else  than  carbon  with  a  few  per  cents  of  earthy  impurities. 

Alcohol,  ether,  chloroform,  phenol,  and  solutions  of  potassium  permanganate 
and  oxidizers  generally,  dissolve  out  small  variable  proportions  of  organic  con- 
stituents. Treated  with  dilute  hydrochloric  acid  and  potassium  chlorate  a 
coal  becomes  brown  and  increases  in  weight  through  oxidation.  A  mixture  of 
concentrated  sulf  uric  and  nitric  acids  dissolves  on  long  digestion  a  considera- 
ble portion  from  bituminous  coal,  which  is  precipitated  on  dilution.  The  resi- 
due of  "  nitro-coal "  contains  more  volatile  matter  than  the  original  and  deflag- 
rates slightly  on  heating ;  it  is  largely  soluble  in  solution  of  sodium  hydrate. 

The  object  of  an  analysis  of  a  given  coal  may  be  for  information  in  respect 
to  (1),  geological  or  mineralogical  investigations ;  (2),  heat  of  combustion,  for 
furnace  purposes  or  steaming;  (3),  the  volume  and  composition  of  combus- 
tible gases  yielded  on  destructive  distillation,  either  without  access  of  air  or 
with  a  limited  supply  of  air  and  steam;  or  (4)  for  special  technical  purposes. 
The  character  of  the  analysis  and  the  methods  chosen  must  therefore  be 
adapted  to  the  end  in  view. 


360  QUANTITATIVE   CHEMICAL   ANALYSIS. 

PROXIMATE   ANALYSIS. 

Here  the  moisture  is  expelled  by  drying  the  coal,  and  the  volatile  hydrocar- 
bons by  ignition  at  a  red  heat;  the  remaining  carbon  is  burned  in  air,  leaving 
the  ash.  The  sulfur  of  the  coal  is  usually  also  determined,  and  occasionally 
the  phosphorus,  nitrogen,  etc.,  these  in  separate  portions  of  the  coal. 

1.  Hygroscopic  moisture  of  the  coal  and  gangue.    The  usual  method   is  to 
heat  one  gram  of  the  powder  in  a  crucible  or  small  dish  to  100°  for  one  hour 
and  call  the  loss  in  weight  the  moisture.    Some  writers  direct  a  temperature 
of  103  o  and  limit  the  time  of  heating  to  fifteen  minutes;  the  Committee  of  the 
American  Chemical  Society  on  Coal  Analysis  recommends  a  temperature  of 
from  104  °  to  107  o  for  one  hour,  the  coal  to  be  contained  in  an  open  porcelain 
or  platinum  crucible,  and  the  heat  furnished  by  a  toluene -bath.    Some  coals 
begin  to  increase  in  weight  if  the  drying  be  prolonged  beyond  an  hour,  prob- 
ably from  oxidation  of  pyrite ;  it  is  said  that  at  100  °  some  of  the  most  volatile 
hydrocarbons  may  escape,  though  this  statement  lacks  confirmation. 

The  percentage  of  moisture  as  determined  by  this  process  is  a  function  not 
only  of  the  hygroscopic  capacity  of  the  coal  but  also  of  the  fineness  of  the  pow- 
der and  the  condition  of  the  atmosphere  at  the  times  of  weighing,  and  cannot 
safely  be  assumed  to  represent  the  average  water -content  of  the  coal  in  the 
massive  state.  Hence  for  a  comparison  of  several  coals  of  the  same  variety, 
some  would  eliminate  the  moisture  entirely  from  the  analyses  and  report  the 
percentages  of  the  other  constituents  referred  to  the  coals  dried  at  100  °  .  A 
more  rational  determination  of  moisture  is  had  by  drying  a  weight  of  say  a 
ton  of  coal,  observing  the  usual  precautions  against  loss  or  gain  of  moisture 
during  the  breaking  of  the  lumps  to  small  fragments.  The  results  of  the  ordi- 
nary analysis  are  calculated  to  the  basis  of  this  determination  by  means  of  the 

i  no  T-     T^"' 
formula  M  '=  M where  TFis  the  percentage  of  moisture  in  the  finely 

powdered  coal,  and  W  that  in  the  large  sample ;  M,  the  percentage  of   any 
constituent  in  the  analysis,  and  M'  the  percentage  in  the  corrected  analysis. 

2.  Matter  volatile  at  a  red  heat.    About  one  gram  of  the  undried  coal  is 
placed  in  a  small  platinum  crucible  provided  with  a  well  fitting  cover.    The 
covered  crucible  is  heated  over  a  Bunsen  burner  for  exactly  three  and  one-half 
minutes,  then,  without  cooling,  over  a  blast-lamp  for  the  same  time.    The  tem- 
perature and  time  of  heating  have  considerable  influence  on  the  result,  hence 
the  specific  directions. 

The  loss  in  weight  of  the  coal  is  conventionally  considered  as  due  to  the  ex- 
pulsion of  the  volatile  hydrocarbons  and  carbohydrates,  plus  the  moisture  as 
determined  in  (1)  which  is  to  be  deducted.  But  in  addition  the  weight  is 
diminished  to  some  extent  by  the  expulsion  of  (1),  a  portion,  approximating 
one -half,  of  the  sulfur  of  the  pyrites  or  marcasite;  (2),  any  free  sulfur;  (3),  the 
combined  water  of  gypsum  and  hydrous  silicates;  (4),  the  carbon  dioxide  of 
calcite ;  (5)  the  escape  of  particles  of  coal  carried  out  with  the  gases,  consid- 
erable in  the  case  of  some  *  dry '  coals;  (6),  the  combustion  of  a  small  amount 
of  the  fixed  carbon  by  air  entering  the  crucible,  or  possibly  from  a  reaction 
with  gypsum.  On  the  other  hand,  the  expulsion  of  the  hydrocarbons  is  prob- 
ably never  complete  on  account  of  the  dissociation  of  a  part  at  a  bright  red 
heat  with  separation  of  carbon. 

3.  Fixed  carbon.    There  remains  from  the  above  ignition  a  mixture  of  car- 
bon, ferrous  sulflde,  and  the  dehydrated  gangue.    The  form  may  be  that  of  a 
loose  powder  indicating  a  non -coking  coal,  or  as  a  swelled  hard  spongy  mass 
indicating  a  coking  coal;  if  between  the  two,  no  positive  conclusion    can  be 
drawn  as  to  which  class  the  coal  belongs. 


COAL.  361 

The  carbon  is  burned  in  the  open  crucible  in  which  a  small  stream  of  oxygen 
may  be  introduced  to  hasten  combustion.  The  loss  in  weight  is  the  fixed  car- 
bon, the  result  increased  by  the  loss  of  the  remainder  of  the  sulfur  of  pyrite 
and  possibly  some  of  the  oxygen  of  the  gypsum,  and  decreased  by  the  con- 
version of  the  iron  of  the  pyrite  to  the  sesquioxide. 

4.  The  ash  of  the  coal  remains  as  a  soft  or  gritty  powder,  tinted  yellow  or 
brown  in  proportion  to  the  content  of  free  ferric  or  manganic  oxide.    The 
composition  varies  greatly,  but  is  mainly  silica  and  alumina,  with  oxides  of 
iron,  calcium,  magnesium  and  manganese,  and  calcium  sulfate.    For  metal- 
lurgical purposes  an  analysis  of  the  ash  may  be  made  according  to  the  usual 
process  for  silicates  insoluble  in  acids,  obtaining  sufficient  material  by  cal- 
cining several  grams  of  the  coal. 

5.  Sulfur.  For   the  determination  of  this  most  important  impurity  several 
methods  have  been  proposed.    Those  most  in  use  are  the  fusion  or  fluxing 
method  and  that  of  Eschka. 

A.  In  the  former  one  gram  of  the  powdered  coal  is  intimately  mixed  with  ten 
grams  of  sodium  carbonate  and  five  grams  of  potassium  nitrate ;  the  mixture  is 
heated  to  fusion  in  a  capacious  platinum  crucible,  when  by  the  oxidizing  action 
of  the  nitrate  the  carbon  burns  to  carbon  dioxide  and  the  sulfur  to  sulfuric 
oxide.    At  the  same  time  the  sodium  carbonate  fluxes  the  silicates  and  con- 
verts the  gypsum  to  calcium  carbonate;  the  iron  is  oxidized  to  ferric  oxide. 
When  cold,  the  mass  is  treated  with  hot  water  and  filtered.    The  filtrate  con- 
tains all  the  sulfur  as  sodium  sulfate,  and  after  acidification,  is  precipitated  by 
barium  chloride.    Since  there  is  always  some  or  all  of  the  silicate  of  sodium 
in  the  lixiviation,  it  is  more  prudent  to  evaporate  the  acidified  solution  to  dry. 
ness  and  redissolve  and  filter  before  precipitation. 

In  another  method  one-half  gram  of  the  powdered  coal  in  a  small  platinum 
dish  is  mixed  with  three  grams  of  sodium  peroxide,  a  little  water  added,  and 
the  liquid  evaporated  to  dryness  and  ignited.  The  evaporation  and  ignition 
are  repeated  with  other  portions  of  the  peroxide,  when  all  the  oxidizable 
matters  should  have  been  oxidized.  The  residue  is  boiled  with  water,  filtered, 
acidified  and  precipitated  as  usual. 

The  greatest  objection  to  fusion  methods  is  that  the  solution  from  which  the 
barium  sulfate  is  precipitated  contains  large  amounts  of  sodium  chloride. 
For  this  reason  the  following  method  devised  by  Eschka  has  largely  displaced 
them. 

B.  One  gram  of  finely  powdered  coal  is  mixed  with  one-  half  gram  of  sodium 
carbonate  and  one  gram  of  powdered  magnesia  (levis)  and  gradually  heated  in 
a  platinum  crucible  or  dish  over  a  snlfurless  flame  with  frequent  stirring,  until 
the  color  shows  that  all  the  carbon  has  been  consumed.    The  crucible  is  cooled, 
a  gram  of  solid  ammonium  nitrate  stirred  in,  and  the  crucible  reheated  until 
the  latter  is  volatilized.    The  mass  remains  as  a  powder,  and  is  transferred  to  a 
beaker  and  boiled  with  water,  filtered,  and  the  sulfuric  acid  precipitated  from 
the  filtrate. 

The  rationale  of  the  process  is  that  the  carbon  is  burned  in  intimate  contact 
with  the  alkali  carbonate  which  immediately  fixes  the  sulfur  dioxide  or  trioxide 
liberated  during  the  combustion.  To  facilitate  the  burning  of  the  coal  there  is 
admixed  a  large  bulk  of  some  infusible  inert  powder,  here  magnesia.  The 
ammonium  nitrate  is  introduced  to  oxidize  any  traces  of  unburnt  carbon  and  to 
convert  any  sodium  sulflde  or  sulfite  to  sulfate.  On  lixiviation  there  passes 
into  solution  sodium  sulfate  and  the  excess  of  carbonate,  also  calcium  sulfate 
if  it  should  not  have  been  entirely  decomposed  by  the  heating  with  sodium 
carbonate  in  the  crucible  or  with  the  solution  during  the  lixiviation.  With 


362  QUANTITATIVE    CHEMICAL    ANALYSIS. 

coals  high  in  sulfur  the  residue  insoluble  in  water  may  retain  a  small  amount 
and  should  be  dissolved  in  dilute  hydrochloric  acid,  filtered,  and  the  filtrate 
tested  by  barium  chloride. 

The  reagents  should  be  examined  for  sulfur  compounds  by  igniting  a 
mixture  of  ten  grams  of  magnesia,  ten  grams  of  ammonium  nitrate,  and  five 
grams  of  sodium  carbonate,  boiling  with  water,  filtering,  acidifying,  and 
precipitating  by  barium  chloride;  if  any  barium  sulfate  falls  it  is  weighed  and 
one -tenth  the  weight  deducted  from  each  analysis. 

A  modification  of  the  above  directions,  somewhat  more  convenient,  dispenses 
with  the  ammonium  nitrate,  and  effects  the  oxidation  to  sulfate  by  the  addition 
of  bromine  water  to  the  lixiviation  —  one  cubic  centimeter  of  the  saturated 
solution  is  usually  sufficient.  It  will  be  noticed  that  on  mixing  the  reagent 
with  the  alkaline  solution  the  bromine  reacts  with  the  alkali,  and  the  solution 
remains  colorless,  but  on  acidification  the  bromine  is  regenerated  and  the 
solution  becomes  light  yellow.  To  dissolve  the  ignition  at  once  in  hydrochloric 
acid  containing  bromine,  as  has  been  proposed,  is  objectionable  for  the 
reason  that  not  only  the  magnesia  but  a  part  of  the  constituents  of  the  ash  as 
well  enter  the  solution  and  may  impurify  the  barium  sulfate. 

Hundeshagen  *  urges  the  substitution  of  potassium  carbonate  for  the  sodium 
carbonate  on  the  ground  that  its  reactive  power  is  superior  in  that  it  requires 
a  higher  heat  for  dehydration ;  the  assertion  is  not  confirmed  by  Handy.f 

Antony  and  Succhesi  propose  that  one  gram  of  coal  be  mixed  with  four 
grams  of  manganese  binoxide  and  two  grams  of  sodium  carbonate.  After 
ignition  for  a  half-hour,  the  mass  is  treated  with  dilute  nitric  acid,  boiled, 
filtered,  and  the  sulfuric  acid  precipitated  as  usual.  They  claim  a  more  rapid 
combustion  results  from  the  substitution  of  the  manganese  binoxide  for 
magnesia  (SMnOa  +  C  =  Mn3O4  +  CO2) . 

Atkinson's  method  is  similar  to  Eschka's,  differing  in  that  the  coal  is  mixed 
with  five  parts  of  dry  sodium  carbonate  and  heated  in  a  platinum  dish  for  an 
hour,  preferably  in  a  muffle.  The  sodium  carbonate  should  not  sinter  or  fuse. 
The  sodium  sulfate  formed  and  the  excess  of  sodium  carbonate  are  lixiviated 
by  water,  filtered,  acidified  and  precipitated  by  barium  chloride  as  usual. 
Neilson  obtained  good  results  by  burning  coal  in  contact  with  calcium  carbon- 
ate alone,  dissolving  in  hydrochloric  acid,  and  precipitating;  some  loss  was 
experienced  in  lixiviating  with  water  alone.  He  proposes  to  mix  the  coal  with 
sodium  carbonate  and  manganous  carbonate,  heat  and  finally  fuse,  the  mangan- 
ese passing  to  oxide  and  acting  as  a  carrier  of  oxygen ;  the  melt  is  dissolved 
in  acid,  evaporated  and  precipitated. 

C.  Fuming  nitric  acid,  nitro- hydrochloric  acid,  and  other  strong  oxidizers 
have  been  used  on  the  presumption  that  all  the  sulfur  can  be  extracted  by  them, 
but  it  appears  well  established  that  a  complete  oxidation  of  the  carbon  is  essen- 
tial to  a  complete  liberation  of  the  sulfur.  Many  coals  are  completely,  though 
slowly,  oxidized  by  potassium  chlorate  in  conjunction  with  nitric  acid,  but  the 
process  offers  no  particular  advantages  over  others  more  speedy  and  certain. 

Sulfur  may  exist  in  coal  in  at  least  five  states  of  combination:  (1)  as  pyrite 
or  marcasite,  sometimes  in  dark  streaks  hardly  to  be  distinguished  from  the 
coal  itself;  (2)  as  gypsum,  rarely  as  barite;  (3)  as  an  organic  compound;! 
(4)  as  basic  ferric  sulfate  ("  misy  ")  coming  from  decomposition  of  pyrite  by 
moist  air;  (5)  rarely  as  free  sulfur  (in  weathered  pyritic  breeze).  It  is  often 
of  practical  interest  to  know  the  combinations  in  which  the  sulfur  of  a  given 


*  Chem.  News,  1892-2-169. 

t  Joarn.  Socy.  Chem.  Ind.  6—360. 

J  Chem.  News,  1888-2—65. 


COAL.  363 

coal  exists  and  a  number  of  plans  for  their  differentiation  have  been 
brought  forward.  None  can  be  considered  satisfactory,  however. 

The  sulfur  combined  as  calcium  sulfate  or  barium  sulf ate  is  left  In  the  ash  on 
burning  the  coal  in  oxygen  at  a  heat  high  enough  to  decompose  any  sulfate  of 
iron  formed  from  pyrite.  If,  however,  the  coal  contains  calcite,  too  high  a 
percentage  of  calcium  sulfate  will  be  found,  due  to  a  reaction  with  pyrite.  Or 
the  calcium  sulfate  may  be  transformed  to  carbonate  by  digestion  with  sodium 
carbonate,  and  the  sulfuric  acid  passing  into  solution  determined,  yet  here 
some  of  the  finely  divided  pyrite  may  also  be  attacked.  On  the  other  hand  if  it 
is  sought  to  oxidize  and  extract  the  pyrite  by  a  mixture  of  bromine  and  solution 
of  sodium  hydrate,  some  of  the  calcium  sulfate  may  also  be  dissolved. 

6.  Phosphorus  is  to  be  looked  for  in  fuel  employed  for  the  smelting  of  Besse- 
mer iron  ores  —  it  is  seldom  higher  than  .1  per  cent.  Lynchenheim*  burns  a 
coal  or  coke  in  a  shallow  platinum  dish,  then  fuses  the  ash  with  sodium  car- 
bonate and  nitrate.  The  fusion  is  dissolved  in  hydrochloric  acid,  the  bases 
converted  to  nitrates  by  evaporation  with  nitric  acid,  and  after  filtering  from 
the  silica,  the  phosphoric  acid  is  precipitated  by  molybdic  solution  and  deter- 
mined as  in  an  iron  ore  (page  356).  Titanic  acid,  a  rare  constituent  of  the 
gangue,  may  be  brought  into  solution  by  fusing  the  ash  with  an  alkali  pyro- 
sulfate,  then  proceeding  for  its  determination  as  directed  for  an  iron  ore. 

As  might  be  expected  from  the  origin  and  nature  of  coal,  various  gases  are 
occluded.  Bedson,f  by  heating  in  vacuo  670  grams  of  coal  dust  to  100  ° , 
obtained  753  cubic  centimeters  of  a  mixture  of  gases,  mainly  nitrogen,  paraffins 
of  the  CnHgn  -j-  2  series,  and  carbon  dioxide. 

Calculation.  As  in  every  analysis  where  one  constituent  is  determined  by  dif- 
ference, the  total  percentage  of  the  proximate  analysis  of  a  coal  is  exactly  100. 
The  proper  location  of  the  sulfur  is  a  problem,  since  the  organic  combinations 
of  sulfur  presumably  pass  off  entirely  with  the  matter  volatile  at  a  red  heat,  the 
sulfur  of  the  pyrite  partly  in  the  volatile  matter  and  partly  in  burning  the  fixed 
carbon,  while  that  of  the  gypsum  remains  in  the  ash.  Since  it  is  not  customary 
to  carry  the  examination  so  far  as  to  attempt  to  distinguish  the  combinations 
of  sulfur,  various  plans  for  distributing  it  have  the  sanction  of  different  writers. 
Some  would  deduct  one-half  the  sulfur  from  the  volatile  matter  and  one-half 
from  the  fixed  carbon,  arguing  that  pyrite  is  the  predominating  combination  of 
sulfur  in  the  ordinary  run  of  coals.  Others  would  deduct  one-third  from  the 
volatile  matter,  fixed  carbon,  and  ash  respectively.  Still  others  would  make 
no  attempt  to  locate  the  sulfur,  allowing  each  of  the  results  on  the  other  con- 
stituents to  retain  whatever  proportion  may  be  included,  and  report  the  total 
.sulfur  separately.  There  are  manifest  objections  to  all  these  schemes. 

It  will  be  apparent  from  the  foregoing  that  a  proximate  analysis  follows  the 
lines  of  the  manufacture  of  coke,  yet  the  differences  in  temperature  and  time 
of  heating,  size  of  particles,  admission  of  air,  etc.,  are  so  great  as  to  advise 
caution  in  pronouncing  on  the  quality  of  a  sample  of  coal  for  this  purpose 
from  an  analysis  alone.  As  a  rule  the  yield  of  coke  from  the  beehive  oven 
is  considerably  greater  than  that  indicated  by  the  percentage  of  fixed  carbon 
and  ash  found  by  analysis. 

The  analysis  of  other  fuels  consisting  mainly  of  fixed  carbon,  such  as  char- 
coal, coke,  or  anthracite,  is  performed  in  practically  the  same  way  as  that  of 
a  coal.  Meade  and  Attix  I  note  that  of  the  matter  volatile  at  redness  when  a 


*  Trans.  Amer.  Inst.  Mining  Engrs.  19—66. 

t  Chem.  News,  1893—2—187. 

t  Journ.  Amer.  Chem.  Socy.  1899—113. 


364  QUANTITATIVE    CHEMICAL   ANALYSIS. 

coke  is  heated  in  the  conventional  analytical  scheme,  the  loss  in  weight  due 
to  combustion  of  carbon  by  air  in  and  entering  the  crucible  may  be  several 
times  the  weight  of  true  volatile  matter,  and  advise  that  the  heating  be  done 
in  a  current  of  nitrogen  or  hydrogen,  or  that  a  correction  be  applied,  found  by 
again  heating  the  crucible  after  the  determination  in  the  usual  way,  when  the 
carbon  burned  by  the  air  is  practically  the  same  as  during  the  previous 
ignition. 

For  the  valuation  of  a  coal  for  the  manufacture  of  illuminating  gas  a  test 
following  the  manufacturing  process  is  in  use  at  some  gas  works.  Three 
samples  of  2.24  Ibs.  each  (1-1000  of  a  gross  ton)  are  submitted  to  distillation. 
The  retort  is  of  iron,  of  the  form  of  a  muffle  five  inches  wide,  four  high,  and 
twenty  -six  long.  It  is  heated  to  bright  redness  in  a  furnace  and  the  sample 
introduced.  The  temperature  is  maintained  as  uniform  as  possible.  The  gases 
rise  in  a  wrought  iron  pipe  two  inches  in  diameter,  and  pass  over  into  a  con- 
denser, a  battery  of  twelve  vertical  pipes  1.6  inch  in  diameter  and  42  inches 
long,  which  serves  to  liquefy  the  tar  and  gas-liquor.  Thence  the  gas  passes 
through  a  scrubber  where  water  takes  up  ammonia  and  sulfurous  acid,  thence 
to  a  purifier  containing  lime  or  ferric  oxide  to  remove  sulfur  compounds, 
finally  to  a  gasometer.  The  quality  of  the  gas  is  tested  by  a  Bunsen's  photo- 
meter and  analysis,  the  tar,  gas-liquor,  and  ammoniacal  water  are  collected 
and  measured,  and  the  coke  weighed.  The  yield  of  gas  from  a  good  quality 
of  coal  is  about  twelve  cubic  feet. 

Ultimate  Analysis. 

The  ultimate  analysis  of  a  fuel  presents  no  great  differences  from  the  elemen- 
tary analysis  of  other  solid  sulfurous  organic  bodies  (page  298).  From  the 
difficulty  of  oxidizing  the  refractory  coke  or  residue  of  fixed  carbon,  the  com- 
bustion is  best  made  in  a  current  of  oxygen.  The  coal  is  contained  in  a  tared 
platinum  or  porcelain  boat  so  that  the  ash  can  be  weighed  and  further  examined 
if  desired.  The  sulfur  and  phosphorus  are  determined  as  described  before. 
The  oxygen  is  found  by  difference,  subtracting  from  100  per  cent  the  sum  of 
the  percentages  of  the  carbon,  hydrogen,  nitrogen,  ash,  and  (assuming  that  all 
the  sulfur  of  the  coal  is  combined  as  pyrite)  five-eighths  of  the  percentage  of 
sulfur. 

(Pyrite  on  ignition  in  oxygen  or  air  passes  to  iron  sesquioxide  which  remains 
With  the  ash.  Hence  from  100  there  should  be  deducted  the  percentage  of 
pyrite  less  the  resulting  iron  sesquioxide  which  has  already  been  deducted  as 
part  of  the  ash.  The  difference  between  the  pyrite  and  sesquioxide  is  five  - 
eighths  of  the  total  sulfur,  since  pyrite  (FeS2,  120.14)  contains  two  atoms  of 
sulfur  (64.14),  and  two  molecules  of  pryrite  (240.28)  yield  one  molecule  of 
ferric  oxide  (Fe2O3,  160).  Hence,  calling  the  percentage  of  sulfur  S,  we  have 


The  calorific  power  of  a  fuel  may  be  computed  from  the  ultimate  analysis 
according  to  the  formula  of  Dulong  — 

Calorific  power  =  8080  C  +  34460  (H  —  2.  )  _|_  2250  S. 

The  determination  by  means  of  the  calorimeter  is  deserving  of  more  atten- 
tion than  has  been  accorded  in  the  past,  especially  since  there  are  several 
calorimeters  on  the  market  that  in  careful  hands  are  capable  of  furnishing 
reliable  data.  A  practical  burning  test  may  well  supplement  the  analysis  of 
a  coal,  conducted  of  course  under  as  nearly  as  possible  the  conditions  that  will 


COAL.  365 

obtain  in  practice,  and  in  association  with  a  similar  experiment  on  a  coal  of 
standard  quality. 

The  specific  gravity  of  a  coal,  and  the  porosity  and  resistance  to  crushing  of 
a  coke  or  charcoal  may  be  required  for  metallurgical  purposes.  The  apparent 
and  true  specific  gravities  and  the  volume  of  the  pores  in  proportion  to  the 
solid  matter  are  calculated  from  the  weight  of  a  lump,  (a)  after  drying;  (b) 
weighed  in  air,  the  pores  filled  with  water;  (c)  weighed  in  water,  the  pores 
filled  with  water. 


366  QUANTITATIVE    CHEMICAL    ANALYSIS, 


NATURAL  WATER. 

Natural  waters  contain  in  solution  various  inorganic  bases  and  acid 
radicals  and  dissolved  gases,  frequently  also  organic  matter  and  suspended 
particles.  Almost  invariably  are  found  lime,  magnesia  and  the  alkalies, 
combined  with  carbonic  and  sulfuric  acids,  and  often  chlorides,  ferrous 
bicarbonate,  silica,  hydrogen  sulflde,  and  in  small  quantities  alumina,  the 
rarer  alkalies,  bromine  and  iodine,  phosphoric  acid,  etc.  If  polluted  by  sewage 
there  are  found  nitrogenous  organic  matter,  ammonia,  nitrates  and  nitrites. 
Suspended  matter  is  usually  clay,  sand  or  limestone  grit,  sometimes  vegetable 
matter. 

The  character  and  proportions  of  the  inorganic  impurities  may  vary  enor- 
mously.* A  few  examples  are  given. 

A  spring  water  from  Sandstone,  Minn.,  contained  total  solid  matter  4.18 
grains  per  gallon,  of  which  1.60  grains  were  calcium  carbonate,  .85  grain  mag- 
nesium carbonate,  and  .40  grain  silica. 

The  Kent  (potable)  water  of  London  contained  30  grains  per  gallon  of  solid 
matter,  of  which  16  grains  were  calcium  carbonate,  and  5  grains  calcium 
sulfate. 

The  Hathorn  Spring  (medicinal)  of  Saratoga,  N.  Y. ;  total  solids  888  grains, 
of  which  610  are  sodium  chloride,  176  magnesium  carbonate,  and  11.45  lithium 
carbonate. 

The  Atlantic  Ocean;  total  solids  2140  grains,  of  which  1671  are  sodium 
chloride,  200  are  magnesium  chloride,  108  potassium  sulfate  and  31  sodium 
bromide. 

The  Dead  Sea;  total  solids  13489  grains,  of  which  1703  are  sodium  chloride, 
4457  magnesium  chloride,  1377  calcium  chloride,  and  683  potassium  chloride. 

The  analysis  of  a  water  should  in  every  case  include  a  determination  of  the 
total  solid  matter,  dissolved  and  suspended.  Beyond  this  the  procedure 
diverges,  the  course  depending  on  the  use  to  which  the  water  is  to  be  put.  If 
for  steaming,  the  nature  and  proportions  of  the  inorganic  solids  will  decide  as 
to  the  liability  of  the  water  to  corrode  the  iron  of  the  boiler,  to  foam  on  boil- 
ing, or  to  deposit  scale.  If  for  drinking,  the  presence  of  pathogenic  germs  or 
the  environment  suitable  for  their  growth  is  indicated  by  an  undue  amount 
of  certain  constituents  characteristic  of  animal  refuse  or  decaying  vegetable 
matters.  Certain  organic  or  inorganic  bodies,  even  in  small  amounts,  will 
condemn  a  water  for  technical  and  manufacturing  purposes.  Medicinal  waters 
call  for  as  complete  an  analysis  as  practicable. 

The  usual  analytical  methods  are  outlined  in  the  following.  The  pronounced 
disadvantages  of  operating  on  material  in  the  form  of  a  highly  dilute  solution 
are  realized  in  many  of  the  determinations,  particularly  as  some  of  the  con- 
stituents are  volatilized  or  altered  in  composition  during  evaporation. 

A.  Analysis  for  technical  purposes. 

1.  Color.  A  water  is  classified  as  to  color  by  viewing  a  white  surface  through 
a  standard  depth.  The  observation  is  made  through  a  tube  of  glass  or  porcelain 
two  inches  in  diameter  and  twenty -four  inches  in  length,  the  ends  closed  by 


*  Muspratt's  Chemistry,  5—133. 


NATURAL    WATER.  367 

disks  of  colorless  glass,  one  cemented  on,  the  other  clamped.  The  tube  is  half 
filled  with  the  water,  held  horizontally,  and  the  comparison  made  between  the 
columns  of  air  and  water. 

For  a  definite  expression  of  color  various  standard  solutions  have  been  pro- 
posed. By  Crookes,  Odling  and  Tidy  are  advised  solutions  of  ferric  and  cupric 
salts  held  in  superimposed  glass  cells,  and  by  Leeds,  Nessler's  solution  tinted 
by  ammonia;  but  of  the  former  it  is  found  that  the  color  of  ferric  chloride 
varies  greatly  with  the  acidity  of  the  solutions,  and  of  the  latter,  that  much 
depends  on  the  temperature  and  concentration  of  the  Nessler's  solution.  Hazen 
recommends  a  solution  of  chloroplatinic  acid  tinged  with  a  cobalt  salt,  the 
standard  being  referred  to  metallic  platinum.  Lovibond's  tintometer  glasses 
(page  261)  are  said  to  be  well  adapted  to  the  purpose. 

The  odor,  if  any,  is  a  clue  to  the  origin  and  present  condition  of  a  sample. 
That  of  sulfu retted  water  or  one  containing  decaying  animal  or  vegetable  mat- 
ter is  pronounced  and  hardly  mistakable. 

The  reaction  with  litmus  and  lacmoid  detects  the  presence  of  an  acid  or 
alkaline  salt  and  whether  the  acidity  is  due  to  carbonic  or  a  mineral  acid. 

The  specific  gravity  of  an  ordinary  water  is  so  near  that  of  distilled  water  that 
for  a  determination  at  least  500  Cc.  is  weighed  in  a  light  flask,  best  at  a  tem- 
perature near  4°  Cent.  Except  for  mineral  and  saline  waters  the  specific, 
gravity  is  not  considered  of  any  great  importance.  Ileichert*  believes  that  the 
specific  conductivity  of  a  natural  water  is  a  characteristic  and  fundamental 
property  whose  determination  should  be  included  in  every  analysis. 

2.  Total  solids.  From  100  to  1000  cubic  centimeters  or  grams  of  the  water  is 
placed  in  a  tared  platinum  dish.    If  the  total  solids  are  less  than  about  500 
grains  per  gallon  the  water  may  be  measured,  if  over  this  amount  the  better 
plan  is  to  weigh.     Should  magnesium  chloride  be  suspected  as  one  of  the  con- 
stituents, one  adds  a  small  weighed  amount  of  sodium  carbonate  to  prevent 
loss   by    dissociation   and    volatilization    of    hydrochloric  acid  during    the 
evaporation, 

The  water  is  evaporated  to  dryness  on  the  water-bath  and  the  residue  heated 
to  from  105°  to  130°,  the  dish  cooled  in  a  desiccator  and  reweighed.  Some 
trouble  may  be  experienced  in  weighing  a  deliquescent  residue,  and  for  waters 
of  this  nature  it  is  better  to  substitute  for  the  platinum  dish  a  broad  glass 
weighing-bottle  with  a  glass  stopper,  plain  or  provided  with  a  stopcock. f 

The  suspended  matter  consists  of  earthy  debris,  clay,  sand,  limestone,  etc., 
and  sometimes  organic  matter.  It  is  usually  filtered  off  and  the  clear  liquid 
used  for  analysis.  If  at  all  considerable  the  filtration  is  done,  best  after  com- 
plete subsidence,  through  asbestos  or  a  thick  tared  paper,  which  is  reweighed 
after  drying  at  100°  .  If  thought  desirable  the  residue  is  examined  chemically 
and  microscopically. 

3.  Loss  on  ignition.  The  diminution  in  weight  on  heating  the  contents  of  the 
dish  to  redness  is  due  to  the  combustion  of  the  organic  matter  by  the  air  or 
nitrates  if  present,  and  the  loss  of  carbon  dioxide  combined  with  iron  oxide, 
lime  and  magnesia,  and  of  some  water  of  crystallization  of  gypsum.    The  carbon 
dioxide  may  be  restored    by  treating  the  ignited  residue  with  solution  of 
ammonium  carbonate,  evaporating,  and  gently  heating  to  remove  the  excess. 

4.  Silica  and  bases.  The  determinations  follow  substantially  the  routine  of 
the  analysis  of  a  soluble  silicate  (page  251).    The  residue  left  after  ignition  is 
treated  with  hydrochloric  acid,  evaporated  to  complete  dryness,  redissolved  in 


*  Chem.  News,  1889—1—180. 

t  Journ.  Anal.  Appl.  Chem.  4—53. 


368  QUANTITATIVE    CHEMICAL    ANALYSIS. 

hot  dilute  acid,  and  filtered;  the  residue  is  silica,  plus  insoluble  sediment  if  the 
water  was  turbid  and  had  not  been  filtered  previous  to  evaporation.  The  fil- 
trate is  heated  with  a  few  drops  of  nitric  acid  to  perduce  ferrous  chloride,  and 
in  succession  the  iron  and  aluminum  are  precipitated  as  hydrates  by  ammonia, 
the  calcium  as  oxalate,  and  the  magnesium  as  ammonium  magnesium  phosphate. 
The  precipitate  of  ferric  and  aluminum  hydrates  is  redissolved  after  weighing 
and  the  iron  determined,  volu metrically  if  large  in  amount,  colorimetrically 
if  small  (by  conversion  to  sulfocyanide  or  ferrocyanide  and  matching  against 
a  solution  of  iron  wire  in  acid  and  similarly  treated).*  Manganese  is  not  a 
common  constituent  of  ordinary  waters;  if  present  it  maybe  precipitated  by 
bromine  in  the  filtrate  from  the  iron  and  aluminum  hydrates,  calcined  to  the 
tetroxide,  and  weighed. 

The  alkalies  can  be  determined  in  the  same  liquid  provided  the  magnesium 
has  been  separated  by  precipitation  by  ammonium  carbonate  instead  of  am- 
monium phosphate,  but  better  in  another  large  volume  of  the  water  concen- 
trated to  a  small  bulk.  The  potassium  and  sodium,  and  lithium  if  present, 
are  separated  as  usual  by  platinic  chloride. 

Muck,f  in  waters  containing  not  more  than  traces  of  iron  and  aluminum, 
determines  the  alkalies  by  evaporating  the  water  with  dilute  sulf  uric  acid  to 
convert  all  the  bases  to  sulfates.  To  avoid  the  use  of  a  large  excess  of  sul- 
f uric  acid,  he  moistens  the  residue  from  evaporation  of  the  water  with  a  mix- 
ture of  three  drops  of  sulfuric  acid  in  100  Cc.  of  alcohol,  then  burns  off  the 
alcohol  and  observes  whether  the  residue  appears  dry;  if  so,  the  above  is  re- 
peated until  the  residue  remains  moist,  showing  a  slight  excess  of  sulfuric  acid. 
The  residue  is  gently  ignited  and  weighed,  and  the  weight  of  the  earthy  sul- 
iates,  calculated  from  the  previous  determination  of  the  earths,  is  deducted; 
the  remainder  is  the  weight  of  the  sulfates  of  the  alkalies. 

The  most  common  of  the  other  metallic  compounds  occasionally  found  in 
natural  waters  are  those  of  lead,  copper,  zinc,  arsenic  and  chromium.  They 
may  come  from  natural  sources  or  from  sewage,  and  are  usually  combined 
with  sulfuric  acid ;  lead  may  be  in  the  form  of  a  metallic  powder  attrited  from 
lead  pipes  by  sand  in  the  water.  Being  more  or  less  poisonous  or  injurious  to 
health,  their  determination  is  of  importance  in  potable  waters.  Usually  occur- 
ring in  minute  quantities,  a  volume  of  several  liters  is  evaporated;  the  deter- 
minations follow  in  the  main  the  well  known  methods  for  separation  and 
determination. 

5.  Acid  radicals.  Carbonic  acid  is  invariably  present  and  may  exist  in  three 
states:  (a),  "bound,"  that  combined  as  calcic,  magnesic,  ferrous  or  alkali 
monocarbonate ;  (b)  "half-bound,"  that  combined  with  the  monocarbonates 
to  form  bicarbonates,  e.  gr.,  CaCO3.H2CO3;  and  (c)  "free,"  that  simply  dis- 
solved in  the  water  and  uncombined  with  a  base.  The  free  and  half-bound 
carbonic  acid  is  expelled  on  boiling  the  water,  the  monocarbonates  of  the  earths 
precipitating  almost  completely.  Following  are  a  few  methods  for  their 
determination.  J 

(1)  Total  carbonic  acid  is  determined  by  adding  to  the  water  an  excess  of 
lime-water,  filtering  the  precipitate  after  it  has  crystallized,  and  weighing  as 
calcium  carbonate.  Baryta-water  has  some  advantages,  but  in  either  case  the 
absorption  of  carbon  dioxide  from  the  air  must  be  guarded  against.  A  volu- 
metric method  is  possible  — a  known  volume  of  standard  lime-water  is  used, 


*  Journ.  Socy.  Chem.  Ind.  8—175. 

t  Chem.  News,  1890—1—38. 

J  Journ.  Amer.  Chem.  Socy.  1901 — 405. 


NATURAL    WATER.  369 

and  after  settling,  an  aliquot  part  of  the  clear  liquid  is  withdrawn  and  titrated 
back  by  standard  acid.  Should  however  a  sulfate  other  than  calcium  sulfate 
be  present,  it  will  also  react  with  calcium  hydrate  (e.  g.,  MgSO4  +  Ca(OH)2  = 
Mg(OH)2  -f-  CaSO4),  and  another  process  must  be  substituted. 

(2)  Jelowitz'   method.    To  the  water  in  a  flask  is  added  neutral  calcium 
chloride  solution.    The  flask  is  connected  by  a  reflux  condenser  to  drying 
tubes,  and  these  to  a  weighed  potash- bulb  as  in  a  combustion  in  the  wet  way 
(page  303).    The  water  is  boiled  until  the  free    and   half -bound  acids  pass 
into  the  potash-bulb  which  is   then  reweighed.    After  again  connecting  the 
train,  the  water  is  acidified  by  sulfuric  acid,  and  the  bound  carbonic  acid 
boiled  out  and  determined  as  before. 

(3)  Vignon's  method.    On  titrating  a  water  by  standard  calcium  hydrate 
and  phenol-phthalein,  the  point  of  neutrality  is  manifested  when  the  free  and 
half- bound  acids  are  saturated.    Another  volume  of  the  water  is  boiled  until 
the  free  and  half-bound  acids  are  expelled  and  the  earthy  carbonates  precip- 
itated; then   the  calcium  and  magnesium   combined    with  sulfuric    acid  and 
chlorine  may  be  determined  by  titration  with  sodium  carbonate  and  phenol- 
phthalein. 

(4)  The  principles  upon  which  Seyler  proceeds  are  that  carbonates  show 
an  alkaline    reaction    toward  phenol-phthalein;    bicarbonates    neutral;  and 
free  carbonic  acid,  an  acid  reaction.    If  a  water  is  neutral  or  acid  toward 
phenol-phthalein,  the  half-bound  equals  the  bound,  and  the  volatile  equals  the 
sum  of  the  bound  and  free.    On  the  other  hand,  a  water  alkaline  to  this  indi- 
cator can  contain  no  free  acid,  and  the  volatile  acid  will  be  less  than  the  fixed 
by  a  certain  amount  determinable  by  titration  with    hydrochloric  acid  and 
phenol-phthalein    until   colorless.    Free    carbon    dioxide    is    determined  by 
titrating  with  sodium  carbonate  and  phenol-phthalein  until  faint  pink,  i.  e.t 
until   the    carbon  dioxide    has    all  combined   with    the   carbonate    to    form 
bicarbonate. 

Chlorine  is  readily  titrated  by  a  weak  standard  solution  of  silver  nitrate  with 
potassium  chromate  as  an  indicator.  Usually  the  water  need  not  be  concen- 
trated, but  if  it  contains  alkali  carbonates  which  would  react  with  the  titrand 
they  are  neutralized  by  first  acidifying  by  nitric  acid,  then  stirring  in  calcium 
carbonate,  finally  boiling  to  remove  carbonic  acid.  On  account  of  the  turbidity 
caused  by  the  silver  chloride  and  its  refusal  to  clot  (it  being  so  small  in 
amount;,  it  is  difficult  to  catch  the  end-point  where  yellow  changes  to  red. 
Mason  advises  to  make  up  a  comparison  liquid  of  a  solution  in  distilled  water 
of  sodium  chloride  (the  proper  proportion  found  by  a  rough  titration  of  the 
water)  and  potassium  chromate,  then  run  in  silver  nitrate  solution  just  short 
of  what  is  required  for  entire  precipitation.  This  liquid  is  practically  of  the 
same  color  and  turbidity  as  the  titrate  near  the  end-point. 

A  gravimetric  determination  of  the  chlorine  as  silver  chloride  is  sometimes 
more  accurate  than  the  volumetric  method. 

Sulfuric  acid  is  precipitated  by  barium  chloride  and  weighed  as  barium  sul- 
fate in  the  usual  way ;  it  is  advisable  that  the  water  be  somewhat  concentrated  by 
evaporation,  acidified,  and  heated  to  boiling  before  precipitation.  Volumetric 
methods  are  rather  tedious  and  not  often  used.  A  photometric  process  is 
based  on  a  comparison  of  the  density  of  the  cloud  produced  by  the  addition  of 
barium  chloride  to  the  unconcentrated  water  with  that  in  corresponding  solu- 
tions of  a  sulfate  of  known  concentration. 

Free  sulfuric  Scid,  often  found  in  waters  from  the  coal  measures  from  the 


*  Chem.  News,  1898—1-46  and  125. 

24 


370  QUANTITATIVE    CHEMICAL    ANALYSIS. 

decomposition  of  pyrite,  is  determined  by  titratiou  with  a  weak  standard  alkali* 
Other  free  acids  are  sometimes  met  with  in  the  drainage  from  factories. 

Hydrogen  sulfide.  Water  containing  this  gas  or  a  sulflde  is  treated  with  an 
excess  of  a  standard  solution  of  arsenious  acid,  the  very  insoluble  arsenious 
sulfide  precipitating.  After  filtering,  the  excess  of  the  arsenic  solution  is 
titrated  by  iodine  and  starch-paste. 

Another  method  is  to  add  the  water  from  a  burette  to  an  arbitrary  small 
volume  (a  Cc.)  of  weak  standard  iodine  solution  until  the  yellow  color  vanishes r 
this  requiring  b  Cc.  of  the  water.  To  the  solution  is  added  starch-paste,  and  it 
is  titrated  to  faint  blue  by  the  iodine  solution  requiring  c  Cc.  Then  a  -{-  c  Cc. 
oxidizes  the  hydrogen  sulflde  in  b  Cc.  of  the  water.  A  blank  titration  is 
made  on  a  -f  b  -f  c  Cc.  of  distilled  water  for  a  correction  for  the  iodine  needed 
to  produce  the  blue  tint. 

Hydrogen  sulflde  reduces  ferric  sulfate  —  Fe2(SO4)3  -f  H2S  =2FeSO4  ~f-  S  -f- 
H2SO4.  The  water  is  acidified  by  sulfuric  acid  and  mixed  with  an  excess  of 
weak  standard  ferric  sulfate  solution,  then  the  ferrous  sulfate  formed  is  titrated 
by  weak  standard  permanganate.  It  is  plain  that  a  water  cannot  be  concen- 
trated for  a  determination  of  hydrogen  sulfide  except  after  making  it  alkaline 
by  sodium  hydrate,  and  here  oxidation  is  apt  to  occur  during  the  evaporation. 

Phosphoric  add.  The  water  is  acidified  by  nitric  acid  and  concentrated  to  a 
small  bulk,  then  boiled  with  addition  of  ferric  chloride  and  ammonium  acetate. 
The  precipitate  contains  all  the  phosphoric  acid  as  ferric  phosphate,  and  is 
filtered  off  and  dissolved  in  nitric  acid.  The  phosphoric  acid  is  then  precip- 
itated by  molybdic  solution  and  the  ammonium  phosphomolybdate  dried  and 
weighed  (page  384). 

A  colorimetric  method  is  based  on  the  yellow  hue  produced  by  potassium 
molybdate  in  a  dilute  solution  of  a  phosphate  acidified  by  nitric  acid.* 

Boracic  acid  is  found  in  small  amount  in  some  waters.  In  the  method  of 
Gooch  the  water  is  made  alkaline  and  evaporated  to  dryness;  the  residue  is 
moistened  with  acetic  acid  and  transferred  to  a  distillation  flask.  The  flask  is 
connected  to  a  condenser,  this  to  a  receiver  containing  a  weighed  amount  of 
calcium  oxide  which  has  been  slaked  and  suspended  in  water.  Into  the  flask 
is  run  some  methyl  alcohol  and  the  mixture  distilled  to  dryness ;  more  alcohol 
is  added  and  again  distilled ;  this  is  repeated  five  times. 

The  free  boric  acid  in  the  flask  unites  with  the  methyl  alcohol  to  form  methyl 
borate  which  passes  over  with  the  alcohol  vapor,  and  meeting  the  lime  reacts  to 
form  calcium  borate.  When  the  distillations  are  over  the  distillate  is  evaporated 
to  dryness  and  the  residue  strongly  ignited,  which  leaves  it  a  mixture  of  calcium 
borate  and  oxide.  The  increase  over  the  weight  of  the  lime  is  boracic  acid. 

6.  Hardness.  In  common  parlance  waters  are  distinguished  as  <  hard »  or 
1  soft '  according  as  they  do  or  do  not  curdle  with  soap.  To  set  a  numerical 
rating  on  this  property,  Dr.  Clark,  following  Accum,  proposed  a  scale  of 
"degrees  of  hardness."  Although  somewhat  empirical,  the  expression 
represents  a  definite  quality  to  the  layman  and  for  certain  purposes  gives  all 
the  information  needed.  The  process,  however,  is  obsolescent. 

Pure  water  shows  an  immediate  and  permanent  lather  when  shaken  with  a 
solution  of  common  soap,  but  when  salts  of  calcium  or  magnesium  are  con- 
tained, as  they  are  in  all  natural  waters,  they  react  with  the  soap  to  form  a 
precipitate  of  calcium  or  magnesium  stearate,  oleate,  and  palmitate;  thus, 
CaCO3.H3CO3  +  2Na(C18H35O2)  (sodium  oleate)  =  2NaHCO3  +  Ca(C18H35O)2 
(calcium  oleate);  (free  mineral  acids  also  decompose  soap,* setting  free  the 


*  Journ.  Amer.  Chem.  Socy.  1901—96. 


NATURAL   WATER.  371 

fatty  acids).  So  that,  varying  with  the  proportions  of  these  salts  must  soap 
solution  be  added  before  a  permanent  lather  appears. 

The  total  hardness  of  a  water  is  expressed  by  the  number  of  cubic  centimeters 
of  a  standard  soap  solution  required  to  precipitate  the  earthy  salts  of  one 
gallon.  On  boiling  a  water  the  calcium,  magnesium,  and  ferrous  bicarbonates 
decompose;  the  mono-carbonates  precipitate  and  no  longer  react  with  the 
soap  solution.  The  volume  of  soap  solution  required  for  a  gallon  of  the  water 
after  boiling  expresses  the  permanent  hardness,  that  due  to  earthy  sulfates, 
chlorides,  etc.  The  difference  between  the  total  and  permanent  is  called  the 
temporary  hardness. 

The  soap  solution  is  made  up  by  dissolving  a  pure  neutral  alkali-fatty-acid 
soap  in  weak  alcohol.  It  is  standarized  against  a  solution  of  calcium  chloride 
(made  by  dissolving  a  weighed  quantity  of  calcium  carbonate  in  hydrochloric 
acid,  evaporating  to  dryness,  and  redissolving  in  water).  The  soap  solution  is 
then  diluted  until  one  cubic  centimeter  exactly  corresponds  to  one  milligram 
of  calcium  carbonate.  Nelson  prepares  the  soap  solution  by  neutralizing  pure 
palmitic  acid  by  normal  sodium  hydrate;  the  number  of  cubic  centimeters  of 
the  normal  alkali  solution  required  times  .050035  is  the  grams  of  calcium 
carbonate  equivalent  to  the  weight  of  the  palmitic  acid  taken. 

The  titration  is  done  on  70  cubic  centimeters  of  the  water  at  15°  Cent,  by  a 
soap  solution  of  which  one  cubic  centimeter  is  equivalent  to  one  milligram  of 
calcium  carbonate.  This  is  tantamount  to  operating  on  one  English  gallon 
(70,000  grains)  with  a  solution  of  the  strength  equaling  one  grain  of  cal- 
cium carbonate  per  cubic  centimeter;  if  the  result  is  to  be  expressed  in  grains 
per  United  States  gallon,  the  quantity  for  the  soap  test  is  58.3  cubic  centimeters. 
The  sample  is  measured  into  a  stoppered  flask  and  the  soap  solution  run  in,  one 
cubic  centimeter  at  a  time,  shaking  after  each  addition  until  the  foam  remains 
permanent  for  five  minutes.  One  Cc.  is  deducted  for  the  amount  required  to 
produce  a  lather  in  distilled  water;  the  remainder  is  reported  as  "degrees  of 
total  hardness."  The  standardization  is  done  in  the  same  manner. 

Calcium  bicarbonate  produces  an  immediate  precipitation,  but  the  reaction 
with  magnesium  bicarbonate  is  slower,  and  one  part  of  magnesium  bicarbon- 
ate is  said  to  consume  as  much  soap  as  1.5  parts  of  calcium  bicarbonate.  If 
for  a  water  containing  principally  calcium  compounds  more  than  16  Cc.  of  the 
titrand  is  required,  or  more  than  7  Cc.  for  one  containing  a  considerable  pro- 
portion of  magnesium  compounds,  another  test  is  made  on  the  water  diluted 
with  distilled  water,  since  the  precipitate  interferes  with  the  exhibition  of  the 
end-point. 

Another  portion  of  the  water  is  boiled  for  some  time,  cooled  and  titrated  as 
before ;  the  result  is  the  permanent  hardness,  and  the  temporary  hardness  is 
found  by  difference. 

The  permanent  hardness  is  determined  by  Hehner*  by  evaporating  to  dryness 
100  Cc.  of  the  water  with  a  measured  volume  (a)  of  fiftieth- normal  sodium 
carbonate,  a  quantity  in  excess  of  that  required  to  react  with  the  calcium  and 
magnesium  salts  other  than  bicarbonates.  The  residue  is  lixiviated  by  water 
and  filtered ;  there  passes  into  solution  the  excess  of  sodium  carbonate  which 
is  then  determined  by  titration  by  N/50  hydrochloric  acid,  requiring  b  Cc.  The 
difference  between  a  and  b  is  termed  the  permanent  hardness  per  100,000  parts 
of  water. 

(Were  distilled  water  subjected  to  this  process,  a  would  equal  b  and  the 
permanent  hardness  would  be  zero.  In  the  case  of  a  natural  water  there  are 


*  Chem.  News,  1898—1—135;  Journ.  Amer.  Chem.  Socy,  1899-369. 


372  QUANTITATIVE    CHEMICAL   ANALYSIS. 

contained  usually  calcium  and  magnesium  bicarbonates,  calcium  sulfate,  and 
sometimes  calcium  and  magnesium  chlorides ;  the  bicarbonates,  which  give  the 
temporary  hardness,  do  not  react  with  the  sodium  carbonate  and  on  evapora- 
tion pass  to  the  monocarbonates,  which  being  insoluble  in  water,  are  left  as  a 
residue  in  the  lixiviation.  But  the  calcium  and  magnesium  chlorides  and  sul- 
fates  react  with  sodium  carbonate  to  form  calcium  and  magnesium  carbonates 
and  sodium  sulfate  and  chloride ;  the  carbonates  remain  insoluble  in  the  lix- 
iviation, and  the  sodium  compounds  are  neutral  to  hydrochloric  acid ;  hence 
according  to  the  quantity  of  sodium  carbonate  reacting,  b  is  decreased  and 
consequently  a  —  b  is  increased.) 

The  total  alkalinity  of  a  water  is  found  by  directly  titrating  100  Cc.  by  N/50 
sulfuric  acid,  using  for  an  indicator  either  lacmoid,  carminic  acid,  or 
azoresorufin.  The  result  is  the  temporary  hardness  per  100,000  parts  of 
water  provided  it  contains  no  sodium  carbonate.  And  if  there  is  no 
sodium  carbonate  in  the  water,  the  amount  of  calcium  bicarbonate  it  con- 
tains can  be  calculated  from  the  volume  of  N/50  sulfuric  acid  used  in  the 
direct  titration.  But  if  there  be,  as  is  usual,  both  calcium  and  magnesium  bi- 
carbonates in  the  water,  the  result  calculated  on  the  former  will  be  inaccurate 
since  the  molecular  weights  of  the  two  compounds  differ  considerably.  How- 
ever, if  the  ratio  in  which  the  two  are  present  is  fairly  well  known,  the  calcu- 
lation can  be  made  accordingly,  or  the  sulfuric  acid  made  proportionally 
stronger. 

In  a  water  that  contains  sodium  carbonate  there  can  be  no  earthy  compounds 
other  than  bicarbonates  and  therefore  no  permanent  hardness;  so  that  in 
Hehner's  process  all  the  original  and  added  sodium  carbonate  is  found  in  the 
lixiviation,  and  b  —  a  is  the  volume  of  acid  equivalent  to  the  original  sodium 
carbonate.  If  c  represents  the  total  alkalinity  as  found  by  direct  titration  by 
N/50  acid,  then  c  —  (6  —  a)  is  the  acid  corresponding  to  the  earthy  bicarbon- 
ates, the  true  temporary  hardness. 

Calculation.  The  custom  of  reporting  the  results  of  a  water  analysis  in 
grains  per  gallon  —  English  or  United  States  as  the  case  may  be  —  is  still 
retained  by  many  chemists  for  reasons  more  or  less  tenable,  but  the  modern 
practice  of  following  a  decimal  system  is  more  in  keeping  with  the  scientific 
trend  and  should  be  adopted  wherever  practicable,  with  the  exception  perhaps 
of  'degrees  of  hardness.'  Since  a  percentage  basis  would  bring  all  the  fig- 
ures beyond  the  decimal  point,  the  results  are  stated  as  parts  either  per  hun- 
dred thousand  or  per  million  of  water ;  gases  as  cubic  centimeters  per  hundred 
or  per  thousand. 

It  is  still  a  matter  of  controversy  whether  the  inorganic  constituents  should 
be  reported  as  elements,  radicals,  or  compounds.  The  first  two  are  nearer  in 
harmony  with  the  theory  of  ionic  dissociation  in  dilute  solutions,  and  relieve 
the  chemist  of  expressing  any  opinion  as  to  the  debatable  matter  of  the 
selective  union  of  bases  and  radicals.  But  from  such  a  tabulation  one  is  not 
informed  as  to  whether  the  water  is  neutral,  or  acid  or  alkaline  and  to  what 
degree,*  whether  any  or  much  temporary  or  permanent  hardness  exists;  arid 
whether  certain  compounds  that  are  believed  to  be  obnoxious  for  boiler  use  or 
manufacturing  purposes  are  present  or  will  separate  when  the  water  is  con- 
centrated. Nor  are  the  facts  that  our  knowledge  of  selective  association  is 
somewhat  vague,  and  that  the  conventional  combinations  can  be  calculated  at 
any  time  from  the  elementary  or  radical  statement,  convincing  arguments  for 
the  adoption  of  the  latter  forms. 

No  absolute  rules  can  be  laid  down  as  to  the  forms  of  combination  that  will 
apply  to  all  waters.  The  conventional  practice  is  to  first  combine  the  alkalies 


NATURAL   WATER.  373 

With  chlorine  and  sulfuric  acid;  second,  the  excess  of  these  acids  with  the 
earths ;  and  lastly  the  remainder  of  the  earths  with  carbon  dioxide.  Variations 
from  the  above  and  more  detailed  rules  will  be  found  in  the  works  on  water 
analysis.  It  must  be  remembered  however  that  certain  compounds  are  incom- 
patible, e.  g.,  an  alkali  carbonate  and  calcium  sulfate. 

B.  Analysis  of  potable  water. 

1.  Total  organic  mattter.  A  number  of  processes  have  been  put  forward  for 
this  determination  by  Forchammer,  Kubel,  Woolf,  Dittmar  and  others,  but 
none  are  quite  satisfactory.  Two  great  difficulties  are  apparent,  one  that  in 
the  organic  matter  of  a  water  are  comprised  many  bodies  of  unlike  character, 
the  other  that  their  proportion  is  very  minute  in  potable  waters  and  oxidation 
during  concentration  is  more  than  possible. 

A.  In  the  procedure  commonly  called  the  "  moist  oxidation  process  "  the 
organic  matter  is  decomposed  by  potassium  permanganate  which  converts  the 
carbon  and  hydrogen  into  carbon  dioxide  and  water.  The  action  of  the  oxidizer 
is  always  incomplete,  however,  and  with  some  organic  compounds  slower  and 
more  imperfect  than  with  others;  thus,  it  is  said  that  there  is  a  notable  differ- 
ence in  reducing  power  between  fresh  animal  matter  and  the  same  in  a  putres- 
cent  state. 

Various  modifications  of  the  details  of  the  process  have  been  exploited 
with  a  corresponding  difference  in  the  oxygen  consumed.  Wanklyn  com- 
pounds a  liter  of  water  with  an  excess  of  standard  potassium  permanganate 
solution,  makes  alkaline  with  potash,  and  distills  rapidly  from  a  glass  retort 
until  900  Cc.  has  passed  over.  The  water  remaining  in  the  retort,  still  colored 
by  unreduced  permanganate,  is  acidified  by  sulfuric  acid,  mixed  with  a  known 
volume  of  standard  ferrous  sulfate,  and  the  excess  of  the  latter  determined  by 
titration  with  permanganate.  The  strength  of  the  permanganate  is  such  that 
one  Cc.  contains  one  milligram  of  available  oxygen.  The  weight  of  oxygen 
consumed  by  the  organic  matter  is  readily  calculated.  Should  the  water  con- 
tain ferrous  compounds,  nitrites,  hydrogen  sulfide,  or  other  reducers,  the 
result  must  be  corrected  therefor. 

Another  method,  wherein  the  oxidation  takes  place  in  an  acid  solution,  is  to 
prepare  a  permanganate  solution  of  which  one  Cc.  contains  .0001  gram  of  avail- 
able oxygen,  and  a  solution  of  oxalic  acid  of  equivalent  strength.  To  200  Cc. 
of  the  water  is  added  10  Cc.  of  dilute  sulfuric  acid  and  enough  permanganate 
to  produce  a  persistent  red  color.  The  mixture  is  boiled  for  15  minutes,  then 
reduced  by  an  excess  of  oxalic  acid  solution,  and  the  remainder  of  the  latter 
titrated  back  by  permanganate.  The  sum  of  the  volumes  of  permanganate  less 
that  of  the  oxalic  acid  is  the  permanganate  reduced  by  the  organic  matter. 

In  another  modification,  two  determinations  are  made  at  80°  Fahr.,  one 
digested  for  16  minutes,  in  which  period  it  is  assumed  that  reducing  compounds 
are  oxidized  but  not  organic  matter;  the  other  for  four  hours  to  oxidize  both. 
The  excess  of  permanganate  is  determined  by  treatment  with  potassium  iodide 
solution  —  K2Mn2O8  +  10KI  =  5I2-f  6K2O  +  2MnO  —  and  the  liberated  iodine 
titrated  by  sodium  thiosulfate  and  starch-paste. 

In  the  method  of  Degener  and  Maercker  the  carbon  is  determined  by  oxidiz- 
ing with  chromic  and  sulfuric  acids  and  absorbing  the  carbon  dioxide  in  a  pot- 
ash bulb  as  in  an  ordinary  moist  combustion.  Hertzfield  improves  the  process 
by  interposing  a  tube  containing  metallic  antimony  which  serves  to  retain  any 
chlorine  liberated  from  the  chlorides  of  the  water.  Burghardt  determines  the 
chromic  acid  reduced  by  the  organic  matter  by  a  back  titration  with  ferrous 
sulfate, 


374  QUANTITATIVE    CHEMICAL    ANALYSIS. 

It  was  found  by  Barnes  that  the  ratio  of  oxygen  consumed  in  the  permanganate 
'  and  bichromate  oxidations  is  distinctly  higher  for  waters  containing  vegetable 
matter  than  for  those  polluted  by  sewage,  independent  of  the  amount  of  organic 
matter  contained. 

B.  Flack's  scheme  for  measuring  organic  matter  by  the  reduction  of  silver 
sulf  ate  to  metallic  silver  has  not  come  into  general  use. 

C.  In  Frankland's  process  for  organic  carbon  and  nitrogen,  one  liter  or  more 
of  the  water  is  evaporated  to  dryness  after  acidification  with  sulf urous  acid 
to  destroy  oxidizing  matters.    The  evaporation  is  done  in  a  glass  dish  care- 
fully protected  from  dust  and  ammonia  fumes.    The  residue  is  scraped  from 
the  dish,  mixed  with  copper  oxide,  and  burned  in  vacuo  in  a  glass  combustion 
tube.    The  products  are  carbon  dioxide,  nitrogen  and  nitric  oxide,  and  are 
transferred  to  a  gas-measuring  tube  and  the  proportion  of  each  determined  by 
the  usual  geometric  methods.    The  carbon  and  nitrogen  in  the  gases  are  cal- 
culated from  the  usual  formulae. 

The  method  requires  complex  apparatus  and  careful  manipulation.  As  to 
the  relative  merits  of  the  Frankland  and  Wanklyn  processes  there  has  been 
much  discussion,  not  always  confined  to  scientific  arguments.  But  it  is  now 
conceded  that  both  are  but  empirical  and  cannot  give  "  more  than  an  approxi- 
mation to  the  quantity  of  certain  substances,  the  presence  of  which  is  only 
circumstantial  evidence  of  the  character  of  the  water." 

2.  Oxygen.  In  water  saturated  with  air  the  dissolved  oxygen  is  a  sensibly 
constant  quantity  at  a  given  temperature  and  pressure.  If  the  water  is  stored 
in  a  closed  vessel  and  remains  at  a  uniform  temperature  there  will  be  neither  a 
loss  or  gain  of  the  gas  provided  the  water  be  pure,  but  if  it  contains  aerobia 
the  oxygen  will  steadily  diminish.  Dupre  adopts  the  number  100  as  a  standard 
signifying  the  quantity  of  dissolved  oxygen  in  water  saturated  at  20°  Cent. 

The  determination  is  made  in  one  of  two  ways,  although  methods  of  the  first 
class  are  no  longer  in  use,  having  been  supplanted  by  the  simpler  volumetric 
methods  of  the  second:  CO  the  oxygen,  with  other  dissolved  gases,  is  expelled 
by  heating  the  water  in  vacuo,  and  the  gases  transferred  to  a  eudiometer  for 
gasometric  analysis;  (2)  the  oxidizing  action  is  measured  by  a  standard  solu- 
tion of  a  reducer,  either  directly  or  by  a  reverse  titration.  It  is  essential  that 
no  air  comes  in  contact  with  water  during  the  operation,  and  that  there  be  no 
loss  through  diffusion. 

(A.)  In  the  method  of  Schuetzenberger  the  water  is  titrated  by  a  standard 
solution  of  the  powerful  reducer  sodium  hyposulflte  (Na2SO3)  ;  the  indicator  is 
indigotin  sulfate  which  is  blue  or  colorless  respectively  when  oxygen  or 
hyposulflte  is  in  excess.  The  process,  somewhat  modified,  is  to  prepare  the 
indicator  by  adding  a  little  indigo  solution  to  water,  then  just  decolorizing  by 
hyposulfite.  Ten  cubic  centimeters  of  the  hyposulfite  solution  is  run  in, 
followed  by  the  water  under  examination  delivered  from  a  burette,  until  the 
liquid  becomes  blue,  then  again  decolorized  by  hyposulfite.  The  hyposulfite  is 
standardized  against  fully  aerated  distilled  water  of  which  the  amount  of 
oxygen  in  solution  is  a  constant  under  a  given  temperature  and  pressure. 
Free  acids  and  alkalies  disturb  the  normal  conduct  of  the  process. 

During  the  mixing  and  titration,  contact  with  the  air  must  be  prevented. 
This  is  accomplished :  (1)  by  arranging  that  these  operations  take  place  in  a  bottle 
through  which  passes  a  current  of  a  non-oxidizing  nearly  insoluble  gas,  i.  e., 
hydrogen.  But  it  has  been  shown  that  dissolved  oxygen  rapidly  diffuses  from 
water  into  hydrogen  gas;  this  difficulty  is  overcome  by  running  the  water  into 
a  measured  volume  of  the  reducing  solution  by  way  of  a  burette  whose  era- 
boachure  Is  beneath  the  surface  of  the  solution,  the  oxygen  being  fixed  before 


NATURAL   WATER.  375 

it  can  come  into  contact  with  the  hydrogen ;  (2)  a  simpler  plan  is  that  of 
Blarez,  who  blankets  the  water  with  a  layer  of  purified  hydrocarbon  oil,  and  in 
adding  the  reagents  takes  care  that  the  orifices  of  the  delivering  vessels  dip 
below  the  layer.  The  titration  is  made  in  a  separatory  funnel  closed  by  a  cork 
holding  the  taps  of  one  or  more  burettes.  The  funnel  is  party  filled  with 
mercury,  then  completely  filled  with  the  water.  As  the  reducer  is  run  in,  the 
tap  of  the  funnel  is  opened  to  allow  a  corresponding  volume  of  mercury  to 
escape.  The  heavy  mercury  allows  a  thorough  mixing  of  the  water  and 
reducer  on  shaking  the  funnel. 

(B.)  In  the  method  of  Mohr  the  water  is  shaken  up  with  recently  precipi- 
tated ferrous  hydrate  which  is  perduced  to  ferric  hydrate  by  absorption  of 
oxygen  —  2Fe(OH)2  +  O  -f  H2O  =  Fe2(OH)6.  The  water  is  made  alkaline 
by  sodium  hydrate,  and  a  measured  volume  of  standard  ferrous  sulfate  run 
in;  after  thorough  agitation,  the  hydrates  are  dissolved  by  acidulation  with 
dilute  sulfuric  acid,  and  the  ferrous  sulfate  titrated  by  permanganate.  A 
correction  for  nitrates  present  is  made  by  titrating  another  portion  of  the  water 
by  ferrous  sulfate  and  permanganate  after  acidification  by  sulfuric  acid. 

Letts  and  Blake  fill  a  globe-shaped  separatory  funnel  completely  with  the 
water;  a  small  fraction  of  the  water  is  withdrawn,  and  a  slightly  less  volume 
of  strong  solution  of  ferrous  sulfate  run  in  through  the  stem.  The  globe  is 
then  filled  up  with  concentrated  ammonia.  After  stoppering  and  agitating, 
the  globe  is  let  stand  for  15  minutes.  It  is  then  reversed,  the  stem  filled  with 
concentrated  sulfuric.  acid,  the  stopcock  opened,  and  the  sulfuric  acid  allowed 
to  diffuse  through  the  liquid.  When  the  iron  hydrates  have  dissolved,  the 
liquid  is  drawn  out  and  titrated  by  permanganate  or  bichromate. 

(C.)  Winkler's*  method  is  similar  to  Mohr's.  Manganous  hydrate  is  precip- 
itated from  a  solution  of  manganous  chloride  by  potassium  hydrate,  but  if  the 
solution  holds  dissolved  oxygen,  a  corresponding  amount  of  the  precipitate 
becomes  manganic  hydrate,  thus  — 

2MnCl2  -f-  4KOH  =  2MnO.H2O  (manganous  hydrate)  -f  4KC1;  and 

2MnCl2  +  4KOH  -f-  O  =  Mn2O3.H20( manganic  hydrate)  -fi  4KC1  +  H2O. 

Manganic  hydrate  when  treated  with  potassium  iodide  and  an  acid  dis- 
solves to  manganous  chloride,  liberating  iodine  — 

Mn203H20  +  2KI  +  6HC1  =  2MnCl2  +  I2  +  2KC1  -f  4H2O. 

Two  atoms  of  iodine  correspond  to  one  atom  of  oxygen  in  the  water. 

The  determination  is  made  in  a  cylinder  nearly  filled  with  the  water ;  by 
long  pipettes  are  introduced  one  cubic  centimeter  each  of  potassium  iodide 
and  potassium  hydrate  solutions.  The  cylinder  is  stoppered  and  agitated. 
Hydrochloric  acid  is  then  introduced  to  dissolve  the  manganese  hydrates,  and 
the  liberated  iodine  is  at  once  titrated  by  sodium  thiosulfate  and  starch. 
Nitrites  also  liberate  iodine  and  must  be  corrected  for,  and  large  amounts  of 
organic  matter  tend  to  retain  iodine. 

(D.)  Thresh's  iodimetric  method. f  Oxygen  in  presence  of  a  carrier,  such  as 
nitrogen  dioxide,  liberates  iodine  quantitatively  from  an  iodide.  The  water  is 
mixed  with  potassium  iodide  and  a  ^weighed  quantity  of  pure  sodium  nitrite,  and 
acidified  by  sulfuric  acid.  First  the  nitrite  reacts  with  the  potassium  iodide 
and  sulfuric  acid  — 

2KI  +  2KNO2  +  2H2S04  =  I2  +  2K2SO4  +N2O2  +  2H20; 
then  the  oxygen  of  the  water  reacts  with  the  excess  of  the  iodide  — 
2RI  +  O  +  H2S04  =  I2  +  K2S04  +  H20. 


*  Berlchte,  1888-2843. 

t  Journ.  Chem.  Socy.  1890—185. 


376  QUANTITATIVE    CHEMICAL   ANALYSIS. 

All  the  operations  are  conducted  under  a  current  of  coal-gas.  The  iodine 
found  by  the  titration  with  thiosulfate  includes  a  that  liberated  by  the  oxygen 
of  the  water,  plus  b  that  liberated  by  a  part  of  the  oxygen  dissolved  in  the 
thiosulfate  solution,  plus  c,  that  formed  by  the  reaction  between  the  iodide  and 
sodium  nitrite.  Corrections  are  made  for  c  by  a  blank  titration  of  potassium 
iodide  plus  sodium  nitrite  plus  sulfuric  acid  dissolved  in  air-free  water;  and 
for  6  by  a  titration  of  ordinary  distilled  water,  on  the  assumption  that  thiosul- 
fate solution  and  water  absorb  equal  amounts  of  oxygen. 

(E.)  Lissonier  titrates  the  water  made  slightly  alkaline,  directly  with  a 
standard  solution  "of  ferrous  tartrate,  using  for  an  indicator  pheno-safranine 
(a  red  dye  — para-amidophenyl-para-amidophenazonium  chloride)  which  is 
decolorized  by  a  ferrous  salt. 

3.  Ammonia.  The  nitrogenous  organic  matter  in  a  water  is  a  complex  mix- 
ture of  a  group  of  bodies  more  or  less  easily  decomposable  and  that  pass  by 
successive  stages  to  ammonia,  nitrous  acid,  and  nitric  acid.  For  these  three 
products  fairly  accurate  methods  oi  determination  are  known. 

(1)  Free  or  already -formed  ammonia.  The  determination  of  this  body,  it 
being  in  so  small  a  proportion  in  potable  waters,  cannot  be  made  by  the  ordi- 
nary gravimetric  or  volumetric  methods.  The  well  known  colorimetric  scheme 
due  to  Nessler  is  sufficiently  delicate.  It  depends  on  the  yellow  to  brown  hue 
produced  by  traces  of  ammonia  in  an  extremely  dilute  colorless  alkaline  solution 
of  potassium  mercuric  chloride.  The  formula  of  Nessler's  solution  prescribed 
by  Wanklyn  is  to  add  to  a  solution  of  35  grams  of  potassium  iodide  and  13 
grams  of  mercuric  chloride  in  800  Cc.  of  hot  water,  a  cold  saturated  solution 
of  the  latter  salt  until  but  little  mercuric  iodide  remains  undissolved;  then  160 
grams  of  potassium  hydrate,  and  water  to  the  volume  of  one  liter.  Finally  a 
little  mercuric  chloride  is  added  and  the  mixture  allowed  to  settle  until  clear 
and  of  a  light  yellow  tint. 

In  this  process  water  entirely  free  from  ammonia  is  required  for  dilution. 
The  traces  present  in  ordinary  distilled  water  pass  off  on  evaporating  to  half  its 
bulk  or  less,  so  that  on  rendering  the  water  slightly  alkaline  and  distilling  until 
the  distillate  shows  no  indication  of  ammonia,  a  further  quantity  distilled  is  prac- 
cally  pure.  A  simpler  method  is  to  redistill  ordinary  distilled  water  after  addi- 
tion of  a  little  sulfuric  acid ;  the  acid  retains  the  ammonia  already  formed  or 
nascent.  And  if  the  sample  of  water  is  to  be  filtered  before  making  the  test,  the 
filter  paper  must  be  purified  before  use  by  copious  washing  with  ammonia-free 
water. 

In  making  the  test,  the  solution  containing  ammonia  or  an  ammonium  salt 
is  poured  into  a  flat -bottomed  test-tube  (Nessler's  tube)  of  clear  glass  marked 
at  50  Cc.  Two  Cc.  of  the  Nessler's  solution  is  added  and  mixed  by  stirring, 
and  the  brown  color  shortly  develops. 

.At  the  same  time  a  comparison  test  is  made  in  the  same  manner  on  ammonia- 
free  water  plus  a  measured  volume  of  standard  solution  of  ammonium  chloride. 
The  tubes  are  held  at  an  angle  over  a  white  porcelain  plate.  On  looking  down 
through  them  it  is  s'een  whether  the  tints  are  identical;  if  so,  the  weight  of  the 
ammonia  in  the  volume  of  the  standard  solution  of  ammonium  chloride  em- 
ployed is  the  weight  of  ammonia  in  the  water;  but  if  they  differ,  as  is  nearly 
always  the  case,  other  tints  for  comparison  are  made  up  with  greater  or  less 
volumes  of  the  ammonium  chloride  solution  as  indicated,  until  an  agreement 
is  had.  With  practice  this  can  be  done  quite  rapidly.  Various  colorimeters 
have  been  recommended  for  the  comparison.  In  all  cases  the  temperature 
of  the  solutions  and  the  order  of  the  mixture  with  the  reagent  should  be  the 
same. 


NATURAL   WATER.  377 

The  details  of  the  determination  in  a  water  as  practiced  by  different  chemists- 
vary  considerably ;  one  plan  is  as  follows :  Two  hundred  Cc.  of  distilled  water, 
made  slightly  alkaline  by  sodium  carbonate,  is  placed  in  a  large  glass  retort 
connected  to  a  condenser  by  cork  fittings.  One  hundred  Cc.  is  distilled  and 
rejected  as  containing  'the  ammonia  from  the  water  and  that  adhering  to  the 
interior  of  the  apparatus.  To  the  residual  water  in  the  retort  is  added  500  Cc. 
of  the  water  for  analysis  and  distilled  at  the  rate  of  about  50  Cc.  in  15  minutes. 
The  distillate  is  received  in  the  Nessler's  tubes,  50  Cc.  in  each,  and  the  first 
four  Nesslerized.  Distributed  through  these  is  practically  all  the  ammonia  in 
the  water,  though  unequally,  the  first  tube  containing  approximately  three- 
fourths  of  all,  the  fourth  practically  none.  The  sum  of  the  ammonia  in  the 
four  is  set  down  as  the  free  ammonia  in  500  Cc.  of  the  water. 

2.  Albumenoid  ammonia.    Wanklyn's  method.    On  digesting  water  contain- 
ing albuminous  matter  with  an  alkaline  solution  of  potassium  permanganate, 
the  nitrogen  is  converted  into  ammonia,  more  or  less  completely  according 
to  the  nature  of  the  bodies  and  the  conditions  of  the  experiment.    It  is  said 
that  by  repeated  distillations  all  the  nitrogen  may  be  so  converted,  but  certain 
bodies,  like  urea,  evolve  ammonia  steadily  and  continuously  for  a  long  time. 

The  method  is  practically  the  same  as  that  for  free  ammonia  with  the  excep- 
tion that  a  strongly  alkaline  permanganate  solution  is  mixed  with  the  water ; 
the  ammonia  determined  in  the  distillates  is  here  the  sum  of  the  free  and 
albumenoid ;  deducting  the  former,  found  by  a  previous  test,  gives  the  latter. 
But  as  the  process  involves  not  merely  a  distillation  of  free  ammonia  as  in  (1), 
but  also  a  chemical  decomposition  of  the  organic  matter,  and  as  this  decompo- 
sition is  never  instantaneous  and  often  very  slow,  the  question  arises  how  far 
to  proceed  with  the  distillation  before  considering  it  practically  complete. 
The  proposal  to  stop  the  distillation  when  the  last  fraction  contains  not  more 
than  one  per  cent  of  the  entire  ammonia  of  the  distillate  would  seem  to  be  an 
ample  allowance. 

3.  Nitrogen  by  Kjeldahl's  method.    Recently  this  process  has  been  applied 
with  success  to  the  determination  of  total  nitrogen  in  water.    Five  hundred 
Cc.  of  the  water   is  mixed  with  ten  Cc.  of  concentrated  sulfuric  acid  and 
evaporated  until  the  acid  becomes  concentrated,  clear  and  nearly  colorless, 
when  a  little  permanganate  may  be  added.    The  residue  is  diluted,  made  alka- 
line by  sodium  hydrate,  and  either  filtered  and  Nesslerized  directly,  or  distilled 
and  the  ammonia  determined  as  usual.    Leffman  and  Beam  determine  free 
ammonia  by  mixing  200  Cc.  of  the  water  with  sodium  hydrate  and  carbonate, 
filtering  through  cotton,  and  Nesslerizing  the  middle  100  Cc.  of  the  filtrate; 
then  deducting  the  result  from  the  nitrogen  by  the  above  method  to  obtain  the 
total  organic  nitrogen.    Drown  and  Martin  boil  500  Cc.  of  the  water  to  300 
Cc.  to  expel  free  ammonia  previous  to  the  Kjeldahl  determination. 

4.  Nitrates.  The  methods  of    Schulze,  Lunge,  Trommsdorf,  etc.,  ordinarily 
adopted  for  the  determination  of  nitrates,  are  most  suitable  for  moderately 
large  amounts  and  can  only  be  used  for  potable  waters  after  concentration  of 
a  large  volume ;  as  this  is  objectionable  on  account  of  the  liability  of  oxida- 
tion of  nitrites  and  for  other  reasons,  they  are  seldom  used. 

(1)  Sprengel's  method.  When  nitric  acid  is  brought  in  contact  with  (color- 
less) phenolsulfuric  acid  there  is  formed  picric  acid  — 

C6H4.OH.SO3H  +  3HNO3  =  C6H2OH(N02)3  •+-  2H2O  -f  H2SO4. 
On  adding  an  excess  of  ammonia,  the  highly  tinctorial  yellow  ammonium 
picrate  is  formed. 

Phenolsulfuric  acid  of  a  definite  composition  is  prepared,  according  to  John- 
son, by  digesting  for  eight  hours  on  the  water  bath,  a  mixture  of  four  parts  of 


378  QUANTITATIVE    CHEMICAL   ANALYSIS. 

crystallized  phenol  with  ten  parts  of  concentrated  sulfuric  acid.  After  cooling 
there  is  added  three  volumes  of  water  and  one  volume  of  concentrated  hydro- 
chloric acid  —  the  latter  heightening  the  delicacy  of  the  reaction. 

The  process  is  to  evaporate. to  dryness  on  the  water  bath  in  small  porcelain 
dishes  ten  Cc.  of  the  water  and  the  same  volume  of  a  standard  solution  of 
potassium  nitrate.  The  accuracy  is  said  to  be  increased  if  a  little  sodium  car- 
bonate be  added  to  prevent  loss  of  nitric  acid.  The  residues  are  stirred  up 
with  one  Cc.  of  the  reagent.  Where  large  amounts  of  nitrate  are  in  the  water 
the  solution  quickly  assumes  a  red  color,  in  good  potable  water  not  for  ten 
minutes.  After  standing  for  16  minutes,  the  solutions  are  washed  into  meas- 
uring jars  and  after  adding  a  slight  excess  of  ammonia,  diluted  to  100  Cc.  The 
yellow  liquids  are  compared  colorimetrically,  and  another  test  made  with  a 
standard  approximating  the  concentration  of  the  water. 

Chlorides  interfere  with  the  reaction  and  are  to  be  removed  by  silver  sulf  ate ; 
or  the  standard  may  have  a  corresponding  addition  of  sodium  chloride  —  sul- 
f ates  have  no  effect.  Should  the  water  be  yellow  originally  it  is  decolorized  by 
aluminum  hydrate  and  filtered. 

(2)  The  Indigo  process.  Sulflndigotic  acid  is  decolorized  by  free  nitrous  and 
nitric  acids,  not  however  in  strict  molecular  proportion.    The  sulfindigotic  acid 
is  made  by  digesting  indigotin  in  fuming  sulfuric  acid,  the  solution  diluted 
somewhat  and  the  strength  determined  by  a  test  with  potassium  nitrate,  then 
further  diluted  until  one  Cc.  is  bleached  by  approximately  one  Cc.  of  N/100 
potassium  nitrate  solution. 

The  process  is  tentative  and  rather  tedious,  and  requires  close  attention  to 
details.  To  20  Cc.  of  the  water  in  a  small  flask  is  added  a  measured  volume 
of  the  indigo  solution,  as  much  as  it  is  thought  will  be  decolorized,  then  a 
volume  of  concentrated  sulfuric  acid  equal  to  that  in  the  flask.  On  heating  for 
five  minutes  in  the  water  bath  either  the  solution  is  completely  decolorized  or 
remains  blue.  The  test  is  repeated  with  greater  or  smaller  amounts  of  indigo 
solution  until  the  equivalent  is  arrived  at — that  is,  when  the  liquid  remains 
but  faintly  blue.  Then  it  is  ascertained  what  volume  of  standard  potassium 
nitrate  solution  will  produce  the  same  tint  with  the  same  volume  of  indigo 
solution  operating  under  the  same  conditions  as  before. 

(3)  Mueller's  method.    Diphenylamin  in  concentrated  sulfuric  acid  gives  a 
blue  coloration  with  nitric  acid.    Into  a  small  test-tube  is  poured  5  Cc.  of  a 
solution  of  the  reagent  (.200  gram  in  a  liter  of  concentrated  sulfuric  acid)  and 
one  Cc.  of  the  water.    Shortly  a  blue  color  is  developed.    Comparison   tests 
are  made  with  solutions  of  potassium  nitrate  ranging  from  one-half  to  five 
milligrams  of  nitric  acid  per  liter.    If  nitrous  acid  is  present  in  the  water  it 
may  be  oxidized  to  nitric  by  potassium  permanganate  and  both  read  as  nitric. 

(4)  Hooker's  method.  A  solution  of  carbazol  (diphenylimid,  C6H4.NH.C6H4) 
in  sulfuric  acid  yields  a  green  color  with  nitric  acid.    One  hundred  Cc.  of  the 
water  is  treated  with  sufficient  silver  sulfate  to  precipitate  the  chlorine  from 
the  chlorides  of  the  water,  together  with  some  aluminum  sulfate,  which  is 
decomposed  by  the  carbonates  of  the  water  with  precipitation  of  aluminum  hy- 
drate which  assists  in  filtration.    The  liquid  is  made  up  to  a  volume  of  110  Cc. 
and  filtered ;  to  two  Cc.  of  the  filtrate  is  added  double  the  volume  of  concen- 
trated sulfuric  acid  and  the  mixture  cooled,  then  one  Cc.  of  a  .04  per  cent  solu- 
tion of  carbazol  in  concentrated  sulfuric  acid.    In  a  few  minutes  a  bright  green 
color  appears.    The  standards  are  the  same  as  in  (3). 

(5)  Reduction  to  ammonia.  Nascent  hydrogen  reduces  nitrous  and  nitric 
Acids  to  ammonia  — 

NaNO3  +  2H  =  NaNO2  -f  H2O ;  and  NaNO2  -f  6H  =  NH8  -f  NaOH  -f  H2O. 


NATURAL  WATER.  379 

For  the  generation  of  hydrogen  in  the  water  various  metals  and  combinations 
of  metals  have  been  proposed;  iron  and  zinc  in  acid  solution,  and  sodium-amal- 
gam, aluminum,  copper-zinc,  and  aluminum-copper-zinc  alloy  in  alkaline 
solution.  The  copper -zinc  couple  is  composed  of  a  sheet  of  zinc  coated  with 
spongy  copper,  made  by  immersing  the  zinc  in  a  solution  of  cupric  sulfate. 
Aluminum  has  largely  taken  the  the  place  of  other  metals  as  it  is  quite  as  effi- 
cient as  any,  convenient,  and  readily  obtained  pure.  The  reaction  is  A12-|- 
SNaOH  =  3Na2O.  A12O3  -f  6H. 

With  50  Cc.  of  the  water  is  compounded  a  little  sodium  hydrate  solution 
(made  from  sodium  and  distilled  water),  and  a  strip  of  aluminum  foil  immersed 
in  the  liquid.  After  standing  for  from  18  to  24  hours  a  portion  of  the  liquid  is 
withdrawn,  and  from  1  to  25  Cc.,  according  to  the  amount  of  ammonia,  Ness- 
lerized.  If  clear  and  colorless  it  is  unnecessary  to  distill  the  ammonia  and  the 
test  is  applied  direct  after  dilution  with  water  (free  from  carbonic  acid  which 
might  precipitate  aluminum  hydrate).  A  correction  is  made  for  nitrous  acid. 
Some  ammonia  is  carried  off  mechanically  by  gaseous  hydrogen;  if  the  con- 
tainer is  a  flask  fitted  with  a  guard  tube  of  dilute  hydrochloric  acid  the  ammonia 
may  be  caught  and  Nesslerized ;  Hazen  instead  makes  a  correction  based  on 
the  theoretical  amount  carried  off.  The  results  by  this  method  as  compared 
with  the  phenolsulfuric  method  are  lower  according  to  Gill,  higher  according 
to  Hazen  and  Clark. 

5.  Nitrites.  The  methods  for  nitrous  acid  are  all  colorimetric,  that  of  titra- 
tion  to  nitrate  with  potassium  permanganate  being  seldom  available  on  account 
of  the  minute  proportion  of  the  nitrite  to  be  determined,  and  the  organic 
matter  accompanying  it. 

(1)  Method  of  Griess.*  Para-amidobenzene-sulfonic  acid   (sulfanilic   acid, 
C6H4.NH2.HS03)  with  alpha-amido-napthalin-acetate  (or  napthylamine,  C10H7. 
NH2)  in  acetic  solution  produce  with  nitrous  acid  the  compound  amid-alpha- 
napthyl-azobenzene-sulfonic  acid,  coloring  the  liquid  pink  to  red  — 

C10H7.NH2  +  C6H4.NH2.HS03  -f  HNO2  ==  C10H6(NH2).N2.C6H4.  HSO3  +  2H2O. 

Two  Cc.  of  solutions  of  each  of  the  above  reagents  are  added  to  25  Cc.  of 
the  water  held  in  a  graduated  cylinder.  One  Cc.  or  more  of  a  standard  solution 
of  sodium  nitrite  (made  from  silver  nitrite  and  sodium  chloride)  is  made  up 
with  pure  water  to  25  Cc.  and  the  same  quantities  of  reagents  added.  The 
comparison  of  the  colors  may  at  first  be  done  by  dilution,  but  finally  by  adjust- 
ing the  concentration  of  the  standard  as  regards  nitrous  acid  to  approximately 
equal  that  of  tfcp  sample  of  water. 

Green  and  Evershed  f  prefer  anilin  hydrochloride  to  sulfanilic  acid  as  the 
color  is  more  rapidly  developed.  Griess  has  also  proposed  metaphenylene - 
diamin  which  with  nitrous  acid  forms  triamido -benzene  (Bismark  brown, 
NH2.C6H4.N:N.C6H3(NH2)2). 

(2)  Trommsdorf's  method  is  based  on  the  liberation  of  iodine  from  zinc  oxide 
by    nitrous    acid  —  Znl2  +  2NaNO2+2H2S04=sI2-fN2O2-f  Na2SO4  +  ZnSO4 + 
2H2O.    In  presence  of  starch-paste  the  liquid  becomes  blue  in  proportion  to 
the  iodine  liberated. 

To  100  Cc.  of  the  water  is  added  3  Cc.  of  a  solution  containing  zinc  iodide 
and  starch  paste,  then  one  Cc.  of  dilute  sulfuric  acid.  Into  four  cylinders  are 
placed  for  comparison  100  Cc.  of  distilled  water  with  one  to  four  Cc.  of  standard 
sodium  nitrite  and  one  Cc.  of  dilute  sulfuric  acid.  Thresh  substitutes  potas- 
sium iodide  for  the  zinc  compound,  and  previous  to  the  test,  shakes  up  the 
water  with  air  to  insure  saturation  with  oxygen.  He  states  that  the  time, 

*  Berichte,  21—1830. 

t  Chem.  News  1892—1—109. 


380  QUANTITATIVE    CHEMICAL   ANALYSIS. 

temperature,  quantities  of  potassium  iodide  and  starch,  and  oxygen  held  by  the 
water  must  be  constant,  when  the  depth  of  color  and  rapidity  of  its  formation 
are  proportional  to  the  nitrous  acid. 

According  to  Musset,  bacteria  give  a  blue  color  with  zinc-starch-iodide  and 
acetic  acid,  hence  the  water  is  to  be  acidified  by  sulfuric  acid  and  distilled,  and 
the  nitrous  acid  determined  in  the  distillate.  Proskauer  removes  bacteria  by 
filtering  through  a  thick  washed  filter  paper. 

6.  Bacteria.  Supplementing  the  chemical  examination,  a  bacteriological  test 
is  valuable,  at  least  as  confirmatory  evidence  of  the  quality  of  a  water.  Many 
chemists  consider  that  a  positive  opinion  as  to  the  quality  can  only  be  formed 
from  a  consideration  of  the  source  of  the  water  and  its  environments,  the 
chemical  composition,  and  the  bacteriological  condition;  yet  others  do  not 
attach  so  much  importance  to  the  last  named  as  to  the  two  former.* 

A  test  can  be  made  by  introducing  a  drop  of  the  water  into  a  medium  of  sterilized 
gelatin,  and  after  a  certain  period,  examining  the  culture  for  pathogenic  germs. 
A  bouillon  culture  may  be  injected  into  the  abdominal  cavity  of  a  small  animal, 
observing  if  disorders  or  death  follows  the  inoculation.  Directions  for  enu- 
merating, recognizing  and  differentiating  the  various  species  will  be  found  in 
works  on  bacteriology.  

Standards.  For  boiler  use  any  considerable  proportion  of  inorganic  matter, 
especially  of  calcium  and  magnesium  compounds,  is  detrimental,  while  a 
moderate  quantity  of  organic  matter  is  of  no  consequence.  Scale  is  formed  by 
the  deposition  of  suspended  particles,  the  dehydration  of  soluble  silicic  acid, 
the  decomposition  of  calcium  and  magnesium  bicarbonates,  and  the  crystal- 
lization of  calcium  sulfate.  Soluble  salts  above  a  certain  proportion  are 
liable  to  cause  the  water  in  a  boiler  to  foam  at  temperatures  above  100  °  . 

Free  sulfuric  acid  above  five  parts  per  hundred  thousand  of  water  causes 
corrosion  of  the  boiler  shell,  and  it  is  said  that  magnesium  chloride  and  sulfate 
are  especially  objectionable  on  this  score  on  the  assumption  that  the  ordinary 
temperature  and  pressure  of  the  water  in  a  boiler  are  together  sufficient  to  dis- 
sociate these  compounds  with  liberation  of  hydrochloric  and  surf  uric  acids 
(e  g.,  MgCla  +  H2O  =  MgO  +  2HC1),  yet  do  not  suspend  the  action  of  the  free 
acids  on  iron. 

In  pronouncing  on  the  quality  of  a  water  for  boiler  purposes,  the  standards 
of  excellence  will  differ  considerably  according  to  the  locality  in  which  the 
water  is  found.  The  supplies  in  many  regions  through  the  Western  States  are 
almost  exclusively  of  a  quality  that  would  be  condemned  nearer  the  coast,  and 
in  the  table  below,  five  or  ten  grains  may  be  added  to  the  figures  there  given. 
The  ft  incrusting  solids  "  include  whatever  is  precipitated  by  the  concentration 
of  the  water  (the  evaporation  never  extends  to  dryness)  in  the  boiler,  namely 
suspended  matter,  silica,  carbonates  of  calcium  and  magnesium,  and  calcium 
sulfate ;  some  would  include  the  magnesia  from  the  sulfate  and  chloride.  All 
of  the  other  inorganic  compounds  are  grouped  as  "  non-incrusting "  or 
"foam-causing". 

The  following  conventional  table  is  applicable  to  the  ordinary  waters  of  the 
Eastern  and  Central  States.  The  quantities  are  in  grains  per  U.  S.  gallon. 

Quality.  Incrusting.    Non-incrusting. 

Good Below  10  Below  10 

Fair  to  passable 10  to  20  1*0  to   15 

Poor  20  to  40  15  to  35 

Very  poor  -.  »  40  to  60  35  to  50 

Unfit  for  use  Above  60  Above  50 


*  Chem.  News,  1893—2—  207. 


NATURAL  WATER.  381 

For  manufacturing  purposes  in  general  the  most  harmful  impurities  are  free 
acids  (drainage  from  factories)  large  amounts  of  total  solids,  especially  if  hard 
toward  soap,  and  organic  matter.  Special  products,  that  depend  for  their  value 
on  qualities  of  taste,  odor,  color,  clearnes|,  etc.,  require  water  free  from  what- 
ever would  modify  or  impair  them. 

The  wholesomeness  of  a  drinking  water  is  not  compromised  by  a  moderate 
amount  of  earthy  and  alkali  compounds  nor  organic  matter.  But  aside  from 
the  natural  aversion  to  the  use  of  water  containing  vegetable  or  animal  refuse 
there  remains  the  fact  that  organic  matter  especially  if  putrescent,  is  an  excel- 
lent medium  for  the  growth  of  pathogens  and  it  is  conceded  that  unwhole- 
some water  is  responsible  for  the  origin  and  spread  of  certain  diseases.  Free 
ammonia,  nitrous  and  nitric  acids,  albuminous  bodies  and  chlorine  are  indica- 
tions of  sewage  contamination,  and  their  presence  above  certain  minimum 
amounts,  intelligently  interpreted,  lead  the  chemist  to  condemn  a  water  on  san- 
itary grounds.  The  subject  is  too  extensive  to  be  considered  at  length  in  this 
place;  reference  may  be  had  to  the  recent  works  on  water  analysis. 

No  absolute  standard  can  be  laid  down  to  which  a  drinking  water  must  con- 
form to  be  considered  wholesome,  and  no  single  analysis  is  a  sufficient  warrant 
for  passing  judgment  where  the  sources  of  the  supply  are  subject  to  modification 
periodically  or  irregularly.  There  must  be  considered  the  geological  strata  per- 
meated by  the  feeders,  the  size  and  character  of  the  well  or  reservoir,  and  in 
surface  waters,  the  area  of  the  water-shed  and  whether  thickly  or  sparsely 
populated,  the  contiguity  of  drains  and  sewers,  and  any  special  conditions  that 
might  allow  temporary  or  permanent  contamination.  A  well  or  stream  may 
receive  organic  impurities  from  drainage  of  the  soil  in  populated  districts  or  by 
infiltration  of  sewage,  storm  water  carrying  in  surface  filth,  roots  of  trees 
reaching  a  well,  dripping  back  from  pumps,  and  dust  and  leaves  blown  in. 

The  color,  odor  and  reaction,  and  an  examination  of  the  sediment  deposited 
on  standing  may  give  valuable  hints  as  to  the  past  or  present  condition. 

In  general  a  potable  water  may  be  considered  passable  when  the  following 
limits  per  100000  parts  of  water  are  not  exceeded. 

(1)  Residue  from  evaporation,  50  parts;    but  this  amount  of   inorganic 
solids  is  often  exceeded  in  undoubtedly  wholesome  waters.    The  less    the 
amount  of  organic  matter  of  any  kind  in  the  residue,  the  safer  the  water. 

(2)  Sodium  chloride,  one  part;  the  source  of  the  water  always  to  be  con- 
sidered. 

(3)  Free  ammonia,  .005  part,  and  albumenoid  ammonia,  .015  part. 

(4)  Of  potassium  permanganate  consumed,  one  part.    Wanklyn  states  that 
a  water  of  first-class  purity  does  not  consume  more  than  .0005  gram  oxygen 
per  liter;  average  drinking  water,  .002  to  .003;  and  dirty  water,  considerably 
more  than  .003. 

(5)  Nitric  acid,  .05  part,  and  nitrous  acid  .0005  part. 

(6)  Iron,  lead,  or  copper,  .2  part. 

In  drawing  a  conclusion  the  above  limits  are  to  be  viewed  in  their  inter- 
relation as  well  as  separately.  Wigner  adopts  a  scale  of  purity,  allowing  one 
unit  for  a  certain  amount  of  each  impurity,  which  he  claims  can  be  applied  to 
every  potable  water. 


382  QUANTITATIVE    CHEMICAL   ANALYSIS. 


FERTILIZERS. 

Commercial  fertilizers  may  be  manures,  vegetable  matter  or  its  ashes,  animal 
or  fish  refuse,  the  waste  products  of  manufactories,  minerals  in  their  natural 
state  or  after  chemical  treatment,  or  naturally  occurring  salts.  The  most  valu- 
able ingredients  of  a  fertilizer  are  nitrogen,  potash  and  phosphoric  acid, 
though  other  bodies  are  esteemed  for  certain  conditions  of  soil  and  plant.  In 
the  valuation  of  a  sample  the  state  of  combination  of  these  bodies  is  of  im- 
portance for  the  reason  that  plants  can  more  readily  assimilate  some  combina- 
tions than  others. 

In  regard  to  analytical  processes  fertilizers  may  be  divided  into  three 
groups:  (1),  organic  material,  containing  more  or  less  inorganic;  (2),  phos- 
phates, natural  or  chemically  altered;  and  (3),  alkali  salts.  The  preliminary 
treatment  and  the  course  of  analysis  differ  to  some  extent. 

The  organic  matter  in  a  fertilizer  is  invariably  destroyed  previous  to  the 
determination  of  the  inorganic  constituents.  If  done  by  ignition  in  air  the 
heat  must  be  kept  under  control  lest  it  become  so  high  as  to  volatilize  some 
alkali  chloride;  many  simply  char  the  sample,  then  lixiviate  the  soluble  alkali 
salts,  burn  the  remainder  to  an  ash,  return  the  solution,  and  evaporate  to 
dryness.  Saturation  of  the  sample  with  sulfuric  acid  before  ignition  facili- 
tates the  combustion,  and  at  the  same  time  allows  a  higher  heat  since  alkali 
sulfates  are  less  volatile  than  chlorides.  Instead  of  calcination,  the  organic 
matter  may  be  oxidized  by  boiling  with  aqua  regia,  fuming  nitric  acid,  nitric 
acid  with  sodium  chlorate,  or  other  strong  oxidizer. 

While  the  determination  of  phosphoric  acid  in  a  phosphate  or  superphos- 
phate presents  no  great  difficulties,  yet  it  so  complicates  the  determination  of 
the  bases  that  its  previous  removal  is  always  to  be  advised  unless  the 
operator  has  had  an  extended  experience  in  the  analysis  of  bodies  of  this 
nature. 

Mineral  salts,  such  as  ammonium  sulfate,  are  analyzed  by  the  usual  methods 
for  commercial  salts  in  a  more  or  less  pure  condition,  and  need  no  special 
mention. 

PHOSPHORIC  ACID. 

In  the  manufacture  of  the  commercial  product  known  as  '  superphosphate  % 
tri-calcic  phosphate,  Cas(PC)4)2,  is  treated  with  sulfuric  acid.  The  proportion 
of  acid  to  phosphate  determines  the  composition  of  the  resulting  product, 
thus  — 

Ca3(PO4)2  4-  H2SO4  =  Ca2H2(PO4)2  (dicalcic  phosphate)  -f  CaSO4. 

Ca3(P04)2  -f-  2H2SO4  =  CaH4(PO4)2  (monocalcic  phosphate)  +  2CaSO4. 

Ca3(PO4)2  +  3H2S04  =  2H3PO4  (phosphoric  acid)  +  3CaSO4. 
The  product  is  of  value  to  the  agriculturist  in  proportion  as  it  is  soluble 
in  water.  But  the  monocalcic  phosphate  suffers  by  age  a  peculiar  retrogression 
or  reversion,  gradually  passing  to  a  form  insoluble  in  water  though  soluble  in 
weak  acids.  The  reversion  is  said  to  be  due  principally  to  a  reaction  between 
monocalcic  and  tri-calcic  phosphates  —  CaH4(PO4)2  -f  Ca3(PO4)2  =  2Ca2H2(PO4)2 
(dicalcic  phosphate).  A  gradual  evolution  of  hydrofluoric  acid  in  mineral 
phosphates  containing  fluorine  is  said  also  to  cause  reversion. 


FERTILIZERS.  383 

We  may  therefore  in  a  commercial  superphosphate  distinguish  three  combi- 
nations of  calcium  with  phosphoric  acid,  viz. 

1.  That  soluble  in  cold  water;  either  free  phosphoric  acid  or  monocalcium 
phosphate,  also  acid  magnesium  phosphate.    This  form  is  the  most  valuable  as 
a  fertilizer. 

2.  That  insoluble  in  water  but  soluble  in  solutions  of  certain  salts  and  in  weak 
acids;  dicalcium  phosphate,  also  iron  and  aluminum  phosphates.    It  is  gener- 
ally considered  of  somewhat  less  value  than  that  of  (1),  but  much  more  so  than 
that  of  (3). 

3.  That  insoluble  in  water  and  weak  acids  but  soluble  in  strong  acids; 
tricalcium  phosphate. 

Total  phosphoric  acid.  The  sample  is  dissolved  in  a  mineral  acid  that  may 
be  hydrochloric,  nitric  or  sulf uric.  When  organic  matter  is  in  the  sample  some 
oxidizer  is  added  to  the  acid  to  destroy  it,  or  it  may  be  burned  by  a  previous 
ignition  in  the  air  provided  there  is  no  organic  phosphorus  present.  The  solu- 
tion is  diluted  with  water  to  a  definite  volume,  and  an  aliquot  part,  containing 
a  convenient  weight  of  phosphoric  acid,  filtered  for  the  determination. 

A  number  of  methods  have  been  proposed  of  which  some  have  become  obso- 
lete on  account  of  their  complexity,  though  yielding  correct  results.  The  best 
known  are  as  follows: 

1.  Phosphoric  acid  forms  insoluble  compounds  with  lead,  silver,  stannic  tin, 
ferric  iron,  and  mercuric  mercury.    Silver  phosphate  precipitates  only  from  a 
neutral  solution  that  may  be  secured  by  nearly  neutralizing  the  solution  by  an 
alkali  and  stirring  in  silver  carbonate.    Stannic  phosphate  is  formed  when 
metallic  tin  is  oxidized  by  hot  concentrated  nitric  acid  containing  phosphoric 
acid.    Ferric  phosphate  is  carried  down,  associated  with  basic  ferric  acetate, 
when  the  solution  of  the  phosphate  is  mixed  with  ferric  chloride  and  precipitated 
hot  by  an  alkali  acetate.    Although  these  methods  afford  a  good  separation  of 
phosphoric  acid  from  calcium  they  are  not  much  in  use  for  fertilizer  analysis. 

2.  The  citric  acid  method.     When  an  acid  solution  of  phosphates  of  calcium, 
magnesium,  iron,  aluminum,  etc.,  is  made  alkaline  by  ammonia  there  forms  a, 
precipitate  which  is  a  mixture  of  the  phosphates  of  the  various  bases.    But  if 
previous  to  the  addition  of  ammonia  there  be  added  enough  of  a  magnesium 
salt  to  combine  with  all  of  the  phosphoric  acid,  and  enough  citric  or  tartaric 
acid  to  form  citrates  or  tartrates  of  the  other  bases,  then  on  supersaturation  by 
ammonia  there  is  precipitated  only  ammonium  magnesium  phosphate. 

The  acid  solution  of  the  phosphate  is  mixed  with  a  sufficiency  of  magnesic 
citrate,  then  with  a  large  excess  of  ammonia,  and  allowed  to  stand  for  several 
hours.  The  precipitate  is  filtered,  washed  with  dilute  ammonia,  and  ignited  to 
the  pyrophosphate  Mg^O?;  if  necessary,  the  pyrophosphate  may  be  dissolved 
in  acid  and  any  residue  of  silica  filtered  and  weighed  and  the  weight  deducted. 

A  volumetric  determination  of  the  precipitate  may  take  the  place  of  ignition 
and  weighing.  It  is  dissolved  in  an  excess  of  standard  acid  and  titrated  back 
by  standard  alkali. 

The  results  of  the  citrate  method  are  fairly  accurate,  the  gain  due  to  the  co- 
precipitation  of  the  phosphates  of  other  bases  of  higher  molecular  weight  than 
magnesium  being  compensated  to  some  degree  by  the  solubility  of  the  precipi- 
tate in  the  citrate  solution.  It  is  said  that  for  each  base  the  magnesium  in  the 
solution  must  bear  a  certain  ratio  to  the  citric  acid.  According  to  Neubauer, 
on  igniting  magnesium  pyrophosphate  there  is  a  loss  by  volatilization  of  phos- 
phoric acid  ranging  from  .6  to  3  per  cent  or  more  of  the  pyrophosphate,  accord- 
ing to  its  weight,  and  the  result  should  be  corrected  accordingly. 


384  QUANTITATIVE    CHEMICAL    ANALYSIS. 

3.  The  Molybdate  method.  Here  the  phosphoric  acid  is  precipitated  as 
ammonic  phosphomolybdate  from  an  acid  solution.  The  free  acid  should 
be  nitric  in  all  cases ;  a  hydrochloric  solution  is  evaporated  to  a  small  bulk 
after  addition  of  concentrated  nitric,  when  the  remaining  chlorine  will  not  be 
in  quantity  great  enough  to  interfere  with  the  precipitation ;  a  sulfuric  solu- 
tion is  nearly  neutralized  by  ammonia,  and  ammonium  nitrate  added. 

On  the  addition  of  a  solution  of  molybdic  acid  in  nitric  acid  and  ammonium 
nitrate  there  forms  a  granular  yellow  precipitate,  slowly  if  allowed  to  stand 
without  agitation,  more  rapidly  if  stirred  and  heated.  After  filtering  and 
washing  with  either  the  diluted  precipitant,  a  weak  solution  of  ammonic  nitrate 
or  sulfate,  or  simply  cold  water,  the  precipitate  may  be  further  treated  in  one 
of  several  ways. 

A.  By  direct  weight.*    A  determination  made  by  drying  and  weighing  the 
yellow  precipitate  is  doubtless  quite  as  accurate  as  by  any  volumetric  method 
depending  on  the  invariability  of  the  composition  of  the  precipitate.    To  insure 
a  definite  composition  close  attention  must  be  paid  to  the  temperature  and 
acidity  of  the  solution  and  reagent,  the  time  of  repose,  agitation  [and  other 
conditions.    The  precipitate  is  filtered  on  a  tared  paper  or  through  a  Gooch 
crucible,  and  the  washing,  or  at  least  the  final  washing,  is  done  with  water. 
According  to  most  authorities  the  precipitate  approaches  the  composition  of 
48MoO3.2P2O5.10NH3. 1 1H2O. 

On  heating  the  precipitate  to  400  to  500°  ,  it  loses  ammonia  and  becomes  a 
molybdenum  phosphate,  containing  4.018  per  cent  of  phosphoric  acid,  of  almost 
a  black  color  and  not  hygroscopic;  overheating  leaves  a  gray  mass  indicating 
the  separation  of  some  molybdic  acid. 

B.  The  Molybdate-magnesia  method.  The  yellow  precipitate  is  dissolved  in 
dilute  ammonia  which  decomposes  it  to  ammonium  phosphate  and  ammonium 
molybdate,  both  freely  soluble.    The  solution  is  nearly  neutralized  by  hydro- 
chloric acid  and  heated  until  any  co-precipitated  silica  or  alumina  separates.    To 
the  filtrate  is  added  a  solution  of  ammonium  magnesium  chloride  and  consider- 
able free  ammonia ;  a  crystalline  precipitate  of  ammonic  magnesic  phosphate 
falls,  which  is  to  be  washed  with  dilute  ammonia,  ignited,  and  weighed  as  mag- 
nesium pyrophosphate,  Mg2P207. 

C.  Pemberton's  method  is  to  obtain  the  phosphate  in  a  nitric  acid  solution, 
neutralize  the  free  acid  by  ammonia,  acidify  with  strong  nitric  acid,  and  add 
ammonium  nitrate.    The  solution  is  then  heated,  and  an  aqueous  solution  of 
ammonium   molybdate    stirred   in.    The  ammonium  molybdate  immediately 
decomposes  with  the  free  nitric  acid  to  form  ammonium  nitrate  and  collodial 
molybdic  acid  which  remains  in  solution.    The  yellow  precipitate  segregates 
quickly,  and  is  filtered  and  washed  with  water.    The  filter  and  precipitate  are 
thrown  into  a  beaker  and  dissolved  in  an  excess  of  standard  potassium  hydrate  — 
(NH4)6(Mo03)24(P04)2  +  46KOH  =  (NH4)4H2(PO4)2  +  (NH4)2MoO4  +  23K2MoO4 
-f  22H2O.    The  excess  of  potassium  hydrate  is  then  titrated  back  by  a  standard 
acid  and  phenol-phthalein. 

D.  The  yellow  precipitate,  formed  under  fixed  conditions,  may  be  dissolved 
in  dilute  ammonia,  acidified  by  sulfuric  acid,  the  molybdic  acid  reduced  by  zinc 
to  a  lower  oxide,  and  the  latter  determined  by  titration  with  standard  perman- 
ganate back  to  the  trioxide  (page  340;. 

E.  Jabert  determines  the  ammonia  of  the  yellow  precipitate  by  decomposing 
the  precipitate  by  potassium  hydrate  solution  and  distilling  the  ammonia  into 


*  Journ.  Amer.  Chem.  Socy.  1896—23. 


FERTILIZERS.  385 

standard  sulfuric  acid.  The  excess  of  sulfuric  acid  is  found  by  titration  with 
standard  alkali.  The  relation  of  ammonia  to  phosphoric  acid  in  the  precipi- 
tate is  claimed  to  be  more  constant  than  the  relation  of  phosphoric  to  molybdic 
acid  or  to  the  entire  precipitate. 

4.  The  Uranium  method.  Leconte  first  proposed  that  the  reaction  between 
uranium  and  phosphoric  acid  be  utilized  as  a  method  for  the  latter  body. 
When  a  salt  of  uranium  and  phosphoric  acid  are  mixed  there  is  precipitated 
hydrogen  uranyl  phosphate,  a  yellow  compound  insoluble  in  water  and  dilute 
acids ;  if  an  ammonium  salt  be  present  in  the  solution  the  precipitate  is  ammo- 
nium uranyl  phosphate.  The  relation  of  uranium  to  phosphoric  acid  is  the 
same  in  both  —  Ur2O3P2O5  +  aq,  and  Ur2O3.2NH4O.P205  +  aq. 

A.  Gravimetric.  The  method  due  to  Button  is  available  only  when  there  is  no 
iron  or  aluminum  in  the  phosphate  solution.    The  solution  is  prepared  so  as 
to  contain  only  acetic  as  a  free  acid,  and  precipitated  at  a  boiling  heat  by 
uranium  acetate.    The  precipitate,  at  first  slimy,  becomes  crystalline  on  wash- 
ing by  decantation  with  hot  water.    On  drying  and  ignition  it  passes  to  uranium 
phosphate  UrPO4.    This  process  has  been  superseded  by  the  volumetric  one 
following. 

B.  Volumetric.  The  titration  of  the  phosphoric  solution  is  done  by  a  stand- 
ard solution  of  uranic  nitrate.    The  end -point  is  observed  by  spotting  a  drop 
of  the  titrate  with  a  drop  of  a  freshly  prepared  solution  of  potassium  ferricy- 
anide,  uranium  ferricyanide  appearing  as  a  chocolate  brown  coloration.    The 
end -point  is  not  manifested  as  sharply  as  could  be  desired.* 

As  applied  to  a  superphosphate  the  solution  in  hydrochloric  acid  is  first  pre- 
cipitated by  magnesic  solution,  citric  acid,  and  ammonia,  thus  separating  iron 
and  other  bases  that  might  interfere  in  the  titration.  The  washed  precipi- 
tate is  dissolved  in  dilute  nitric  acid,  nearly  neutralized  by  ammonia,  and  enough 
ammonium  acetate  added  that  all  the  free  acid  may  be  acetic.  The  solution 
is  then  heated  to  boiling  and  titrated  by  uranium  nitrate  that  has  been  set 
against  sodium  or  ammonium  phosphate  or  ammonic  magnesic  phosphate. 

The  process  is  rapid,  and  fairly  accurate  when  certain  precautions  have  been 
observed,  among  which  is  that  the  standardization  and  the  titration  be  done  on 
the  same  material  and  under  like  conditions. 

Soluble  phosphoric  acid.  For  the  determination  of  the  soluble  phosphates  of 
a  superphosphate  a  large  weight  of  the  sample  is  treated  with  cold  water. 
Some  direct  to  lixiviate  several  times  with  small  quantities  of  water,  others 
(believing  that  iron  and  aluminum  phosphates  are  soluble  in  a  concentrated 
solution  of  monocalcium  phosphate)  treat  with  a  large  volume  at  once.  The 
mixture  is  filtered  through  a  dry  paper  and  the  residue  washed  with  water  until 
the  filtrate  and  washings  reach  a  certain  volume;  from  this  an  aliquot  part  is 
withdrawn  for  the  determination  which  may  be  made  according  to  any  of  the 
methods  outlined  for  total  phosphoric  acid. 

If  there  be  no  other  free  acid  in  the  sample  the  amount  of  free  phosphoric 
acid  may  be  determined  volumetrically.  Kalman  and  Messels  f  state  that  if  a 
solution  of  acid  calcium  phosphate  be  titrated  by  caustic  alkali  and  phenol- 
phthalein,  too  high  results  will  be  obtained  on  the  assumption  that  one  mole- 
cule of  alkali  saturates  one-half  molecule  of  phosphoric  acid ;  but  if  now  the 
liquid  be  filtered  and  the  filtrate  titrated  back  by  standard  hydrochloric  acid 
and  methyl  orange,  the  result  of  this  titration  will  be  as  much  too  low  as  the 


*  Crookes,  Select  Methods,  2d  Ed.  524. 
t  Chem.  News,  1895-2-28. 

25 


386  QUANTITATIVE    CHEMICAL    ANALYSIS. 

former  was  too  high,  and  the  arithmetical  mean  of  the  two  will  be  the  correct 
acidimetric  value. 

Another  method  is  that  of  titrating  the  solution  directly  by  standard  potas- 
sium hydrate  and  methyl  orange ;  when  the  red  color  disappears  there  re- 
mains monocalcium  phosphate.  If  now  there  be  added  to  the  titrate  some 
calcium  chloride,  the  reaction  CaH4(PO4)2  +  2CaCl2  =  Ca3(PO4)2  +  4HC1,  or 
2KH2PO44-3CaC]2  =  Ca3(P04)2  +  2KCl  +  4HCl  ensues,  and  the  titration  is 
continued  with  phenol-phthalein.  With  methyl  orange  as  indicator  one  mole- 
cule of  potassium  hydrate  neutralizes  one  molecule  of  phosphoric  acid  forming 
KH2PO4,  while  with  phenolphthalein  the  ratio  is  as  one  to  one -half. 

Reverted  phosphoric  acid.  The  determination  of  this  form  of  phosphate  is 
essentially  conventional  and  should  be  carried  on  under  conditions  sanctioned 
by  the  majority  of  chemists  engaged  in  this  line  of  work.*  The  problem,  as 
yet  not  solved  with  entire  satisfaction,  is  to  find  some  liquid  that  will  extract 
the  reverted  phosphate  without  attacking  the  insoluble  phosphoric  compounds. 
Among  the  reagents  that  have  been  proposed  are  very  dilute  solutions  of  or- 
ganic acids,  ammonium  oxalate,  ammonium  sulfate,  etc.,  but  ammonium  citrate 
has  been  generally  adopted.  The  solution  advised  by  the  American  Associ- 
ation of  Official  Agricultural  Chemists  is  strictly  neutral  and  of  the  specific 
gravity  of  1.09  at  20  °  Cent.  Into  100  cubic  centimeters  of  the  citrate  solution, 
heated  to  65° ,  is  dropped  the  filter  containing  the  residue  from  the  water  ex- 
traction; the  flask  is  stoppered  and  vigorously  shaken,  then  heated  on  the 
water  bath  to  65°  for  one -half  hour.  The  solution  is  filtered  and  washed  with 
water  at  65  °  . 

The  citrate  solution  may  be  treated  in  different  ways:  (1),  the  phosphoric 
acid  may  be  determined  in  an  aliquot  part;  (2),  a  portion  united  with  a  corre- 
sponding part  of  the  water  solution  and  the  phosphoric  acid  determined  in  the 
mixture;  or  (3),  it  may  be  rejected,  and  the  difference  between  the  total  acid 
of  the  sample  and  that  of  the  water-soluble  plus  insoluble  acid  called  the 
reverted  acid.  The  reason  for  the  last  named  course  is  the  belief  that  the  large 
amount  of  organic  matter  such  as  citric  acid  would  hinder  complete  precipita- 
tion by  the  usual  reagents ;  it  has  been  found,  however,  that  as  regards  the 
molybdate  process,  citric  acid  in  moderate  quantity  is  without  practical  effect  — 
probably  different  kinds  of  organic  matter  have  an  unequal  power  as  regards 
holding  up  phosphoric  acid. 

Ross  proposes  to  destroy  the  citric  acid  and  other  organic  matter  by  evapo- 
rating the  solution  with  concentrated  sulfuric  acid  and  mercuric  oxide,  diluting 
and  neutralizing  by  ammonia,  acidifying  by  nitric  acid,  and  precipitating  by 
molybdate  solution. 

Insoluble  phosphate.  The  residue  from  the  citrate  extraction  is  ignited  to 
burn  organic  matter,  then  dissolved  in  hydrochloric  acid,  or  at  once  dissolved 
in  aqua  regia,  and  the  phosphoric  acid  determined  in  an  aliquot  part  by  one  of 
the  usual  methods. 

POTASH. 

The  only  precipitant  for  potassium  in  common  use  is  chloroplatinic  acid.  In 
favor  of  this  reagent  it  may  be  said  that  it  is  almost  specific  for  potassium,  no 
other  common  metal  except  ammonium  forming  a  precipitate  insoluble  in  water 
or  alcohol;  the  high  combining  weight  of  platinum  reduces  the  effect  of 
mechanical  errors;  the  precipitate  is  fairly  insoluble  in  dilute  alcohol,  and 
easy  to  filter  and  wash.  Against  it  is  the  coarsely  crystalline  character  of  the 


*  Chem.  News,  1892— 1—209  and  221. 


FERTILIZERS.  387 

potassium  platinchloride,  rendering  it  liable  to  occlude  other  compounds,  and 
its  decomposition  on  ignition. 

In  the  usual  run  of  fertilizers  there  are  found  more  or  less  calcium,  iron, 
aluminum,  magnesium,  and  other  bases,  and  sulfuric  and  phosphoric  acids.  It 
has  been  considered  the  better  plan  to  remove  these  from  the  solution  before 
precipitating  the  potassium,  though  by  the  adoption  of  certain  precautionary 
measures  their  disturbing  influence  may  be  brought  to  a  negligible  minimum. 

Usually  the  sample  is  extracted  by  water  only,  as  it  is  considered  that  all  the 
agriculturally  valuable  potassium  will  be  withdrawn  thereby.  The  removal  of 
the  bases  and  acids  and  destruction  of  organic  matter  may  be  done  in  the 
entire  solution  or  in  the  aliquot  part  withdrawn  for  the  determination.  The 
reagents  for  precipitation  of  the  bases  and  acids  may  be  barium  chloride  with 
barium  hydrate  followed  by  ammonium  carbonate  and  oxalate;  barium  chloride 
followed  by  ammonium  carbonate;  ammonium  oxalate  and  ammonia;  barium 
oxalate  in  hydrochloric  acid  and  a  little  hydrogen  peroxide  (to  peroxidize 
ferrous  salts)  followed  by  ammonia,  etc. 

The  filtrate  after  one  of  these  separations  is  evaporated  to  dryness,  best  after 
addition  of  a  little  sulfuric  acid  to  convert  the  potassium  salt  to  the  sulfate  not 
volatile  at  redness.  Should  the  filtrate  contain  soluble  organic  matter  or  am- 
monium salts,  it  is  generally  ignited  in  the  air.  The  residue  is  taken  up  with 
water,  filtered,  acidified  by  hydrochloric  acid,  and  is  ready  for  precipitation. 

A  moderate  excess  of  chloroplatinic  acid  is  added,  and  the  liquid  evaporated 
to  a  thick  syrup.  This  is  taken  up  by  80  per  cent  alcohol,  and  filtered  on  a  tared 
paper  or  a  Gooch  crucible.  The  washing  is  done  first  by  alcohol,  then  a  solu- 
tion of  ammonium  chloride  saturated  with  potassium  platinchloride  (for  the 
purpose  of  decomposing  adhering  impurities  to  soluble  compounds),  finally  by 
alcohol.  The  filter  and  precipitate  are  dried  at  100°  to  110°  and  weighed. 
After  weighing,  the  filter  is  replaced  in  the  funnel  and  washed  with  hot  water; 
any  insoluble  impurities  remain  and  may  be  weighed  and  deducted. 

Other  methods  of  treating  the  precipitate  are  to  dissolve  it  in  hot  water, 
filter,  evaporate  to  dryness  in  a  small  tared  platinum  dish  and  weigh;  to  decom- 
pose it  by  ignition  to  potassium  chloride  and  platinum,  lixiviate  the  former  and 
weigh  the  latter ;  etc. 

According  to  Moore  the  precipitation  may  take  place  in  presence  of  iron, 
aluminum  and  other  chlorides,  purifying  the  precipitate  by  washing  succes- 
sively with  hydrochloric  acid  in  alcohol,  ammonium  chloride  solution,  and  85 
per  cent  alcohol. 

Perchloric  acid  forms  with  potassium  a  finely -crystalline  powder  of  potassium 
perchlorate,  KC1O4.  Although  a  specific  precipitant  for  potassium,  its  use  is 
somewhat  limited  by  the  necessity  of  the  absence  of  sulfates.  The  acid  solution 
of  potassium  chloride  is  evaporated  to  a  small  bulk,  mixed  with  a  moderate 
excess  of  perchloric  acid  solution,  more  in  presence  of  phosphates,  and  evapo- 
rated to  a  syrupy  consistence.  The  syrup  is  diluted  with  alcohol,  filtered  on 
asbestos,  washed  by  decantation,  dried  at  130 o  and  weighed.* 

Phosphomolybdicacid  has  been  proposed  as  a  precipitant,  the  potassium  phos- 
phomolybdate  being  much  less  soluble  than  the  corresponding  compounds  of 
other  bases  with  the  exception  of  ammonium;  it  contains  about  five  per  cent  of 
its  weight  of  potassium.  The  reagent  is  prepared  by  decomposing  ammonium 
phosphomolybdate  by  aqua  regia  which  breaks  up  the  ammonium  to  nitrogen 
and  water.  The  potassium  is  obtained  as  nitrate  in  a  concentrated  slightly  acid 


*  Chem.  News,  1896—1—17,  Jonrn.  Amer.  Chem.  Socy.  1897—33. 


388  QUANTITATIVE    CHEMICAL    ANALYSIS. 

solution  and  the  reagent  added.  The  mixture  is  evaporated  to  dryness  at  not 
over  50°  and  taken  up  by  a  liquid  compounded  of  a  dilute  solution  of  the  pre- 
cipitant plus  sodium  nitrate  and  saturated  with  potassium  phosphomolybdate. 
The  liquid  is  filtered  on  two  balanced  papers,  washed  with  the  same  liquid  as 
was  used  for  solution  of  the  residue,  and  the  papers  dried  and  weighed.  The 
object  of  the  double  filter  is  to  neutralize  the  gain  due  to  the  solids  in  the 
adhering  washing  fluid. 

NITROGEN. 

The  nitrogen  of  a  fertilizer  may  be  in  one  or  more  of  several  combinations : 
(1),  as  ammonia  or  a  salt  of  ammonium;  (2),  as  nitrate,  rarely  as  nitrite;  (3), 
asaproteid;  (4),  as  other  organic  compounds.  The  nitrogenous  fertilizers 
are  dried  blood,  flesh,  fish  or  animal  refuse,  guano,  oil-cake,  barnyard  manure, 
and  commercial  nitrates  or  salts  of  ammonium. 

The  determination  of  nitrogen  by  the  absolute,  soda-lime,  and  Kjeldahl 
methods  and  modifications  follows  the  usual  routine,  with  a  few  changes  to 
suit  the  material  in  hand.  Of  the  three  the  Kjeldahl  is  most  in  use,  the  abso- 
lute method  but  seldom. 

The  nitrogen  of  an  ammonium  salt  is  determined  by  suspending  the  sample 
In  a  moderately  concentrated  solution  of  sodium  hydrate  and  distilling  the 
ammonia  into  standard  sulf  uric  acid ;  the  excess  of  acid  is  titrated  back  by 
standard  alkali.  Should  the  sample  contain  also  nitrogenous  organic  matter, 
for  the  sodium  hydrate  is  substituted  magnesia  which  has  less  tendency  to  de- 
compose the  organic  matter  with  formation  of  ammonia.  Ammonium  magne- 
sium phosphate  is  not  completely  decomposed  by  distillation  with  magnesia 
unless  it  has. previously  been  dissolved  in  sulfuric  acid. 

Nitric  acid.  The  combined  nitric  acid  in  a  fertilizer  may  be  determined  in 
several  ways. 

(1).  Reduction  of  the  nitrate  to  nitrogen  and  measuring  its  volume. 

(2).  Reduction  of  the  nitrate  to  nitric  oxide  by  a  reducing  agent  (e.  g.,  ferrous 
chloride),  and  determining  either  the  iron  oxidized,  the  volume  of  nitric  oxide, 
or  the  reducing  power  of  the  nitric  oxide. 

(3).  Conversion  of  the  nitrate  to  ammonia  and  its  determination.  The  re- 
duction may  be  effected  by  ignition  with  soda-lime,  boiling  with  concentrated 
sulfuric  acid,  by  nascent  hydrogen,  or  electrolytically. 

(4).  Measuring  directly  or  indirectly  the  oxidizing  power  of  the  nitric  acid. 
There  are  included  various  colorimetric  methods  depending  on  the  formation 
or  destruction  of  a  color  of  some  reagent  held  in  aqueous  solution. 

The  determination  by  conversion  into  nitrogen  by  ignition  with  cupric  oxide, 
conversion  to  ammonia  by  sulfuric  acid  or  soda-lime,  and  by  colorimetric 
methods  have  been  described  elsewhere  (pp.  307  and  377). 

A.  Reduction  to  nitric  oxide.  la  all  determinations  by  methods  on  this  prin- 
ciple, air  must  be  rigorously  excluded  since  nitric  oxide  readily  combines  with 
oxygen.  The  operation  may  be  conducted  in  vacuo,  or  under  hydrogen  or 
carbon  dioxide  with  special  precautions  to  insure  the  freedom  of  the  gas  from 
traces  of  air. 

In  the  method  of  Pelouze  the  nitrate  is  boiled  in  a  glass  retort  with  a  known 
volume  of  a  standard  solution  of  ferrous  chloride  in  dilute  hydrochloric  acid, 
under  a  current  of  hydrogen.  The  liquid  is  cooled  while  the  gas  still  passes, 
and  the  iron  remaining  as  ferrous  chloride  is  titrated  by  standard  potassium 
bichromate;  or  the  ferric  chloride  formed  may  be  titrated  by  a  standard  reduc- 
ing solution.  The  weight  of  nitric  acid  is  calculated  from  the  equation 
2  tf N03  +  6FeCl2  -f  CHC1  =  N2O2  +  3Fe2Cl6  -f-  4H20. 


FERTILIZERS.  389 

Schultze-Tieman.  A  small  flask  A,  Fig.  175,  contains  the  concentrated  solu- 
tion of  the  nitrate  and  is  closed  by  a  two-hole  stopper. 
From  the  stopper  pass  two  long  glass  tubes  bent 
downward,  one,  B,  dipping  into  a  beaker  filled  with  a 
solution  of  ferric  chloride  in  hydrochloric  acid,  the 
other,  C,  bent  upward  at  the  orifice  to  enter  the  mouth 
of  a  gas-measuring  tube  D  filled  with  a  solution  of 
sodium  hydrate  and  standing  in  a  trough  E  of  the 
same  liquid.  There  are  short  rubber  joints  in  the 
tubes  B  and  C  that  may  be  closed  by  a  pinch-cock  or 
compression  by  the  fingers. 

The  nitrate  solution  is  boiled  until  ail  air  is  ex- 
pelled from  the  flask  and  tubes;  C  is  then  closed  and 
the  ferrous  chloride  solution  in  F  drawn  in  by  mo-  Fig.  175. 

mentarily  stopping  the  boiling.  B  is  closed  and  C  opened,  and  the  nitric  oxide 
allowed  to  pass  into  D,  the  last  traces  being  carried  over  in  the  steam.  Any 
carbon  dioxide  or  hydrochloric  acid  accompanying  the  nitric  acid  is  absorbed 
by  the  sodium  hydrate. 

The  measuring  tube  is  transferred  to  a  trough  of  water,  the  volume  of  nitric 
oxide  read,  and  the  weight  of  nitric  acid  calculated. 

Various  modifications  of  the  above  fairly  accurate  but  rather  difficult  process 
have  been  published. 

Schloessing.*  The  concentrated  solution  of  the  nitrate  is  mixed  with  a  cold 
solution  of  ferrous  chloride  and  hydrochloric  acid  in  a  flask  through  which  is 
conducted  a  current  of  pure  carbon  dioxide.  The  nitric  oxide  evolved  on 
boiling,  with  perhaps  some  hydrochloric  acid,  is  passed  into  a  gas-tube  con- 
taining potassium  hydrate  solution,  the  hydrochloric  and  carbonic  acids  being 
retained  by  the  alkali.  The  residual  nitric  oxide  is  then  mixed  with  a  slight 
excess  of  oxygen,  forming  nitric  acid;  this  is  absorbed  in  water  and  titrated 
by  standard  alkali. 

Morse  and  Linn  measure  the  reducing  power  of  nitric  oxide.  The  nitrate  is 
decomposed  by  acid  ferrous  chloride  in  a  flask  in  a  current  of  carbon  dioxide. 
The  nitric  oxide  is  caught  in  a  strong  solution  of  standard  potassium  per- 
manganate which  reoxidizes  the  gas  to  nitric  acid  and  retains  it  —  3K2Mn2O8  -f- 
5N2O2  =  5N2O5  -|-3K2O  -f-  6MnO.  A  known  volume  of  the  permanganate  solu- 
tion receives  the  gas,  and  after  the  operation  is  concluded  there  is  added  of 
standard  oxalic  acid  solution  a  volume  which  would  reduce  all  of  the  original 
permanganate,  and  the  excess  of  oxalic  acid  titrated  by  standard  permanganate. 

B.  In  the  process  of  Goochand  Gruener  the  nitrate  is  decomposed  by  hydro- 
chloric acid  in  presence  of  manganous  chloride ;  the  chlorine  evolved  is  passed 
into  a  solution  of  potassium  iodide  liberating  an  equivalent  of  iodine  which  is 
then  titrated  by  standard  sodium  thiosulfate  and  starch. 

C.  Gattner's  method.  If  a  nitrate  be  heated  with  phosphorus  acid,  sulf  uric 
acid  and  ammonium  chloride,  the  nitric  acid  is  reduced  to  nitrogen  — 

2KN08  +  P203  +  2NH4C1  =  2N2  +  P2O5  +  2KC1  +  4H2O. 

The  decomposition  is  effected  in  a  flask  connected  to  a  washing  flask  con- 
taining sodium  hydrate  solution,  this  to  a  cylinder  fitted  up  like  a  wash -bottle 
and  completely  filled  with  water.  The  exit-tube  of  the  cylinder  enters  an 
empty  measuring  jar.  The  sample  of  nitrate  with  the  phosphorus  acid  and 
ammonium  chloride  is  placed  in  the  evolution  flask,  dilute  sulfuric  acid  run  in, 
and  the  mixture  warmed.  The  nitrogen,  with  some  hydrochloric  acid  vapor, 


*  Chem.  News,  1893-2-40. 


390  QUANTITATIVE    CHEMICAL   ANALYSIS. 

passes  through  the  alkali  which  absorbs  the  acid,  into  the  cylinder  forcing  out 
an  equal  volume  of  water  into  the  measuring  jar.  Finally  the  volume  of  water 
in  the  jar  is  read ;  this  may  be  assumed  to  equal  the  volume  of  the  nitrogen, 
but  to  correct  for  solubility,  etc.,  a  previous  experiment  on  pure  potassium 
nitrate  is  made  the  basis  of  the  calculation.  The  temperature  of  the  water  at 
the  close  of  the  operation  must  be  brought  to  that  at  the  beginning. 

D.  Ulsch.  The  sample  is  allowed  to  stand  for  a  time  in  contact  with  reduced 
iron  (iron  in  fine  powder)  and  dilute  sulf  uric  acid.    The  nascent  hydrogen  con- 
verts the  nitric  acid  to  ammonia  —  N2O5  +  16H  =  2NH3  -f  5H2O.    Then  an  ex- 
cess of  magnesia  is  stirred  in,  and  the  freed  ammonia  distilled  into  dilute 
sulfuric  acid.    The  process  is  modified  by  Schmidt  who  proposes  a  mixture  of 
powdered  iron  and  zinc  and  acetic  acid  instead  of  iron  and  sulfuric  acid,  and 
by  Krueger  who  substitutes  tin  in  spongy  form,  and  a  solution  of  stannous 
chloride  in  hydrochloric  acid.    The  Halle  Station  advocates  a  mixture  of  zinc 
dust  and  iron  filings  and  solution  of  sodium  hydrate. 

E.  Monnier.  The  solution  of  the  nitrate  is  mixed  with  tartaric  acid  and 
brought  in  contact  with  a  certain  quantity  of  sodium-amalgam  in  a  special 
apparatus.    The  hydrogen  evolved  by  the  oxidation  of  the  sodium  in  the  amal- 
gam is  passed  into  a  gas-measuring  tube  and  measured.    A  determination  of 
the  hydrogen  evolved  by  the  same  amount  of  sodium  amalgam  with  tartaric 
acid  is  made ;  the  difference  between  the  two  readings  is  the  volume  of  hydro  - 
gen  that  is  combined  with  the  nitrogen  of  the  nitrate  to  form  ammonia,   this 
uniting  with  tartaric  acid  to  form  ammonium  tartrate  — 

7Na2  +  2NaN03  -f  9H2C4H4O6  =  SNa^HA  +(NH3)2C4H4O6  +  6H2O. 


Tor  the  separation  of  nitric  and  ammoniacal  nitrogen  from  organic  nitrogen, 
the  nitric  acid  is  determined  by  one  of  the  common  methods ;  then  another 
portion  of  the  material  is  percolated  by  a  weak  solution  of  tannin  which  com- 
bines to  insoluble  compounds  with  all  the  soluble  organic  nitrogenous  matter, 
while  the  nitrates  and  ammonia  compounds  are  dissolved.  The  nitrogen  is 
determined  in  the  filtrate  and  residue. 


Hygroscopic  water*  is  determined  as  usual  by  drying  at  100°  to  110°.  In 
the  absence  of  combined  water,  the  loss  on  igniting  the  sample  to  redness  in 
the  air  is  organic  matter  and  carbon  dioxide,  and  on  deducting  the  latter, 
iound  by  another  experiment,  the  difference  may  be  put  down  as  organic  matter. 

Carbon  dioxide.  Most  mineral  phosphates  carry  calcium  carbonate  in  varying 
quantities.  The  carbon  dioxide  is  best  determined  by  boiling  the  sample  with 
dilute  sulfuric  acid,  passing  the  gas  into  potash  bulbs  (preceded  by  a  drying 
tube),  and  noting  the  increase  in  weight. 

Silica.  The  usual  method  for  the  determination  of  this  compound  must  be 
modified  where  fluorine  is  a  constituent,  since,  on  dissolving  the  sample  in 
hydrochloric  acid  and  evaporating,  part  of  the  silica  will  combine  with  the 
fluorine  and  volatilize  as  hydrofluosilicic  acid.  The  phosphate  is  fused  with 
sodium  carbonate  and  the  melt  extracted  with  water;  sodium  silicate,  fluoride, 
aluminate  and  carbonate  enter  the  solution.  After  filtering  from  calcium  car- 
bonate, the  filtrate  is  heated  with  ammonium  carbonate,  when  most  of  the 
silica  and  aluminic  hydrate  precipitate.  The  remainder  of  the  silica  may  be 


*  Chem.  News,  1891-1-100, 114,  and  122. 


FERTILIZERS.  391 

thrown  down  by  a  solution  of  zinc  oxide  .in  ammonia.    The  separation  of  the 
silica  from  the  alumina  and  zinc  oxide  presents  no  difficulties. 

Fluorine.  Here  the  problem  is  the  separation  of  fluorine  from  silica, 
alumina,  and  calcium  phosphate.  The  sample  is  fused  with  sodium 
carbonate  and  silica  and  the  fusion  extracted  with  water.  The  silica  is 
precipitated  by  ammonium  carbonate  and  the  liquid  filtered.  In  the  filtrate 
the  fluorine,  phosphoric  acid,  and  carbonic  acid  are  precipitated  as  calcium 
compounds  by  addition  of  calcium  chloride. 

After  filtration  the  precipitate  is  dried  and  evaporated  with  dilute  acetic  acid 
which  converts  the  calcium  carbonate  into  soluble  calcium  acetate ;  the  cal- 
cium fluoride  and  phosphate  are  not  affected.  After  lixiviation  there  remains 
a  mixture  of  calcium  fluoride  and  calcium  phosphate,  which  is  ignited  and 
weighed.  The  precipitate  is  then  dissolved  in  acid  and  the  phosphoric  acid 
determined  gravimetrically.  From  the  phosphoric  acid  is  calculated  the  cor- 
responding calcium  phosphate,  and  this  subtracted  from  the  total  weight  leaves 
that  of  the  calcium  fluoride.  From  the  calcium  fluoride  is  calculated  the 
weight  of  the  fluorine.* 

Calcium,  owing  to  the  association  of  phosphoric  acid,  cannot  be  precipitated 
as  oxalate  from  the  acid  solution  by  ammonium  oxalate  and  ammonia.  But  if 
the  solution  be  allowed  to  remain  faintly  acid  the  precipitate  will  be  nearly 
pure  calcium  oxalate,  while  the  loss  by  incomplete  insolubility  will  not  be 
serious. 

•    The  precipitation  of  calcium  by  sulf  uric  acid  and  alcohol  gives  good  results, 
and  the  filtrate  may  be  used  for  the  determination  of  iron  and  aluminum. 

Iron  and  aluminum.  In  a  direct  precipitation  by  ammonia  or  ammonium 
acetate,  the  precipitate  of  iron  and  aluminum  phosphates  contains  calcium 
phosphate,  which,  however,  can  be  held  up  in  great  measure  by  a  sufficient 
quantity  of  ammonium  chloride.  It  is  well  after  filtering,  to  dissolve  the  pre- 
cipitate in  acid  and  repeat  the  precipitation;  or  to  previously  remove  the 
calcium  by  sulfuric  acid  and  alcohol. 

On  washing  the  phosphates  of  iron  and  aluminum  there  is  lost  some  of  the 
combined  phosphoric  acid,  and  hence  if  the  precipitate  is  weighed  as  the  normal 
phosphates  the  calculated  results  for  the  metals  will  be  too  low.  The  only  safe 
procedure  is  to  dissolve  the  weighed  precipitate  and  determine  the  phosphoric 
acid  and  call  the  difference  iron  and  aluminum  oxides. f 

Krug  and  McElroy  obtain  a  nitric  acid  solution  of  the  metals  and  remove 
the  phosphoric  acid  by  precipitation  as  ammonium  phosphomolybdate.  The 
filtrate  is  treated  by  ammonia  in  the  cold,  the  ammonic  molybdate  remaining 
in  solution  while  ferric  and  aluminic  hydrates  precipitate.  A  repetition  of  the 
process  is  advised  in  presence  of  much  calcium. 

Sodium.  In  the  determination  of  potassium  the  filtrate  from  the  potassium 
platinchloride  contains  the  sodium  as  chloride  with  the  excess  of  chloroplatinic 
acid.  The  filtrate  and  washings  are  evaporated  in  a  wide -mouth  Erlenmeyer  to 
dryness.  By  adapting  a  cork  carrying  two  bent  tubes  a  current  of  moist 
hydrogen  is  passed  over  the  residue  which  is  kept  at  100  o .  When  the  residue 
has  become  black  from  the  reduction  of  the  sodium  -platinchloride  and  chloro- 
platinic acid  to  metallic  platinum,  a  little  water  is  added,  evaporated  to  dry- 
ness,  and  hydrogen  again  transmitted.  The  moistening  and  evaporation  are 
repeated  until  the  reduction  is  complete,  proved  by  the  aqueous  solution  no 


*  Wiley,  Agricultural  Anal.  2-130. 
t  Chem.  News,  1897—1—150. 


392  QUANTITATIVE    CHEMICAL   ANALYSIS. 

longer  showing  a  yellow  tint.    The  sodium  chloride  is  lixiviated  from  the 
residue  by  water,  evaporated  to  dryness,  and  weighed. 

Sulfocyanic  add  is  found  in  some  samples  of  superphosphate.    This  com- 
pound is  desulf  urized  by  potassium  permanganate  as  in  the  equation  — 

5HCNS  -f  3K2Mn208  -f  7H2SO4  =  5HCN  +6KHSO4  +  6MnSO4  +  4H2O. 
but    secondary  reactions  may  intervene  in  the  titration  by  standard  perman- 
ganate, hence  a  parallel  titration  is  always  made  on  a  potassium  sulfocyanide 
solution  of  ascertained  strength  and  the  results  of  the  assay  calculated  on  that 
basis. 


THE   ALCOHOLS. 


THE  ALCOHOLS. 

Alcohol  is  the  generic  term  for  a  number  of  organic  bodies  derived  from  hy- 
drocarbons by  the  substitution  of  hydroxyl  for  hydrogen,  and  when  acted  on  by 
an  acid  split  up  to  form  an  ether  and  water;  thus  — 

2CH3OH  (methyl  alcohol)  =CH3.O.CH3  (methyl  ether)  +HOH. 

Methyl  alcohol  is  a  colorless  volatile  liquid  of  a  specific  gravity  of  .802  and 
boiling  point  about  5Q°  Cent.  It  forms  a  definite  compound  with  calcium 
chloride  and  reacts  with  oxalic  acid  to  form  methyl  oxalate,  these  properties 
availed  for  the  purification  of  the  commercial  article.  Wood  spirit  is  a  complex 
liquid  containing  methyl  alcohol,  acetone,  aldehyd,  methyl  acetate,  dimethyl  and 
allyl  alcohols,  etc.  The  best  commercial  grade  contains  about  95  per  cent  of 
methyl  alcohol,  commoner  articles  from  75  to  90  per  cent,  and  the  crude  from 
40  to  60. 

Ethyl  alcohol  or  shortly  alcohol,  may  be  considered  as  the  hydrate  of  ethyl, 
C2H5.OH;  it  is  a  colorless  mobile  liquid  boiling  at  78.4  °,  and  of  a  specific 
gravity  of  .7395  at  15.5/15.50  Cent.  Crude  alcohol  ('  feints ')  is  a  mixture'of 
alcohol  and  water,  and  contains  also  from  traces  up  to  one  per  cent  or.  more  of 
aldehyd,  acetic  acid,  oily  and  resinous  bodies,  etc. 

Amyl  alcohol  C5HU.OH,  is  a  colorless  liquid  boiling  at  132  o  }  and  of  a  specific 
gravity  of  .8184.  It  is  the  main  constituent  of  the  complex  liquid  known  as 
fusel  (fousel)  oil,  a  deleterious  concomitant  of  newly  made  liquors.* 

The  methods  of  determination  given  below  apply  to  the  commercial  ethyl 
alcohols  and  also  to  the  distillates  from  alcoholic  beverages,  practically  mix- 
tures of  alcohol  and  water. 

Physical  methods. 

1.  By  density.  The  percentage  of  alcohol  can  be  found  with  reasonable 
accuracy  from  the  specific  gravity  in  a  pure  mixture  of  alcohol  and  water,  and 
also,  though  with  a  proportionately  greater  error,  in  mixtures  containing  mod- 
erate amounts  of  other  constituents.  Since  there  is  a  considerable  contraction 
in  volume  when  alcohol  and  water  are  mixed  (100  volumes  of  absolute  alcohol 
plus  60  volumes  of  water  making  only  154  volumes),  the  proportion  of  alcohol 
does  not  vary  inversely  with  the  gravity;  so  that  the  percentage  of  alcohol 
corresponding  to  any  given  gravity  must  be  found  from  tables  based  on  direct 
experiments. 

Any  of  the  usual  methods  of  observing  the  specific  gravity  may  be  employed ; 
the  simplest  is  by  the  alcoholometer,  a  hydrometer  graduated  to  show  per- 
centages of  absolute  alcohol  V/V\at  60/60  <=>  Fahr.  Since  the  other  constit- 
uents of  commercial  alcohol  are  in  comparatively  small  amounts,  a  gravity  test 
is  sufficiently  exact  for  a  commercial  assay,  but  in  fermented  beverages  the  ex- 
tractive matter,  sugar,  etc.,  preclude  its  employment  without  previous  distil- 
lation. 

The  distillation  is  made  from  a  glass  or  metal  retort  or  distillation-flask 
provided  with  a  well-cooled  worm.  For  the  stronger  wines  and  distilled 
liquors  the  receiver  is  connected  to  the  condenser  air-tight  and  is  provided 


*  Journ.  Anal.  Chem.  4—29. 


394  QUANTITATIVE   CHEMICAL  ANALYSIS. 

with  a  water-seal;  these  precautions  against  evaporation  of  alcohol  from  the 
distillate  are  unnecessary  in  distilling  the  lighter  wines  and  beers,  where  the 
receiver  need  only  be  loosely  closed  by  a  cork  or  the  body  of  the  condenser. 
But  invariably  the  connection  of  the  still  to  the  condenser  must  be  steam-tight, 
and  the  worm  ample  in  surface  to  insure  complete  condensation. 

Beer  and  the  lighter  wines  are  distilled  without  other  preparation  than  to 
neutralize  any  free  acid  by  potash  or  calcium  carbonate.  Stronger  wines  are 
diluted  with  one  or  two,  and  distilled  liquors  with  fl^e  to  ten  volumes  of  water. 
The  distillation  need  never  be  carried  to  dryness,  for  if  the  alcoholic  strength 
of  the  original  is  low  or  made  so  by  dilution,  practically  all  the  alcohol  will  pass 
over  into  the  first  half  or  two-thirds  of  the  distillate.  A  graduated  cylindrical 
receiver  is  convenient  for  observing  approximately  the  ratio  of  the  distillate  to 
the  original  volume. 

The  quantity  of  liquid  to  be  distilled  and  the  capacity  of  the  apparatus  de- 
pends on  the  method  of  taking  the  gravity  of  the  distillate;  usually  from  100  to 
300  Cc.  will  suffice. 

The  same  principle  is  applied  in  a  different  way  in  an  old  method  now  but 
little  in  use.  It  depends  on  the  increase  in  gravity  of  wine  or  beer  when  the 
alcohol  is  removed  and  replaced  by  an  equal  volume  of  pure  water.  The  gravity 
is  observed  at  15°  Cent,  and  a  measured  volume  boiled  until  all  the  alcohol  is 
driven  off.  The  liquid  is  then  made  up  to  exactly  the  original  volume  with 
distilled  water  reducing  the  temperature  to  15°,  and  the  gravity  again 
observed. 

Now  had  the  original  volume  of  wine  been  distilled  to  dryness,  the  distillate, 
containing  all  the  alcohol  and  water  of  the  wine,  would  have  a  specific  gravity  g 
from  which  the  proportion  of  alcohol  contained  could  be  ascertained  by  refer- 
ence to  the  published  tables  of  the  gravity  of  all  concentrations  of  dilute 
alcohol.  The  value  of  g  may  be  computed  from  the  above  data,  it  equaling 
G  + 1  —  G',  where  G  is  the  gravity  in  the  first  observation,  and  G',  that  of  the 
second.  Several  sources  of  error  vitiate  the  accuracy  of  the  method. 

2.  Several  other  physical  attributes  can  be  applied  for  the  determination  of 
alcohol.    Traube  *  assays  spirit,  up  to  ten  per  cent  by  weight,  by  the  '  stalag- 
mometer '.    This  is  a  small  pipette  with  a  capillary  orifice  delivering  the  con- 
tents in  uniform  drops.    The  number  of  drops  of  water  at  15  o  Cent,  is  called 
«,  the  size  of  the  drops  diminishing  and  their  number  increasing  as  alcohol  is 
contained  in  the  water.    Traube's  table,  compiled  on  the  basis  of  a  =  100  at 
16° ,  shows  the  number  of  drops  corresponding  to  each  .2  per  cent  of  alcohol 
by  weight  at  temperatures  of  10  °  to  30°.    Thus,  a  ten  per  cent  spirit  at 
15°  furnishes  148  drops;  at  30 o,  155  drops.    Wines  and  beers  are  distilled 
before  testing;  the  small  amounts  of  ethereal  oils,  glycerol,  etc.,  coming  over 
are  without  influence  on  the  number  of  drops. 

3.  Rakowitch,  for  an  approximate  determination,  proposes  to  measure  the 
expansion  of  a  measured  volume  of  chloroform  through  absorption  of  alcohol 
on  shaking  up  with  a  spirit.    The  increase  is  said  to  be  in  direct  ratio  to  the 
percentage  of  alcohol. 

4.  The  boiling  point  of  a  spirit  varies  inversely  with  the  proportion  of  alcphol 
contained;  thus,  absolute  alcohol  boils  at  174®  Fahr.,  a  five  per  cent  spirit  at 
205° ,  water  at  212  o .    The  barometric  pressure  must  of  course  be  considered 
in  a  determination.    Moderate  amounts  of  extractive  matter  do  not  materially 
affect  the  boiling  point. 

6.  The  temperature  of  the  vapor  of  a  spirit  is  lower  in  proportion  to  the 


*  Berlchte,  20-2824;  Ohem.  News,  1888—1—39. 


THE    ALCOHOLS.  395 

concentration  of  alcohol  in  the  vapor.  Thus,  if  the  vapor  temperature  is  210  ° 
Fahr.,  the  condensed  vapor  will  contain  13  per  cent  of  alcohol  and  the  concen- 
tration of  the  original  spirit  is  one  per  cent ;  while  if  the  vapor  temperature  is 
170°  ,  the  distillate  will  contain  93  per  cent  of  alcohol  and  the  original  liquid 
92  per  cent  (Gruening). 

6.  Various  other  methods  are  based  on  viscosity,  dilatation  by  heat,  vapor 
density,  etc. 

Chemical  methods. 

These  depend  for  the  most  part  on  the  conversion  of  alcohol  into  acetic  acid 
by  strong  oxidizers,  or  further  into  carbon  dioxide  and  water.  One  molecule 
of  alcohol  yields  one  molecule  of  acetic  acid  or  two  molecules  of  carbon  diox- 
ide. Owing  to  the  high  reducing  power  of  alcohol,  the  weight  that  can  be 
treated  in  a  determination  is  comparatively  very  small,  a  serious  disadvantage. 

1.  Roese's  process.  *  About  five  grams  of  the  spirit,  diluted  with  water  to  a 
concentration  of  about  one  per  cent  of  alcohol,  is  treated  with  50  Cc.  of  a  one- 
per  cent  solution  of  potassium  permanganate  and  20  Cc.  of  concentrated  sul- 
furic  acid.    The  alcohol  is  immediately  oxidized  to  carbon  dioxide  and  water. 
The  excess  of  permanganate  is  determinable  by  reduction  with  standard  oxalic 
acid,  the  excess  of  the  latter  titrated  back  by  permanganate. 

2.  Treated  with  potassium  bichromate  and  dilute  sulf uric  acid,  alcohol  is 
oxidized,  first  to  aldehyd,  then  to  acetic  acid.    Bourcart  heats  the  alcohol  with 
dilute  sulf  uric  acid  and  a  weighed  amount  of  potassium  bichromate  in  a  sealed 
tube  for  two  or  three  hours.    The  reaction  is 

3C2H60  +  2K2Cr2O7  +  8H2SO4  =  3HC2H3O2  -f  2^804  +  2Cr2(SO4)3  +  11H2O. 
The  excess  of  bichromate  is  determined  by  the  addition  of  potassium  iodide, 
when  iodine  is  liberated  — 

K2Cr2O7  +  6KI  +  7H2SO4  =  3I2  -f  4^804  -f  Cr2(SO4)3  -f-  7H2O. 

The  iodine  is  titrated  by  sodium  thiosulfate. 

A  modification  of  the  above  employs  chromic  acid  in  sulfuric  acid  as  an 
oxidizer,  heated  for  five  minutes  in  a  flask  to  98  °  Cent,  (at  which  temperature 
it  is  claimed  there  is  no  reaction  between  the  chromic  and  sulfuric  acids  at  the 
specified  concentrations),  reducing  the  excess  of  the  chromic  acid  by  ferrous 
sulf  ate,  and  titrating  back  by  permanganate  or  bichromate. 

Small  quantities  of  ethyl  alcohol  can  be  converted  by  oxidizers,  such  as 
potassium  permanganate,  ammoniacal  solution  of  copper  oxide,  etc.,  into 
acetic  acid,  the  liquid  distilled,  and  the  acid  determined  by  titration.  An 
amount  of  spirit  containing  not  above  .1  gram  of  alcohol  is  compounded  with  a 
solution  of  bichromate  of  potassium  in  sulfuric  acid  and  digested  in  a  closed 
flask  at  100°  for  two  hours.  To  prevent  further  oxidation  of  the  acetic  acid, 
the  excess  of  chromic  acid  in  the  liquid  is  reduced  to  chromium  sulfate  by 
metallic  zinc,  then  the  liquid  is  distilled  to  dryness,  water  added  to  the  residue 
and  again  distilled.  The  united  distillates  are  titrated  by  sodium  hydrate  and 
phenol-phthalein. 

If  any  sulfuric  acid  is  carried  over  into  the  receiver  mechanically  it  will  react 
with  the  alkali  and  be  counted  as  acetic  acid.  To  correct  for  this  the  distillate 
is  tested  before  titration  by  neutral  barium  chloride,  any  sulfuric  acid  produc- 
ing a  precipitate  of  barium  sulfate,  at  the  same  time  liberating  hydrochloric 
acid  equivalent  in  neutralizing  power.  The  barium  sulfate  is  filtered  off  and 
weighed,  and  for  each  233  parts  is  deducted  92  parts  of  alcohol. 

(The  calculation  by  which  this  proportion  is  arrived  at  is  as  follows :  in  round 
numbers  the  molecular  weight  of  barium  sulfate  is  233;  of  sulfuric  acid  98;  of 


*  Zeits.  angew,  1888-31;  Chem.  Zeit.  1891—847;  Journ.  Amer.  Chem.  Socy.  1898—293. 


396  QUANTITATIVE    CHEMICAL   ANALYSIS. 

acetic  acid  60;  and  of  ethyl  alcohol  46.    Let  w  be  the  weight  of  sulfuric  acid 
neutralized  by  one  Cc.  of  the  standard  alkali,  then  -75-  is   the    corresponding 

weight  of  acetic  acid  neutralized  by  one  Cc. 

98 
One  gram  of  barium  sulf ate  is  formed  from  -^-  gram  of  sulfuric  acid,  and 

98 
23Bw  is  the  volume  of  alkali  required  to  neutralize  the  latter.    The  weight  of 

acetic  acid  is  then  -^~X  233^,  grai»s.    Since   60  parts   of   acetic   acid    are 

yielded  by  46  of  alcohol,  ^-X^||^-X^  =^  grams    of   alcohol    corre- 
sponding to  one  gram  of  barium  sulf  ate.) 

3.  Monell  describes  a  colorimetric  method.  A  mixture  of  cobaltic  nitrate, 
solution  and  an  alcoholic  solution  of  ammonium  sulfocyanide  has  a  deep  blue 
color  which  disappears  when  the  mixture  is  diluted  with  a  sufficient  proportion 
of  water.  To  a  measured  volume  of  the  reagent  is  added  the  sample  until  the 
tint  is  but  faint,  then  a  mixture  of  alcohol  and  water  is  tentatively  made  up  so 
that  a  volume  equal  to  that  of  the  standard  will  produce  the  same  tint. 


An  admixture  of  methyl  alcohol  with  water,  as  in  distillates  or  high-grade 
wood  spirits,  can  be  assayed  by  specific  gravity  in  the  same  way  as  for  ethyl 
alcohol.  But  the  impurities  in  crude  wood  spirit  vary  too  much  to  allow  any 
reliable  conclusions  to  be  drawn  from  the  specific  gravity. 

Strong  oxidizers  convert  methyl  alcohol  first  to  formic  acid,  then  to  carbon 
dioxide  and  water  — 

3CH3O  -f  3O  =  2HCHO2  +  H2O ;  and  HCH02'+  O  =  CO2  -f  H2O. 
With  potassium  bichromate  and  sulfuric  acid  there  is  entire  oxidation  to 
carbonic  acid.  If  standard  bichromate  solution  be  employed  the  chromic  acid 
in  excess  may  be  reduced  by  standard  ferrous  sulfate  and  the  excess  of  the 
latter  titrated  back  by  bichromate.  Since  under  these  conditions  ethyl  alcohol 
is  oxidized  only  to  acetic  acid,  mixtures  of  methyl  and  ethyl  alcohols  may  be 
treated  by  these  reagents,  the  weight  of  bichromate  that  has  reacted  determined 
as  above,  and  the  acetic  acid  distilled  and  titrated  by  standard  alkali. 

In  the  assay  of  wood  spirit  the  methyl  alcohol  may  be  determined  by  conver- 
sion to  methyl  iodide  — 

5CH4O  +  31  +  PI2  =  5CH3I  (methyl  iodide)  -f  H3PO4  +  H2O. 

Phosphorous  iodide  is  placed  in  a  dry  flask  and  the  wood  spirit  dropped  in, 
followed  by  a  solution  of  iodine  in  hydriodic  acid.  After  heating  for  some 
time  to  80° ,  the  liquid  is  distilled,  finally  passing  a  current  of  air  through  the 
apparatus  to  carry  the  vapor  and  what  remain^  dissolved  in  the  liquid  into  the 
receiver.  The  distillate  is  shaken  up  with  water  in  a  graduated  tube,  and  the 
volume  of  methyl  iodide  read.  Any  methyl  acetate  in  the  spirit  is  also  decom- 
posed to  form  methyl  iodide,  and  its  amount,  calculated  from  a  previous  deter- 
mination, is  to  be  deducted.  Acetone  also  is  found  in  the  distillate,  but  may 
be  washed  out  by  water,  correcting  for  the  methyl  iodide  dissolved  in  this 
operation. 

Instead  of  measuring  the  volume  of  the  product  it  can  be  decomposed  by 
solution  of  sodium  hydrate  in  alcohol  (CH8I-f  NaOH  =  CH8OH-fNaI),  the 


THE   ALCOHOLS.  397 

alcohol  evaporated  off,  the  aqueous  solution  of  the  residue  acidified  by  nitric 
acid,  and  the  iodine  of  the  sodium  iodide  titrated  by  standard  silver  nitrate 
solution. 

For  a  determination  of  small  amounts  of  methyl  alcohol  in  ethyl  alcohol,  the 
former  is  concentrated  by  three  successive  distillations  into  a  comparatively 
small  quantity  of  the  latter.  By  this  operation  certain  impurities  are  elim- 
inated. In  each  distillation  about  two-thirds  of  the  liquid  is  brought  over.  In 
the  first  the  liquid  is  made  alkaline;  in  the  second  the  first  distillate 
is  made  acid;  and  in  the  third  the  second  distillate  is  dehydrated  by 
potassium  carbonate.  In  the  final  distillate  the  ethyl  and  methyl  alcohols 
are,  after  dilution,  determined  by  specific  gravity  and  also  by  the  bichro- 
mate process,  distilling  and  titrating  the  acetic  acid.  Should  no  methyl  alcohol 
have  been  present  in  the  sample  the  results  will  practically  agree,  otherwise 
the  latter  test  will  show  proportionally  lower  since  methyl  alcohol  is  oxidized 
to  carbon  dioxide,  not  acetic  acid. 

The  basis  of  another  method  is  that  anhydrous  methyl  alcohol  forms  a  com- 
pound with  dry  calcium  chloride  (CaClg.iCI^O)  which  is  not  broken  up  at  the 
temperature  of  100®.  The  sample  is  first  dehydrated  by  distillation  from 
anhydrous  potassium  carbonate,  and  the  distillate  allowed  to  stand  for  some 
time  over  dry  calcium  chloride.  On  again  distilling,  the  methyl  alcohol  com- 
pound remains  in  the  flask  while  the  ethyl  alcohol  passes  over;  on  treating  the 
residue  with  water  it  is  decomposed  to  methyl  alcohol  and  calcium  chloride. 

The  specific  gravities  of  the  iodides  of  methyl  and  ethyl  differ  considerably, 
and  a  determination  can  be  made  by  preparing  the  iodides  by  compounding  the 
spirit  with  iodine  and  red  phosphorus.  The  gravity  of  the  mixed  iodides  is 
observed  by  the  usual  methods,  and  the  proportion  of  the  constituents  calcu- 
lated.* 


Amyl  alcohol.  This  compound  is  oxidized  to  valeric  acid  by  chromic  acid  — 
C5Hn.OH  +  O2  =  HC5H902  +  H20. 

From  a  dilute  aqueous  solution,  that  may  also  contain  ethyl  alcohol  up  to  a 
certain  percentage,  and  extractive  matter,  the  amyl  alcohol  is  extracted  by 
shaking  three  times  with  purified  chloroform.  The  chloroformic  solution  is 
washed  with  water  to  remove  ethyl  alcohol  and  other  soluble  matters,  then 
digested  with  a  solution  of  potassium  bichromate  in  sulfuric  acid.  Valeric 
acid  is  formed  and  is  separated  from  the  excess  of  chromic  and  sulfuric  acids 
by  distillation;  the  distillate  is  chloroform  and  water  and  contains  all  the 
valeric  acid  and  usually  also  some  hydrochloric  acid  from  a  reaction  between 
chloroform  and  chromic  acid.  Digestion  with  barium  carbonate,  removal  of 
chloroform  by  evaporation,  and  filtration,  leaves  a  solution  of  only  barium 
valerate  and  chloride. 

The  solution  is  evaporated  and  the  residue  weighed ;  it  is  dissolved  in  water 
and  the  solution  divided  into  two  equal  parts.  In  one  is  determined  the  barium, 
in  the  other  chlorine.  From  the  weight  of  barium  is  subtracted  the  weight  of 
barium  calculated  to  combine  with  the  chlorine,  and  from  the  remainder  is 
calculated  the  equivalent  weight  of  valeric  acid  and  of  amyl  alcohol. 

Dupre  proceeds  to  oxidize  the  previously  distilled  alcohol  by  chromic  and 
sulfuric  acids,  reduces  the  excess  of  chromic  acid  by  zinc,  and  distills.  In  the 
distillate  the  volatile  acids  are  neutralized  by  sodium  hydrate,  the  solution 


*  Zeits  Angew.  1898-125. 


398  QUANTITATIVE    CHEMICAL   ANALYSIS. 

concentrated,  and  the  sodium  salts  decomposed  by  sulf uric  acid.  The  acids 
are  again  distilled  and  in  the  distillate  converted  to  barium  salts,  then  the 
combined  barium  determined.  Since  barium  valerate  contains 
40.41  per  cent  of  barium,  and  barium  acetate  (coming  from  the 
ethyl  alcohol)  53.72  per  cent,  the  proportion  may  be  approximately 
calculated  (page  171). 

From  ethyl  alcohol  diluted  with  water  to  a  prescribed  concen- 
tration, amyl  alcohol  is  extracted  by  chloroform  which  proportion- 
ately increases  in  volume.  Roese's  apparatus,  Fig.  176,  has  a 
lower  bulb  of  a  capacity  of  20  Cc.  to  the  lowest  mark  on  the  stem. 
Above  the  mark  the  tube  is  graduated  in  l-20th  Cc.  to  26  Cc.  The 
upper  bulb  has  a  capacity  of  about  200  Cc.  and  is  marked  for  a 
total  content  of  120  Cc.  Chloroform  at  20  °  Cent,  is  poured  in  up 
to  the  20  Cc.  mark,  then  100  Cc.  of  the  sample  to  be  tested  (previ- 
ously diluted  to  a  concentration  of  30  per  cent  of  alcohol),  and  a 
little  dilute  sulf  uric  acid.  The  apparatus  is  stoppered  and  well 
shaken,  the  temperature  of  the  liquid  brought  to  20°  ,  and  the  in- 
crease in  volume  of  the  chloroform  read  on  the  scale . 

UStutzer  and  Maul  concentrate  fusel  oil  into  a  smaller  bulk  of 
alcohol  by  fractional  distillation  before  the  test  is  made.  Hertz- 
feld  substitutes  carbon  tetrachloride  for  chloroform,  and  brine  for 
water  when  diluting  the  spirit.  Various  other  modifications  have 
Fig.  176.  Vs  been  made  in  the  process  which  like  all  others  for  this  deter- 
mination is  unsatisfactory  at  best. 

Bardy  mixes  the  sample  with  brine  and  extracts  the  isobutylic  and  amylic 
alcohols  by  carbon  disulfide.  The  solution,  containing  also  some  ethyl  alcohol, 
is  in  turn  extracted  by  a  little  monohydrated  sulf  uric  acid.  Through  the  acid 
is  blown  a  current  of  air  to  remove  any  carbon  disulflde;  it  is  then  mixed  with 
glacial  acetic  acid  and  boiled  under  a  reflux  condenser.  The  ethers  of  the 
higher  alcohols  are  now  floated  by  brine,  and  their  volume  measured  in  a  narrow 
graduated  tube. 

If  crude  alcohol  be  diluted  with  water  to  a  concentration  of  30  per  cent  alcohol 
and  shaken  up  with  chloroform,  in  the  lower  layer  are  amylic  alcohol,  acetal, 
aldehyd,  and  isobutyl  alcohol;  in  the  upper  are  ethyl  alcohol,  tertiary-butyl 
alcohol,  and  acetic  acid. 

The  number  of  drops  of  a  pure  ethylic  spirit  of  given  concentration  delivered 
from  a  pipette  with  capillary  orifice  is  increased  by  fusel  oil  contained ;  thus,  a 
spirit  containing  .01  per  cent  of  the  oil  forms  1.6  more  drops  than  a  pure  spirit. 
The  test  is  made  more  decisive  by  concentrating  the  oil  into  a  small  volume  of 
alcohol ;  the  spirit  is  diluted  to  20  per  cent  of  alcohol  and  saturated  with  salt, 
when  the  oil  and  part  of  the  alcohol  float  and  can  be  removed,  diluted,  and 
distilled  to  one  third.  The  distillate  contains  all  the  oil  and  is  tested  against 
spirit  dosed  with  known  proportions  of  the  pure  oil. 

The  height  to  which  a  spirit  rises  in  a  capillary  tube  is  diminished  by  the 
presence  of  fusel  oil.  The  *  capillarometer '  is  an  open  glass  tube  of  .8  milli- 
meter bore  with  a  scale  from  0  to  50  millimeters  divided  in  half-millimeters. 
The  spirit  is  diluted  with  water  to  20  per  cent  of  alcohol  and  the  rise  noted. 
Blyth  found  that  pure  alcohol  of  this  concentration  rose  to  50  Mm.,  that  con- 
taining one  per  cent  of  fusel  oil  to  only  43.3  Mm. 

For  the  higher  alcohols  in  distilled  liquors  the  coloration  produced  by  sul- 
furic  acid  is  compared  with  that  given  by  an  alcoholic  solution  of  iso-butyl 
alcohol.  To  100  Cc.  of  the  distillate  is  added  a  little  anilin  and  phosphoric  acid 
and  heated  for  an  hour  under  a  reflux  condenser;  the  anilin  phosphate  forms  a 


THE   ALCOHOLS.  399 

non-volatile  compound  with  the  aldehyds.  The  liquid  is  then  distilled  to  dry- 
ness  and  the  sulfuric  acid  test  applied  to  the  distillate. 

For  the  extraction  of  the  higher  alcohols  from  an  aqueous  or  alcoholic  solu- 
tion, common  salt  is  dissolved  in  the  liquid  until  the  specific  gravity  reaches 
1.1.  It  is  then  extracted  by  four  portions  of  carbon  tetrachlorlde.  The  united 
extracts  contain  the  higher  alcohols  and  some  ethyl  alcohol.  To  wash  out  the 
latter,  the  carbon  tetrachloride  is  shaken  first  with  saturated  brine,  then  with  a 
saturated  solution  of  sodium  sulfate.  The  liquid  is  heated  under  a  reflux 
condenser  with  chromic  and  sulfuric  acids  to  oxidize  the  higher  alcohols  into 
their  corresponding  acids,  and  then  distilled,  the  organic  acids  and  usually  a 
little  mineral  acid  passing  over  with  the  carbon  tetrachloride.  In  the  distillate 
the  mineral  acid  is  neutralized  by  barium  hydrate  and  methyl  orange ;  then  the 
organic  acids  are  titrated  by  standard  barium  hydrate  and  phenolphthalein. 

The  hydroxyl  group  combined  with  the  radical  of  the  higher  alcohols  may  be 
determined  by  acetylation.  Acetyl  chloride  dissolved  in  chloroform  reacts  with 
the  alcohol  to  form  a  neutral  ester,  liberating  a  molecule  of  hydrochloric  acid; 
thus  — 

(1) . . ..C5HU.OH  (amyl alcohol)  +  C2H30. 01  =  C5HnO.C2H3O  (amyl acetate) -f 
HC1. 

When  a  chloroformic  solution  of  acetyl  chloride  is  treated  with  water  there 
are  formed  hydrochloric  and  acetic  acids  — 

(2)  ....  C2H3O.C1  +  H20  =  HC1  +  HC2H3O2. 

Hence  if  equal  volumes  of  acetyl  chloride  be  acted  on,  the  one  by  an  equiva- 
lent of  amyl  alcohol,  the  other  by  water,  and  the  resulting  liquids  be  titrated 
by  standard  alkali  — 

(1)  ....  HC1  -f  KOH  =  KC1  +  H2O ;     and    (2),    HC1  +  HC2H3O2  +  2KOH  = 
KC2H3O2  +  2H2O. 
the  latter  will  require  double  the  volume  of  standard  alkali  of  the  former. 

In  practice,  the  alcohol  is  treated  with  an  excess  of  a  chloroformic  solution  of 
acetyl  chloride,  and  the  excess  of  the  latter  decomposed  by  water.  An  equal 
volume  of  the  reagent  is  decomposed  by  water.  Both  are  titrated  by  standard 
alkali ;  the  former  will  require  for  neutralization  a  less  volume  of  alkali  than 
the  latter  in  proportion  to  the  amount  of  alcohol  reacting  with  the  acetyl 
chloride. 

When  a  higher  alcohol  is  heated  in  contact  with  soda-lime  to  a  temperature 
of  300  ° ,  hydrogen  is  quantitatively  evolved  from  the  alcoholic  hydroxy-groups, 
e.g.— 

CieHss-OH  (cetyl  alcohol)  +  NaOH  =  NaCi6HsiO2  (sodium  palmitate)  -f  2H2. 
The  alcohol  and  soda-lime  are  held  in  a  glass  tube  set  vertically  in  an  air-bath 
and    connected  at   the   top  with  a  U-form  gas -measuring  tube  filled  with 
mercury. 

The  rapidity  with  which  a  commercial  ethyl  alcohol  loses  the  color  imparted 
by  a  little  potassium  permanganate  is  roughly  in  proportion  to  the  amount  of 
impurities  present.  When  two  or  three  drops  of  centinormal  permanganate  are 
added  to  ten  cubic  centimeters  of  pure  alcohol,  reduction  takes  place  in  about 
ten  minutes.  This  test  is  of  use  to  those  requiring  a  fairly  pure  alcohol  for 
manufacturing  purposes. 

Acetone  is  contained  in  crude  methylene  or  wood  spirit  to  the  extent  of  20  per 
cent  or  more,  but  is  largely  eliminated  during  the  refining  processes.  Commer- 
cial ethyl  alcohol  contains  much  less  than  commercial  methyl  alcohol,  though 
always  an  appreciable  quantity. 


400  QUANTITATIVE    CHEMICAL    ANALYSIS. 

A  characteristic  reaction  of  acetone  is  that  taking  place  with  iodine  and  an 
alkali,  yielding  iodof orm  and  an  acetate  — 

CH3.CO.CH3  -f  3I2  +  4KOH  =  CHI3  (iodoform)  -f.  CH3.COOK  +  SKI  +  3H2O. 

Iodof  orm  is  a  yellow  powder  melting  at  119°,  slightly  volatile  at  ordinary 
temperatures,  and  soluble  in  alcohol  and  ether. 

For  a  gravimetric  determination,  one  Cc.  of  commercial  wood  spirit  is  mixed 
-with  an  excess  of  sodium  hydrate,  then  with  iodine  and  potassium  iodide,  both 
in  concentrated  aqueous  solutions.  The  iodoform  separates  as  a  powder  and 
is  extracted  by  ether,  the  ether  allowed  to  evaporate  spontaneously,  and  the 
iodoform  dried  over  sulf uric  acid  and  weighed.  No  free  iodine  enters  the  ether 
as  all  the  excess  has  been  converted  by  the  potassium  hydrate  into  potassium 
iodate  and  iodide.  Should  the  sample  of  spirit  contain  above  1.5  per  cent  of 
acetone,  it  is  to  be  diluted  with  water  previous  to  the  test.  A  minute  amount 
of  potassium  iodide  will  dissolve  in  the  ether,  and  some  tarry  matter  from  a 
crude  spirit,  both  increasing  the  weight  of  the  iodoform;  these  are  in  a  meas- 
ure neutralized  by  the  retention  of  iodoform  in  the  aqueous  solution  and  its 
volatility.  The  efficiency  of  the  method  in  presence  of  aldehyd  or  ethyl  alcohol 
and  for  crude  spirit  has  been  questioned  by  Vignon.* 

The  application  of  this  reaction  to  volumetric  processes  has  been  the  subject 
of  much  investigation  and  controversy.  All  the  volumetric  methods  rest  on  the 
direct  or  indirect  measurement  of  the  residual  iodine  from  a  known  weight 
compounded  with  the  acetone. 

Messinger  compounds  20  to  30  Cc.  of  normal  potassium  hydrate  with  1  to  15 
Cc.  of  wood  spirit  in  a  stoppered  flask.  A  measured  excess  of  N/5  iodine  solu- 
tion is  run  in  and  the  flask  well  shaken.  The  mixture  is  acidified  by  hydro- 
chloric acid  and  the  excess  of  iodine  reduced  by  sodium  thiosulfate  solution 
with  starch  paste  as  indicator,  then  the  residual  thiosulfate  titrated  back  by 
standard  iodine. 

According  to  Vignon,  in  the  presence  of  water  two  reactions  may  follow 
the  bringing  together  of  acetone,  iodine,  and  alkali,  viz. :  — 

CH3COCH3  +  3I2  +  NaOH  =  CH3CONaO  -f  CHI3  -f  3NaI  +  3H20 ;  and 
3I2  +  6NaOH  =  5NaI  +  NaIO3  +  3H2O. 

and  to  the  extent  of  the  latter  reaction  the  amount  of  iodine  to  convert  the 
acetone  to  iodoform  must  be  increased ;  it  is  favored  by  the  presence  of  methyl 
and  ethyl  alcohol  and  retarded  by  aldehyd. 

Squibb, f  for  the  assay  of  commercial  acetone^  instead  of  the  standard  solu- 
tion of  iodine  would  liberate  the  element  in  situ  from  sodium  hypochlorite  and 
potassium  iodide,  thus  NaOCl  +  2KI  +  H20  =  I2-fNaCl  +  2KOH.  The  method 
is  to  mix  the  acetone  with  an  alkaline  solution  of  potassium  iodide,  then  titrate 
the  mixture  by  standard  sodium  hypochlorite.  The  end-point  is  where  a  blue 
color  is  developed  when  a  drop  of  the  titrate  is  mixed  with  a  drop  of  starch 
paste  containing  sodium  bicarbonate.  He  confirms  the  statement  that  ethyl 
alcohol  or  small  amounts  of  paraldehyd  do  not  interfere. 

Kebler  J  modifies  the  above  by  mixing  the  acetone  with  potassium  iodide, 
sodium  hydrate,  and  excess  of  standard  sodium  hypochlorite.  The  solution  is 
acidified  by  hydrochloric  acid  and  the  residual  iodine  determined  by  adding 
an  excess  of  standard  thiosulfate  and  titrating  back  by  standard  sodium  hypo- 
chlorite and  starch-paste  — 

2Na2S203  +  NaOCl  -f  2HC1  =  Na2S406  +  3NaCl  +  H2O. 


*  Chem.  News,  1890—1—166. 

t  Journ.  Amer.  Chem.  Socy.  1896—1068. 

J  Idem,  1897-316. 


THE   ALCOHOLS.  401 

Aldehyds  are  found  in  small  proportions  in  all  samples  of  commercial  alcohol. 
For  some  purposes,  as  where  the  alcohol  is  to  be  the  solvent  of  certain  dye- 
stuffs,  any  considerable  amount  of  aldehyd  is  highly  objectionable. 

With  acid  sodium  sulflte  aldehyds  form  crystalline  compounds  that  are  sol- 
uble in  water  and  alcohol  but  nearly  insoluble  in  a  concentrated  solution  of  the 
reagent;  thus  — 

CaHsOH  (acetic  aldehyd)  -fNaHSO3  =  Na(C2H3)SO3  (sodium  ethylidene  sulflte) 
+  H20. 

This  reaction  can  be  applied  to  samples  comparatively  rich  in  aldehyds, 
washing  the  precipitate  by  a  concentrated  solution  of  the  reagent,  then  distilling 
with  a  dilute  acid. 

The  determination  of  the  small  quantities  normal  to  a  purified  alcohol  or  a 
beverage  has  not  yet  been  accomplished  satisfactorily.  It  is  attempted  colori- 
metrically  by  Guyon,  applying  the  reappearance  of  color  in  an  acid  solution  of 
fuchsin  previously  decolorized  by  sodium  sulflte;  the  comparison  is  made 
against  a  standard  solution  of  acetic  or  ethylic  aldehyd  in  dilute  alcohol.  It  is 
doubtful,  however,  whether  the  coloration  reproduced  is  proportionate  to  the 
aldehyd. 

Wine  and  spirits  may  contain  as  bases  ammonia,  the  pyridens,  the  amids, 
and  certain  alkaloids.  After  distilling  the  sample  with  a  little  phosphoric  acid, 
the  distillate  is  mixed  with  a  dilute  solution  of  sodium  carbonate,  and  free  and 
liberated  ammonia  distilled  and  Nesslerized  (page  376),  the  result  being  con- 
sidered as  due  to  ammonia  and  amids.  On  again  distilling  with  the  addition 
of  permanganate,  the  ammonia  found  is  a  partial  yield  of  the  nitrogen  in  the 
pyridins  and  alkaloids  —  the  process  is  never  more  than  approximate. 

For  the  determination  of  impurities,  other  than  acetone,  in  commercial 
methyl  alcohol,  Barillot  mixes  10  Cc.  of  the  sample  with  15  Cc.  of  a  solution  of 
sodium  bisulfite.  After  cooling,  the  mixture  is  agitated  with  exactly  20  Cc.  of 
chloroform.  Acetone  does  not  increase  the  volume  of  the  chloroform,  but  other 
impurities  (benzols,  methylol,  diallyl,  etc.)  enter  it  without  condensation  of 
volume.  A  special  tube  similar  to  that  of  Roe*se  (Fig.  176)  is  used  to  measure 
the  expansion  of  the  choloroform.  Ordinary  methyl  alcohols  of  good  quality 
show  from  one  to  five  per  cent  of  impurities,  strong  smelling  samples  from  ten 
to  twenty  per  cent. 

Fermented  Beverages. 

These  are  manufactured  by  converting  the  sugar  of  a  saccharine  liquid  to 
alcohol  and  carbonic  acid  through  the  action  of  certain  organisms.  Cane  sugar 
passes  first  to  invert  sugar,  then  to  alcohol,  but  glucose  directly  CeHi2O6  = 
2C2HeO  +  2COs.  The  starches  of  malt  and  potatoes  are  transformed  by  the 
ferment  diastase  to  maltose,  this  by  yeast-ferment  to  glucose.  Besides  alcohol 
and  carbonic  acid  there  are  formed  in  the  fermentation  small  amounts  of  the 
higher  alcohols,  succinic  acid,  glycerol,  acetic  acid,  etc. 

Pure  wine  is  the  fermented  juice  of  the  grape,  but  certain  additions,  as  of 
alcohol  and  sugar,  are  considered  legitimate.  The  normal  constituents  of 
wine  are  water,  alcohol  from  six  to  twelve  per  cent,  sugar,  tannin,  glycerol, 
succinnicacid,  coloring  matter,  and  traces  of  many  other  organic  and  inorganic 
bodies.  Beer,  made  from  malt,  hops  and  a  starchy  cereal,  contains  alcohol 
from  three  to  ten  per  cent,  carbonic  acid,  malt  extract,  bitter  principles  from 
the  hop,  constituents  of  the  water  of  brewing,  etc.  Distilled  liquors  should  be 
only  water,  alcohol  from  30  to  60  per  cent,  and  traces  of  volatile  organic  ethers, 
etc.,  but  various  artificial  coloring  and  flavoring  matters  are  not  uncommon. 
Liqueurs  and  cordials  contain  large  amounts  of  sugar  and  essential  oils. 

26 


402  QUANTITATIVE    CHEMICAL    ANALYSIS. 

The  specific  gravity  is  a  function  of  the  ratio  of  the  alcohol  to  the  water  andf 
extractive  and  mineral  matters  contained,  hence  the  gravities  of  fermented 
liquors  are  usually  above  unity,  while  distilled  liquors  are  usually  below. 
Although  affording  no  specific  information  it  is  useful  in  corroborating  con- 
clusions drawn  from  the  results  of  other  determinations.  It  is  perhaps  most 
accurately  observed  by  the  Westphal  balance,  approximately  by  the  oenometer. 

Acidity.  All  natural  wines  react  acid  from  free  tartar ic  or  succinic  acids 9 
sometimes  from  potassium  bitartrate  or  acetic  acid ;  in  beer  the  volatile  acid  is 
acetic,  the  non-volatile  lactic  and  succinic;  and  in  cider,  malic.  Occasionally 
sulf urous  and  salicylic  acids  are  found. 

The  determination  of  total  acidity  is  made  as  usual  by  titration  with  weak 
standard  alkali  and  a  suitable  indicator  —  litmus  paper  for  highly  colored 
samples. 

Practically  all  of  the  acetic  acid  passes  over  on  repeated  distillation  with 
water,  more  readily  in  vacuo,  and  may  be  titrated  in  the  distillate ;  sulf  urous 
acid  also  distills,  but  is  largely  converted  to  sulfuric  by  the  action  of  air  on 
the  vapor.  In  the  residue  are  the  fixed  acids  that  may  be  directly  titrated  after 
dilution. 

Sulfurous  acid  may  exist  in  wine  either  as  such  or  as  an  aldehyd  compound. 
The  former  is  titrated  by  iodine  and  starch  after  acidification  by  sulfuric  acid, 
either  directly  in  the  wine  or  after  distillation  under  carbon  dioxide  gas.  The 
total  sulf  urous  acid  is  then  determined  in  another  portion  of  the  wine  by  de- 
composing the  aldehyd  compound  by  an  alkali,  e.  g.,  K(C2Hs)SO3+  KOH  == 
K2SO3  +  (C2H3)CH,  acidifying,  arid  titrating  as  before.  The  difference  in  the 
volumes  of  titrand  represents  the  aldehyd -sulf  urous  acid. 

A  separation  of  tartaric  acid  and  potassium  bitartrate  depends  on  the  insolu- 
bility of  the  latter  in  strong  alcohol.  One  hundred  Cc.  of  the  wine  is  evapo- 
rated to  a  thin  syrup,  and  alcohol  added  as  long  as  a  precipitate  forms.  In  a 
few  hours  the  bitartrate  (with  some  extractive,  etc.)  is  filtered,  washed  with 
strong  alcohol,  dissolved  in  water  and  titrated  by  an  alkali.  In  the  filtrate  the 
tartaric  acid  is  precipitated  by  calcium  acetate  as  calcium  tartrate,  which  is 
determined  gravimetrically.  Or  to  the  wine  is  added  sufficient  potassium 
hydrate  to  neutralize  about  one-fifth  of  the  free  acids,  then  five  volumes  of 
alcohol;  all  the  tartaric  acid  precipitates  as  potassium  bitartrate  together  with 
that  already  existing  as  such.  The  precaution  of  limiting  the  alkali  is  to  avoid 
the  formation  of  soluble  potassium  tartrate. 

Cider  is  evaporated  to  one- tenth  its  volume  and  the  potassium  bitartrate  and 
calcium  salts  thrown  down  by  an  equal  volume  of  alcohol.  The  filtrate  is 
made  slightly  alkaline  by  lime-water,  whereupon  calcium  malate  separates  and 
is  purified  by  dilute  nitric  acid  from  which  calcium  bimalate  crystallizes. 

Salicylic  acid  is  sometimes  used  as  a  preservative  for  beer.  Following  con- 
centration at  a  low  heat  the  acid  is  extracted  by  ether  or  a  mixture  of  ether  and 
gasoline.  After  washing  and  evaporating  the  solvent  the  acid  may  be  deter- 
mined colorimetrically  by  the  violet  color  struck  with  ferric  chloride. 

The  carbonic  acid  in  a  beer  or  sparkling  wine  is  always  in  excess  of  a  saturated 
solution.  The  cork  of  the  bottle  is  pierced  by  a  champagne-tap  or  other  de- 
vice allowing  only  a  slow  outflow  of  the  gas  which  is  led  through  some  form  of 
absorbent  and  determined  gravi metrically  or  volumetrically.  The  gas  remain- 
ing in  solution  is  boiled  out,  with  the  addition  of  a  little  tannin  to  prevent 
frothing,  into  bulbs  holding  barium  hydrate  solution;  the  precipitated  barium 
carbonate  is  determined  directly  or  by  difference.  A  vacuum  pump  may  be 
employed  in  the  operation  with  advantage. 

The  extractive  matter  of  wine  or  beer  is  the  non- volatile  organic  matter  con- 


THE   ALCOHOLS.  403 

tained.  In  wine  it  usually  ranges  from  1.6  to  3  per  cent,  sometimes  as  high  as 
5  per  cent  or  more  ;  some  include  the  glycerol  in  the  extractive,  others  not. 
The  solution  -density  of  the  extractive  matter  is  taken  as  1.039. 

The  customary  method  of  determination  —  by  evaporation  and  weighing  the 
residue  and  deducting  the  ash  —  is  beset  by  the  difficulty  common  to  all  sacchar- 
ine fluids,  the  attainment  of  a  constant  weight  without  danger  of  decomposi- 
tion of  the  residue.  Where  this  method  is  adopted  it  is  important  that  the 
dish  used  has  a  level  flat  bottom  and  is  of  a  breadth  proportionate  to  the  vol- 
ume of  wine  evaporated  ;  best  so  large  that  the  thickness  of  the  layer  of  wine 
does  not  exceed  one  millimeter.  After  evaporating  10  to  50  Cc.  of  wine  or  5  to 
10  Cc.  of  beer,  the  residue  is  dried  at  100  °  to  fairly  constant  weight  —  it  is  said 
that  after  three  hours  any  further  loss  is  due  to  volatilization  of  glycerol.  If 
it  be  desired  to  include  this  body  in  the  extractive,  a  little  standard  baryta- 
water  is  added  before  evaporation.  Sweet  wines  are  better  diluted  before 
evaporation. 

Another  method  is  that  of  boiling  off  the  alcohol  and  other  volatile  matter, 
making  up  to  the  original  volume,  and  taking  the  specific  gravity  at  15.5°. 
Tables  showing  the  percentage  of  extractive  corresponding  to  the  gravity  will 
be  found  in  works  on  wine  analysis.  According  to  Schultze  and  Hager,  the 
percentage  of  extract  for  beer  is  260  times  the  specific  gravity  less  one,  and  for 
wine  is  220  times  the  specific  gravity  less  one. 

Riegler  observes  the  refractive  index  of  the  wine,  then  boils  until  the  alcohol 
is  driven  off,  cools,  makes  up  to  the  original  bulk  and  again  observes  the  re- 
fraction. One  gram  of  extractive  in  JOOCc.  of  wine  increases  the  refraction 
over  that  of  water  by  .00145,  while  the  same  proportion  of  alcohol  raises  it  by 
.00068.  If  B  be  the  refraction  of  distilled  water  ;  Rr  the  refraction  of  the  wine  ; 
R"  that  of  the  boiled  and  diluted  wine  ;  x,  the  percentage  of  extractive  ;  and  y 
the  per  cent  of  alcohol;  then  R'  =  .ft  +.00145  x  +.00068  y;  and  .R" 
.00145  x.  Whence 

B"  —  R  R'—R" 


Astringent  matter.  The  usual  methods  for  the  determination  of  tannin  in 
aqueous  extracts  may  be  applied  to  wine.  Loewenthal's  is  no  doubt  most  in 
use  and  is  recommended  by  Vogel;  others  give  preference  to  various  precipi- 
tation methods.  Nessler  and  Earth  remove  albuminous  matters  by  the  addition 
of  a  large  proportion  of  alcohol  to  the  wine,  then  concentrate  the  filtrate,  and 
precipitate  by  ferric  chloride  and  sodium  acetate  in  a  conical  graduated  tube. 
After  standing  24  hours,  the  volume  of  the  precipitate  is  read  on  the  gradua- 
tions, one  cubic  centimeter  corresponding  to  .033  per  cent  of  tannin  in  the 
wine.  Girard  notes  the  increase  in  weight  when  strips  of  purified  sheep  -gut, 
previously  soaked  in  water,  are  left  in  contact  with  the  wine  for  a  day  or  two. 

Nitrogenous  matters.  The  total  nitrogen  of  wine  or  beer  is  easiest  determined 
by  Kjeldahl's  method,  evaporating  the  sample  with  an  excess  of  sulfuric  acid, 
boiling,  etc.  (page  306). 

In  distilled  liquors,  Mohler  determines  the  ammonia  corresponding  on  the 
one  hand  to  the  amids  and  saline  ammonia  by  distillation  with  sodium  carbon- 
ate; then  that  corresponding  to  the  pyridin  bases  and  alkaloidal  matter  by  con- 
tinuing the  distillation  with  permanganate.  In  each  case  the  distillate  is, 
Nesslerized. 

Boed  lander  and  Traube  propose  the  determination  of  the  peptones  of  wine 
from  their  influence  on  the  constant  of  capillarity.  The  apparatus  is  a  pipette 
of  peculiar  construction,  the  lower  orifice  greased  on  the  sides.  For  pure  water 
the  number  of  drops  to  a  specified  volume  at  a  given  temperature  is  practically 


404  QUANTITATIVE    CHEMICAL   ANALYSIS. 

constant,  while  even  as  little  as  .02  per  cent  of  peptone  perceptibly  increases 
the  number.  Albumin  and  gelatin  have  comparatively  little  effect  in  this  way. 
Sugar.  In  unadulterated  wine  the  sugar  is  wholly  glucose.  Cane  sugar  is 
legitimately  added  to  champagne  during  manufacture,  but  is  usually  completely 
inverted  during  the  long  period  of  aging.  Cordials  and  liqueurs  are  heavily 
charged  with  sucrose. 

The  carbohydrates  of  wine  are  chiefly  grape  sugar  with  some  tartaric  acid 
and  certain unfermentable  bodies;  perfectly  fermented  wine  is  nearly  optically 
neutral,  while  any  unfermented  sugar  is  usually  laevo-rotatory.  To  arrive  at 
the  quantities  of  these  constituents  a  somewhat  complicated  process  is  neces- 
sary, the  wine  being  polarized  after  clarification  by  lead  subacetate  and  sodium 
carbonate,  and  also  after  inversion  and  fermentation  by  yeast. 

Ethers.  The  bouquet  and  flavor  of  a  wine  depend  largely  on  the  volatile 
esters,  the  taste  on  the  fixed.  Owing  to  their  minute  quantity  the  determina- 
tion is  not  very  satisfactory.  All  react  with  a  caustic  alkali  with  the  produc- 
tion of  an  alkali  salt  and  alcohol,  e.  g., 

CH3.C2H302(methyl  acetate)  -f  NaOH  =  CH3OH  (methyl  alcohol)  +  NaC2H302. 
The  amount  of  alkali  neutralized  in  the  reaction  corresponds  to  the  weight  of 
the  ester. 

To  the  wine  is  added  a  measured  volume  of  standard  alkali,  the  mixture 
boiled  under  a  reflux  condenser,  and  the  excess  of  alkali  titrated  back  by 
standard  acid.  The  result  is  calculated  to  acetic  ester  and  so  expressed.  In 
spirits,  bodies  of  the  type  of  aldehyd  and  furfurol  also  react  with  alkali,  but 
Mohler  finds  that  on  distillation  with  anilin  and  syrupy  phosphoric  acid  the 
volatile  ethers  pass  over,  while  furfurol  and  aldehyd  remain. 

A  separation  of  the  volatile  from  the  fixed  esters  is  done  by  distilling  the 
exactly  neutralized  wine  from  a  retort  until  nine-tenths  has  passed  over.  To 
both  the  distillate  and  residue  is  added  a  measured  volume  of  decinormal 
potash,  and  after  standing  for  a  time,  the  residual  alkali  is  determined  by  back 
titration  with  an  acid,  using  as  indicator  phenol-phthalein  for  the  distillate,  and 
blue  litmus  paper  for  the  (highly  colored)  residue. 

Coloring  matter.  The  coloring  agents  of  wine  may  be  either  natural  (oenolin) 
or  artificial,  the  latter  harmless  or  deleterious.  The  intensity  of  the  color  is 
arbitrarily  expressed  as  degrees  of  the  'vino -colorimeter  '. 

The  most  common  of  the  artificial  dyes  used  to  heighten  the  color  of  natural 
wine  or  simulate  it  in  factitious  articles,  are  f uchsin,  cochineal,  logwood  and 
magenta.  Schemes  for  the  detection  of  these  and  others  are  based  on  the  de- 
portment of  the  wine  to  acids,  alkalies,  oxidizing  and  reducing  agents,  etc. 
Kagnoul  states  that  if  5  Cc.  of  a  strong  solution  of  soap  in  water  be  mixed  with 
an  equal  volume  of  water  and  from  10  to  20  drops  of  wine  added,  the  natural 
coloring  matter  will  be  destroyed  and  the  mixture  becomes  colorless,  while  the 
color  if  artificial  will  remain. 

Other  means  of  differentiation  are  by  the  spectroscopic  bands  and  by  absorp- 
tion in  silk,  stearic  acid,  fuller's  earth,  etc.  Nessler  and  Earth  determine 
rosanilin  dyes  by  agitating  the  wine  with  ether  and  ammonia;  the  ether  layer, 
containing  the  greater  part  of  the  dye,  is  removed  and  evaporated  in  a  capsule 
with  a  thread  of  white  wool  which  absorbs  it.  Similar  threads  are  dyed  by 
ethereal  solutions  of  different  amounts  of  rosanilin,  and  the  relation  between 
the  colors  is  a  rough  quantitative  index  of  the  proportion  of  the  dye. 

Furfurol.  This  compound  is  not  a  natural    product  of  fermentation  but  is 
believed  to  come  from  excessive  local  heat  during  the  manufacture.    Like  the 
esters  it  is  saponified  by  an  alkali,  forming  a  pyromucate  and  f urfuryl  acohol  — 
2C4H3COOH  -f  KOH  =  C4H3O.  COOK  +  C4H3O.CH2OH. 


THE    ALCOHOLS.  405 

It  may  be  roughly  determined  colorimetrically  by  the  red  color  developed 
in  a  solution  of  anilin  in  glacial  acetic  acid,  the  color  attaining  a  maximum  in 
thirty  minutes.  The  test  is  said  to  be  exceedingly  delicate.  Other  reagents 
for  the  purpose  are  rosanilin  hydrochloride  with  sodium  bisulfite  in  dilute 
sulf  uric  acid,  and  xylidin  in  glacial  acetic  acid. 

Inorganic  matter.  The  ash  of  normal  wine  consists  chiefly  of  potassium  com- 
bined as  carbonate,  sulf  ate,  phosphate  and  chloride;  sodium  as  chloride;  calci- 
um as  phosphate  and  carbonate.  Plastered  wines  (those  clarified  by  calcium 
sulf  ate)  leave  an  ash  high  in  sulfates.  The  determination  is  made  as  usual  by 
evaporation  and  ignition  of  the  residue.  Sweet  wines  leave  so  much  carbon  on 
charring  that  to  avoid  loss  of  alkalies  on  calcination  the  char  should  be  lixivi- 
ated before  burning.  The  proportion  of  the  constituents  of  the  ash  may  be 
determined  by  the  ordinary  methods  of  mineral  analysis. 

GLYCEROL. 

Glycerol  (glycerine)  may  be  considered  as  a  triatomic  alcohol  having  the 
formula  C3H5(OH)3.  It  is  a  colorless,  viscid,  odorless  fluid  of  neutral  reaction 
and  has  a  specific  gravity  of  about  1.265.  Fixed  at  ordinary  temperatures, 
it  volatilizes  completely  at  160  o.  It  is  quite  hygroscopic  and  mixes  in  all 
proportions  with  water  and  alcohol,  but  is  sparingly  soluble  in  ether.  On 
boiling  the  solution  in  water  of  a  strength  of  70  per  cent  or  over  there  is  a 
perceptible  loss  of  glycerol  whose  vapor  tension  is  64  Mm.  of  mercury  at  100  ° 
and  760  Mm.  Peculiar  compounds  known  as  glycerates  are  formed  with  the 
alkalies,  earths,  and  lead  oxide. 

Physical  methods  of  assay.  For  reasonably  pure  aqueous  solutions  various 
physical  methods  can  be  applied. 

1.  For  specific  gravity,  tables  have  been  drawn  up  by  Gerlach,  Skalweit, 
Strohmer  and  others  *  that  agree  quite  well.    The  determination  in  a  dilute 
solution  presents  no  special  difficulties,  but  where  the  sample  is  fairly  con- 
centrated the  viscous  fluid  may  inclose  and  retain  air-bubbles  that  rise  so 
slowly  that  one  may  have  to  wait  for  hours  till  they  disappear;  by  care  in 
pouring  into  the  flask  this  may  be  avoided  largely  or  entirely. 

2.  The  refractive  index  of  pure  glycerol  at  12. 5 o  Cent,  is  1.4742;  of  a  one 
per  cent   solution  is  -1.334:2,  water  at   this  temperature    registering  1.3330. 
Hence  with  a  ref  ractometer  reading  to  thousandths,  a  determination  accurate 
within  one  per  cent  is  possible. 

3.  In  the  vaporimeter  pure  glycerol  has  a  vapor  tension  of  66,  and  a  one  per 
cent  solution  740,  both  at  100  °  Cent,  and  760  Mm.  of  mercury.    The  average 
difference  is  nearly  7  millimeters  for  one  per  cent  of  glycerol. 

4.  According  to  Deiss  a  given  mixture  of  anhydrous  phenol  and  aqueous 
glycerol  always  absorbs  the  same  quantity  of  water  up  to  the  point  of  turbidity. 
Ten  grams  of  the  sample  is  mixed  with  six  grams  of  crystallized  phenol  and  the 
mixture  titrated  at  11°  Cent,  with  a  solution  of  50  grams  of  phenol  in  a  liter 
of  water.    Finally  on  continual  stirring  a  permanent  turbidity  remains.    Under 
these  circumstances  anhydrous  glycerol  requires  28.15  Cc.,  and  a  commercial 
article  containing  28.15  per  cent  of  glycerol  with  71.85  per  cent  of  water  would 
require  zero  Cc.    Adopting  the  formula  page  16,  JTis  the  percentage  of  pure 
glycerol;  r,  the  percentage  of  28.15  per  cent  glycerol;  a  is  29.15;  b  is  zero; 
and  d  the  volume  of  the  titrand  used. 

Chemical  methods.  1.  On  digestion  with  chromic  and  sulfuric  acids  of  cer- 


*  Journ.  Socy.  Chem.  Ind.  1889—424. 


406  QUANTITATIVE    CHEMICAL    ANALYSIS. 

tain  concentrations,  the  oxygen  of  the  former  consumes  the  glycerol  to  water 
and  carbon  dioxide  — 

3C3H8O3  -f  7K2Cr2O7  +  35H2SO4  =  7Cr2(SO4)3  +  9C02  +  14KHSO4  +  40H20. 

The  sample  of  glycerine  is  heated  with  a  standard  solution  of  potassium  bi- 
chromate in  sulfuric  acid.  When  the  reaction  is  over  the  excess  of  the  oxi- 
dizer  is  reduced  by  a  known  weight  of  ferrous  chloride,  and  the  excess  of  the 
latter  titrated  back  by  standard  bichromate.  Or  the  chromic  solution  may  be 
standardized  by  addition  of  an  excess  of  potassium  iodide,  two  atoms  of  iodine 
being  liberated  for  each  atom  of  available  oxygen  — 

K2Cr2O7  -f  7H2SO4  +  6KI  =  4K2SO4  -f  Cr2(S04)s  -f  7H2O  -f-  3I2. 
and  the  iodine  determined  by  titration  by  sodium  thiosulfate  and  starch-paste ; 
the  chromic  acid  remaining  after  the  reaction  with  glycerol  is  determined  by 
the  same  process. 

Several  colorimetric  methods  have  been  proposed  with  the  green  color  of 
chromic  sulfate  as  a  basis,  this  formed  by  reduction  of  chromic  acid  by  glycerol 
in  a  hot  acid  solution. 

Since  the  carbon  dioxide  is  evolved  in  direct  ratio  to  the  glycerol,  it  may  be 
collected  and  weighed  or  measured  and  the  glycerol  calculated  therefrom. 
Some  claim  this  to  be  more  accurate  than  the  volumetric  process.  Of  course 
if  the  sample  contains  other  organic  matter  decomposable  by  the  acid  mixture 
the  results  will  be  correspondingly  high  by  either  process. 

2.  In  an  acid  solution  glycerol  reacts  with  the  oxygen  of  a  permanganate  to 
form  carbon  dioxide  and  water  — 

5C8H803  +  7K2Mn2O8  -j-  28EI2SO4  =  14KHS04  +  UMnS04  -f  15CO2  +  41H2O. 

The  solution  of  glycerol  is  held  in  a  flask  arranged  as  in  Fig.  166,  and  after 
the  addition  of  an  excess  of  potassium  permanganate  solution  and  concentrated 
sulfuric  acid,  is  boiled  until  all  the  gas  has  passed  into  the  potash  bulb  B 
through  the  drying  tube  D.  From  the  weight  of  the  carbon  dioxide  is  calcu- 
lated that  of  the  glycerol. 

3.  Glycerol  is  converted  into  oxalic  acid,  carbon  dioxide  and  water  when 
boiled  with  a  permanganate  and  an  alkali  — 

C3H803  +  2K2Mn2O8  =  K2C204  -f  4H2O  +  4MnO  +  K2CO3. 
The  solution  of  glycerol  is  made  strongly  alkaline  by  potassium  hydrate,  and 
an  excess  of  strong  solution  of  potassium  permanganate  run  into  the  hot  so- 
lution until  permanently  red.  The  excess  of  permanganate  is  reduced  by 
sulfurous  acid  or  hydrogen  peroxide,  and  the  now  colorless  liquid  filtered  from 
the  precipitated  manganic  oxide.  After  boiling  off  the  excess  of  the  reducer, 
the  oxalic  acid  in  the  nitrate  is  determined  by  acidifying  and  titrating  by  per- 
manganate, or  otherwise. 

4.  Triacetin  is  formed  when  glycerol  is  heated  with  acetic  anhydride  — 

2C3H8O3  +  3(C2H3O)2O  =  2C3H5(O.C2H3O)3  -f  3H2O. 
Glycerol    Acetic  anhydride    Triacetin 

And  on  heating  triacetin  with  caustic  soda  it  is  saponified  with  the  produc- 
tion of  sodium  acetate  and  glycerol  — 

C3H5(O.C2H30)3  (triacetin)  +  3NaOH  =  C3H8O3  (glycerol)  +3NaC2H3O2. 
The  concentrated  glycerol  is  boiled  with  an  excess  of  acetic  anhydride  for 
some  hours  in  a  flask  topped  by  an  inverted  condenser.  The  product  is  diluted 
with  warm  water,  converting  the  excess  of  acetic  anhydride  to  acetic  acid 
(page  315),  and  the  solution  of  triacetin  and  acetic  acid  filtered  from  a  resi- 
due containing  most  of  the  impurities  of  the  original  glycerin.  The  free 
acetic  acid  is  exactly  neutralized  with  caustic  soda  and  phenol- phthalein;  a 


THE   ALCOHOLS.  407 

known  volume  of  standard  caustic  soda  is  added,  the  solution  boiled  for  a  short 
time,  and  the  excess  of  alkali  titrated  back  by  standard  acid.  From  the  above 
equations  may  be  calculated  the  percentage  of  glycerol  in  the  sample.  With 
impure  samples  the  results  are  said  to  be  much  in  excess  of  the  truth. 

6.  In  an  alkaline  solution  glycerol  forma  esters  with  benzoyl  chloride  that 
are  fairly  insoluble,  containing  one,  two  or  three  groups  of  the  benzoyl  radical, 
thus  C3H5.(OH)2.C7H5O2;  C3H5.OH.(C7H5O2)2  andC3H5.(07H5O2)3.  It  is  said 
that  .1  gram  of  glycerol  yields  .385  gram  of  the  mixed  ethers  dried  at  110° .  It 
is  to  be  remembered  that  benzoyl  chloride  forms  insoluble  compounds  with 
many  other  organic  bodies. 

6.  According  to  Wanklyn  and  Johnstone  *  glycerol  and  hydriodic  acid  react 
as  expressed  by  the  equation  C8H8O3  +  5HI  =  C8H7I  -f-  2I2+  3H2O. 


Separation.  Volatile  organic  bodies  may  be  evaporated  or  distilled  from 
glycerol  after  fixing  the  latter  by  the  addition  of  lime  and  alcohol,  limiting 
the  heat  to  that  of  boiling  water. 

Since  pure  glycerol  volatilizes  completely  at  160® ,  organic  bodies  not  vola- 
tile or  decomposed  at  this  temperature  are  left  with  inorganic  matter.  To 
prevent  decomposition  of  the  glycerol  into  polymers  less  volatile,  this  tempera- 
ture should  not  be  exceeded.  A  preferable  plan  is  to  distill  in  vacuo;  for 
small  quantities  a  simple  method  of  purification  is  to  place  the  impure 
glycerol  in  one  end  of  a  bent  glass  tube,  exhaust  the  air  and  seal  the  orifice, 
then  distill  into  the  other  end. 

Glycerol  is  found  in  small  quantity  in  most  soaps,  and  in  "  glycerin  toilet 
soaps "  may  reach  to  twenty  per  cent  or  more.  For  a  determination,  the 
aqueous  solution  of  a  large  weight  of  the  soap  is  decomposed  by  sulfuric 
acid,  and  the  filtrate  from  the  fatty  acids  neutralized  and  concentrated  by 
evaporation  at  a  low  heat.  Or  the  excess  of  the  sulfuric  acid  may  be  pre- 
cipitated by  barium  carbonate,  the  filtered  solution  mixed  with  alcohol  and 
evaporated  with  an  occasional  addition  of  alcohol.  From  the  residue  the 
glycerol  is  extracted  by  alcohol  and  ether  and  determined  in  one  of  the  usual 
ways. 

Glycerol  is  an  invariable  constituent  of  wine,  coming  from  a  secondary  re- 
action in  the  fermentation  whereby  glucose  yields  succinic  and  carbonic  acids 
and  glycerol.  The  normal  proportion  of  glycerol  to  alcohol  is  said  to  be  from 
7  to  14  of  the  former  to  100  of  the  latter. f 

The  usual  course  of  the  determination  is  to  evaporate  the  wine  to  dryness 
with  calcium  hydrate.  Strong  alcohol  will  dissolve  the  glycerol  from  the 
residue,  leaving  the  sugar  and  succinic  acid  as  calcium  sucrate  and  succinate. 
On  evaporation  of  the  alcohol  the  glycerol  is  left  in  a  fairly  pure  state  except 
when  glucose  has  been  added  to  the  wine.  For  further  purification  it  is  dis- 
solved in  absolute  alcohol,  mixed  with  ether  and  the  precipitate  filtered  off,  the 
solution  evaporated,  and  the  residue  weighed. 

In  the  method  of  Oliveri  and  Spica  the  glycerol  is  distilled  and  the  distillate 
titrated  by  permanganate.  The  wine  is  heated  on  the  water  bath  until  the 
alcohol  is  driven  off,  and  tannin  and  other  precipitable  bodies  are  thrown  down 
by  basic  lead  acetate.  The  excess  of  lead  is  removed  by  sodium  carbonate, 
And  the  filtrate  evaporated  to  a  small  volume,  then  distilled  in  vacuo,  or  in 


*  Chem.  News,  1891—1—251. 

t  Journ.  Amer.  Chem.  Socy.  1898—881. 


408  QUANTITATIVE    CHEMICAL    ANALYSIS. 

steam  at  a  high  temperature.  The  distillate  is  heated  to  100°  and  standard 
permanganate  dropped  in  to  permanent  redness,  then  the  excess  of  perman- 
ganate titrated  back  by  standard  oxalic  acid.  Or  the  glycerol  may  be  deter- 
mined colorimetrically. 

The  analysis  of  a  crude  glycerol  for  technical  purposes  comprises  determina- 
tions of  the  specific  gravity,  ash,  fatty  acids,  organic  impurities,  and  sulfur 
compounds.  To  distinguish  crude  from  refined  glycerine,  the  aqueous  solution 
is  tested  by  silver  nitrate  and  basic  lead  acetate,  neither  of  which  produces  a 
precipitate  in  the  pure  article. 

For  the  manufacture  of  nitro-glycerine  a  specially  pure  grade  is  demanded 
which  should  meet  the  following  requirements.* 

(1).  Aminimum  specific  gravity  of  1.261  at  15°  Cent. 

(2).  Should  nitrify  well. 

(3).  After  nitrification  the  separation  of  the  nitroglycerine  should  be  sharp 
within  half  an  hour  without  the  separation  of  flocculent  matter,  nor  should  any 
white  flocculent  matter  (due  to  fatty  acids')  be  formed  when  the  nitrated 
glycerol  is  thrown  into  water  and  neutralized  with  carbonate  of  soda. 

(4).  Should  be  free  from  lime  and  chlorine,  and  contain  only  traces  of 
arsenic,  sulfuric  acid,  etc. 

(5).  Should  not  leave  more  than  .25  per  cent  inorganic  and  organic  residue 
together  when  evaporated  in  a  platinum  dish  without  ebullition  (about  160  o  ) 
or  partial  decomposition. 

(6).  The  silver  test  fair. 

(7).  The  glycerol  when  diluted  one-half,  should  give  no  deposit  or  separation 
of  fatty  acids  when  nitric  peroxide  gas  is  passed  through  it. 

(8).  A  practical  nitrifying  test  is  made,  following  the  usual  process  of  man- 
ufacture on  the  large  scale.  The  sample  of  glycerol  is  from  25  to  50  grams  and 
the  mixed  acids  in  proportion.  The  nitroglycerine  is  decanted  from  the  acids, 
washed  with  water  and  solution  of  sodium  carbonate,  and  the  volume  measured 
in  a  graduated  tube.  Especially  to  be  observed  are  the  yield  of  the  product, 
its  rapidity  of  separation  from  the  acids,  and  the  absence  of  fiocculent  matter. 


*  Journ.  Anal.  Appl.  Ohem.  189S-273. 


THE  VEGETABLE  ALKALOIDS. 


409 


THE  VEGETABLE  ALKALOIDS. 

These  are  a  class  of  organic  bodies  of  complex  formulae,  all  containing  car- 
bon, hydrogen  and  nitrogen  and  the  majority  also  oxygen.  A  number  have  been, 
produced  artificially  from  the  pyridin  bases  (CnH2n— sN)  as  a  starting  point, 
and  from  the  general  relationship  between  the  two  classes,  it  has  been  pro- 
posed to  define  alkaloids  as  vegetable  organic  bodies  that  are  derivatives  of 
pyridin.* 

All  the  alkaloids  are  bases  of  a  relatively  weak  character  and  unite  additively 
with  acids  to  form  salts.  Most  are  solid  and  fixed,  a  few  liquid  and  volatile . 
Characteristic  features  are  a  bitter  taste  and  a  marked  disturbing  influence  on 
the  animal  economy;  the  greater  number  are  active  poisons  in  small  doses. 
Although  classed  together,  marked  differences  are  observed  in  both  the  chemi- 
cal and  physical  properties  of  the  members;  some  are  quite  stable  and  may  be 
subjected  to  various  analytical  operations  without  decomposition,  while  others 
are  rapidly  broken  down,  even  hot  water  alone  decomposing  some  varieties. 

Many  of  the  alkaloids  undergo  hydrolysis  on  boiling  with  a  dilute  solution 
of  a  fixed  alkali  with  the  production  of  an  organic  acid,  usually  one  of  the 
aromatic  series,  and  a  base ;  thus  —  « 

CwHigNOs  (piperine)   -f  KOH  =  CsHuN  (piperidine)  -f  K2Ci2H904  (potassium 

piperate). 

Ci7H2iNO4  (cocaine)  -j-  KOH  -f- H2O  =  CgHisNOs    (ecgonine)  +  CH4O    (methyl 
alcohol)  +  KC7Hg02  (potassium  benzoate). 

Alkaloids  do  not  exist  in  plants  in  the  free  state,  but  are  usually  combined 
with  organic  acids  as  tannates,  malates,  meconates,  etc.  Some  plants  contain 
but  one  variety,  while  in  others,  notably  cinchona,  the  poppy,  and  aconite  root, 
several  are  contained  in  widely  different  amounts.  Usually  the  alkaloids  asso- 
ciated in  one  plant  have  several  properties  in  common,  yet  for  various  reasons 
but  one  or  two  may  have  an  extended  therapeutic  use,  the  others  being  com- 
paratively inactive  or  otherwise  unsuitable ;  for  example,  of  the  twenty-one 
known  alkaloids  of  opium,  but  one  or  two  have  an  extended  use  in  medicine. 

A  list  of  the  best  known  varieties  follows. 

Source.      Radical  formula.      Form.         Properties. 
Amorphous    Febrifuge 


Name. 

Source. 

Quinine 

Cinchona  bark 

Cinchonidine 

M 

Cinchonine 

(i 

Morphine 

Opium 

Codeine 

u 

Narcotine 

it 

Curarine 

Woorara 

Strychnine 

Nux  vomica 

Brucine 

M 

Cocaine 

Coca 

Nicotine 

Tobacco 

Caffeine 

Coffee,  tea 

Crystalline 


Ci7Hi9N03 
Ci8H21N08 
C^H^NO? 


Ci7H2iNO4 

CioHi4N2 


Oily  liquid 
Crystalline 


Narcotic 


Paralytic  poison 
Tetanic  poison 
Convulsive  poison 
Local  anesthetic 
Violent  poison 
Stimulant 


*  Allen,  Coml.  Org.  Anal.  3  —  2  — 162  et  seq. 


410 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


Name. 

Source.       Radical  formula.       Form. 

Properties. 

Theobromine 

Coca 

C7H8N402 

Crystalline 

Stimulant 

Aconitine 

Aconite 

CffiHtfNOu 

11 

Violent  poison 

Berberine 

Barberry 

C2oH17N04 

« 

Tonic 

Hydrastine 

ii 

C21H21N06 

tt 

it 

Pipeline 

Pepper 

Ci7Hi9N03 

« 

Conine 

Hemlock 

CgHnN 

Oily  liquid 

Paralytic  poison 

Pilocarpine 

Jaborandi 

CnHi6N202 

Crystalline 

Depressive 

Physostigmine 

Calabar-bean 

Ci5H2iN302 

ii 

Mydriatic 

Veratrine 

Wild  Hellebore 

CszHssNOn 

Amorphous 

Poison 

Atropine 

Night-shade 

Ci7H23N03 

Crystalline 

Mydriatic 

Emetine 

Ipecac 

Ci5H22N02 

Amorphous 

Emetic 

Colchicine 

Meadow  saffron 

C22H25N06 

SI 

Poison 

The  exact  formulae  of  some  of  the  above  are  yet  unknown. 

Most  alkaloidal  salts  are  soluble  in  water  and  alcohol,  but  insoluble  in  ether, 
chloroform,  and  light  petroleum.  They  are  decomposed  by  alkalies  and  the 
earths,  the  freed  alkaloid  separating  from  the  liquid  as  a  voluminous  flocculent 
precipitate,  in  some  cases  soluble  in  excess  of  the  precipitant. 

Qualitative  tests.  Evidence  as  to  the  presence  of  an  alkaloidal  base  in  a  solu- 
tion may  be  furnished  by  a  precipitate  forming  on  addition  of  an  alkali,  the 
separation  of  a  crystalline  salt  on  acidification  with  a  mineral  acid,  etc. 

Color  reactions .  Decomposition  products  are  formed  through  the  action  of 
certain  oxidizing,  reducing,  and  dehydrating  reagents.  Many  of  the  products 
shoV  brilliant  and  characteristic,  though  often  fugitive,  colors  with  a  small 
fragment  of  the  alkaloid  or  a  drop  of  its  solution,  but  it  is  important  that  the 
alkaloid  be  free  from  other  organic  matter.  Among  the  numerous  reagents 
that  have  been  described  are  concentrated  sulfuric  acid,  nitric  acid,  bromine 
water,  solution  of  iodine,  and  concentrated  sulfuric  acid  containing  traces  of 
nitric  acid,  potassium  chlorate,  potassium  bichromate,  molybdic  acid,  cane- 
sugar,  etc. 

For  example,  a  solution  of  ammonium  vanadate  in  sulfuric  acid  produces 
brown  colors  with  aconitine,  morphine,  narceine,  codeine,  solanine,  and 
piperine;  blue  with  apomorphine  and  antipyrine;  green  with  colchicine,  quini- 
<Jine,  and  conine;  violet  with  strychnine  and  papaverine;  yellow  withcinchonine, 
quinine,  and  physostigmine;  red  with  kairine,  veratrine,  brucine  and  atropine; 
and  no  color  with  nicotine  and  caffeine.  A  brown  color  is  also  given  with 
digitalein,  a  green  with  salicylic  acid,  and  a  purple  with  antifebrin. 

Odor.  On  oxidizing  an  alkaloid  with  fuming  nitric  acid,  evaporating  to  dry- 
ness,  and  treating  the  residue  with  an  alcoholic  solution  of  potassium  hydrate, 
peculiar  odors  are  exhaled  by  a  few  varieties,  as  cocaine,  eserine,  delphenium. 

Many  of  the  alkaloids  and  their  salts  in  aqueous  solution  are  strongly  dextro- 
or  laevo-rotatory  toward  polarized  light,  but  for  several  reasons  the  quanti- 
tative determination  is  seldom  attempted  by  this  process. 

The  physiological  effect  is  sometimes  useful  as  a  confirmatory  test.  A  peculiar 
benumbing  or  tingling  of  the  lips  and  tongue  follows  contact  with  aconitine 
or  cocaine,  and  after  administration  of  one  of  the  inydriatic  alkaloids  —  atro- 
pine, conine,  cocaine,  etc.  — the  pupil  of  the  eye  is  dilated,  while  physostigmine 
and  members  of  the  nux  vomica  species  contract  it.  The  intensely  toxic  power 
of  many  varieties  may  be  exhibited  by  the  administration  of  a  solution  or  a 
subcutaneous  injection  to  one  of  the  smaller  animals.  The  effect  varies  with 
the  nature  of  the  alkaloid,  and  the  experimenter  may  observe  changes  in  the 
ease  and  rate  of  respiration,  a  paralytic  action  on  the  nervous  system,  contrac- 
tion of  the  muscles,  rise  or  fall  in  temperature,  or  circulatory  disturbance  from 


THE  VEGETABLE  ALKALOIDS. 


411 


dilatation  of  the  arteries,  or  stimulus  or  depression  of  the  heart  in  systole  or 
diastole.  Blow-flies  have  been  successfully  used  for  such  experiments,  and 
the  addition  of  a  drop  of  a  solution  of  certain  alkaloids  to  stagnant  water 
immediately  paralyzes  and  shortly  destroys  the  infusoria.  It  is  said  that  on 
the  addition  of  a  drop  of  solution  of  muscarine  to  weak  brine  holding  the 
recently  excised  heart  of  a  frog,  the  pulsations  ceased,  but  were  re-estab- 
lished, after  a  lapse  of  four  hours,  by  the  addition  of  a  drop  of  solution  of 
atropine  —  a  striking  illustration  of  the  physiological  antagonism  of  the  two 
alkaloids. 

Quantitative  analysis.  A  crude  drug  as  received  by  the  manufacturing  phar- 
macist may  be  far  below  or  above  the  average  in  alkaloidal  strength,  from 
differences  in  soil  and  climate  at  the  place  of  growth,  age,  manner  of  collection 
and  preservation,  hygroscopic  condition,  etc.  For  the  preparation  of  a  tincture 
or  a  fluid  or  solid  extract  of  definite  alkaloidal  content,  either  the  drug  itself 
must  be  assayed  to  decide  the  proper  volume  of  menstruum  to  yield  a  prepara- 
tion conforming  to  the  official  article,  or  the  extract,  that  it  may  be  concen- 
trated if  below  or  diluted  if  above  the  standard.  Usually  both  are  assayed, 
the  latter  as  a  precaution  against  possible  mistakes  during  manufacture. 

A  generally  applicable  method  of  analysis  follows,  to  be  modified  to  suit 
particular  cases. 

1.  The  first  step  is  the  extraction  of  the  alkaloid,  in  as  pure  a  condition  as 
practicable,  from  the  other  constituents  of  the  plant.  The  following  table  * 
shows  the  solvent  action  of  water,  alcohol  and  ether  on  the  general  constit- 
uents of  vegetable  matter  and  indicates  the  nature  of  the  bodies  likely  to 
accompany  the  alkaloid  when  one  of  these  is  used  for  extraction. 

Water.  Alcohol.  Ether. 

Alkaloidal  salts Soluble.  Soluble.  Insoluble. 

Other  salts  of  inorganic 

acids Mostly  soluble.     Mostly  insoluble.     Insoluble. 

Other  salts  and  organic 

acids Soluble.  Soluble.  Mostly  insoluble. 

Free  organic  acids Soluble.  Soluble.  Mostly  insoluble. 

Tannins    and     coloring 

matters Soluble.  Soluble.  Variable. 

Sugars Soluble.  Soluble.  Insoluble. 

Gums       and       pectous 

bodies Soluble.  Mostly  insoluble.     Insoluble. 

Albuminoids,  etc Soluble.  Insoluble.  Insoluble. 

Starch Soluble  in  hot        Insoluble.  Insoluble. 

water. 

Cellulose Insoluble.  Insoluble.  Insoluble. 

Resins Insoluble.  Soluble.  Variable. 

Fixed  oils Insoluble.  Sparingly  soluble.     Soluble. 

Essential  oils Insoluble.  Soluble.  Soluble. 

Chlorophyll  Insoluble.  Soluble.  Soluble. 

As  the  alkaloid  is  usually  in  combination  with  an  organic  acid,  it  is  liberated 
by  the  intervention  of  a  stronger  base  which  may  be  an  alkali  or  an  earth,  the 
latter  preferable  on  account  of  its  sparing  solubility. 

After  mixing  the  powdered  drug  with  lime  or  other  base,  the  mass  is  treated 
with  alcohol,  hot  or  cold,  strong  or  diluted,  according  to  the  nature  of  the 


Allen,  Coml.  Org.  Anal.  3-2-151. 


412  QUANTITATIVE    CHEMICAL   ANALYSIS. 

alkaloid  and  the  vegetable  matter.  The  solution  of  the  alkaloid  and  other 
bodies  is  filtered  (unless  the  extraction  has  been  done  by  percolation)  and 
evaporated  to  dryness,  leaving  a  more  or  less  impure  alkaloidal  residue. 

Dialysis  has  been  recommended  to  separate  the  crystalline  active  principles 
of  opium,  aconite  root  and  belladonna,  but  the  process  has  no  great  advantages 
over  others  more  expeditious. 

Volatile  alkaloids  may  be  extracted  by  mixing  the  drug  with  water  and  excess 
of  lime  or  baryta  and  distilling  into  dilute  hydrochloric  acid.  The  distillate  is 
concentrated  and  the  alkaloidal  chloride  decomposed  by  an  alkali  and  extracted 
by  ether. 

2.  The  next  step  is  the  separation  of  the  alkaloid  contained  in  the  residue 
from  the  various  other  extractives.     The  residue  is  weighed,  dissolved  in  a 
dilute  acid,  the  solution  filtered,  made  alkaline,  and  the  filtrate  agitated  with 
several  portions  of  ether  or  chloroform  which  abstract  the  alkaloid,  leaving 
the  impurities  in  the  dilute  acid.    But  where  certain  impurities  are  in  the 
residue   that   are   fairly  soluble  in  both  dilute  acid  and  the  organic  solvent, 
a  more  extended  process  must  be  carried  out.    It  is  based  on  the  principle  that, 
as  a  rule,  free  alkaloids  are  insoluble  in  water  but  soluble  in  ether,  chloroform 
and  petroleum  ether,  while  their  salts  are  soluble  in  water  but  insoluble  in  the 
organic  liquids.    Hence  some  coloring  matters,  oils,  fats,  resins,  chlorophyll, 
etc.,  are  extracted  from  an  acid  solution  of  an  alkaloidal  salt  by  shaking  with 
ether,  and  after  super-saturating  the  decanted  aqueous  solution  with  an  alkali, 
the  alkaloid  is  extracted  by  ether,  leaving  other  impurities  —  extractive  matter, 
crystalline  salts,  gums  —  behind.    If  yet  impure,  the  alkaloid  may  be  alter- 
nately passed  from  ether  to  an  aqueous  acid  and  back  to  ether,  though  a  not 
inconsiderable  loss  is  suffered  in  each  transition. 

Another  plan  is  to  precipitate  the  neutral  solution  by  basic  lead  acetate,  filter, 
add  a  slight  excess  of  ammonia,  again  filter,  warm  until  the  excess  of  ammonia 
is  dissipated,  and  finally  precipitate  the  excess  of  lead  by  hydrogen  sulflde. 
Coloring  matter,  tannin,  and  organic  acids  are  carried  down  in  the  neutral  or 
alkaline  solution,  mechanically  or  in  combination  with  lead.  A  small  loss  of 
alkaloids  is  inevitable. 

In  a  method  due  to  Lloyd,*  the  concentrated  impure  alkaloidal  solution  is 
ground  in  a  mortar  with  a  paste  of  a  mixture  of  ferric  hydrate  and  sodium  bi- 
carbonate, then  triturated  with  chloroform.  The  clear  liquid  is  decanted  and 
the  residue  washed  several  times  with  small  volumes  of  chloroform,  and  the 
united  liquid  and  washings  evaporated  to  dryness  at  a  gentle  heat.  The  resi- 
due is  extracted  by  two  per  cent  sulfuric  acid  and  the  filtered  solution  made 
slightly  alkaline  by  ammonia,  then  extracted  by  chloroform.  Aluminum  or 
chromium  hydrate  may  be  substituted  for  ferric  hydrate. 

Filtration  through  a  limited  amount  of  bone-black  will  withdraw  coloring 
matters,  tannic  and  malic  acids,  and  some  other  bodies,  but  generally  some  of 
the  alkaloid  is  retained  as  well.  In  dealing  with  a  solution  containing  only  a 
minute  weight  of  an  alkaloid,  a  large  proportion  of  bone  -black  will  take  up  all 
the  alkaloid,  to  be  afterward  recovered  by  boiling  with  alcohol. 

The  volatile  alkaloids  may  be  extracted  or  purified  by  distillation  in  a 
current  of  steam,  receiving  the  distillate  in  dilute  hydrochloric  or  sulfuric 
acid. 

3.  Having  carried  the  purification  as  far  as  can  safely  be  done,  the  weight 
of  the  alkaloid  may  be  determined  by  evaporation,  precipitation  or  volumetri- 
cally.    The  simplest  method  is  to  evaporate  the  solution  to  dryness,  heat  to 


*  Journ.  of  Pharm.3-21— 1144;  Journ.  Amer.  Chem.Socy.  1892—162. 


THE  VEGETABLE  ALKALOIDS.  413 

the  boiling  point  of  water  and  weigh,  but  many  alkaloids  are  too  volatile  or 
easily  decomposed  by  heat  to  allow  of  this  procedure.  If  an  appreciable 
weight  of  impurity  is  suspected,  the  residue  is  dissolved  and  determined  as 
follows.  It  is  treated  with  neutral  alcohol,  filtered,  and  at  once  titrated  by  a 
standard  acid  with  a  suitable  indicator.  The  impurities  of  the  residue  are 
seldom  of  either  an  acid  or  basic  character.  The  standard  acid  is  not  stronger 
than  tenth- normal  (some  use  as  dilute  as  hundredth-normal)  since  the 
weight  of  the  total  residue  from  the  amount  of  drug  usually  taken  for  analysis 
is  not  likely  to  exceed  100  milligrams. 

An  objection  to  direct  titration  is  that  the  color  of  the  indicator  may  be 
modified  or  overpowered  by  coloring  matter  dissolved  by  alcohol  from  the 
residue,  and  here  a  reverse  titration  by  standard  alkali  is  resorted  to  with 
advantage.  In  some  cases  the  plan  may  be  followed  of  adding  a  drop  of 
methyl  orange  solution  and  some  ether  to  the  aqueous  or  diluted  alcoholic 
solution,  and  proceeding  with  the  titration  by  standard  acid,  constantly 
stirring  the  titrate  during  the  operation.  The  coloring  matter  largely  enters 
the  ether  leaving  the  subnatant  aqueous  solution  comparatively  light  colored. 

Since  alkaloids  differ  so  widely  in  strength,  no  one  indicator  is  suitable  for 
the  alkalimetric  titration  of  all.*  A  number  of  special  indicators  have  been 
recommended,  among  them  extracts  of  logwood  and  Brazilwood,  haematoxylin, 
Porrier's  blue,  gallein,  iodeosin,  phloxin,  etc.,  each  well  adapted  to  particular 
alkaloids.  Litmus  and  phenol-phthalein  are  available  for  the  stronger  bases  like 
quinine,  nicotine  and  morphine,  but  the  weaker  bases,  as  the  minor  derivatives 
of  the  poppy,  are  neutral  to  them,  so  that  the  acid  in  their  neutral  salts  may  be 
titrated  as  if  it  were  uncombined.  Most  of  the  mineral  acid  salts  of  the  alka- 
loids are  neutral  to  methyl  orange,  and  any  free  acid  in  a  solution  of  one  of 
these  may  be  neutralized  by  its  aid.  Allen  states  that  the  quinine  sulfate  of 
commerce  (C2oH24N202)2.H2SO4.aq.  is  neutral  to  Brazilwood,  logwood  and  cochi- 
neal, distinctly  alkaline  to  litmus,  and  strongly  alkaline  to  methyl  orange ;  and 
in  titrating  quinine  by  standard  sulfuric  acid  the  point  of  neutrality  is  reached 
with  the  former  indicators  at  (C2oH24N202)2.H2SO4,  and  with  methyl  orange  at 

C20H24N202H2S04. 

A  committee  appointed  by  the  American  Pharmaceutical  Association!  to  in- 
vestigate the  question  of  indicators  in  the  titration  of  alkaloids,  studied  the 
application  of  Brazilwood,  cochineal,  haematoxylin,  lacmoid,  tropaeolin  00,  and 
indeosin;  they  experienced  considerable  difficulty  in  securing  some  of  the 
indicators,  especially  tropaeolin  00  and  indeosin,  of  a  satisfactory  quality;  of 
the  former  none,  with  perhaps  the  exception  of  one  sample,  was  sufficiently 
sensitive.  The  conclusions  of  the  committee  were 

11 1,  Haematoxylin  is  the  indicator  par  excellence  for  titrating  alkaloids. 
Brazilwood  and  cochineal  compare  favorably  with  haematoxylin  but  are  not  as 
reliable  in  some  cases ;  nor  do  they  appear  to  be  quite  as  sensitive. 

2.  Pure  alkaloidal  material  can  be  titrated  with  satisfactory  results,  except- 
ing the  cinchona  alkaloids.    Such  anomalous  results  were  obtained  with  the 
cinchona  alkaloids  that  we  are  inclined  to  think  that  the  nature  of  these  alka- 
loids is  not  fully  understood. 

3.  The  estimation  of  alkaloids  by  means  of  volumetric  solutions  can  only  be 
carried  out  in  laboratories  where  daily   determinations  are  made.     If  the 
operator  is  not  constantly  in  touch  with  his  end-reaction  tints  he  will  be  unable 
to  secure  satisfactory  results. 


*  Druggists  Circular,  1900-214 ;  Allen,  Coml.  Org.  Anal.  3—3—80. 
t  Proceedings,  1896. 


414  QUANTITATIVE    CHEMICAL    ANALYSIS. 

4.  The  gravimetric  results  based  on  process  b  are  quite  satisfactory,  and  it 
it  is  with  this  process  that  the  average  worker  will  obtain  the  most  concordant 
results.  While  the  volumetric  process  yields  good  results  in  the  hands  of  ex- 
tremely careful  workers  and  under  the  most  favorable  conditions,  yet  we  feel 
convinced  from  our  work  that  the  method  has  not  been  sufficiently  evolved  to 
recommend  it  for  general  use." 

The  process  b  referred  to  in  (4)  is  in  essence  the  extraction  of  the  dried  drug 
by  a  mixture  of  ether  and  chloroform  made  ammoniacal.  A  portion  of  the 
decanted  liquid  is  extracted  by  acidulated  water  which  converts  the  alkaloid  to 
a  salt;  the  acid  solution  is  made  alkaline  by  ammonia,  and  the  alkaloid  ex- 
tracted by  a  mixture  of  chloroform  and  ether.  The  solution  is  evaporated  to 
dryness  at  a  low  heat  and  the  residue  weighed ;  then  dissolved  in  hot  alcohol, 
diluted  with  water  and  titrated  by  standard  acid,  preferably  a  reverse  titration 
by  standard  alkali. 

Simpler  methods  answer  for  alkaloidal  tinctures  and  solid  and  fluid  extracts. 
In  that  of  Farr  and  Wright,  a  volume  of  25  to  50  Cc.  of  a  tincture  is  evap- 
orated on  the  water-bath,  with  addition  of  water,  until  the  alcohol  is  dissipated. 
What  remains  is  acidified  and  filtered  into  a  separatory  funnel,  washing  with 
acidulated  water.  The  liquid  is  made  alkaline  by  ammonia,  and  the  alkaloid 
extracted  by  chloroform.  To  remove  ammonia,  the  chloroformic  solution  is 
washed  with  water  until  the  washings  are  neutral;  then  the  alkaloid  titrated  by 
N/20  hydrochloric  acid  and  indeosin. 


A  number  of  reagents  precipitate  compounds  of  more  or  less  definite  compo- 
sition and  degree  of  insolubility.    The  precipitant  may  be 

1.  An  alkali  or  alkali  carbonate,  liberating  the  base  of  the  alkaloidal  salt  by 
combination  with  the  acid,  thus 
(C22H23NO7)2.H2SO4    (narcotine  sulfate)  +  2KOH  =  2C22H23NO;   (narcotlne)  -+- 


A  few  alkaloids  are  sufficiently  insoluble  and  stable  for  a  gravimetric  deter- 
mination and  may  be  filtered,  dried  and  weighed. 

2.  Picric  acid.  Many  of  the  picrates  are  insoluble,  or  nearly  so,  in  water  and 
an  excess  of  the  acid,  forming  light  yellow,  crystalline  precipitates.    The  com- 
bination is  very  suitable  for  the  cinchona  group,  nicotine,  brucine  and  strych- 
nine; they  should  be  in  the  form  of  sulfates  in  a  concentrated  but  only  slightly 
acid  solution,  and  thrown  down  by  an  equivalent  of  a  saturated  solution  of 
(sparingly  soluble)  picric  acid.    Instead  of  weighing  the  precipitate  it  is  ad- 
vised that  it  be  decomposed  by  an  alkali  and  the  alkaloid  extracted  by  alcohol. 

3.  Tannic  acid  precipitates  most  alkaloids,  but  the  tannates  are,  as  a  rule, 
somewhat  soluble  in  even  very  dilute  or  weak  acids  and  ammonia.    Like  the 
corresponding  picrates,  the  moist  tannates  may  be  decomposed  by  a  strong 
base,  here  lead  oxide  or  carbonate  forming  insoluble  lead  tannate  ;  the  dried 
precipitate  is  extracted  by  hot  alcohol. 

4.  Phospho-molybdic  acid  in  nitric  acid.  The  phospho-molybdates  are  yellow 
and  amorphous,  and  generally  quite   insoluble.    Ammonium  salts  and  many 
organic  compounds  are  also  precipitated  by  the  reagent  and  must  be  absent 
from  the  solution.    The  compounds  are  not  of  a  sufficiently  constant  compo- 
sition for  direct  weighing,  but  may  be  decomposed  by  treatment  with  dilute 
ammonia  with  the  formation  of  ammonium  phosphomolybdate   (soluble  in 
ammonia)  and  separation  of  the  free  alkaloid  which  can  be  extracted  from  the 
residue  by  alcohol.    Phospho-tungstic    and     phospho-antimonic    acids    give 
analogous  precipitates,  the  strychnine  and  quinine  compounds  with  the  former 
reagent  being  particularly  insoluble. 

5.  lodtne.  A  saturated  solution  of  iodine  in  potassium  iodide  precipitates 


THE  VEGETABLE  ALKALOIDS.  415 

most  alkaloids  from  a  dilute  sulfuric  solution  as  addition  compounds  of  com- 
plex formulae,  e.  g.,  with  caffeine  is  formed  caffeine  hydriodide  tetra-iodide, 
C8HioN4O2.HI.l4.  The  precipitates  are  yellow  or  reddish  brown,  flocculent, 
usually  amorphous,  though  crystalline  when  formed  in  alcoholic  solutions,  and 
are  only  slightly  soluble  in  water  but  more  readily  in  presence  of  an  excess  of 
the  precipitant.  After  filtering,  the  precipitate  may  be  dissolved  in  sulfurous 
acid,  which  converts  the  iodine  into  hydriodic  acid,  and  the  sulfate  of  the 
alkaloid  extracted  or  otherwise  dealt  with. 

The  compound  known  as  herapathite  after  its  discoverer  Herapath,  is  an 
iodosulfate  of  quinine  said  to  have  the  formula  (C2oH22N2O2)4(H2SO4)3(HI)2l4  -f- 
xHQO.  It  is  a  brown  crystalline  precipitate  thrown  down  when  tincture  of 
iodine  is  compounded  with  a  solution  of  quinine  sulfate ;  the  reaction  may  be 
applied  for  the  separation  and  determination  of  quinine  or  as  a  qualitative 
test.  When  precipitated  by  iodine  directly  the  compound  is  liable  to  vary  in 
composition,  and  De  Vrij  has  proposed  to  substitute  the  iodosulfate  of  qui- 
noidine  (an  uncrystallizable  alkaloid  of  cinchona)  claiming  that  a  definite  iodo  - 
sulfate  of  quinine  is  invariably  formed  thereby. 

Bismuth  potassium  iodide  with  solutions  strongly  acidified  by  sulfuric  acid 
yields  orange-red  precipitates,  the  majority  insoluble  in  cold  water,  and  the 
reagent  has  been  highly  recommended  for  strychnine  and  the  cinchona  alkaloids. 
Mercuric  potassium  iodide  forms  light-yellow  flocculent  iodomercurates  in 
solutions  free  from  alcohol  and  acetic  acid ;  many  of  these  are  quite  insoluble 
in  dilute  acids  and  become  crystalline  on  standing,  but  their  composition  is  not 
definite.  The  precipitates  may  be  filtered,  suspended  in  water,  decomposed  by 
hydrogen  sulflde,  and  after  filtering  from  the  precipitated  sulflde,  treated  with 
an  alkali  carbonate  and  extracted  by  ether  or  chloroform. 

6.  Gold  chloride.  Yellow  crystalline  chloraurates  or  combinations  of  an  alka- 
loidal  hydrochloride  with  auric  chloride.     The  precipitates  are  well  suited  for 
quantitative  determinations  though  somewhat  unstable  and  rapidly  decomposed; 
metallic  gold  remains  on  ignition.    Mercuric  chloride  reacts  in  a  similar  man- 
ner, as  does  platinic  chloride. 

7.  Potassium  permanganate.  Alkaloidal  permanganates  are,  as  a  rule,  very 
unstable,  some  decomposing  immediately  with  deposition  of  manganic  hydrate, 
others  more  slowly,  while  a  few,  as  of  cocaine  and  narceine,  are  comparatively 
permanent. 

Other  reagents  are  bromine,  gallic  acid,  tannin,  potassium  chromate  and 
bichromate,  cadmium  potassium  iodide,  tartaric  acid,  potassium  ferricyanide, 
etc.,  but  most  of  these  are  better  suited  for  qualitative  tests  than  quantitative 
determinations. 

A  number  of  the  above  reagents  have  been  applied  volumetrically,  but  several 
difficulties  are  met  with,  namely  that  the  composition  of  the  precipitates  is  far 
from  constant,  varying  with  the  conditions  of  the  experiment  and  necessitating 
the  conduction  of  a  parallel  determination  with  each  analysis;  that  with  equiv- 
alent amounts  of  alkaloid  and  precipitant  a  condition  of  equilibrium  obtains 
where  the  addition  of  either  will  produce  a  precipitate;  and  that  from  the 
absence  of  suitable  indicators,  the  end-point  must  be  taken  where  the  precip- 
itate ceases  to  fall,  and  as  the  precipitate  remains  suspended  for  a  considerable 
time,  a  small  volume  must  be  filtered  off  and  tested  after  each  addition  of  the 
titrand.  The  tedious  filtration  may  be  avoided  by  adopting  the  method  of 
reverse  titration,  or  after  filtering  and  washing,  the  precipitate  may  be  sus- 
pended in  water  and  titrated  directly  by  a  suitable  reagent. 

A  free  alkaloid  may  be  dissolved  in  a  measured  volume  of  standard  acid  and 
precipitated  by  a  neutral  reagent  such  as  Wagners  or  Meyers,*  and  after  filter- 

*  Jonrn.  Amer.  Chem.  Socy.  1900—18. 


416  QUANTITATIVE    CHEMICAL   ANALYSIS. 

ing,  the  remaining  free  acid  in  the  filtrate  titrated  by  standard  alkali  and 
phenol  -phthalein. 

An  alkaloid  forming  a  stable  hydrochloride  may  be  dissolved  in  hydrochloric 
acid,  the  solution  evaporated  to  dryness  to  remove  the  excess  of  acid,  the 
residue  taken  up  by  water,  and  the  hydrochloric  acid  combined  with  the  alkaloid 
determined  by  titration  with  silver  nitrate  and  potassium  chromate.  The 
weight  of  the  alkaloid  is  calculated  from  the  formula  of  the  hydrochloride. 

Several  of  the  alkaloids  may  be  determined  iodimetrically  with  good  results. 
A  known  volume  of  standard  mercuric  potassium  iodide  or  of  iodine  in  potas- 
sium iodide  is  added  to  the  solution  of  the  alkaloid  and  the  liquid  made  up  to  a 
definite  volume.  Filtration  is  done  through  asbestos,  and  the  residual  iodine 
in  an  aliquot  part  of  the  filtrate  found  by  titration  with  sodium  thiosulf ate ;  or 
the  washed  precipitate  may  be  suspended  in  water  and  titrated  directly. 

Prescott  and  Gordin  make  the  point  that  where  the  ordinary  solution  of 
iodine  in  potassium  iodide  is  added  at  once  and  in  large  excess  to  the  solution 
of  the  alkaloid  there  is  precipitated  the  highest  periodide  only,  which  is  not  the 
case  if  added  a  little  at  a  time.  A  preliminary  test  shows  by  the  color  of  the 
supernatant  fluid  what  must  be  the  ratio  between  the  two  solutions  that  only 
the  highest  periodide  will  form.  The  proper  volume  of  iodine  solution  is 
mixed  at  once  with  the  alkaloidal  solution,  the  mixture  filtered,  and  the  excess 
of  iodine  determined  in  an  aliquot  part  of  the  filtrate  by  titration  with  thio- 
sulf ate.  

The  separation  of  the  alkaloids  is  a  problem  of  far  greater  di faculty  than  the 
separation  of  the  inorganic  bases,  for  besides  lacking  the  stability  of  the  latter, 
the  alkaloids  and  their  precipitable  compounds  have  not  that  degree  of  insolu- 
bility that  allows  a  nearly  perfect  separation  of  the  metals.  In  practice  the 
alkaloids  to  be  isolated  are  usually  members  of  a  group  derived  from  one 
natural  source,  and  the  separation  is  consequently  more  difficult  since  they  have 
many  properties  in  common  and  a  similar  deportment  toward  solvents  and  pre- 
cipitants. 

By  reason  of  the  varied  degrees  of  solubility  of  the  precipitates  in  water, 
dilute  acids  and  alkalies,  organic  solvents,  and  an  excess  of  the  precipitating 
solution,  several  of  the  precipitants  named  above  may  be  of  service.  Thus,  the 
tannates  of  picrotoxin  and  agaricine  are  readily  soluble  in  an  excess  of  tannic 
acid  solution,  those  of  colchicine  and  digitaline  insoluble  but  dissolved  by  a 
dilute  alkali,  while  those  of  aconitine  and  papaverine  are  insoluble  in  either. 

The  compounds  of  a  few  of  the  alkaloids  (as  narcotine,  papaverine,  narceine) 
with  weak  acids  decompose  rapidly  with  separation  of  the  base,  and  if  there  be 
added  to  a  solution  of  any  of  their  salts  an  excess  of  sodium  acetate  and  the 
mixture  allowed  to  stand  for  sometime,  all  the  alkaloid  will  be  obtained  as 
a  precipitate. 

The  volatile  alkaloids  (nicotine,  conine,  the  coniceines,  lobeline,  sparteine, 
lopeline,  piturine,  and  spigeline)  may  be  distilled  from  those  non-volatile  and 
not  decomposed  at  the  temperature  of  distillation.  The  operation  is  best  con- 
ducted in  vacuo. 

The  diversity  in  the  comparative  solubilities  of  free  alkaloids  in  different 
menstrua  often  permits  a  fair,  more  rarely  a  nearly  complete  separation  by  the 
process  of  extraction  by  an  immiscible  solvent,  favored  by  the  ease  with  which 
tree  alkaloids  are  converted  into  their  salts  and  vice  versa.  A  general  scheme* 
for  the  separation  of  alkaloids  and  the  organic  bodies  usually  accompanying 
them  is  given  opposite,  though  the  correctness  of  some  of  the  statements  has 


*  Allen,  Coml.  Org.  Anal.  3—159. 


THE  VEGETABLE  ALKALOIDS. 


417 


5 

II 


27 


418  QUANTITATIVE    CHEMICAL   ANALYSIS. 

been  questioned.    The  scheme  can  be  modified  to  advantage  when  the  nature 
and  relative  proportions  of  the  members  present  are  known. 


Special  methods.  As  illustrations  of  the  special  methods  in  use  for  the  deter- 
mination of  the  medicinally  valuable  alkaloids  in  crude  drugs,  a  few  examples 
are  given.  The  first  is  a  favored  method  for  the  assay  of  aconite  root. 

1.  About  30  grams  of  the  powdered  root  is  percolated  with  160  Cc.  of  alcohol 
of  a  specific  gravity  of  .890.  The  percolate  is  evaporated  to  a  small  bulk  on  the 
water  bath,  and  after  cooling,  diluted  with  a  little  water  and  15  Cc.  of  decinor- 
mal  sulfuric  acid.  After  filtering  and  washing  the  precipitate  with  dilute  acid, 
the  united  filtrate  and  washings  is  shaken  twice  with  chloroform  to  remove 
coloring  matter,  the  alkaloidal  salts  remaining  with  the  acid.  Any 
traces  of  alkaloid  that  may  have  passed  into  the  chloroform  are  reclaimed  by 
shaking  it  with  a  little  dilute  acid  which  is  then  united  with  the  main  portion, 
The  acid  solution  is  now  made  slightly  alkaline  by  potassium  carbonate  and 
the  free  alkaloids  extracted  by  chloroform;  the  chloroform  is  washed  once 
with  water  to  remove  any  potassium  salts  it  may  have  taken  up,  then  evaporated 
to  dryness,  and  the  somewhat  impure  residual  alkaloids  weighed. 

The  separation  of  the  associated  alkaloids  is  not  an  easy  matter  as  a  rule;  a 
method  proposed  by  Wright  and  perfected  by  Allen  for  the  determination  of 
the  crystallizable  (medicinally  active)  alkaloids  in  the  residue  is  more  feasible. 
The  principle  is  that  of  hydrolyzing  the  alkaloids  by  heating  under  pressure 
with  an  alcoholic  solution  of  potassium  hydrate,  yielding  the  potassium  salts 
of  several  organic  acids  —  thus  aconitine  gives  potassium  benzoate,  potassium 
acetate  and  aconine  — 


OJ2  4-  2KOH  =  KC7H5O2  -f  KG2H3O2 
Aconitine  Potassium  Potassium  Aconine 

benzoate      acetate 

The  alcohol  is  evaporated  and  the  residual  liquid  acidified  by  hydrochloric 
acid;  the  liberated  benzole  acid  is  then  extracted  by  ether.  The  ether  is  washed 
with  water  until  free  from  mineral  acid,  stirred  up  with  water,  and  the  organic 
acid  titrated  by  weak  standard  alkali  or  baryta-  water  and  phenol-phthalein. 

2.  A  well-known  method  for  the  determination  of  quinine  in  the  various  kinds 
of  cinchona  bark  is  due  to  Schmidt.  He  macerates  for  24  hours,  twenty  grams 
of  the  finely  powdered  and  air-  dried  bark  in  a  mixture  of  ten  Cc.  of  ten  per 
cent  ammonia,  twenty  Cc.  of  ninety  per  cent  alcohol,  and  170  Cc.  of  ether.  Of 
the  clear  solution  100  Cc.  is  mixed  with  27  Cc.  of  water  and  three  or  four  Cc. 
of  normal  hydrochloric  acid,  and  set  aside  for  24  hours  to  allow  spontaneous 
evaporation.  The  liquid  is  further  concentrated  on  the  water-bath,  maintaining 
a  slightly  acid  reaction  by  hydrochloric  acid,  or  if  too  strongly  acid  by  the  base 
cinchonine. 

After  cooling,  the  liquid  is  exposed  to  the  air  until  any  coloring  matter  has 
separated,  and  after  filtering,  two  or  three  grams  of  potassium  sodium  tartrate 
added  to  precipitate  the  quinine  and  cinchonidine  as  tartrates  ;  the  mixture  is 
warmed  on  the  water-bath  for  fifteen  minutes  and  allowed  to  stand  for  twenty- 
four  hours,  then  filtered  and  washed  with  the  minimum  of  cold  water  and 
drained.  The  filtrate  and  washings  are  measured  in  order  that  a  correction 
may  be  applied  for  the  solubility  of  the  tartrates  therein. 

The  precipitate  is  dissolved  in  very  dilute  hydrochloric  acid,  and  foreign 
matters  extracted  by  shaking  out  several  times  with  ether  (in  which  the  alka- 
loidal hydrochlorides  are  insoluble)  ;  the  acid  solution  is  then  made  alkaline 
by  sodium  hydrate  and  the  precipitated  alkaloids  extracted  by  ether.  The 


THE  VEGETABLE  ALKALOIDS.  419 

ethereal  solution  on  evaporation  leaves  a  residue  of  quinine  and  cinchonidine 
to  be  weighed  after  drying  at  100°  to  110°  . 

The  mixed  alkaloids  are  now  washed  with  a  saturated  solution  of  cincho- 
nidine in  ether  which  dissolves  the  quinine  but  not  the  cinchonidine,  nor  does 
any  cinchonidine  separate  from  the  ethereal  solution.  The  residue  of  cincho- 
nidine is  washed  with  a  little  pure  ether,  dried  and  weighed,  and  the  quinine 
found  by  difference,  both  corrected  for  the  solubility  of  their  tartrates. 

Cinchonidine  sulfate  is  a  common  impurity  of  commercial  quinine  sulfate, 
and  an  empirical  method  for  its  determination  is  based  on  the  greater  volume 
of  solution  of  ammonia  required  to  dissolve  cinchonidine  than  quinine.  In  prac- 
tice, a  weighed  amount  of  the  sample  to  be  tested  is  macerated  with  ten  times 
its  weight  of  water  at  15°  and  the  solution  filtered;  the  filtrate  is  a  saturated 
solution  of  quinine  sulfate  (one  gram  dissolves  in  about  740  Cc.  of  water)  con- 
taining also  all  the  cinchonidine  sulfate  present  in  the  sample  since  the  propor- 
tion rarely  exceeds  one  per  cent.  A  measured  portion  is  titrated  by  standard 
ammonia  until  the  precipitate  of  quinine  and  cinchonidine  at  first  formed  has 
just  dissolved  to  a  clear  fluid.  The  volume  of  ammonia  in  excess  of  that  re- 
quired in  a  parallel  determination  on  pure  quinine  sulfate  is  in  proportion  to 
the  cinchonidine  sulfate  in  the  sample. 

Another  method  for  this  determination  is  that  known  as  the  '  optical  assay  * 
depending  on  the  principle  that  the  salts  of  both  quinine  and  cinchonidine  are 
laevo-rotatory  to  polarized  light,  the  former  to  a  greater  extent  than  the  latter. 
The  procedure  recommended  by  Koppeschaar  is  to  convert  the  mixture  of  the 
alkaloids  into  tartrates,  filter  and  dry  the  precipitate,  and  dissolve  .4  gram  in 
20  Cc.  of  dilute  hydrochloric  acid.  The  angle  of  rotation  is  observed  in  the 
polarimeter  by  monochromatic  light,  and  the  percentages  of  the  alkaloidal 

tartrates  calculated  by  the  general  formula  for  mixtures,  X=  100  . 

a  —  b 

Here  X  is  the  percentage  of  quinine  tartrate ;  a,  the  specific  rotatory  power  of 
quinine  tartrate  ( — 220.07°);  6,  that  of  cinchonidine  tartrate  ( — 137.67°); 
and  d,  that  of  the  mixture  polarized.  The  value  of  d  is  found  from  the  equa- 
tion d  =  _ — —,  where  a  is  the  angular  rotation,  and  L  the  length  in  deci- 
2Zi 

meters  of  the  observation  tube  of  the  polarimeter  (since  of  the  mixed  tartrates 
there  was  dissolved  .4  gram  in  20  Cc.  of  acid  making  a  two  per  cent  solution, 
and  the  angle  of  rotation  observed  through  a  tube  L  decimeters  long). 

3.  For  the  determination  of  nicotine  in  tobacco,  advantage  is  taken  of  the 
volatile  nature  of  the  alkaloid  for  its  separation  from  cellulose,  proteids, 
pectic  and  other  organic  acids,  gum-resins,  inorganic  matter,  etc.  The  method 
of  Kiessling,  somewhat  modified,  is  as  follows  :  — 

Of  powdered  tobacco  leaves,  one  hundred  grams  is  mixed  with  slaked  lime 
and  distilled  in  a  current  of  steam  until  the  vapor  is  no  longer  alkaline;  nico- 
tine distills  together  with  some  ammonia  formed  by  the  action  of  the  lime  on 
nitrogenous  matter.  The  distillate  is  slightly  acidulated  by  sulfuric  acid, 
evaporated  to  50  Cc.  (nicotine  sulfate  is  not  volatile),  made  slightly  alkaline, 
and  the  nicotine  extracted  by  six  portions  of  ether.  The  ethereal  solution  is 
titrated  by  tenth-normal  sulfuric  acid,  and  the  nicotine  calculated  from  the 
volume  required.  The  results  are  usually  somewhat  too  high.  As  a  check,  the 
titrated  solution  is  evaporated  to  dryness,  weighed,  and  the  nicotine  sulfate 
dissolved  from  the  residue  of  ammonium  sulfate  (slightly  soluble  in  ether)  by 
absolute  alcohol.  The  weight  of  the  residue  is  deducted  from  the  total 
weight,  nicotine  sulfate  forming  the  difference.  Or  the  sulfuric  acid  com- 


420  QUANTITATIVE    CHEMICAL   ANALYSIS. 

bined  with  the  nicotine  may  be  titrated  by  standard  alkali  and  phenol- 
phthalein,  since  nicotine  is  neutral  to  this  indicator. 

4.  In  morphiometry  the  alkaloids  of  the  poppy  (papaver  somniferum  —  the 
exudation  of  the  capsules  of  the  plant)  are  extracted  from  resins,  gums,  col- 
oring matter,  etc.,  followed  by  their  separation,  or  more  usually  the  isolation 
of  morphine  alone. 

In  all  of  the  numerous  methods  it  will  be  noticed  how  minutely  the  directions 
are  laid  down  in  order  that  the  unavoidable  losses  through  solubility,  etc.,  may 
be  kept  to  a  minimum. 

Teschemacher  and  Smith  *  direct  that  13  grams  of  pulverized  opium  be  com- 
pletely exhausted  with  warm  water  and  filtered.  The  extract  is  evaporated  to 
the  consistency  of  a  syrup  at  a  heat  not  exceeding  90  o ,  poured  into  a  small 
flask  and  the  dish  rinsed  with  a  little  water.  There  are  added  four  Cc.  of  alco- 
hol sp.  gr.  .820,  and  forty  grams  of  ether,  the  flask  corked,  and  the  contents 
mixed  by  a  gentle  rotation;  then  3.3  grams  of  ammonia  sp.  gr.  .935  is  introduced 
and  the  flask  well  shaken  to  precipitate  the  alkaloid  in  a  granular  form.  After 
standing  for  eighteen  hours  to  complete  the  precipitation,  the  liquid  is  filtered 
by  the  aid  of  the  pump,  and  the  precipitate  washed  until  the  washings  pass 
uncolored,  first  with  a  saturated  (.33  per  cent)  solution  of  morphine  in  alcohol 
containing  five  per  cent  of  ammonia,  then  with  a  saturated  (.04  per  cent)  solu- 
tion of  morphine  in  water.  The  precipitated  alkaloids  are  dried  at  a  low  heat, 
finally  at  100®. 

In  order  to  remove  the  narcotine,  codeine,  papaverine  and  narceine  from  the 
morphine,  the  precipitate  is  ground  in  a  mortar  and  digested  and  washed  sev- 
eral times  with  benzene.  The  residue  is  morphine  impurifled  by  considerable 
coloring  and  other  organic  matter.  After  drying  it  is  weighed  and  titrated  by 
standard  hydrochloric  acid  of  such  a  concentration  that  ten  Cc.  neutralizes  one 
gram  of  pure  morphine  with  litmus  as  indicator.  It  has  been  proposed  to 
simplify  the  assay  by  omitting  the  extraction  by  benzene,  since  litmus  is 
stronger  than  the  precipitated  alkaloids  other  than  morphine. 


*  Chem.  News,  62—93  and  103. 


THE   TANNINS.  421 


THE  TANNINS. 

The  tannins  are  a  group  of  amorphous  astringent  organic  bodies  of  vegetable 
origin  having  many  properties  in  common,  yet  differing  greatly  in  others. 
They  are  characterized  by  their  power  to  unite  with  gelatin  to  form  non- 
putrescible  compounds,  their  value  in  the  arts  depending  chiefly  on  this  prop- 
erty. Sources  of  tannin  of  commercial  importance  are  oak,  hemlock  and 
chestnut  barks,  canaigre,  sumach,  gambler,  myrobalans,  gall-nuts,  etc.  Many 
of  these  contain  two  or  more  varieties  of  tannin. 

All  are  composed  of  carbon,  hydrogen  and  oxygen.  Various  schemes  for 
classifying  the  different  varieties  have  been  devised.*  That  of  Trimble  divides 
them  into  two  groups,  (1)  the  gallo-tannins,  containing  about  50  per  cent  of 
carbon  and  3  to  4.5  per  cent  of  hydrogen,  and  (2)  the  oak-tannins,  containing 
about  60  per  cent  of  carbon  and  5  per  cent  of  hydrogen.  Proctor  makes  three 
classes,  namely  — 

"  1.  Derivatives  of  catechol,  which  yield  no  bloom,  and  usually  give  greenish- 
blacks  with  ferric  acetate,  and  which  include  hemlock,  mimosa,  cutch, 
gambler,  quebracho,  etc. 

2.  Derivatives  of  pyrogallol,  which  deposit  bloom  in  leather,  give  bluish - 
blacks  with  iron,  and  embrace  galls,  sumach,  divi-divi,  myrobalanes,  pome- 
granite  rind,  etc. 

3,  Tannins  which  contain  both  pyrogallol  and  catechol,  such  as  oak-bark  and 
valonia,  and  which,  as  is  well  known,  yield  bloom  and  give  blue -blacks  with 
iron." 

Regarding  the  chemical  constitution,  it  is  said  that  all  varieties  contain  the 
benzene  ring,  while  Reinitzer  considers  it  probable  that  they  contain  oxyhy- 
drogen  groups  of  a  phenolic  character. 

The  tannins  are  soluble  in  water,  alcohol,  alcohol-ether,  glycerol  and  acetone, 
but  are  nearly  insoluble  in  ether,  chloroform,  benzol,  gasoline  and  carbon 
disulflde.  With  alkalies  they  oxidize  and  darken ;  with  nitric  acid  are  oxidized 
to  oxalic  acid,  almost  quantitatively  at  a  certain  strength  of  the  acid,  a  dis- 
tinction from  gallic  acid  which  is  oxidized  to  isotrichlorglyceric  acid.  Insol- 
uble precipitates  are  afforded  with  gelatin,  benzoyl  chloride,  and  many  organic 
bases  and  metallic  compounds. 

Extraction.  For  the  extraction  of  the  tannin  from  barks  and  other  sources, 
the  solvent  is  almost  invariably  water.  It  is  usual  to  weigh  such  an  amount 
of  the  powdered  tan-ware  that  of  an  infusion  measuring  one  liter,  100  Cc.  shall 
yield  on  evaporation  a  residue  of  .6  to  .8  gram.  The  temperature  of  the 
water  during  the  extraction  has  an  influence  on  the  amount  of  tannin  taken  up, 
boiling  water  often  abstracting  less  than  water  at  a  lower  temperature.  The 
infusion  is  made  either  by  heating  the  powdered  tan-ware  with  water  in  a 
flask  and  filtering ;  or  by  percolation,  when  the  first  half  of  the  water  used  is 
at  50° ,  the  remainder  at  100*5  ;  the  two  are  mixed,  cooled  to  room  tempera- 
ture, and  made  up  to  a  liter.  The  percolation  process  is  to  be  preferred,  and 
several  forms  of  apparatus  have  been  designed  especially  for  the  purpose* 


*  Journ.  Amer.  Chem.  Socy.  1898—34;  Prescott,  Organic  Anal.  466. 


422  QUANTITATIVE    CHEMICAL    ANALYSIS. 

acting  continuously  or  intermittently.*  An  aliquot  part  of  the  clear  infusion 
is  withdrawn  for  the  tannin  assay. 

The  color  of  the  tan-liquor  may  be  expressed  in  terms  of  equal- colored 
standard  solutions  of  tannin,  anilin  dyes,  and  the  like,  but  more  satisfactorily 
according  to  the  units  of  Lovibond's  tintometer  (page  261).f 

Determination  of  tannin.  Of  the  multitude  of  methods  that  have  been  ad- 
vocated it  would  appear  that  none  directly  measures  tannin  —  practically,  the 
substance  which  in  a  cold  solution  converts  hide  to  leather.  No  two  varie- 
ties of  tannin  have  precisely  the  same  deportment  with  reagents,  and  further, 
the  reactions  are  not  limited  to  tannin  alone  but  include  considerable 
amounts  of  other  constituents  of  the  extract  such  as  gallic,  ellagic,  gallamic, 
glauco-melonic  acids,  etc.,  that  while  possessing  many  of  the  chemical 
properties  of  tannin  are  of  less  or  no  value  as  tanning  material.  A  few  methods 
are  measurably  free  from  these  sources  of  error. 

The  methods  that  have  been  proposed  for  the  determination  of  tannin  in 
aqueous  solution  may  be  divided  into 

1.  Precipitation  by  a  metallic  salt  or  other  compound,  weighing  the  precipi- 
tate, or  igniting   it  and  weighing  the  inorganic    residue ;  or  the  reaction  is 
applied  volumetrically. 

2.  Precipitation  by  an  organic  body,  weighing  the  precipitate,  or  calculating 
from  the  specific  gravity  of  the  solution  before  and  after  precipitation ;  or  the 
reaction  applied  volumetrically. 

3.  Oxidation  of  the  tannin  by  a  reagent  in  solution  or  in  the  solid  form,  with 
a  correction  for  other  oxidizable  matters. 

4.  Absorption  of  the  tannin  in  a  solid  reagent,  determining  it  by  increase  in 
weight  of  the  reagent,  or  by  the  difference  in  weight  of  the  residues  left  on 
evaporation  of  the  original  liquid  and  the  filtrate,  or  by  the  difference  between 
•their  specific  gravities. 

5.  Colorimetric  methods  based  on  the  colors  produced  by  the  addition  of 
certain  reagents,  e.  g.t  ferric  salts. 

6.  Methods  based  on  the  refraction  of  light. 

7.  Methods  based  on  the  rotation  of  polarized  light. 

Of  the  many  proposed  methods  but  two  can  be  said  to  have  gained  the  con- 
fidence of  the  tanning  chemist,  namely  the  Loewenthal  and  the  hide-powder 
methods  and  their  modifications.  For  many  years  the  former  was  considered  to 
afford  the  nearest  approach  to  accuracy  obtainable  in  the  tannin  assay,  but 
more  recently  the  hide -powder  method  has  the  favor  of  most  chemists  as  more 
nearly  agreeing  with  tannery  practice. 

1.  Loewenthals  method  is  based  on  the  oxidation  of  tannin  by  permanganic 
acid,  it  being  converted  into  a  number  of  oxidation  products;  the  nature  of  the 
reaction  is  somewhat  obscure.  The  oxidation  proceeds  slowly  and  irregularly, 
and  gallic  acid  and  other  organic  bodies  of  the  leach  also  reduce  permangan- 
ate. On  these  accounts  a  simple  titration  of  the  extract  would  be  futile,  and 
the  practice  of  the  process  is  complicated  by  the  necessary  modifications. 

Sulfindigotic  acid  in  a  dilute  sulfuric  acid  solution  is  rapidly  and  completely 
oxidized  by  potassium  permanganate,  the  blue  color  passing  to  a  pure  yellow. 
And  where  a  solution  of  a  mixture  of  sulfindigotic  acid  and  tannin  is  titrated 
by  permanganate,  the  influence  of  the  adjunction  of  the  former  causes  the  oxi- 
dation of  the  latter  to  proceed  much  more  rapidly  than  when  alone,  and  for  this 
object  snlflndigotic  acid  is  introduced  into  the  leach,  the  two  being  simulta- 
neously oxidized  during  the  titration. 


*  Jonrn.  Socy.  Chem.  Ind.  1896—385. 
t  Leather  Manufacturer,  1895-83. 


THE    TANNINS.  423 

To  correct  for  the  action  on  permanganate  of  the  organic  non-tannins,  two 
titrations  are  made ;  the  first  on  the  extract  directly,  the  second  on  another 
portion  of  the  extract  from  which  the  tannin  has  been  removed  by  absorption 
in  gelatine  or  hide-powder.  The  details  of  the  process  have  been  considerably 
modified  by  different  chemists ;  in  essence  it  is  as  follows. 

There  are  required  three  solutions  of  reagents ;  potassium  permanganate  that 
has  been  standardized  against  oxalic  acid;  sulflndigotic  acid  in  diluted  sulfuric 
acid;  and  gelatine  in  water.  A  volume  Fof  the  infusion  of  the  bark  to  be  as- 
sayed is  largely  diluted  with  water,  a  measured  volume  of  the  sulflndigotic  acid 
mixed  in,  strongly  acidified  by  sulfuric  acid,  and  titrated  by  permanganate  until 
the  end  point  is  shown.  The  titrand  is  run  in  by  drops  or  by  single  cubic 
centimeters,  and  the  first  appearance  of  a  faint  pink  tinge  at  the  edge  of  the 
yellow  liquid  is  taken  as  the  end-point.  This  titration  will  require  a  cubic 
centimeters  of  the  permanganate. 

Another  volume  Fof  the  leach  is  treated  with  a  measured  volume  of  the 
gelatin  solution  to  precipitate  tannin,  and  the  excess,  of  gelatine  thrown  down 
by  saturating  the  liquid  with  common  salt.  Dilute  sulfuric  acid  is  added  and 
a  limited  weight  of  finely  powdered  barytes  or  kaolin  to  facilitate  the  filtration. 
The  mixture  is  diluted  to  a  definite  volume  with  water  and  a  part  filtered 
through  a  dry  paper;  a  measured  portion  of  the  filtrate  is  titrated  as  before 
and  the  volume  of  the  titrand  required  is  calculated  to  the  original  volume  F 
of  the  leach,  this  amonting  to  b  Cc. 

Then  a,  the  volume  of  permanganate  oxidizing  the  tannin,  sulflndigotic  acid, 
and  gallic  acid  and  associates,  minus  6,  the  volume  oxidizing  the  sulflndigotic 
acid  and  gallic  acid  and  associates,  leaves  c,  the  volume  oxidizing  the  tannin. 
A  correction  is  also  to  be  made  for  the  oxidizable  impurities  of  the  gelatin 
solution,  found  by  a  direct  titration.  Uniformity  in  the  rate  of  addition  of  the 
titrand,  rapid  and  continuous  stirring  during  the  titration,  and  a  uniform  tem- 
perature of  the  titrate  are  essential  for  concordant  results. 

It  has  been  found  that  62.36  grams  of  the  tannin  of  oak  bark  reduces  the 
same  weight  of  permanganate  as  63  grams  of  oxalic  acid,  and  the  weight  of 
tannin  may  be  calculated  from  this  datum,'  but  as  there  is  some  uncertainty 
regarding  the  ratio,  and  as  the  tannins  of  other  tan-wares  have  different  ratios, 
it  is  best  to  make  the  results  comparative  and  report  that  the  sample  assayed 
contains  tannin  equivalent  to  so  many  grams  of  oxalic  acid,  or  that  it  con- 
sumes so  many  grams  of  oxygen  (of  permanganate)  per  hundred  grams  of  bark. 

2.  By  hide  powder.  The  methods  now  most  in  favor  are  based  on  the 
absorption  of  tannin  by  powdered  raw  hide.  It  will  be  seen  that  the  scheme 
follows  in  a  measure  the  technic  of  tanning  and  therefore  gives  comparative 
results.  While  it  cannot  be  denied  that  certain  substances  other  than  tannin 
are  also  absorbed,  yet  the  same  is  also  the  case  in  the  tanning  process.  Like 
all  other  methods  this  must  be  regarded  as  yielding  only  an  approximate 
determination  of  tannin  itself. 

The  powdered  hide  is  prepared  by  several  manufacturing  chemists.  Ununi- 
formity  in  quality  is  said  to  be  the  greatest  source  of  errors  in  analysis,  and 
some  recommend  mixing  the  powder  with  an  equal  weight  of  shredded  filter 
paper  to  distribute  the  particles  of  powder  uniformly  in  the  container.  The 
physical  condition  of  a  lot  of  hide  powder  indicates  its  quality  to  some  ex- 
tent, but  the  absorbent  power  for  tannin,  and  freedom  from  matters  soluble 
in  water  but  precipitated  by  tannin,  must  always  be  ascertained  before  putting 
it  into  use. 

As  substitutes  for  hide  have  been  advocated  the  middle  coat  of  .the  intestine 
of  the  sheep,  finely  shredded  rabbit  skin,  asbestos  fiber,  formalized  gelatin, 


424  QUANTITATIVE    CHEMICAL    ANALYSIS. 

ungummed  silk,  coagulated  albumen,  and  other  absorbents,  but  none  have  dis- 
placed hide.  Procter  states  that  the  sawdust  from  the  buffalo-hide  pickers  (a 
waste  product  of  tanneries),  after  purification  by  sulfurous  acid,  is  equal  in 
absorbing  power  to  hide  powder,  though  too  fine  for  use  in  the  percolation 
process. 

For  the  absorption  of  the  tannin  the  infusion  4s  either  (1),  shaken  up  with 
the  hide  powder  and  the  liquid  filtered,  or  (2),  is  percolated  through  a  column 
of  the  powder. 

(1).  In  the  "  shake  method  "  from  50  to  100  Cc.  of  the  clear  cold  tannin 
infusion  is  evaporated  to  determine  total  solids.  To  another  equal  volume  is 
added  washed  hide  powder  in  several  portions,  shaking  well  after  each  addi- 
tion. The  liquid  is  filtered  and  a  convenient  fraction  evaporated  and  weighed  ; 
the  weight  is  calculated  for  the  total  volume  and  corrected  for  the  moisture 
brought  in  by  the  moist  hide  powder.  The  process  is  rather 
a  tedious  one  but  may  be  hastened  by  the  use  of  some  me- 
chanical device  for  mixing  the  hide  with  the  infusion; 
Fiebing  employs  the  well  known  **  milk  shake  "  apparatus. 

(2).  In  the  "percolation  method  "  the  infusion  is  perco- 
lated through  a  cylinder  filled  with  hide  powder.  The  first 
50  Cc.  passing  through  is  rejected,  since  the  hide  takes  up 
gallic  acid  until  it  has  come  in  contact  with  tannin;  the  next 
50  Cc.  is  evaporated  to  dryness  and  the  residue  weighed. 

Schreiner's  percolator  is  shown  in  Fig.  177.  The  conical 
bulb  A  is  filled  with  hide  powder  and  pressed  on  the  rubber 
stopper  B.  The  infusion  is  poured  into  C,  and  rising  through 
A  empties  into  a  beaker  below  D.  The  object  of  the  bulb  E 
is  to  collect  the  first  flow,  which  is  not  to  be  used  for  the 
determination.  After  E  is  filled,  the  remainder  of  the  per- 
Fig.  177.  colate  flows  directly  to  D,  not  mixing  with  that  in  E.  Vari- 

ous modifications  endeavor  to  obviate  the  weak  feature  of  the  apparatus, 
namely  the  difficulty  of  packing  A  uniformly  close. 

Hurty,*  operating  on  a  ten  per  cent  solution  of  an  extract,  evaporates  100 
Cc.  to  obtain  total  solids.  The  remainder  of  the  solution  is  filtered  giving  a 
filtrate  (A)  of  which  100  Cc.  is  evaporated  to  obtain  the  weight  of  the  soluble 
solids.  Hide  powder  is  purified  from  soluble  constituents  precipitable  by  tan- 
nin by  percolating  100  Cc.  of  (A)  giving  a  percolate  (B)  which  is  reserved  for 
testing;  the  entire  removal  of  soluble  matter  of  the  hide  is  tested  by  running 
through  it  a  little  of  (A)  and  mixing  the  filtrate  with  a  further  portion  of  (A) 
whereupon  the  mixture  should  remain  clear.  If  so,  100  Cc.  of  (A)  is  perco- 
lated and  evaporated  to  obtain  the  weight  of  non-tannins.  That  all  the  tannin 
has  been  absorbed  by  the  hide  is  proved  by  running  through  5  Cc.  of  (A)  and 
mixing  with  some  of  (B) ;  any  tannin  contained  is  precipitated. 

The  weight  of  the  residue  from  (1)  or  (2)  is  subtracted  from  that  obtained 
from  an  equal  volume  of  the  original  infusion,  the  difference  being  the  weight 
of  tannin  absorbed. 

Instead  of  weighing  the  residues  left  on  evaporation,  the  specific  gravities 
of  the  original  extract  and  the  percolate  may  be  determined,  the  difference 
being  proportional  to  the  tannin  withdrawn.  Each  gram  of  gallotannic  acid  in 
100  Cc.  of  the  extract  is  considered  to  increase  the  density  by  .004,  this  hold- 
ing good  up  to  a  total  of  five  per  cent. 

3.  Absorption  by  gelatin.  As  gelatin  and  tannin  unite  to  form  an  insoluble 
compound,  more  readily  in  presence  of  alum  or  sodium  chloride,  it  would 


*  Journ.  Amer.  Chem.  Socy.  1898—64 ;  Leather  Manufacturer,  9—10. 


THE   TANNINS.  425 

appear  that  a  simple  and  accurate  method  of  assay  would  be  that  of  weighing 
the  precipitate,  but  not  only  does  the  composition  vary  according  to  the  nature 
of  the  tannin,  but  many  manipulative  difficulties  are  encountered.  Collin  and 
Benoist  propose  a  volumetric  method  in  which  a  standard  solution  of  gelatin  is 
titrated  by  the  tannin  solution  using  methylene  blue  as  an  indicator. 

Of  the  many  other  methods  that  have  been  put  forward  it  is  unnecessary  to 
speak  in  detail  since  none  are  in  common  use.  A  number  of  reagents  combining 
with  tannin  to  form  insoluble  compounds  have  been  advocated,  (1)  weighing 
either  the  precipitate  or  the  inorganic  base  left  on  ignition,  or  (2) ,  computing 
from  the  specific  gravities  of  the  solution  before  and  after  precipitation.  Lead 
carbonate  and  acetate,  antimony  tartrate,  copper  and  zinc  oxides,  ammonium 
salts  of  copper,  nickel,  and  zinc,  certain  alkaloids,  certain  anilin  dyes,  etc.,  are 
precipitants,  but  the  constancy  of  the  composition  of  the  respective  precipitates 
formed  cannot  be  assured  even  when  produced  under  fixed  conditions. 

Volumetric  methods  are  based  on;the  precipitation  of  the  tannin  by  a  reagent  or 
combination  of  reagents;  after  filtering  the  precipitate  is  dissolved  or  suspended 
in  water,  and  the  reagent  combined  with  tannin  is  titrated  by  a  volumetric  solu- 
tion ;  or  the  precipitant  may  itself  be  a  volumetric  solution,  and  the  excess  in  the 
filtrate  or  an  aliquot  part  titrated.  Reagents  for  this  purpose  are  solutions  of 
gelatin,  lead  acetate,  methylene  blue  alone  or  with  gelatin,  potassium  antimony 
tartrate  with  safranin,  Porrier's  green  4JE,  etc.,  but  here  the  same  objection 
applies.  Modifying  Loewenthal,  the  tannin  may  be  precipitated  by  ferric 
acetate,  zinc  acetate,  or  the  like,  and  the  precipitate  dissolved  and  titrated  by 
permanganate ;  or  the  solution  may  be  titrated  before  and  after  precipitation. 
Colorimetric  methods  are  based  on  the  green  or  blue  hues  struck  with  ferric 
solutions  by  the  tannins,  and  a  polarimetric  method  has  been  described. 

Tanning  extracts.*  These  contain  water  and  the  soluble  matter  of  the  bark 
or  wood.  For  example,  extract  of  hemlock  bark  contains  from  45  to  55  per 
cent  of  water,  15  to  30  of  tannin,  12  to  18  of  non-tannins,  2  to  10  of  '  reds',f  and 
1  to  1.5  of  ash.  Various  additions  are  made  to  certain  extracts  for  technical 
reasons. 

1.  Water  is  determined  by  evaporating  two  to  four  grams  of  the  extract  in  a 
platinum  dish,  and  drying  at  100°  to  constant  weight. 

2.  Ash,  by  burning  the  residue  from  (1)  in  a  place  free  from  draughts,  since 
the  ash  is  very  light  and  easily  blown  away. 

3.  Soluble  matter.  From  20  to  25  grams  of  the  extract  is  diluted  with  water 
to  one  liter ;  some  would  reduce  the  weight  of  extract  to  about  five  grams  per 
liter,  claiming  that  a  much  greater  residue  is  left  with  the  prescribed  quanti- 
ties.   After  heating  the  liquid  to  90  ° ,  a  part  is  quickly  filtered  thraugh  cotton. 
The  filtrate  contains  all  constituents  soluble  in  hot  water;  it  is  allowed  to  cool 
and  100  Cc.  drawn  out  by  a  pipette  into  a  tared  dish,  taking  care  to  shake  up 
the  liquid  before  drawing  out,  since  a  part  of  the  matter  soluble  only  in  hot 
water  separates  as  the  liquid  cools.    The  liquid  is  evaporated  to  dryness  and 
the  residue  dried  at  100°    and  weighed;  it    is  the    matter  soluble  in  hot 
water,  a. 

The  remainder  of  the  liter  is  cooled,  filtered  clear,  and  100  Cc.  evaporated 
and  the  residue  dried  and  weighed;  it  is  the  matter  soluble  in  cold  water,  b. 

The  difference  between  a  and  b  is  the  matter  soluble  in  hot  water  only,  the 
'reds.'J 


*  The  Leather  Manufacturer,  1895—64. 

t  Analyst,  1898-33. 

t  Proctoi  Leather  Indust.  Text-book,  56. 


426  QUANTITATIVE    CHEMICAL   ANALYSIS. 

A  part  of  the  filtrate  is  used  to  determine  the  tannin  and  non-tannins  by 
evaporating  and  weighing  the  liquid  before  and  after  nitration  through  hide- 
powder,  as  before  described. 


Valuation  of  leather.*  The  sample  is  finely  divided  by  rasping,  and  the  powder 
treated  as  follows.  , 

1.  The  specific  gravity  is  determined,  most  conveniently  by  finding  the  volume 
of  mercury  displaced  by  a  given  weight  of  the  powder.    The  result  is  calculated 
on  the  basis  of  18  per  cent  (the  average)  content  of  water.    The  mean  specific 
gravity  of  94  samples  of  leather  showed  1.012. 

2.  Water  is  determined  by  drying  at  105  o  to  constant  weight. 

3.  Ash.  The  inorganic  matter  is  determined  by  incinerating  a  portion  in  a 
platinum  crucible.    According  to  Eitner  it  is  generally  about  one  per  cent,  over 
three  per  cent  indicating  fraudulent  weighting.    The  ash  may  be  examined 
qualitatively  or  quantitatively  for  barium,  calcium  or  lead  sulfates,  sand,  clay, 
etc. 

4.  Oil  and  Grease.  These  may  be  either  natural  or  incorporated.    From  five  to 
ten  grams  of  the  powder  is  extracted  in  a  Soxhlet  apparatus  by  petroleum  ether. 
The  extract  is  evaporated  and  the  residue  weighed ;  it  may  contain  besides  oil 
and  grease,  any  paraffin  or  resin  that  may  have  been  in  the  leather,  detected  by 
qualitative  tests. 

5.  Tannins.  The  residue  from  the  above  is  dried  and  heated  with  water,  fil- 
tered, and  the  aqueous  extract  evaporated  and  weighed.    In  the  residue  are  the 
tannins  and  their  products,  and  also  glucose,  dextrin,  and  soluble  salts  used  to 
weight  the    leather.    A  portion  is  incinerated,  the    ash    being   the    soluble 
mineral  salts,  to  be  examined  qualitatively  for  sulf  uric  acid,  chlorine,  and  cer- 
tain metals.    The  remainder  of  the  residue  is  powdered  and  exhausted  with  cold 
water,  and  the  tannin  and  coloring  matter  precipitated  by  magnesia  and  lead 
carbonate ;  the  filtrate  is  tested  for  glucose  and  dextrin  by  Fehlings  solution ; 
this  reagent  however  will  throw  down  some  precipitate  from  all  extracts. 

6.  Nitrogen.  The  total  nitrogen  of  the  leather  is  determined  by  one  of  the 
usual  methods  and  calculated  to  dry  ash-free  leather. 

7.  Hide-substance.  From  the  nitrogen  found    in   (6)   the  amount    of  pure 

L.Nl 
hide-substance  H  may  be  calculated  from  the  equation  H  —  N^  ,  where  L  is 

the  dry  ash -free  leather  substance;  Nl,  the  nitrogen  inL;  and  2V7>,  the  nitrogen 
in  the  dry,  ash -free  hide  as  prepared  for  tanning,  the  percentage  of  nitrogen 
being  fairly  constant  for  any  one  kind  of  hide.  Deducting  H  from  the  pure 
leather  gives  the  combined  tannin.  It  is  said  that  a  complete  tannage  is  real- 
ized only  where  the  hide  substance  has  combined  with  its  own  weight  of  tannin. 
(The  equation  is  founded  on  the  absence  of  nitrogen  in  tannin  and  like 
bodies.  When  a  hide  is  tanned  a  certain  proportion  of  the  tannin,  etc.,  of 
the  liquor  enters  it  and  the  nitrogen  content  is  proportionately  reduced. 
Hence  the  proportion  — 

Per  cent  of  nitrogen  in  dry,  ash-free  hide  substance  :  per  cent  of  nitrogen  in 
the  same  after  tanning  :  :  per  cent  of  dry,  ash -free  leather  :  per  cent  of 
dry,  ash-free  hide  substance  in  the  leather  — that  is,  Nb  :  Nl  :  :  L  :  H). 


*  Proctor  Text-book  of  Tanning;  Journ.  Socy.  Chem.  Ind.  1898-164. 


THE   CARBOHYDEATES.  427 


THE  CARBOHYDRATES. 

The  carbohydrates  are  a  class  of  organic  bodies  composed  of  carbon,  hydro- 
gen and  oxygen,  the  two  latter  being  in  the  atomic  ratio  of  two  to  one.  They 
may  be  divided  in  three  groups. 

1.  The  glucoses,  comprising  the  sugars  containing  from  three  to  nine  atoms 
of  carbon  in  the  molecule.  The  most  common  of  the  members  is  dextrose, 


2.  The  saccharoses,  sugars  of  the  formulae  CigH&Oii  and  CieH&Oie.    The  best 
known  member  is  cane  sugar,  C^H^On. 

3.  The  starches  and  isomers,  comprising  the  starches,  celluloses,  dextrin, 
inulin,  glycogen,  and  the  natural  gums. 

The  saccharoids  are  non-fermentable  saccharine  bodies  whose  hydrogen  and 
oxygen  atoms  are  not  in  the  water-ratio;  mannite,  CeH^Oe,  is  a  typical 
member. 

The  Sugars. 

The  most  familiar  of  this  family  are  sucrose,  from  the  sugar  cane,  white  beet 
and  sorgo  ;  dextrose  from  fruits  or  made  artificially  by  the  hydrolysis  of  starch  ; 
and  the  sugar  of  milk.  Other  sugars,  derived  from  vegetable  or  animal  sources, 
are  of  less  practical  importance.  Most  varieties  of  sugar  are  crystalline  and 
anhydrous,  and  all  are  soluble  in  water. 

Sucrose  (saccharose,  cane-sugar,  saccharon),  Ci2H22Ou.  The  process  of  man- 
ufacture is  essentially  the  expression  of  the  juice  of  the  sugar  cane  or  white 
beet,  clarification  by  chalk  or  lime,  decolorization  by  bone-black,  and  crystal- 
lization. Commercial  white  sugar  is  nearly  pure  sucrose;  so  rare  is  adultera- 
tion that  it  is  claimed  that  the  ordinary  granulated  white  sugar  is  the  purest 
manufactured  food  substance  of  commerce.  Brown  sugars,  unrefined  or  partly 
refined,  contain  considerable  water,  glucose,  albuminoid  and  other  organic 
matter,  and  inorganic  salts. 

Sucrose  crystallizes  in  rhombic  prisms  that  melt  at  160°  Cent,  and  decom- 
pose at  a  higher  temperature  with  the  formation  of  caramel  and  other  bodies. 
It  is  freely  soluble  in  water,  but  insoluble  in  absolute  alcohol  ;  the  aqueous  solu- 
tion rotates  the  plane  of  a  ray  of  polarized  light  to  the  right  73.8  °  at  20  <=>  Cent. 
Sucrose  ferments  with  yeast  giving  alcohol  and  carbonic  acid,  and  when  boiled 
with  a  dilute  acid  is  transformed  to  invertose  —  a  mixture  of  dextrose  and 
levulose.  It  does  not  reduce  metallic  salts  in  alkaline  solution. 

Invertose,  CeHisOe,  is  found  in  many  fresh  fruits.  It  is  uncrystallizable  and 
differs  from  sucrose  in  that  the  solution  reduces  certain  metallic  salts  to  lower 
oxides  or  to  the  metallic  state,  and  in  having  a  left-handed  rotation  for  polar- 
ized light  of  26  °  .  It  is  fermentable  by  yeast. 

Dextrose,  CeHisOe.  Found  in  honey  and  many  fruits,  and  extensively  manufac- 
tured by  the  hydrolysis  of  starch  by  dilute  sulfuric  acid.  It  is  less  soluble  in 
cold  water  and  less  sweet  than  sucrose,  reduces  metallic  salts  and  rotates  polar- 
ized light  53  °  to  the  right. 

Levulose,  CeH^Oe-  Found  in  honey  and  many  fruits  and  can  be  made  artifi- 
cially by  the  hydrolysis  of  inulin.  It  is  uncrystallizable  and  rotates  polarized 
light—  93.70  .  It  reduces  metallic  salts. 


428  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Milk  sugar,  Ci2H22Ou.H2O.  Crystallizes  in  four-sided  prisms,  is  less  soluble  in 
water  than  sucrose,  reduces  metallic  salts,  and  has  a  dextro- rotation  of  polar- 
ized light  of  52.50. 

Maltose,  CiaEfoOn.H^O.  Formed  in  the  action  of  malt  infusion  on  starch.  Crys- 
tallizes in  hard  white  crusts  of  fine  needles,  and  is  much  less  soluble  in  water 
than  dextrose.  It  is  fermentable  and  reduces  metallic  salts.  Has  a  dextro- 
rotation  of  135.3°. 

Raffinose,  CisH^Oie.  Occurs  in  molasses  from  beet  root  sugar,  and  in  manna, 
barley,  etc.  Crystallizes  in  long  needles  and  is  readily  soluble  in  cold  methyl 
alcohol.  On  hydrolysis  it  is  converted  into  dextrose,  levulose  and  galactose. 


From  sugar  cane  the  juices  can  be  expressed  by  a  small  roller-mill.  Sugar 
beets  are  rasped  or  drilled  and  the  sugar  extracted  from  the  resulting  pulp  by 
water  or  alcohol,  diffusion  taking  place  through  the  cell  walls  quite  rapidly  pro- 
vided the  pulp  is  sufficiently  comminuted.  Another  plan  is  to  dry  the  pulp  and 
extract  the  sugar  in  a  Soxhlet  apparatus  by  hot  water  or  dilute  alcohol. 
A  simple  method  yielding  an  extract  ready  for  polarization,  is  that  of  heating 
the  pulp  in  a  closed  flask  with  water  and  a  little  lead  acetate  and  sufficient 
calcium  carbonate  to  neutralize  any  free  organic  acids. 

Having  obtained  evidence  of  the  presence  of  a  sugar  in  a  solution,  it  may  be 
isolated  by  precipitating  the  albumenoid  bodies  by  heating,  the  dextrin  and 
gummy  matters  by  alcohol,  and  the  organic  acids,  tannin,  etc.,  by  lead  acetate. 
The  excess  of  lead  is  removed  by  hydrogen  sulfide,  and  the  filtered  solution 
concentrated ;  on  evaporation  to  dryness,  the  sugar  crystallizes  or  is  left  as 
an  amorphous  mass  according  to  the  variety.  A  sugar  can  be  separated  from 
colloidal  associates  by  the  process  of  dialysis,  but  the  operation  is  slow 
and  tedious  ordinarily,  though  it  may  be  considerably  shortened  by  special 
appliances. 

The  crystalline  or  amorphous  residue  is  then  to  be  identified  by  the  action 
toward  polarized  light,  reduction  of  metallic  salts,  susceptibility  to  fermenta- 
tion, and  other  tests. 


The  principal  methods  for  the  determination  of  sugar  are  as  follows:  — 

1 .  From  the  specific  gravity  of  the  solution.    When  in  a  clear  aqueous  solution, 
free  or  nearly  so  from  other  dissolved  matter,  the  proportion  of  a  sugar  may 
be  found  from  the  density,  observed  by  one  of  the  customary  methods.    With 
proper  care  this  plan  is  quite  as  accurate  as  that  of  evaporating  the  solution  and 
weighing  the  residue.    Perrier  states  that  at  a  constant  temperature  and  volume 
each  gram  of  saccharose  in  aqueous  solution  increases  the  densimetric  expres- 
sion by  a  constant  value  up  to  a  concentration  of  forty -five  per  cent  of  sac- 
charose; above  forty-five  per  cent  the  increase  is  less  regular.    Tables  of  the 
relation  between  the  specific  gravity  and  concentration  of  solutions  of  sucrose 
have  been  compiled  by  Balling,  Scheibler,  Gerlach,  and  others  and  will  be 
found  in  most  books  of  chemical  tables  and  works  on  sugar  analysis. 

2.  By  fermentation.  On  digestion  with  a  ferment  most  of  the  sugars  are 
broken  up,  the  major  products  being  alcohol  and  carbonic  acid,  e.  g.,  C^H^On  -j- 
H2O  =  4C2H6O  +  4CO2.    Under  certain  conditions  the  products  are  practically 
constant  in  proportion  to  the  sugar  and  may  be  used  as  a  means  of  its  deter- 
mination.   During  the  fermentation  free  oxygen  must  be  excluded,  and  an 
ample  time  allowed  for  the  decomposition,  longer  for  saccharose  than  for 
maltose  and  dextrose.    The  most  suitable  concentration  for  the  sugar  solution 


THE    CARBOHYDRATES. 


429 


is  said  to  be  about  eight  percent,  and  the  temperature  of  digestion  35°, 
Sucrose  yields  about  49  per  cent  of  carbon  dioxide  and  dextrose  about  46.5  per 
cent. 

Of  the  two  products,  carbon  dioxide  is  the  one  usually  determined  as  it  offers 
fewer  manipulative  difficulties  than  the  alcohol.  One  form  of  apparatus  is 
shown  in  Fig.  178.  A  flask  A  contains 
the  sugar  solution  and  a  small  amount  of 
a  saccharomycete  of  a  rapidly  diffusive 
variety.  The  flask  is  topped  by  a  cork 
bearing  two  glass  tubes,  one  B  closed  by 
a  short  rubber  tube  and  a  glass  rod,  the 
other  D  joined  to  an  absorption  flask  E 
containing  a  clear  solution  of  barium 
hydrate,  protected  from  contact  with  the 
external  air  by  a  Bunsen  valve  F  or  other 
arrangement.  The  apparatus  is  kept  in 
a  warm  place  until  the  fermentation  is 
over  —  usually  from  24  to  48  hours  —  the  Fig.  178. 

carbon  dioxide  passing  into  the  barium  hydrate  precipitating  an  equivalent  of 
barium  carbonate.  The  tube  B  is  pushed  down  into  the  liquid  and  what  re- 
mains of  the  gas  in  the  generating  flask  is  expelled  by  heating  the  solution  and 
passing  a  current  of  pure  air.  The  precipitate  barium  carbonate  is  then  col- 
lected and  weighed  as  usual.  It  is  always  best  to  accompany  the  analysis  by 
another  on  about  the  same  weight  of  pure  sugar,  since  in*practice  the  reaction 
never  proceeds  exactly  according  to  the  above  simple  equation. 

Other  methods  for  the  determination  of  the  carbon  dioxide  may  be  followed, 
or  the  alcohol  formed  may  be  distilled  and  determined  in  the  distillate  by  spe- 
cific gravity  or  other  process. 

3.  By  polarization.  A  solution  of  a  sugar  rotates  the  plane  of  a  ray  of  polar- 
ized light  to  an  extent  determined  by  (1),  the  kind  of  sugar;  (2),  the  concen- 
tration of  the  solution;  (3),  the  length  of  the  solution  traversed  by  the  ray; 
and  (4),  the  temperature  of  the  solution.  Sucrose,  dextrose,  milk-sugar,  and 
maltose  turn  the  plane  to  the  right  (dextrogyrate),  while  levulose  and  invertose 
turn  it  to  the  left  (laevogyrate). 

The  specific  rotatory  powers  of  the  different  sugars  are  stated  by  Landoldt 
as  follows,  the  concentration  ten  grams  in  100  Oc.  of  water,  the  temperature 
20°  Cent.,  and  the  light  the  D  or  sodium  ray. 

Sucrose -+-  66.58°.  Levulose —    70.47°. 

Milk  sugar +  52.53°.  Invertose —  20.02°. 

Maltose +138.10°.  Raffinose +104.50°. 

Dextrose +   62.74°. 

The  sugars  derived  from  natural  sources  are  either  dextro-  or  laevo-rotatory, 
but  the  same  varieties  formed  artificially  may  consist  of  molecules  of  both  right 
and  left-handed  rotation.  The  syrups  prepared  by  dissolving  some  of  the 
sugars  in  cold  water  exhibit  an  abnormally  high  or  low  rotation,  coming  to 
the  normal  after  standing  for  a  time,  or  immediately  on  heating  to  the  boiling 
point. 

The  influence  of  the  temperature  of  the  sugar  solution  on  the  saccharimeter 
reading  is  not  of  practical  importance  within  the  usual  range  of  laboratory 
temperatures  except  for  levulose  and  invertose.  Each  degree  Centigrade  of 
rise  in  the  temperature  of  a  solution  of  14  grams  of  levulose  in  100  Cc.  of  water 
reduces  the  angular  deviation  by  .172°;  this  peculiarity  is  applied  in  the 
analysis  of  mixed  sugars. 


430  QUANTITATIVE    CHEMICAL    ANALYSIS. 

The  solution  of  sugar  to  be  polarized  must  be  perfectly  clear  and  quite  or 
nearly  colorless,  and  be  free  from  albuminous  and  like  bodies  that  have  also  a 
rotatory  power.  Several  reagents  are  in  use  for  clarifying  and  decolorizing  the 
solutions  and  precipitating  foreign  matters  that  interfere,  Basic  lead  acetate 
precipitates  most  organic  acids  and  the  precipitate  carries  down  colloidal 
bodies  and  coloring  matters.  Gill  states  that  the-  excess  of  the  reagent  af- 
fects the  rotatory  power  of  levulose,  but  that  the  error  can  be  overcome  by 
certain  precautions.  Edson  recommends  normal  lead  acetate  as  having  some 
advantages  over  the  basic  acetate.  In  all  cases  only  a  slight  excess  should  be 
used. 

Bone-black  is  an  eminent  decolorizing  agent 'but  should  be  employed  with 
caution,  using  as  little  as  will  accomplish  the  clarification,  as  it  adsorbs  a  small 
amount  of  sugar.  It  has  been  proposed  to  percolate  the  syrup  through  a  col- 
umn of  bone-black,  avoiding  the  error  due  to  retention  of  sugar  by  rejecting 
the  first  issue  of  the  percolate.  Aluminum  hydrate  suspended  in  water  acts 
chiefly  or  entirely  in  a  mechanical  way,  and  alone  is  suited  only  for  the  purer 
syrups.  Acid  mercuric  nitrate  has  a  coagulating  action  and  is  preferable  for 
syrups  containing  much  albuminous  matter. 

The  saccharimeter  and  its  operation  are  described  on  page  165. 

4.  By  reduction  of  metallic  salts.  Fehlings  solution.  When  an  alkaline 
solution  of  a  cupricsalt  is  boiled  with  a  solution  of  a  sugar  mutual  decomposi- 
tion takes  place.  The  exact  nature  of  the  reaction  is  not  known,  but  in  general 
on  the  one  hand  the  cupric  salt  is  reduced  and  cuprous  oxide  (Cu2O) 
separates  (unless  retained  in  solution  by  certian  reagents),  and  on  the  other  the 
sugar  is  oxidized  by  the  cupric  salt  and  alkali  to  form  oxalic,  formic,  lactic 
and  other  acids  and  decomposition  products.  As  the  reaction  proceeds  the 
deep  blue  color  of  the  copper  solution  gradually  lightens  as  the  copper  passes 
to  the  cuprous  state,  until,  if  the  sugar  is  in  excess,  the  solution  becomes 
light  yellow,  colored  only  by  the  decomposition  products  or  impurities  of  the 
sugar.  Under  certain -uniform  conditions,  i.  e.,  when  the  reagent  is  of  a  given 
composition  and  is  in  large  excess  throughout  the  reaction,  the  sugar  solu- 
tion of  a  certain  approximate  concentration,  and  the  rate  of  mixing  the  two, 
and  the  initial  and  subsequent  temperatures  concordant,  the  reactions  between 
the  copper  salt,  sugar  and  alkali  are  practically  regular,  and  the  weight  of 
the  precipitated  cuprous  oxide  bears  a  definite  relation  to  the  weight  of  the 
sugar.  But  it  must  be  remembered  that  each  variety  of  sugar  has  its  own 
specific  reducing  coefficient  and  that  many  other  organic  bodies  react  in  a 
similar  way. 

Of  the  common  sugars,  glucose,  milk  sugar,  and  maltose  reduce  Fehlings 
solution,  though  in  different  ratios,  while  sucrose  has  no  action  on  it  and  can 
only  be  determined  after  conversion  to  invertose. 

The  typical  Fehlings  solution  is  compounded  of  copper  sulfate,  sodium 
hydrate,  and  potassium  tartrate,  all  dissolved  in  water.  The  object  of  the  tar- 
trate  is  to  hold  in  solution  the  copper  which  would  otherwise  be  precipitated 
as  cupric  hydrate  by  the  alkali.  The  tartrate  may  be  replaced  by  other  com- 
pounds that  contain  the  hydroxyl  group,  such  as  glycerol,  mannite,  etc. 

Many  recipes  have  been  published  for  compounding  the  solution,  modifying 
the  original  proportions  of  the  reagents  directed  by  Fehling.  In  that  of  O'Sul- 
livan  below,  the  copper  sulfate  and  alkali -tartrate  solutions  are  made  up 
separately  and  only  united  just  before  using,  since  the  mixture  slowly  decom- 
poses on  keeping. 


THE    CARBOHYDRATES.  431 


Fehlings  solution. 

A. 

Crystallized  cupric  sulf ate,  powdered 69.278  grams. 

Sulf uric  acid,  concentrated One  Cc. 

Dissolve  in  water  and  make  up  to One  liter. 

B. 

Potassium  sodium  tartrate,  crystallized 356  grams. 

Sodium  hydroxide 100  grams. 

Dissolve  in  waterand  make  up  to One  liter. 

Equal  volumes  of  A  and  B  are  mixed  for  the  determination. 

The  modification  of  the  original  solution  due  to  Eossel  substitutes  glycerol 
for  the  tartaric  acid  on  the  grounds  that  a  more  stable  solution  results.  In 
Soldaini's  solution  no  organic  matter  is  contained;  it  is  made  up  of  a  solution 
of  the  carbonate  and  hydrate  of  copper  in  potassium  carbonate.  The  superi- 
ority asserted  is  the  lessened  action  of  the  reagent  on  any  cane-  or  other  non- 
reacting  sugar  that  may  be  present.  For  small  amounts  of  invert  sugar  in 
sucrose  an  addition  of  potassium  sulfate  is  made,  the  amount  of  sugar  solution 
added  is  greater,  and  the  time  of  boiling  reduced. 

Since  the  reaction  is  greatly  modified  by  the  conditions  of  the  experiment, 
arbitrary  directions  must  be  closely  followed.  Those  of  Defren  are  substan- 
tially as  follows. 

The  sugar  solution  is  clarified  and  other  reacting  bodies  removed  by  lead 
acetate,  or  other  reagent,  and  the  excess  of  lead  by  potassium  sulfate.  After 
filtering,  the  solution  is  diluted  until  there  is  contained  approximately  one- 
half  of  one  per  cent  of  sugar,  the  volume  observed,  and  the  solution  filled  into 
a  burette.  Of  the  copper  and  potassium  tartrate  solutions  supra,  15  Cc.  of 
each  are  mixed,  diluted  with  50  Cc.  of  freshly-boiled  distilled  water,  and  heated 
on  the  water-bath  for  five  minutes.  Twenty -five  Cc.  of  the  sugar  solution  is 
run  into  the  hot  mixture,  and  the  whole  kept  in  the  water-bath  for  12  to  15 
minutes.  The  mixture  at  first  turns  green,  then  brownish,  and  finally  dingy 
red.  It  is  filtered  by  suction  and  the  cuprous  oxide  washed  by  hot  water  until 
the  washings  are  no  longer  alkaline.  The  precipitate  is  ignited  and  weighed 
as  cupric  oxide  and  the  weight  of  sugar  calculated  from  the  equations  below, 
w  being  the  weight  of  cupric  oxide. 

Weight  of  dextrose  =  w  (.4400  -j-  .000037  to) 
Weight  of  maltose  =  w  (.7215  -f  .000061  w) 
Weight  of  lactose  =  w  (.6270  -f-  .000053  to) 

The  weight  of  the  precipitate  of  cuprous  oxide  can  be  found  by  several  plans. 
A  resume  may  be  of  interest. 

1.  The  filter,  with  the  precipitate  inclosed,  may  be  burned  in  an  open  cruci- 
ble, and  the  residue  ignited  until  all  the  cuprous  oxide  has  become  cupric  oxide, 
which  is  weighed.    With  the  small  amounts  of  precipitate  usually  obtained 
oxidation  is  complete  after  a  short  ignition. 

2.  The  precipitate  may  be  caught  on  a  tared  paper  or  a  Gooch   crucible,  and 
after  drying  at  100  ° ,  weighed  as  cuprous  oxide.  It  is  said  that  as  it  is  impossible 
to  free  the  filter  from  the  last  traces  of  the  filtrate  by  washing  with  water  alone, 
a  double  filter  should  be  used,  the  outer  paper  having  been  trimmed  to  an  equal 
weight  to  that  of  the  inner;  this  on  the  supposition  that  each  will  retain  the 
same  amount  of  the  filtrate.    As  one  paper  may  not  retain  the  finely  divided 
precipitate  it  is  advised  to  use  a  quadruple  filter,  the  two  outer  counterbalanced 
by  the  two  inner. 


432  QUANTITATIVE    CHEMICAL    ANALYSIS, 

On  igniting  the  precipitate  in  hydrogen,  the  vapor  of  formic  acid,  or  other 
reducing  gas,  there  is  left  metallic  copper.  Here  the  nitration  is  done 
through  a  thin  tube  of  hard  glass  of  the  shape  of  a  calcium  chloride 
drying  tube  Fig.  158,  plugged  with  asbestos  and  glass-wool.  After  drying 
and  weighing  the  tube,  the  filtration  is  proceeded  with,  the*  precipi- 
tate washed  with  water,  then  with  alcohol  and  ether,  and  the  contents 
dried  by  conducting  a  current  of  air  through  the  tube  for  a  short  time.  Then 
the  tube  is  connected  to  a  hydrogen  generator  and  heated  moderately;  a  short 
exposure  effects  the  reduction  to  the  metal.  The  tube  is  cooled  while  the  gas 
still  passes,  and  is  ready  for  weighing. 

3.  The  cuprous  oxide  may  be  dissolved  in  nitric  acid  and  the  copper  precipi- 
tated electrolytically .    Although  consuming  more  time  than  the  other  methods, 
it  has  several  advantages  and  is  preferred  by  many  chemists. 

4.  Ehrmann  would  stir  the  washed  oxide  into  a   concentrated  solution  of 
sodium  platinic  chloride,  the  cuprous  oxide]  decomposing  this  compound  with 
deposition  of  metallic  platinum  which  is  filtered  off  and  weighed.    Superior 
accuracy  has  been  claimed  over  the  method  of  reducing  the  cuprous  oxide  to 
metallic  copper  and  weighing,  by  reason  of  the  higher  weight  of  platinum, 
though  if   the    equation    Na2PtCl6  +  2Cu2O  -f  4HC1  =  2Cu2Cl4  +  Ft  -f  2NaCl  + 
SHsO  is  correct,  a  less  weight  will  be  obtained. 

5.  Similarly  Gedult  directs  to  stir  the  precipitate  into  an  ammoniacal  solution 
of    silver  chloride  —  2 AgCl  +  Cu2O  =  Ag2-f  CuO  +  CuCl2  —  the    cupric  oxide 
formed  is  held  in  solution  by  the  ammonia.    The  deposited  silver  is  filtered  off 
and  weighed. 

6.  More  rapid  is  the   volumetric  method  of   Sidersky  who  dissolves  the 
cuprous  oxide  in  a  known  volume  of  standard    sulf uric    acid    and  a  slight 
excess  of  potassium  chlorate  — 

6Cu2O  -f  12H2S04  +  2KC103  =  K2SO4-{-  llCuS04  +  CuCl2  -f  12H2O 
—  after  which  the  uuneutralized  acids   (sulf uric  and  chloric)   are  titrated  by 
standard  alkali. 

7.  In  another  volumetric  method  the  precipitate  is  dissolved  in    an  acid 
solution  of  ferric  sulfate  and  the  ferrous  sulfate  formed  —  Cu2O  -f  Fe2(SO4)s  -j- 
H2SO4  =  2FeSO4  -f-  2CuS04  +H2O  —  titrated  by  standard  potassium    perman- 
ganate. 

8.  Politis  accurately    measures  the  volume  of  the  copper  solution  boiled 
with  the  sugar,  and  determines,  not  the  weight  of  cuprous  oxide  precipitated, 
but  that  of  the  copper  remaining  in  solution.    After  filtering  and  washing  the 
precipitate,  the  filtrate  and  washings  is  mixed  with  potassium  iodide  (Cu2CJ4 
-}-4KI  =  Cu2l2-f  4KCl-f  I2),and  the  iodine  set  free  is  titrated  by  potassium 
thiosulfate  and  starch-paste.    Instead  of  filtering,  the  solution  after  precipi- 
tation may  be  made  up  to  a  definite  volume  and  an  aliquot  part  of  the  clear 
liquid  withdrawn  for  testing. 

Volumetric  methods.  The  sugar  solution  is  diluted  to  a  definite  volume  so 
chosen  that  the  sugar  shall  be  at  a  concentration  of  one  to  two  per  cent,  and 
a  burette  filled  with  it.  Beneath  is  placed  a  porcelain  dish  supported  over 
a  Bunsen  burner.  An  accurately  measured  volume  of  Fehlings  solution  is 
run  into  the  dish  and  heated  to  boiling.  The  sugar  solution  is  run  in  slowly 
until  the  blue  color  nearly  disappears,  then  the  titration  cautiously  continued 
until  a  small  filtered  portion  shows  no  coloration  with  a  solution  of  potassium 
ierrocyanide  acidulated  with  acetic  acid.  If  lead  acetate  has  been  used  for 
clarification  the  lead  must  be  removed  before  the  titration  since  it  interferes 
with  the  end-reaction. 


THE    CARBOHYDRATES.  433 

The  formulae  for  compounding  Fehlings  solution  aim  to  secure  a  standard 
solution  of  which  one  cubic  centimeter  reacts  with  a  definite  weight  of  sugar  — 
usually  one  milligram.  But  as  the  reaction  is  complex,  varying  with  the  con- 
ditions of  the  test,  and  as  in  the  titration  each  successive  addition  of  sugar 
encounters  a  solution  weaker  in  copper,  the  reaction  slackens  continually,  and 
it  is  evident  that  a  parallel  determination  on  as  pure  a  sugar  as  can  be  ob- 
tained is  to  be  preferred  to  a  reliance  on  the  equivalent  of  the  solution  or  a 
computation  from  a  formula.  This  also  applies  to  the  tables  of  equivalents  of 
the  different  sugars  for  given  volumes  of  Fehlings  solution.* 

According  to  Soxhlet,  .500  gram  of  sugar  in  a  one  per  cent  solution  reduces 
the  following  volumes  of  undiluted  Fehlings  solution  — 

Dextrose 105.2  Cc.  Galactose 98.0  Cc- 

Invertose 101.2  Cc.  Levulose 97.2  Cc. 

Milk  sugar ......  74.0  Cc.  Maltose 64.2  Cc. 

Pavy  combines  ammonia  with  the  fixed  alkali  of  Fehlings  solution  by  adding 
ammonia  until  strongly  ammoniacal.  With  this  solution  the  cuprous  tartrate 
is  reduced  by  the  sugar  solution  to  form  soluble  cuprous  ammonium  tartrate, 
and  the  solution  remains  clear  throughout  the  boiling,  the  blue  color  fading  as 
the  copper  passes  to  the  cuprous  state.  Since  a  hot  solution  of  a  cuprous 
salt  is  readily  oxidized  by  the  air,  the  titrate  is  held  in  a  flask  whose  cork  is 
pierced  by  the  burette  tip,  and  also  by  a  glass  tube  which  leads  the  steam  and 
ammonia  vapors  out  of  the  flask  into  a  dilute  acid  in  order  that  the  operator 
may  not  be  inconvenienced ;  a  Bunsen  valve  prevents  any  regurgitation  should 
the  titrate  become  cooled  momentarily.  A  simpler  plan  is  that  of  Allen  who 
blankets  the  titrate  with  a  layer  of  paraffin  oil. 

Instead  of  ammonia,  Causse  would  compound  potassium  ferrocyanide  with 
Fehling's  solution,  believing  that  this  reagent,  while  answering  the  purpose  of 
ammonia,  has  less  action  on  sugar  and  its  associates. 

Allen  and  Gaud's  solution  is  simply  copper  sulfate  dissolved  in  an  excess  of 
ammonia;  while  Gerrard  holds  up  the  cuprous  oxide  by  potassium  cyanide. 
When  using  the  latter  the  end-point  (the  disappearance  of  the  blue  color) 
must  not  be  overstepped,  since  any  excess  of  the  sugar  solution  produces  a 
green  tint. 

Knapp's  solution.  Mercuric  cyanide  in  alkaline  solution  is  reduced  by  sugars 
with  the  formation  of  a  precipitate  of  metallic  mercury.  The  hot  one  per  cent 
solution  of  the  salt  is  titrated  by  the  dilute  sugar  solution  until  a  drop  of  the 
mixture  gives  no  cloud  with  a  drop  of  stannous  chloride,  or  shows  no  decided 
brown  tint  when  exposed  to  the  fumes  of  ammonium  sulfide.  Sacchse  prefers 
an  alkaline  solution  of  mercuric  potassium  iodide,  using  a  solution  of  stannous 
potassium  chloride  for  an  indicator,  applied  as  above.  These  two  solutions 
are  useful  in  special  determinations  and  in  qualitative  tests,  but  for  general 
work,  Fehling  has  the  preference.  The  following  table  shows  the  relative 
action  of  different  sugars  on  Fehlings  (undiluted),  Knapps  and  Sacchses 
solutions,  taking  dextrose  as  100  for  a  basis. 

Fehling.  Knapp.  Sacchse. 

Dextrose 100.  100.  100. 

Invertose 9(5.2  99.0  124.5 

Levulose 92.4  102.2  148.6 

Milk  sugar 70.3  64.9  70.9 

Galactose 93.2  83.0  74.8 

Milk  sugar,  Invert.     96.2  90.0  85.5 

Maltose  61.0  63.8  65.0 


*  Chem.  News,  1894—1—233;  Journ.  Anal.  Chem.  4—329. 


434  QUANTITATIVE   CHEMICAL    ANALYSIS. 

4.  By  precipitation.  Reducing  sugars  from  crystalline  precipitates  with 
phenylhydrazine  (CeH^Os),  which  are  termed  osazones;  usually  a  designation 
of  the  sugar  is  prefixed,  thus  glucosazone,  lactosazone,  maltosazone.  One 
molecule  of  the  reagent  unites  with  one  molecule  of  a  sugar  to  form  a 
normal  hydrazone,  water  being  eliminated;  but  if  the  phenylhydrazine  be 
in  excess,  it  reacts  with  the  hydrazone  to  form  an  osazone,  e.  g.,  glucosazone, 
CH2.OH(CH.OH)3.C.CH  :  N2.(NH. €4^)2.  The  melting  points  of  the  products 
of  the  various  sugars  differ  considerably,  ranging  from  160°  to  205°. 

The  sugar  solution  is  moderately  heated  with  an  excess  of  an  acetic  acid 
solution  of  phenylhydrazine,  the  compound  falling  as  a  crystalline  precipitate, 
but  slightly  soluble  in  water  though  readily  in  alcohol.  After  washing  with 
water  the  precipitate  is  dried  and  weighed.  The  results  are  rather  low  from 
the  solubility  of  the  precipitate. 

Carbohydrates,  with  the  exception  of  the  pentoses,  when  macerated  and  dis- 
tilled with  a  dilute  mineral  acid,  develop  furfurol,  a  volatile  aldehyd  of  the 
composition  C^sO.COH.  The  sugar  solution  is  mixed  with  dilute  hydrochloric 
acid  and  distilled  until  all  of  the  furfurol  produced  has  come  over  (accompanied 
by  weak  acid)  as  shown  by  a  test  with  sodium  acetate  which  strikes  with  it  a 
red  color.  The  furfurol  may  then  be  precipitated  by  stirring  with  phenylhy- 
drazine, the  two  reacting  to  form  a  crystalline  precipitate  of  furfurolhydrazone, 
The  precipitate  may  be  dried  at  60°  and  weighed,  or  dissolved  in  alcohol,  the 
solution  evaporated  to  dryness,  dried  and  weighed.* 

Or  the  furfurol  may  be  titrated  by  the  same  reagent  in  standard  solution, 
using  anilin  acetate  for  an  indicator. 

Phlorglucin  produces  with  furfurol  in  acid  solution  a  cherry-red  color,  the 
liquid  showing  a  characteristic  absorption  band  in  the  spectrum. f  After  a 
short  time  a  brown  precipitate  falls  which  has  the  composition  CieH^Oe.  The 
precipitate  can  be  collected,  washed  with  water,  dried  and  weighed.  Since  it 
is  somewhat  soluble  in  water  a  correction  is  made  for  the  amount  retained  in 
solution. 

Some  varieties  of  sugar  form  definite  compounds  with  certain  metals,  not- 
ably lead  and  those  of  the  earths.  The  solubility  of  such  precipitates,  however, 
prevents  any  extended  application  of  these  reactions  to  quantitative  work 
though  a  separation  from  other  organic  bodies  may  sometimes  be  accomplished 
with  passable  results.  The  sugar  may  be  regenerated  by  suspending  the  pre- 
cipitate in  water  and  passing  a  current  of  hydrogen  sulfide  or  carbon  dioxide 
to  precipitate  the  base. 

Inversion.  A  notable  property  of  sucrose  and  some  other  sugars  is  that  of 
becoming  hydrolyzed  or  « inverted '  by  the  action  of  dilute  acids  or  other  re- 
agents. The  inversion  is  far  more  prompt  on  heating,  and  results  in  the 
division  of  the  sucrose  into  a  mixture  of  equal  parts  of  dextrose  and  levulose, 
thus 

C12H22On  (sucrose)  -|-  H2O  =  C6H12O6  (dextrose)  -f  C6H12O6  (levulose). 
342  parts  of  sucrose  giving  360  parts  of  the  mixture  called  'invertose.'  The 
levulose  of  the  mixture  however  differs  somewhat  in  its  properties  from  levu- 
lose derived  from  natural  sources.  The  inversion  of  milk  sugar  produces  dex- 
trose and  galactose ;  of  maltose  gives  sucro-dextrose ;  and  of  melitose  gives 
dextrose  and  eucalyn. 

The  acid  takes  no  direct  part  in  the  reaction,  acting  only  as  an  excitant. 
Since  at  ordinary  temperatures  levulose  has  the  greater  rotatory  power,  the  in- 


*  Journ.  Anal.  Chem.  1893—190. 
t  Berichte,  18%— 1202. 


THE    CARBOHYDRATES.  435 

vertose  is  laevo-rotatory,  but  as  the  temperature  is  increased  the  rotations 
become  more  nearly  equal  until  at  about  88  o  Cent,  they  are  identical  and  the 
invertose  shows  no  action  toward  polarized  light. 

Inversion  is  a  great  aid  in  sugar  analysis  since  it  supplies  two  definite,  cor- 
related values  and  gives  data  for  the  elimination  of  the  effect  of  certain  optic- 
ally active  associated  bodies  whose  rotation  is  not  affected  by  heating  with 
acid.  The  same  applies  to  determinations  by  means  of  Fehlings  solution  on 
which  sucrose  has  no  action. 

The  reaction  given  above  expresses  essentially  the  change  wrought  in  inver- 
sion, but  secondary  reactions  always  occur.  For  this  reason  it  is  necessary  to 
follow  a  certain  routine  to  obtain  corresponding  results  in  analysis.  The 
strength  of  the  sugar  solution,  time  and  temperature  of  heating,  and  other 
factors  modify  the  yield  of  invertose  to  a  considerable  extent.  The  directions 
of  Clerget  are  substantially  as  follows. 

The  normal  weight  of  sugar  used  for  the  saccharimeter  reading  (here  16.471 
grams)  is  dissolved  and  made  up  with  water  to  100  Cc.,  then  with  strong  hydro- 
chloric acid  to  HOCc.  The  liquid  is  heated  to  68°  Cent.,  so  adjusting  the 
source  of  heat  that  the  thermometer  shall  attain  this  temperature  in  fifteen 
minutes.  The  liquid  is  then  cooled  rapidly  to  the  temperature  of  the  room, 
filtered  if  necessary,  and  polarized,  adding  ten  per  cent  to  the  reading  to  com- 
pensate for  the  volume  of  acid.  The  percentage  of  sucrose  originally  present 
is  found  from  the  proportion  S  :  100  :  :  the  algebraic  difference  between  the  two 
readings  :  the  theoretical  difference  —  .5  t;  here  8  is  the  percentage  of  su- 
crose; t° ,  the  temperature  of  the  polarized  liquid;  and  the  theoretical  differ- 
ence is  taken  to  be  either  144°  or  more  usually  142.4  o .  These  figures  are 
derived  from  the  fact  that  a  normal  weight  of  pure  sucrose  in  the  standard 
volume  of  water  reads  on  the  sugar-scale  saccharimeter  100°  to  the  right,  and 
after  inversion,  44®  or  42.4  o  (according  to  conditions)  to  the  left;  both  at  a 
temperature  of  zero. 

The  same  method  is  pursued  when  the  invertose  is  to  be  determined  by 
means  of  Fehlings  solution,  neutralizing  the  acid  before  the  test. 

Although  dilute  hydrochloric  acid  is  the  usual  hydrolyzing  agent  some  prefer 
zinc  with  hydrochloric  acid,  stannous  chloride,  etc.  The  action  of  yeast  is 
slower  than  that  of  acid  but  it  has  some  advantages.  O'Sullivan*  directs  that 
with  50  Cc.  of  the  neutral  syrup  there  be  incorporated  of  brewers  yeast  about 
one-tenth  of  the  weight  of  the  sugar,  and  the  mixture  heated  on  the  water- 
bath  for  four  hours;  then  cooled  to  15.5°,  clarified  by  alumina,  and  diluted  to 
100  Cc.  A  portion  of  the  solution  is  polarized  at  once,  and  another  portion 
after  several  hours  have  elapsed  to  ascertain  if  the  conversion  was  complete 
in  the  first  instance.  Invertase  is  in  some  respects  superior  for  the  purpose  to 
yeast. 

Sucrose  is  also  slowly  inverted  by  many  organic  and  inorganic  salts.    The 

rate  of  inversion  may  be  expressed  by  the  formula  K=  —  log.  — _ —   where  A 

t  a.  —X 

is  the  amount  of  sugar  originally  present;  x  that  inverted  up  to  any  time  t  as 
measured  by  the  difference  in  rotation ;  and  K,  the  constant  or  coefficient  of 
inversion.!  

Separation  of  sugars  and  impurities.  1.  By  lixiviation.  Cane  sugars  and 
the  better  grades  of  beet  sugars  may  be  assayed  on  the  principle  that  sucrose 


*  Journ.  Chem.  Socy.  57—834. 

t  Journ.  Amer.  Chem.  Socy.  1896—120  and  693. 


436  QUANTITATIVE    CHEMICAL    ANALYSIS. 

is  completely  insoluble  in  a  mixture  of  anhydrous  alcohol  and  ether,  and  in 
dilute  alcohol  that  has  been  previously  saturated  with  sucrose.  Four  solutions 
are  prepared:  (A)  alcohol  of  80  per  cent,  containing  five  per  cent  of  acetic 
acid:  (B)  alcohol  of  88  per  cent;  (C)  alcohol  of  94  percent;  and  (D)  absolute 
alcohol  mixed  with  half  its  volume  of  anhydrous  ether.  The  first  three  are 
saturated  with  sucrose  by  digestion  over  the  crystals  for  several  days  at  the 
normal  temperature  of  the  laboratory. 

A  certain  weight  of  the  dried  sugar  is  placed  in  a  long  tube  similar  to  a 
Mohr's  burette,  and  percolated  successively  with  the  four  solutions,  avoiding 
any  decrease  or  increase  in  the  temperature  of  the  first  three  which  might 
precipitate  some  of  the  sucrose  they  hold,  or  cause  them  to  dissolve  some  of 
the  sucrose  of  the  sample.  The  pure  crystallized  sugar  remaining  is  then 
dried  and  weighed,  or  dissolved  and  polarized. 

Casamajor,  reversing  this  process,  determines  any  starch  sugar  that  may 
be  in  commercial  cane  sugar  by  extracting  the  dried  sample  several  times  with 
a  saturated  solution  of  starch-sugar  in  methyl  alcohol.  The  residual  glucose 
is  washed  rapidly  with  pure  methyl  alcohol,  dried  and  weighed. 

2.  If  to  a  mixture  of  sucrose  and  a  reducing  sugar  there  be  added  a  limited 
proportion  of  lead  protoxide,  the  reducing  sugar  preferentially  unites  with  it, 
and  where  the  proportions  of  the  two  sugars  are  approximately  known  a  fair 
separation  is  possible. 

To  a  mixture  of  sucrose,  invertose,  and  dextrose  or  levulose,  Winter  adds 
ammoniacal  lead  acetate  and  dilutes  largely.  All  three  sugars  form  compounds 
with  lead  oxide,  but  only  the  sucrose-lead  compound  is  freely  soluble  and 
passes  into  the  filtrate;  the  sucrose  can  be  regenerated  by  transmitting  a  cur- 
rent of  carbon  dioxide  which  precipitates  the  lead  as  carbonate  and  leaves  the 
sucrose  in  solution  to  be  determined  by  the  usual  methods.  The  precipitate  of 
the  lead  compounds  of  invertose  and  dextrose  or  levulose  is  suspended  in 
water  and  treated  with  carbon  dioxide  which  decomposes  the  dextrose-lead 
compound  only,  giving  on  filtration  a  solution  of  dextrose  and  a  residue  of  lead 
carbonate  plus  the  lead-levulose  compound.  The  latter  is  suspended  in  water 
and  a  current  of  hydrogen  sulfide  passed,  when  all  the  lead  becomes  sulflde 
while  the  levulose  enters  into  solution.  The  separation  is  only  fair  at  best. 

3.  But  in  most  cases  no  direct  separation  is  attempted.    Indirect  methods 
are  based  largely  on  calculations  from  the  variation  of  the  constants  of  polar- 
ization, reducing  power  toward  metallic  salts,  etc.,  determined  on  the  sample 
intact  and  also  after  some  physical  or  chemical  change  has  been  wrought.    A 
few  examples  are  appended. 

A.  Given  a  mixture "  of  only  sucrose  and  optically  inactive  matter.    A  sac- 
charimeter  weight  (26.048  or  16.471  grams)  is  dissolved  and  polarized,  read- 
ing a  divisions  on  the  sugar-scale,  the  percentage  of  sucrose. 

If  another  equal  weight  were  dissolved,  inverted  by  acid,  and  polarized, 
the  reading  would  be  6  divisions;  were  the  sample  entirely  sucrose,  a  would 
be  100°,  and  6,  —  42.4  o;  that  is,  the  divergence  would  be  K=U2A°  at 
zero  Cent.,  and  at  any  higher  temperature  t° ,  would  be  K—.5  t.  But  as 
sucrose  formed  only  part  of  the  sample,  the  percentage  shown  directly  by  a, 
b  lessens  as  the  ratio  of  a  to  100  diminishes. 

B.  A    mixture   of  sucrose  and  dextrose.    Proceeding  as  above,  the  per- 
centage of  sucrose  8  =  10°  ^~~ 6)  and  of  dextrose  J=   100c 

XL  o.Ooo  A. 

In  this  case  the  reading  a  is  the  sum  of  the  (right-handed)  rotations  of  the 
two  sugars,  and  after  inversion,  the  reading  b  is  the  left-handed  rotation  of 
invertose  diminished  by  the  right-handed  of  the  dextrose;  in  other  words, 


THE    CARBOHYDRATES.  437 

each  reading  is  affected  by  the  same  quantity  c,  the  rotation  due  to  dextrose. 
Hence  if  the  percentage  of  sucrose  in  the  mixture  by  represented  by  S;  the 
reading  before  inversion  by  a  and  after  inversion  by  b;  the  rotatory  power  of 
the  dextrose  by  c;  and  the  divergence  of  sucrose  and  invertose  by  K;  then 

190  (a  —  6") 
8  :  100  :  :  (a  — c)  —  (6  — c)  :  K;  whence  S  =  ^ -• 

The  first  polarization  minus  the  percentage  of  sucrose  equals  the  rotatory 
power  of  the  dextrose  expressed  in  degrees  of  the  sugar-scale,  that  is,  a  — 
S  =  c,  for  if  the  sample  contained  100  per  cent  of  sucrose  the  rotation  would 
be  100,  hence  a  sample  containing  S  per  cent  will  read  S  divisions. 

One  gram  of  dextrose  is  equivalent  to  3.055°  on  the  sugar-scale,  therefore 
the  weight  of  dextrose  d  for  a  deviation  c  is  given  by  the  proportion  3.055  :  c : : 
1  :  d;  and  since  the  weight  A  of  the  sample  was  polarized,  the  percentage 
of  dextrose  D  is  found  from  the  proportion  A  :  d  : :  100  :  D;  whence  D  = 

100       y  C 

~A        3.055* 

The  same  method  can  be  applied  to  mixtures  of  sucrose  with  other  sugars 
whose  gyrodynat  is  not  changed  in  the  process  of  inversion. 

C.  A  mixture  of  sucrose  and  invertose  may  be  analyzed  by  polarizing  at  a 
moderate  temperature  and  again. at  87.6°  at  which  point  invertose  has  no  rotat- 
ory power.    The  calculation    is  simple.    Or  the  mixture  may  be  boiled  with 
Fehlings    solution  which  is  decomposed  by  the  invertose  only,  then  another 
portion  inverted,  neutralized,  and  determined  as  before ;  the  difference  in  the 
weights  of  cuprous  oxide  is  that  corresponding  to  the  invertose  from  the 
sucrose. 

D.  For  a  mixture  of  dextrose  and  maltose,  Sieben  determines  the  total  re- 
ducing power  of  the  mixture  toward  Fehlings  solution,  then   the  reducing 
power  toward  an  acetic  acid  solution  of  cupric  acetate  from  which  only  dex- 
trose   precipitates    cuprous    oxide,   maltose    not   being  decomposed   by  this 
reagent. 

E.  If  a  mixture  of  cane  and  milk  sugars  be  boiled  with  a  two  per  cent  solu- 
tion of  citric  acid,  only  the  former  will  be  inverted ;  another  reagent  for  the 
purpose  is  benzoic  sulflnide. 

F.  For  sucrose,  dextrose  and  levulose,  Sieben  has  proposed  a  method  based 
on  these  principles :  sucrose  does  not  precipitate  Fehlings  solution  as  do  dex- 
trose and  levulose;  sucrose  is  converted  into  equal  parts  of  dextrose  and 
levulose  by  inversion;  and  levulose  is  destroyed  on  heating  with  hydrochloric 
acid  of  a  strength  that  will  invert  sucrose.    In  practice  the  dextrose  plus  levu- 
lose are  first  determined  by  Fehlings  solution ;  then  the  sucrose  is  inverted 
and  the  total  reducing  sugars  found  in  the  same  way,  and  also  by  polarization ; 
and  finally,  the  levulose  is  destroyed  by  long  heating  with  hydrochloric  acid  of 
a  particular  strength,  and  the  remaining  dextrose  is  determined  by  Fehlings 
solution.    These  data  are  sufficient  for  computing  the  proportions  of  each 
sugar,  but  that  the  levulose  can  be  eliminated  without  some  change  taking 
place  in  the  dextrose  is  very  questionable.* 

G.  The  determinations  of  a  mixture  of  dextrose  and  invertose  may  be  accom- 
plished by  calculation  from  the  unlike  action  of  Fehlings  and  Sacchses  solutions 
(page  433)  on  these  sugars.    If  a  represents  the  volume  of  Fehlings  solution 
reduced  by  one  gram  of  dextrose ;  a',  that  reduced  by  one  gram  of  invertose ; 
and  d,  that  reduced  by  a  given  volume  of  the  solution  of  the  two  sugars ;  and  if 
6,  6',  and  d'  represent  the  corresponding  figures  for  Sacchses  solution ;  and  x 


*  Journ.  Anal.  Appl.  Chem.  5—401. 


438  QUANTITATIVE    CHEMICAL    ANALYSIS. 

and  y  the  weights  of  dextrose  and  invertose  respectively  in  the  given  volume 
of    the    sugar    solution:    then  d  =  a  x  -f-  a'  y,   and    df  =b  x  -f  b'y;  whence 

cc=  a'd  ~  b'd 
a'b  —  a'b 


The  analysis  of  a  commercial  raw  or  refined  sugar  is  made  on  the  following 
lines. 

1.  Determination  of  water.    On  account  of  the  ready  decomposability  of  the 
sugars  the  heat  is  limited  to  75  o  or  80  °  .    In  the  case  of  syrups  it  is  better 
to  soak  up  a  suitable  weight  of  the  sample,  diluted  if  quite  viscous,  in  a  porous 
medium  such  as  sand,  blotting  paper,  or  the  like. 

At  best  this  conventional  method  is  tedious  and  uncertain  from  the  unstable 
and  easily  oxidizable  nature  of  sugar,  and  it  has  been  found  by  tests  on  pure 
sugar,  weighed,  moistened  and  dried,  that  desiccation  in  vacuo  leaves  much 
more  nearly  the  original  weight  of  the  sugar  than  if  under  atmospheric  pres- 
sure. Thorne  and  Jeffers*  describe  an  apparatus  In  which  the  weighed  sugar 
is  distributed  in  a  coil  of  filter  paper  by  means  of  a  little  water;  the  roll  is 
dried  in  a  slow  current  of  highly  rarifled  dry  carbon  dioxide  at  a  heat  of  from 
650  to  700  furnished  by  a  bath  of  the  vapor  of  boiling  methyl  alcohol,  About 
six  to  ten  hours  drying  is  needed.  They  find  that  both  the  prescribed  temper- 
ature and  the  atmosphere  of  rarifled  carbon  dioxide  are  essential  to  correct 
results. 

2.  Determination  of  the  inorganic  constituents.    These  are  principally  the 
fixed  alkalies  and  earths  combined  as  organic  salts,  but  may  also  be  in  part 
sand,  clay,  etc.    On  incineration  of  the  sugar  there  are  formed  at  first  caramel 
and  like  bodies ;  at  a  higher  heat  a  carbonaceous  mass  remains,  this  finally 
burning  to  an  ash  composed  largely  of  carbonates  of  the  alkalies  resulting  from 
the  decomposition  of  the  organic  salts.    Simple  incineration  of  the  sugar  is  apt 
to  be  tedious  for  the  reason  that  a  comparatively  low  heat  must  not  be  exceeded 
for  fear  of  loss  of  some  of  the  bases  by  volatilization.    A  common  practice  is  to 
moisten  the  char  with  sulf  uric  acid,  dry  and  ignite  in  air  whereupon  the  carbon 
readily  burns;  there  are  left  sulfates  of  the  bases,  and  a  conventional  deduc- 
tion of  ten  per  cent  of  its  weight  is  made  for  the  sulfate  radical.    A  more 
rational  procedure  is  to  report  the  result  as  so  much  •  sulfated  ash.'f 

The  char  burns  more  readily  when  the  sugar  has  been  dissolved  in  a  little 
water  and  the  solution  imbibed  in  a  weighed  quantity  of  clean  sand.  Cour- 
tonne  recommends  ferric  oxide  for  the  purpose,  but  the  liability  of  reduction 
to  a  lower  oxide  on  ignition  with  carbon  is  against  the  use  of  this  compound. 
To  render  the  char  porous,  Boyer  heats  the  sugar  to  carmelization  in  a  platinum 
capsule,  adds  a  solution  of  benzoic  acid  in  alcohol,  and  ignites;  the  vapors 
from  the  decomposition  of  the  acid  cause  the  mass  to  be  spongy  and  easily 
burned. 

Laugier  attempts  the  reproduction  of  the  compounds  as  they  exist  in  the 
sugar.  The  sample  is  treated  with  dilute  sulfuric  acid  which  discomposes  the 
organic  salts  and  sets  free  the  organic  acids  thereof,  and  these  are  extracted 
from  the  syrup  by  ether.  Another  equal  weight  of  the  sample  is  burned  to  an 
ash,  on  this  poured  the  ethereal  solution,  evaporated  to  dryness,  and  weighed. 
The  organic  acids  decompose  the  carbonates  of  the  ash. 

An  analysis  of  the  ash  is  not  often  asked  for;  it  is  made  by  the  usual 
methods  of  inorganic  analysis. 


*  Journ.  Socy.  Chem.  Ind  1898—114. 
f  School  of  Mines  Quart.  Vol.  2,  No.  1. 


THE    CARBOHYDRATES.  439 

3.  Organic  acids.  These  are  set  free  on  treating  the  concentrated  solution  of 
the  sugar  with  dilute  sulfuric  acid,  and  may  be  extracted  by  ether,  the  ethereal 
solution  mixed  with  a  little  water,  and  titrated  by  weak  standard  alkali. 

4.  The  insoluble  matter,  usually  clay  or  sand,  is  determined  by  dissolving 
the  sample   in  water,  filtering,  washing  with  water,  drying  the  residue  and 
weighing. 

5.  Free  acid  in  appreciable  quantity  is  rarely  found ;  it  may  be  determined 
by  direct  titration  by  weak  standard  alkali  and  litmus. 

6.  The  determination  of  the  sucrose  and  invert  sugar  is  made  either  by  the 
polariscope  or  Fehlings  solution  as  before  described,'  first  in    the  aqueous 
solution,  then    after    inversion,  and  the  results    calculated    from  the    usual 
formulae. 


Commercial  starch-sugar  or  glucose  is  a  complex  mixture  composed  mainly 
of  dextrose  and  maltose,  with  dextrin,  unfermentable  carbohydrates,  and  inor- 
ganic matter  including  a  trace  of  sulfuric  acid.  It  is  extensively  manufactured 
by  hydrolyzing  corn  starch  or  potato  starch  by  dilute  sulfuric  acid,  removing 
the  acid  as  calcium  sulfate  by  means  of  chalk,  and  evaporating  to  a  syrup  or  to 
crystallization.  There  are  found  in  the  market  two  forms,  one  a  thick  color- 
less syrup  called  "corn  syrup"  or  " confectioners  glucose"  of  a  specific 
gravity  of  about  1.4  and  containing  about  23  per  cent  of  water;  the  other  a 
white  granular  solid,  '*  grape  sugar."  Both  kinds  are  largely  used  in  the  prep- 
aration of  the  cheaper  grades  of  confectionery,  syrups,  preserves,  etc.  The 
crystallized  glucose  is  mainly  dextrose,  and  is  less  sweet  than  cane  sugar,  the 
ratio  being  about  1  to  1.53. 

Water  is  determined  in  the  usual  way  by  drying  at  100  °  ;  the  last  traces  are 
removed  by  moistening  with  absolute  alcohol  and  redrying.  Corn  syrup  is 
best  dehydrated  by  diluting  with  weak  alcohol,  imbibing  the  liquid  in  a  weighed 
quantity  of  sand,  and  drying,  finally  moistening  with  strong  alcohol  and  re- 
drying. 

The  specific  gravity  observation  presents  some  difilculties  from  the  viscous 
nature  of  the  syrup.  A  fairly  accurate  method  is  that  of  diluting  a  weighed 
quantity  with  water  to  a  known  volume  and  observing  the  gravity  of  the  solu- 
tion. 

Inorganic  matter,  chiefly  calcium  sulfate,  generally  runs  below  one  per  cent, 
and  is  left  on  burning  in  a  platinum  dish.  To  prevent  the  inconvenient  swell- 
ing up  of  the  mass  on  carbonizing,  the  syrup  or  concentrated  solution  may  be 
dropped  into  a  red  hot  platinum  dish  (page  105).  The  presence  of  calcium  sul- 
fate or  other  calcium  compound  in  the  ash  differentiates  commercial  starch- 
sugar  from  the  sugar  from  natural  sources. 

The  determination  of  the  other  constituents,  with  perhaps  sucrose  added 
during  manufacture  to  augment  the  sweetness  of  the  product,  cannot  be  made 
with  any  assurance  that  the  results  are  more  than  fair  approximations.  It  is 
said  that  in  samples  of  commercial  glucose  the  relation  between  the  optically 
active  and  copper-reducing  constituents  is  a  constant  and  may  serve  to  indicate 
the  nature  of  the  saccharine  bodies  present. 

Possibly  the  most  satisfactory  method  is  to  dissolve  the  sample  in  a  little 
water  and  precipitate  the  dextrin  by  strong  alcohol,  then  decant  the  solution 
and  weigh  the  dextrin  and  ash.  On  distilling  the  alcohol  there  is  left  an  aqueous 
solution  of  dextrose,  maltose,  carbohydrates  and  possibly  sucrose ;  it  is  diluted 
with  water  and  five  aliquot  parts  withdrawn. 

The  first  is  fermented  by  yeast  to  destroy  all  but  the  unfermentable  carbo  - 
hydrates  and  these  determined  by  the  polariscope ;  the  second  is  boiled  with 


440  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Fehlings  solution,  the  cuprous  oxide  precipitated  corresponding  to  the  dex- 
trose and  maltose ;  the  third  is  inverted  by  acid  and  boiled  with  Fehlings 
solution,  the  increase  in  weight  of  the  cuprous  oxide  over  the  preceding 
coming  from  the  inverted  sucrose;  the  fourth  is  polarized  directly;  and  the 
fifth  after  inversion.  Knowing  the  specific  rotation  of  each  sugar,  the  pro- 
portions of  dextrose  and  maltose  can  be  deduced  by  a  somewhat  complicated 
calculation. 

Another  method  determines  the  rotary  power  [a]o  and  the  reducing  power 
toward  cupric  tartrate,  K;  also  the  specific  gravity  and  the  ash,  from  which 
data  may  be  calculated  the  total  organic  matter  8.  Then  the  percentages  of 
maltose  M,  of  dextrose  D,  and  of  dextrin  d  are  found  from  the  formulae 

[a]D-f  1.421T— 195  S.K 

Af=S.  -  27>2 ;     -0  =  100-  —  .61;  andd  =  #—  (M+D).     A  cor- 

rection  is  to  be  made  for  the  action  of  the  acid  on  the  dextrin. 

The  total  nitrogen  is  ascertained  by  the  Will-Varrentrapp  or  the  Kjeldahl 
method  or  byNessler's  test.  The  product  of  the  result  by  the  factor  6.25  is  the 
albuminoid  matter. 

Free  acid  should  not  exceed  traces,  found  by  titration  by  weak  standard 
alkali. 


Expressed  honey  is  essentially  a  clear  concentrated  solution  of  various 
sugars,  principally  dextrose  and  levulose  or  invert  sugar,  together  with  the 
saccharoid  mannite  (a  hexatomic  alcohol),  and  small  quantities  of  wax,  min- 
eral matter,  organic  acids,  etc.  The  percentage  of  water  ranges  from  15  to  25. 
It  is  said  that  the  honey  derived  from  flowers  is  laevo-gyrate,  that  from  conifers 
dextro- gyrate,  and  from  both,  either  indifferent  or  weakly  right  or  left  handed 
as  may  be. 

The  principal  adulteration  is  by  admixture  of  starch  sugar,  though  an  entirely 
factitous  article  is  said  to  have  been  manufactured  from  flavored  and  slightly 
tinted  glucose,  inclosed  in  cells  of  paraffin.  The  analysis  follows  the  lines  of 
that  for  starch-sugar,  the  principal  tests  being  designed  to  detect  adulteration 
with  that  body. 

The  ash  in  genuine  honey  should  not  exceed  5  per  cent,  and  if  over  3  per  cent 
should  be  tested  for  calcium  sulfate,  absent  from  genuine  honey  but  an 
almost  invariable  concomitant  of  glucose.  A  factitious  honey  made  up  of 
dextrose  and  levulose  has  been  found  on  the  market;  as  stated  by  Hehner  there 
was  no  phosphoric  acid  in  the  ash,  an  invariable  constituent  of  the  ash  of  gen- 
uine honey. 

The  matter  insoluble  in  cold  water  is  tested  by  iodine  for  starch,  a  blue  color 
pointing  to  the  presence  of  flour.  After  fermentation  with  yeast,  the  residual 
unfermentable  matter,  principally  carbohydrates,  should  not  exceed  8  per  cent. 
Dextrin  from  added  starch-sugar  may  be  precipitated  by  strong  alcohol,  or  the 
honey  may  be  fermented  and  the  dextrin  and  carbohydrates  polarized,  then 
heated  with  dilute  acid  and  the  latte/  found  by  Fehlings  solution. 

STARCH. 

The  starches  are  the  major  constituent  of  cereals,  forming  over  half  their 
weight.  A  microscopic  examination  of  the  starch  from  different  plants  reveals 
marked  peculiarities  of  structure,  and  the  size  of  the  granules  (from  .02  to  .10 
Mm.  in  diameter)  and  their  configuration,  the  position  and  shape  of  the  hilum, 
and  the  appearance  under  polarized  light  in  conjunction  with  a  selenite  plate 


THE    CARBOHYDRATES.  441 

often  afford  a  clue  to  the  origin  of  the  sample.  Muter  *  divides  the  starches 
into  five  groups  differentiated  by  their  microscopic  structure,  the  examination 
made  by  a  .4- inch  objective  and  B  eye-piece,  water  immersion  and  oblique  il- 
lumination; viz.:  (1),  the  potato  group;  (2),  the  leguminous  starches;  (3),  the 
wheat  group;  (4),  the  sago  group;  and  (5),  the  rice  group. 

To  starch  is  assigned  the  empirical  formula  (CeHioO5)n  ;  the  molecular  weight 
is  undoubtedly  very  high.  It  is  composed  of  two  allied  bodies,  granulose,  col- 
ored blue  by  iodine,  and  pseudo- cellulose,  colored  pale  yellow  by  this  reagent; 
the  two  may  be  separated  by  dilute  chromic  acid  which  dissolves  the  former 
only.  The  blue  color  struck  with  iodine  by  a  cold  acid  solution  of  granulose  is 
a  delicate  and  characteristic  test  of  starch ;  the  formula  of  the  compound  is 
said  to  be  (C24H4oO2oI)4.HI. 

The  starch  of  commerce  is  a  white,  tasteless  and  odorless  powder  agglu- 
tinated in  the  form  of  irregular  fragments,  containing  from  16  to  28  per  cent  of 
water,  and  a  little  fat,  and  mineral  and  nitrogenous  matters.  It  is  insoluble  in 
alcohol  and  ether  and  in  cold  water,  but  in  water  heated  to  above  60  °  the 
granules  swell  and  burst,  and  a  perfectly  colloidal  solution  results  which  is 
highly  dextro-rotatory  (190  o  to  200  o  ) ,  but  has  no  reducing  action  on  copper 
salts.  Starch  is  also  soluble  in  hot  glycerol,  and  in  cold  hydrochloric  acid  of 
1.2  specific  gravity,  with  some  alteration  however. 

On  heating  starch  to  160°  to  200°  or  for  a  limited  time  with  a  dilute  acid  or 
a  solution  of  invertase,  it  is  converted  into  dextrin  (British  gum).  Dextrin  is 
an  isomer  of  starch  and  is  found  in  the  market  as  a  light  yellow  amorphous 
powder.  It  is  of  a  gummy  nature,  readily  soluble  in  water,  and,  like  starch,  is 
converted  into  oxalic  acid  when  heated  with  nitric  acid,  a  distinction  from 
ordinary  gums  which  form  mucic  acid.  Dextrin  gives  no  blue  color  with 
iodine,  rotates  polarized  light  to  the  right  200.4® ,  does  not  precipitate  metallic 
salts,  and  is  converted  into  dextrose  by  heating  with  a  dilute  acid.  Its  insol- 
ubility in  alcohol  is  applied  in  analysis  as  a  means  of  separation  from  sugars, 
etc.,  usually  by  compounding  the  concentrated  aqueous  solution  with  a  large 
volume  of  alcohol,  the  dextrin  precipitating  as  a  cohesive  mass. 

Separation.  An  approximate  mechanical  separation  from  gluten,  cellulose, 
etc.,  can  be  made  by  washing  the  powdered  material  on  a  closely  woven  sieve 
with  cold  water ;  a  milky  liquid  passes  through  holding  the  starch  granules  in 
suspension  and  leaving  other  insoluble  constituents  on  the  sieve.  The  liquid 
is  allowed  to  settle  and  the  starch  collected  and  determined  in  the  usual 
way. 

Mixtures  of  starch  with  sugars  or  other  soluble  bodies  can  be  parted  by 
lixiviation  with  cold  water. 

Asboth  f  is  the  author  of  a  method  of  separation  which  under  certain  condi- 
tions is  perhaps  the  most  accurate  of  any.  The  basis  is  the  formation  of  a 
compound  of  barium  oxide  and  starch  of  the  formula  (C6H10O5)4.BaO,  soluble 
in  water  but  reprecipitated  by  alcohol.  The  starch  is  obtained  in  solution  by 
heating  with  water,  and  to  the  clear  liquid  there  are  added  a  measured  volume 
(an  excess)  of  a  standard  solution  of  baryta,  and  alcohol  up  to  a  definite  vol- 
ume. When  the  precipitate  has  subsided,  an  aliquot  portion  of  the  clear  liquid 
is  withdrawn  and  the  residual  baryta  determined  by  titration  with  standard 
hydrochloric  acid.  In  the  analysis  of  cereals  the  starch  is  first  freed  from  fatty 
matter  by  ether,  the  residue  rubbed  with  cold  water  and  the  emulsion  poured 
off  and  treated  as  above.  Spence  J  states  that  when  a  volume  of  50  Cc.  is  used 


*  Allen,  Coml.  Org.  Anal.  1-408. 
t  Chem.  Zeit.  1888—693  and  1889—591. 
I  Journ.  Socy.  Chem.  Ind.  1888—77. 


442  QUANTITATIVE    CHEMICAL    ANALYSIS. 

for  one  gram  of  starch  the  baryta  solution  should  not  be  weaker  than  about 
fifth-normal.  From  numerous  criticisms  of  the  method  it  is  probable  that  it 
is  trustworthy  only  for  certain  material  and  in  the  hands  of  practiced  opera- 
tors. 

Amylogen,  the  soluble  starch  produced  by  heating  the  commercial  variety  in 
a  closed  vessel  to  100°,  is  precipitated  from  its  aqueous  solution  by  alcohol, 
ammonium  lead  acetate  (as  Ci2HisPb2Ou),  and  several  other  reagents.  Burk- 
hardt  states  that  if  alcohol  be  added  to  a  solution  until  faint  turbidity  ensues 
and  the  mixture  warmed  and  treated  with  tannic  acid,  all  the  starch  will  sep- 
arate on  cooling  as  a  flocculent  starch-tannic-acid  compound ;  the  acid  can  be 
removed  by  washing  the  precipitate  with  alcohol. 

All  the  methods  for  the  determination  of  starch  in  mixtures  without  its  sep- 
aration depend  on  the  principle  that  starch  is  converted  into  dextrose  by  the 
action  of  certain  excitants.  However,  this  conversion  never  affords  the 
theoretical  amount  of  dextrose  but  only  93  to  97  per  cent,  the  remainder  being 
bodies  of  the  nature  of  dextrin.  Concerning  the  latter  it  is  claimed  that  but 
three  simple  carbohydrates,  possibly  in  molecular  aggregates,  exist  in  the 
solution  of  a  starch  product  hydrolized  by  acids. 

Rolfe  and  Defren  *  from  a  study  of  the  hydrolysis  of  starch  deduce  that  the 
first  change  is  to  amylo-dextrin  (CseH^Osi.H^O),  then  by  successive  stages 
through  malto-dextrin,  maltose  and  dextrose,  ultimately  to  dextrose  entirely. 
As  the  conversion  proceeds  the  rotatory  power  of  the  product  diminishes; 
thus,  taking  no  account  of  reversion  products,  at  195°  of  the  polariscope  the 
dextrin  is  100  per  cent  and  the  maltose  and  dextrose  none;  at  129  °  the  dextrin 
has  decreased  to  27.5  per  cent,  dextrose  has  been  formed  to  the  extent  of 
28.4  per  cent,  and  maltose  to  44.1  percent,  its  maximum;  and  at  53.5°,  both 
dextrin  and  maltose  have  disappeared,  the  dextrose  becoming  100  per  cent. 

The  usual  amylolytic  agent  is  dilute  hydrochloric  acid.  The  starch-bearing 
substance  is  simply  boiled  with  a  suitable  amount  of  the  acid,  filtered  from 
insoluble  matter,  and  the  dextrose  determined  by  Fehlings  solution.  But  in 
some  cases  the  results  may  be  highly  erroneous  since  starch-free  bodies  — 
e.  g.t  the  sheath  of  the  kernels  of  maize  —  when  treated  as  above  yield 
copious  precipitates  of  cuprous  oxide.  The  same  holds  where  other  acids, 
as  nitric,  oxalic,  or  salicylic,  are  substituted  for  hydrochloric;  in  fact,  the 
investigations  of  Stone  show  that  the  pentosan  that  occurs  in  all  feed-stuffs 
behaves  exactly  as  does  starch  in  any  of  the  methods  of  inversion  by  acids, 
and  in  Asboth's  precipitation  method  as  well. 

Starch  is  hydrolyzed  when  heated  with  water  for  several  hours  at  a  pressure 
above  atmospheric.  Should  sugar  be  present,  a  trace  of  tartaric  or  citric 
acid  is  added  to  prevent  its  decomposition. 

A  method  in  common  use  is  based  on  the  activity  of  invertase,  a  ferment 
which  has  no  effect  on  the  pentosans,  a  property  often  of  great  advantage. 
As  the  preparation  of  invertase  itself  is  a  rather  tedious  process  and  the 
product  loses  its  power  on  keeping,  a  freshly  prepared  aqueous  extract  of 
malt  is  usually  substituted,  answering  the  purpose  though  subject  to  a  cor- 
rection for  the  starch  and  sugar  it  contains ;  the  extract  is  made  by  steeping 
ground  malt  in  water  and  filtering.  Maercker's  revised  methodf  directs  heating 
the  amyliferous  body  with  water,  and  after  cooling  somewhat,  with  a  little  of 
the  malt  extract.  The  mixture  is  acidified  by  tartaric  acid  and  heated  under  a 
pressure  of  several  atmospheres  in  an  autoclave,  then  cooled  and  filtered  from 


*  Technology  Quarterly,  1897— Mar. 
t  Zeits.  anal.  21—617. 


THE    CARBOHYDRATES.  443 

cellulose,  etc.  In  the  filtrate  the  dextrin  and  maltose  are  converted  to  dextrose 
by  boiling  with  dilute  hydrochloric  acid.  The  malt  extract  is  treated  in  the 
same  way  to  ascertain  the  proper  deduction  for  its  starch  and  sugar.  Any  fat 
contained  in  the  sample  is  previously  removed  by  extraction  by  ether. 

Later  methods  omit  the  heating  in  an  autoclave.  Hibbard*  has  devised  a 
method  similar  to  the  above  especially  adapted  to  fodder,  cattle  foods,  and  the 
like.  He  prepares  an  extract  by  soaking  malt  in  water  containing  from  15  to  20 
per  cent  of  alcohol  for  a  preservative.  The  powdered  substance  is  compounded 
with  water  and  a  little  of  the  extract,  and  the  mixture  heated  to  boiling,  then 
cooled  somewhat,  more  extract  added,  and  again  boiled.  After  cooling,  the 
liquid  is  tested  by  iodine  solution  to  detect  unconverted  starch;  if  found, 
the  above  treatment  is  repeated.  The  solution  is  now  filtered  through  fine 
muslin  and  an  aliquot  part  boiled  with  a  small  volume  of  hydrochloric  acid  in 
a  narrow-necked  flask.  The  solution  is  cooled,  nearly  neutralized  by  sodium 
hydrate,  and  the  dextrose  determined  by  Fehlings  solution  with  a  correction 
for  the  malt  extract  used. 

It  is  said  that  at  any  period  in  the  conversion  of  starch  by  diastase  the 
product  behaves  as  a  mixture  simply  of  maltose  and  dextrin,  and  that  the 
rotatory  power  bears  a  constant  ratio  to  the  cupric  reducing  power,  so  that  one 
can  be  calculated  from  the  other,  f  Wein's  table,  revised  by  Krug,  for  the 
weight  of  starch  corresponding  to  different  weights  of  copper  oxide  from 
Fehling's  test  will  be  found  in  the  Journ.  Amer.  Chem.  Socy.  1897 — 452. 

Various  other  ferments  induce  hydrolysis,  such  as  amylopsin  (contained  in 
pancreatic  juice)  and  taka- diastase.  ChittendenJ  has  obtained  good  results 
with  neutralized  human  saliva,  which  contains  the  ferment  ptyalin,  followed  by 
dilute  hydrochloric  acid;  an  advantage  over  malt  extract  is  that  there  is 
needed  no  correction  for  starch  and  sugar  contained. 

A  number  of  attempts  have  been  made  for  the  colorimetric  determination  of 
starch  utilizing  the  intense  blue  color  struck  with  free  iodine,  but  as  yet  the 
exact  composition  of  the  starch-iodine  compound  has  not  been  established.  A 
solution  of  ery  thro -dextrin  shows  a  red  color  with  iodine,  and  one  of  cellulose 
a  violet  tint. 


In  the  analysis  of  commercial  starch  made  from  potatoes,  wheat  or  corn, 
there  are  to  be  determined  the  water,  ash  and  proteids,  and  the  starch,  the 
latter  by  one  of  the  methods  described  or  simply  by  difference.  The  origin  of 
the  sample  may  be  ascertained  by  a  microscopic  examination. 

Determination  of  water.  On  account  of  the  facility  with  which  starch  is  con- 
verted into  dextrin,  the  drying  is  conducted  in  vacuo  or  a  current  of  some 
neutral  gas,  first  at  a  low  heat,  finally  to  near  100°  . 

Bloch,  §  for  the  approximate  determination  of  moisture  in  commercial  sam- 
ples, has  devised  a  '  f eculometer  '  on  the  principle  that  ten  grams  of  pure  dry 
potato  starch  forms  when  mixed  with  water  a  sort  of  hydrate  of  a  volume  of 
17.567  Cc.,  this  volume  varying  inversely  with  the  percentage  of  water  in  a  sam- 
ple. The  apparatus  is  a  measuring  tube,  the  upper  open  end  expanded  to  a  fun- 
nel for  introducing  the  starch  and  water;  the  lower  part,  22  Cm.  long  and  16 


*  Oil,  Paint,  and  Drug  Reporter,  1895—24. 

t  Journ.  Amer.  Ohem.  Socy.  1895—587  and  1896—536. 

t  Journ.  Anal.  Chem.  1888—153. 

§  Journ.  of  Applied  Chem.  1874—73. 


444  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Mm.  internal  diameter,  is  closed  at  the  bottom  and  marked  with  a  scale 
graduated  in  degrees  from  zero  at  the  bottom  to  100°  at  17.567  Cc.  For  a 
test,  ten  grams  of  the  powdered  starch  is  washed  into  the  lower  tube  with 
cold  water  and  after  settling,  its  height  is  read  on  the  scale,  which  equals 
the  percentage  of  dry  starch  in  the  sample.  The  difference  between  100  and 
the  reading  is  the  moisture  contained;  about  24  per  cent  is  the  maximum. 
If  the  sample  is  adulterated  or  spoiled  the  hydrate  will  not  readily  settle 
and  it  cannot  be  tested  in  the  instrument. 

Another  method  is  that  of  Scheibler.*  When  starch  powder  containing  11.4 
per  cent  of  moisture  is  shaken  up  with  alcohol  of  .8339  sp.  gr.  (containing  90 
per  cent  of  alcohol)  the  density  of  the  latter  remains  unchanged,  but  if  the 
starch  contains  less  than  this  percentage  of  moisture  water  is  absorbed  from 
the  alcohol  and  its  gravity  lowered  proportionally,  and  if  containing  over  11.4 
per  cent  it  gives  up  water  to  the  alcohol.  One  hundred  Cc.  of  alcohol  of  the 
above  strength  is  mixed  with  half  its  weight  of  starch  in  powder,  the  mixture 
filtered,  and  the  specific  gravity  of  the  filtrate  observed.  A  table  (loc.  cit.) 
shows  the  percentage  of  moisture  corresponding  to  different  gravities  from 
.8226  to  .8798,  a  difference  of  about  .0009  being  equivalent  to  one  per  cent  of 
moisture. 

The  ash  is  determined  by  simple  ignition  in  air.  It  is  composed  mainly  of 
phosphates  of  the  alkalies  and  earths,  and  silica. 

The  proteids  are  deduced  from  the  nitrogen  found  by  the  methods  of  ultimate 
analysis. 

For  practical  purposes  the  starch  may  be  estimated  by  difference  closely 
enough.  If  it  is  desired  to  determine  it  directly,  the  sample  is  heated  to  100  o 
with  eight  to  ten  times  its  volume  of  water  containing  about  .5  gram  of  hydro- 
chloric acid  gas;  the  digestion  is  continued  for  three  or  four  hours.  The 
reaction  is  assumed  to  be  CeHioOs  +  H2O  =  CeHiaOe,  162  parts  of  starch  pro- 
ducing 180  parts  of  dextrose.  But  practically  only  about  176  parts  of  dextrose 
are  formed.  Guichard  boils  the  sample  for  several  hours  (under  a  reflux  con- 
denser to  prevent  evaporation)  with  a  mixture  of  one  volume  of  concentrated 
nitric  acid  with  nine  volumes  of  water.  The  results  by  acid  inversion  are 
more  exact  than  in  the  case  of  more  complex  bodies,  such  as  the  cereals. 

MALT. 

On  exposing  barley  to  moist  air  in  a  moderately  warm  place  the  grains 
sprout;  during  the  germination  starch  is  converted  to  dextrin  and  glucose,  and 
there  is  generated  a  small  amount  of  a  peculiar  ferment  known  as  diastase. 

In  the  process  of  malting,  the  barley  is  covered  with  water  and  allowed  to 
'  spire »  until  the  plumules  have  reached  about  one-half  inch  in  length.  Then 
the  germination  is  arrested  by  *  killing » the  grain  by  heating  to  32  o  •  it  is  then 
dried  at  about  55  ° ,  sometimes  as  high  as  75  o  to  80  °  . 

The  objects  of  subjecting  the  barley  to  the  process  of  malting  are  the  disso- 
lution of  the  cellulose  forming  the  cells  in  which  the  starch  granules  are 
inclosed,  and  the  consequent  liberation  of  the  starch;  the  breaking  down  of 
the  nitrogen  constituents  of  the  corn;  and  the  production  of  diastase  for 
future  service  in  the  mash  tub.  When  the  original  barley  is  inferior  or  the 
malting  has  not  been  carried  out  on  proper  lines,  the  cellulose  surrounding  the 
starch  granules  is  not  dissolved,  and  it  is  hardly  possible  for  the  diastase  to 
convert  the  encysted  starch  at  ordinary  temperatures. 

The  formula  of  diastase  has  not  as  yet  been  satisfactorily  established,  but 


*  'Biederman's  Chem.  Kal.  312. 


THE    CARBOHYDRATES.  445 

investigations  point  to  its  being  a  complex  body  whose  activity  depends  on  a 
principle  called  maltin-.  One  part  of  diastase  will  convert  as  high  as  2000  parts 
of  starch  into  dextrin  and  maltose,  the  latter  the  chief  product.  It  may  be 
prepared  by  extracting  ground  malt  with  tepid  water  and  heating  the  wort 
to  about  75°  to  coagulate  albumin,  and,  after  filtering,  precipitating  the 
diastase  by  alcohol  in  the  form  of  white  amphorous  flakes.  These  are  washed, 
first  with  diluted  alcohol  then  with  absolute  alcohol,  and  dried  in  vacuo  at 
ordinary  temperatures. 

Omitting  the  water  contained,  the  following  analyses  record  successive 
stages  in  the  practice  of  malting.* 

After 

After         U  days         Dried  Malt 

Barley.      steeping,    steeping.        malt.  dust. 

Starch  and  dextrin 80.42  81.12  70.09  72.03  43.68 

Sugars 2.56  1.56  12.14  11.01  11.35 

Crude  fiber 4.69  5.22  5.03  4.84  9.67 

Proteids 9.83  9.83  10.39  9.95  26.90 

Ash  2.50  2.27  2.35  2.17  8.40 

An  analysis  of  malt  is  of  value  to  the  brewer  as  indicating  the  quality, 
flavor  and  brilliancy  of  the  beer  or  ale  made  from  it.  Moisture  exceeding  four 
or  five  per  cent  points  to  insufficient  drying  of  the  malt  or  improper  storage ; 
a  low  per  cent  of  sugar  is  evidence  that  the  germination  of  the  barley  was 
prematurely  checked,  while  a  high  (over  17)  percentage  argues  the  sprouting  to 
have  been  too  rapid.  To  the  soluble  proteids  of  the  malt  used  is  credited  much 
of  the  nutritive  value  of  a  beer.  Unmodified  starch  ('  steeliness')  is  but 
slowly  and  incompletely  converted  in  the  routine  process  of  brewing  and  tends 
to  haze  or  cloud  the  beer:  it  should  not  exceed  7  per  cent.  The  diastatic 
power  of  the  wort  is  a  measure  of  the  capacity  of  the  malt  to  convert  starch 
beyond  what  is  self-contained.  From  the  acidity,  which  should  not  be  over 
.7  per  cent,  may  be  judged  the  age  of  the  malt.  The  percentages  of  free 
maltose,  malto-dextrins  and  dextrin  determine  the  condition,  flavor  and  atten- 
uation of  the  beer  when  the  mashing  is  done  under  fixed  conditions.  Finally, 
the  color  indicates  the  heat  of  drying  and  determines  to  a  great  extent  the 
color  of  the  beer. 

The  complete  analysis  of  a  malt  may  be  made  in  the  following  manner 
although  opinions  differ  as  to  which  and  how  many  of  these  determinations  are 
really  necessary  to  fix  the  quality  or  selling  price. 

1.  Moisture.  From  four  to  five  grams  is  weighed  in  the  form  of  grains  and 
bruised  in  a  mill.    After  transferring  to  a  watch  glass,  the  heating  is  done  at  a 
temperature  of  100  °  to  105  ° ,  best  in  a  current  of  dry  hydrogen,  assuming 
constant  weight  when  the  deviation  does  not  exceed  .25  per  cent.    On  account 
of  the  hygroscopic  nature  of  malt,  protection  from  the  air  during  the  weigh- 
ings is  important. 

2.  The  ash  remains  on  burning  —  it  is  mainly  potassium  and  magnesium 
phosphates  and  silica. 

3.  The  acidity  is  determind  by  extracting  the  sample  with  cold  water  and 
titrating  by  a  weak  standard  alkali.    The  result  is  calculated  to  lactic  acid. 

4.  Extractive  matter  is  found  by  a  process  of  mashing.    Quite  a  number  of 
methods  have  been  put  forward,  differing  in  details,  but  all  agreeing  in  the  gen- 
eral conduct,  namely,  the  heating  of  a  large  weight  of  the  malt  with  water  for 
a  given  time  at  a  given  temperature,  filtering,  and  determining  the  matter  in 


Journ.  Franklin  Inet.  1900—198. 


446  QUANTITATIVE    CHEMICAL    ANALYSIS. 

solution  either  from  the  specific  gravity  or  by  evaporating  a  portion  to  dryness 
and  weighing  the  residue.  The  extract  is  also  used  for  other  determinations. 

Jalowetz'  method.  •  Fifty  grams  of  salt  is  ground  in  a  mill,  then  washed  into 
a  weighed  beaker  of  500  Cc.  capacity  with  200  Cc.  of  water  at  45  o  Cent. 
The  beaker  is  heated  in  the  water-bath  for  one  half  hour  at  45  ° .  The  heat  is 
then  increased  at  the  rate  of  one  degree  per  minute  up  to  70  ° ,  and  kept  at 
this  point  until  a  drop  of  the  liquid  removed  to  a  porcelain  plate  gives  only 
a  weak  red  or  pale  yellow  color  to  a  drop  of  iodine  solution.  The  time  of 
heating  is  termed  the  "time  of  saccharizing  ".  The  mash  is  cooled  and  to 
it  is  added  200  Cc.  of  cold  water,  and  then  made  up  with  water  to  exactly 
450  grams.  Part  of  the  mash  is  filtered  through  a  large  dry  ribbed  paper 
and  the  specific  gravity  of  the  wort  found  by  a  picnometer  at  the  temperature 
of  17.60.  Cent. 

The  calculation  of  the  extractive  matter  in  the  malt  is  from  the  following 
proportion :  —  the  weight  of  extractive  in  100  grams  of  wort  :  the  weight  of 
water  in  100  grams  of  wort  :  :  weight  of  extractive  in  50  grams  of  malt :  weight 
of  water  in  the  total  mash. 

Let  e  represent  the  grams  of  extractive  in  100  grams  of  wort  as  found  from 
the  specific  gravity  of  the  wort;  then  100  —  e  is  the  grams  of  water  in  100  grams 
of  wort.  If  the  percentage  of  water  in  the  malt  as  found  by  (1)  supra  is  w, 

w  w 

then  -g  is  the  grams  of  water  in  50  grams  of  malt  and  400  -j-  g-  expresses  the 

total  weight  of  water  in  the  mash.    And  if  E  be  the  percentage  of  extractive 

E 
in  the  malt,  then  7,  is  the  grams  of  extractive  in  50  grams  of  malt.     Whence  the 

E               w                     800e  +  ew 
proportion  e  :  1 00  —  e  :  :  -g  :  400  -j-  -g ;  and  E  =    1QQ 

Heron's  method  *  is  in  considerable  use.  Fifty  grams  of  the  ground  malt  is 
quickly  weighed  and  covered  with  400  Cc.  of  water  at  68  °  ;  the  mixture  is  kept 
at  65°  to  66°  for  one  hour  with  occasional  stirring.  After  cooling  to  15.5  o, 
the  mash  is  made  up  to  515  Cc.  (the  15  Cc.  is  an  allowance  for  the  volume  of 
the  grains),  filtered,  and  the  gravity  of  the  nitrate  taken  at  15.5  o  Cent.  The 
color  and  flavor  of  the  wort  are  noted. 

Miller's  modification.!  Fifty  grams  of  the  ground  malt  in  a  tared  copper 
beaker  is  covered  with  200  Cc.  of  water  at  40  ° ,  and  the  mixture  heated  to  60  ° 
and  kept  at  that  temperature  for  20  minutes  with  constant  stirring;  then 
tested  by  iodine  for  starch  and  ery  thro -dextrin.  If  a  coloration  is  noted  the 
mash  is  further  heated,  not  above  70°,  until  no  coloration  is  shown.  After 
cooling,  water  is  added  to  the  weight  of  450  grams  plus  the  tare  of  the  beaker  — 
that  is,  a  total  of  400  Cc.  of  added  water.  Filtering  clear,  the  percentage  of 
extractive  matter  in  the  wort  is  found  from  the  specific  gravity  by  the  tables  of 
Schultze.  The  percentage  of  extractive  matter  in  the  malt  itself  should  be 

400 

represented  by  -^-  of  that  in  the  wort,  but  from  Miller's  experiments  he  con- 
cludes that  while  this  fraction  may  represent  the  amount  of  extract  afforded 

438 

the  brewer,  yet  the  absolute  amount  obtainable  is  higher  —  about  -g^—  If,  in- 
stead of  deducing  the  extractive  from  the  density  of  the  wort,  an  aliquot  part  is 
evaporated,  dried,  and  weighed,  the  temperature  must  be  restricted  to  about 


*  Journ.  Socy.  Chem.  Ind.  7—259. 

t  Jouin.  Amer.  Chem.  Socy.  1894—353. 


THE    CARBOHYDRATES.  447 

75  ° ,  since  a  higher  temperature  will  cause  decomposition  of  the  maltose  and 
loss  in  weight. 

The  extract  as  obtained  from  the  above  is  reserved  for  the  following  deter- 
minations. 

5.  The  diastatic  power  of  the  extract  is  gauged  by  the  proportional  weight  of 
starch  in  aqueous  solution  that  it  will  convert  to  maltose  and  dextrin,  and  the 
ratio  is  taken  as  a  basis  for  an  expression  of  the  diastatic  capacity.    Several 
methods  have  been  proposed. 

One  of  these  follows  the  lines  of  Lintner's  scheme  for  the  valuation  of  sam- 
ples of  impure  diastase.  The  directions  are  to  measure  out  from  a  solution 
of  the  sample  of  known  concentration  a  number  of  equal  volumes,  and  to  each 
add  ten  Cc.  of  a  two  per  cent  solution  of  pure  potato  starch  and  allow  the  mix- 
tures to  ferment.  Then  to  each  is  added  five  Cc.  of  Fehlings  solution  and  the 
liquid  boiled ;  where  sugar  has  been  formed  by  the  action  of  diastase  on  starch 
in  excess  of  what  is  required  to  decompose  all  the  Fehlings  solution,  the  super- 
natant liquid  will  be  yellow,  but  where  less  sugar  has  been  formed  the  excess 
of  copper  will  color  the  liquid  blue.  Intermediate  will  be  found  one  of  the 
tests  that  is  colorless  (or  but  faintly  blue  or  yellow),  showing  that  starch  was 
inverted  in  quantity  to  exactly  correspond  to  five  Cc.  of  Fehlings  solution,  and 
from  this  datum  the  weight  of  the  starch  can  be  computed.  Lintner  proposed 
to  designate  as  100  the  capacity  of  the  most  active  specimen  of  diastase  he  was 
able  to  prepare,  namely  of  which  .00012  gram  hydrolyzed  a  weight  of  starch  the 
products  just  sufficient  to  combine  with  five  Cc.  of  Fehlings  solution,  operat- 
ing under  the  above  conditions,  and  the  capacities  of  other  specimens  by 
proportional  figures. 

A  simpler  method  is  to  slowly  add  the  wort  from  a  burette  by  single  cubic 
centimeters  to  a  hot  mucilage  of  starch  of  known  concentration.  The  point 
where  all  the  starch  has  been  hydrolyzed  is  found  by  testing  with  iodine 
solution. 

An  old  approximate  test  is  that  of  digesting  two  equal  weights  of  bread  taken 
from  one  loaf,  one  with  a  measured  portion  of  the  extract,  the  other  with  an 
equal  volume  of  water.  After  filtering,  equal  volumes  of  the  filtrates  are 
evaporated,  dried,  and  weighed;  the  difference,  less  the  weight  of  extractive 
matter  in  the  wort,  is  the  amount  of  bread  made  soluble. 

6.  Maltose  and  dextrose  are  determined  by  Jalowetz  in  the  wort  prepared  as 
in  (4).    Of  this  30  Cc.  is  diluted  with  water  to  200  Cc.  and  25  Cc.  withdrawn 
and  treated  by  Fehlings  solution.    From  Wein's  tables  is  found  the  corre- 
sponding sugar  —  m.    Then  in  the  30  Cc.  of  the  wort  there  are  8m  grams  of 
sugar.    Let  the  weight  of  the  extract  in  100  Cc.  of  the  wort  be  e  grams,  and 
the  density  of  the  wort  d,  then  the    extractive  in  30  Cc.  or  30  d  grams  is 

—  d.e.    Whence.  3  d.  e:  8m:  :  E  :  M;  orflf=l^? 
100  .3  d.e 

The  separate  determination  of  the  maltose,  dextrose  and  dextrin  may  be 
done  in  several  ways.  One  of  these  is  founded  on  the  right-handed  rotation 
of  the  three,  the  copper-reducing  power  of  maltose  and  dextrose,  and  their 
practically  complete  decomposition  on  fermentation  by  yeast,  the  dextrin 
being  unaffected.  Assuming  that  the  dextrogynat  of  the  dextrins  is  193,  of 
dextrose  53,  and  of  maltose  138,  and  that  the  reducing  power  of  dextrose  is 
to  that  of  maltose  as  1  to  .62 ;  then,  calling  the  weight  of  dextrose  2>,  of 
maltose  M,  and  of  dextrin  d,  the  reducing  sugars  R  may  be  represented  as 
-R  =  Z>-f.62  M;  the  polarization  P  before  treatment  with  yeast  P=53Z)-f- 
138  M  +  193d;  and  the  polarization  after  fermentation  P'  =  193  Z>.  From 


448  QUANTITATIVE    CHEMICAL   ANALYSIS. 

P' 
these    equations    it    may     be    derived    that    ^==19^;    D  =  R  —  .62  M;    and 

P  —  P'—  53B 

105.14 

The  sugars  formed  during  the  process  of  malting  may  be  extracted  from  the 
malt  by  cold  water  and  determined  in  one  of  the  usual  ways. 

7.  The  proteids  in  the  malt  are  deduced  by  determining  the  nitrogen  by  the 
method  of  Kjeldahl  or  otherwise,  Nesslerizing  the  distillate  (page  376)  as  the 
percentage  of  malt  is  low.    The  product  of  the  nitrogen  multiplied  by  the 
factor  6.25  represents  the  albuminous  matter. 

8.  Unmodified  starch  is  determined  by  mashing  50  grams  of  the  malt  as  in  (4), 
then  boiling  vigorously  for  an  hour.    After  cooling  to  65  o,  50  Cc.  of  a  ten 
percent  cold  water  extract  of  malt  is  added  and  the  whole  kept  at  65°  for 
an  hour.    It  is  then  cooled  and  diluted  to  515  Cc.,  filtered  and  the  specific 
gravity  taken,  allowing  for  the  cold  water  extract  added.    The  specific  gravity 
is  calculated  to  percentage   of  extractive  matter,  and  the  difference  between 
this  result  and  that  from  (4)  is  called  unmodified   starch.    The  additional 
features  of  this  process  over  those  of  (4)  —  namely,  the  boiling  and  addition 
of  extra  diastase  —  brings  the  unmodified  starch  into  solution. 

9.  The  residue  of  ( brewers  grains9  from   (4)  may  be  washed,    dried    and 
weighed. 

10.  The  color  and  flavor  of    the  wort  are  noted.    Lovibond  *  proposes    to 
register  the  color  of  malt  extracts  by  preparing  a  wort  by  mashing  100  grams 
of  crushed  malt  in  850  Cc.  of  water  at  74  o,  and  measuring  the  color  in  depths 
of  one  to  nine  inches. 

Grossman  f  suggests  the  following  example  as  a  suitable  form  for  reporting 
the  analysis  of  a  malt,  mashed  under  standard  conditions :  — 

Free  maltose,  fermentable » 33.30 

Ready-formed  sugars,  fermentable  ....  14.08 

Malto-dextrins,  unfermentable  (  maltose>  3-°  I , 4.90 

I  dextrin,  1.9/ 

Free  dextrin,  unfermentable 13.40 

Albuminoids 2.21 

Ash 1.60 

Acid  (as  lactic  acid) 51 

Total  dry  extract 70.00 

Unmodified  starch , 7.00 

Moisture 1.90 

Grains 21.10     100.00 

Diastatic  capacity,  30;  color  of  wort,  pale;  flavor,  good. 

CELLULOSE. 

The  celluloses  are  a  group  of  allied  carbohydrates  that  form  the  largest  con- 
stituent of  plant  tissues.  When  vegetable  fiber  is  treated  successively  by 
alcohol  and  ether,  hot  water,  a  weak  solution  of  an  alkali,  bromine  water,  and 
finally  with  alcohol  and  weak  lye,  cellulose  remains  in  a  state  of  approximate 
purity,  in  the  form  of  a  colorless  or  pure  white  amorphous  solid,  retaining  to 
some  extent  the  structural  form  of  the  plant  cell,  devoid  of  taste  or  odor, 
having  the  specific  gravity  of  1.5,  and  the  empirical  formula  C6H10O5. 

A  peculiar  property  of  cellulose  is  the  presence  of  a  certain  definite  amount  — 


*  Journ.  Socy.  Chem.  Ind.  1898—207. 
t  Journ.  Amer.  Chem.  Socy.  16—559. 


THE    CARBOHYDRATES.  449 

from  six  to  twelve  per  cent  —  of  water  of  condition,  the  proportion  being  quite 
independent  of  the  physical  form  of  the  species,  and  said  to  depend  on  the  oxy- 
groups  of  the  molecule,  for  as  these  are  suppressed  by  combination  with  nega- 
tive radicals  to  form  cellulose-esters,  the  product  has  a  decreasing  capacity  for 
water.  Wood-pulp  for  the  manufacture  of  paper  is  sold  on  the  basis  of  a 
content  of  ten  per  cent  of  moisture. 

Cellulose  is  remarkable  for  its  resistance  to  nearly  all  simple  solvents.  It  is 
soluble  however  in  a  few  reagents,  though  less  readily  after  dehydration  (as  by 
soaking  in  alcohol),  namely  concentrated  solution  of  zinc  chloride  at  60°  to 
100°,  and  a  strongly  acid  solution  of  this  reagent  in  the  cold;*  ammoniacal 
solutions  of  cupric  and  cuprous  chlorides;  sulfuric  acid  of  1.62  specific  gravity. 
On  dilution  of  the  solvent  the  dissolved  cellulose  is  reprecipitated  as  a  gelatin- 
ous hydrate  which  is  soluble  in  strong  nitric  acid  and  alkali  solutions  and  is 
more  readily  hydrolyzed  by  hot  dilute  acids  and  alkali  solutions.  Weak  lyes 
of  sodium  hydrate  (of  one  or  two  per  cent  alkali)  are  without  sensible  action 
even  when  boiling,  but  a  ten  per  cent  solution  hydrates  cellulose,  and  on  wash- 
ing the  product  with  water  a  hydrate  Ci2H2oOi0.H2O  is  left;  a  peculiar  alkali  - 
cellulose -xanthate  is  formed  by  the  action  of  carbon  disulfide  on  this  hydrate. 
A  gummy  substance  isomeric  with  starch  is  the  result  of  treatment  by  diluted 
sulfuric  acid,  and  with  nitric  acid  of  certain  high  concentrations  are  produced 
the  well  known  nitro-celluloses. 

Cellulose  is  less  susceptible  to  oxidation  and  hydrolysis  than  the  other  car- 
bohydrates. It  is  not  fermentable  by  yeast.  On  prolonged  boiling  with  a  di- 
lute acid  it  is  gradually  converted  into  hydrocellulose.  Through  the  action  of 
moderately  dilute  nitric  acid  there  are  formed  oxycellulose  (acting  as  an  acid 
toward  coal  tar  dyes),  and  oxalic  acid.  On  heating  cellulose  to  100°  with 
acetic  anhydride  it  is  dissolved  to  a  triacetate  CeHr^HsO^Os,  which  is  precip- 
itated by  highly  diluting  the  solution  with  water. 

On  account  ol  its  negative  qualities  the  separation  from  other  bodies  is 
nearly  always  made  by  extracting  the  latter  by  suitable  solvents  and  drying 
and  weighing  the  residue.  This  residue  is  called  cellulose  by  some,  though 
more  appropriately  designated  by  the  more  comprehensive  term  ( crude  fibre ' 
since  it  is  always  somewhat  impure  —  in  point  of  fact  no  exact  quantitative 
method  of  isolating  cellulose  in  a  pure  state  from  other  vegetable  constituents 
is  as  yet  known ;  and  this  is  the  more  to  be  regretted  since  its  determination  is 
but  seldom  called  for  except  when  so  associated. 

From  sugars  and  other  matter  soluble  in  cold  water,  cellulose  may  be  sepa- 
rated by  simple  lixiviation.  From  starch,  boiling  the  mixture  with  weak  sul- 
furic acid  converts  the  starch  into  sugar  which  may  be  determined  by  the 
polariscope,  or  otherwise,  while  the  cellulose  is  left  ready  to  be  dried  and 
weighed.  If  the  use  of  an  acid  is  objectionable,  the  starch  may  be  converted 
by  malt  extract.  Honig  would  heat  the  mixture  with  anhydrous  glycerol  to 
210  ° ,  cool,  and  add  alcohol  and  ether  which  precipitate  both  the  starch  and 
cellulose;  then  boil  with  dilute  hydrochloric  acid  to  convert  the  starch  to 
dextrose. 

A  direct  method  for  the  determination  of  cellulose  in  presence  of  vegetable 
matter  is  to  heat  the  sample  with  a  ten  per  cent  solution  of  potassium  hydrate 
to  180  °  for  an  hour,  then  cool  the  mixture  and  acidify  by  dilute  sulfuric  acid, 
cellulose  hydrate  precipitating;  on  making  alkaline  by  a  slight  excess  of  sodium 
hydrate  there  is  dissolved  all  but  the  cellulose.  The  liquid  is  filtered,  and  the 
residue  dried  and  weighed,  then  burned  and  the  ash  deducted.  But  the  com- 


*  Chem.  News,  1894—1-174. 

29 


450  QUANTITATIVE    CHEMICAL    ANALYSIS. 

plete  insolubility  of  cellulose  hydrate  in  dilute  alkali  solution  is  very 
doubtful. 

Lange  states  that  cellulose  remains  undecomposed  when  heated  with  a 
highly  concentrated  solution  of  sodium  hydrate  to  a  temperature  upwards  of 
200°,  while  under  these  conditions  other  plant  constituents  become  soluble. 
After  lixiviation  with  water  and  filtering,  the  residue  is  washed,  best  in  a  cen- 
trifuge* by  alcohol  and  ether,  dried  and  weighed. 

For  a  determination  of  cellulose  in  bread,  Hoenigf  heats  two  grams  with  60  Cc 
of  a  solution  of  potassium  hydrate  in  diluted  glycerol.  At  about  130  °  a  vigorous 
action  begins,  increasing  up  to  about  160  o .  The  heat  is  raised  to  180  <=>  and  the 
liquid  poured  into  boiling  water.  After  stirring  well  and  allowing  to  settle,  the 
supernatant  liquid  is  removed  by  upward  filtration  through  a  linen  cloth  tied 
over  a  funnel.  The  residual  fiber  is  boiled  with  water  and  filtered,  then  washed 
with  weak  hydrochloric  acid,  alcohol,  and  ether,  dried  and  weighed.  It  is  said 
that  only  traces  of  nitrogenous  bodies  are  left  with  the  cellulose. 

An  official  method  for  the  determination  of  crude  fiber  directs  to  extract  the 
pulverized  substance  with  ether,  and  b.oil  the  residue  under  a  condenser  for  30 
minutes  with  water  containing  1.25  per  cent  of  sodium  hydrate;  the  residual 
crude  fiber  is  washed,  dried  at  110  °  and  weighed,  then  incinerated  and  the 
ash  deducted. 

Stone  proposes  the  following  scheme  for  vegetable  fibers. 

1.  From  50  to  100  grams  of  the  powdered  material  is  boiled  under  a  reversed 
condenser  for  two  hours  with  half  a  liter  of  strong  alcohol.   The  sugars  are  dis- 
solved, and  after  filtering  and  distilling  the  alcohol,  the  residue  is  dissolved 
in  water  and  further  examined. 

2.  The  matter  insoluble  in  alcohol  is  next  treated  with  500  Cc.  of  cold  water 
to  dissolve  soluble  starch  and  dextrin,  and  filtered  through  linen.     The  filtrate 
is  evaporated  to  a  small  bulk  and  an  aliquot  part  inverted  and  the  total  car- 
bohydrates determined  by  Fehlings  solution.    From  another  aliquot  part  the 
soluble  starch  may  be  precipitated  by  baryta  and  the  dextrin  determined  in  the 
filtrate  by  inversion  and  Fehlings  solution. 

3. 'The  matter  insoluble  in  cold  water  is  dried  and  weighed  and  a  portion 
boiled  with  water  to  render  starch  soluble.  The  filtered  solution  is  then  di- 
gested with  a  fresh  infusion  of  malt  at  65°  until  iodine  gives  no  color;  after 
filtering,  the  maltose  is  converted  into  dextrose  by  heating  with  ten  per  cent 
hydrochloric  acid.  The  dextrose  is  then  polarized  or  otherwise  determined 
and  the  result  calculated  back  to  starch,  an  allowance  being  made  for  the 
sugar  in  the  malt  extract. 

4.  The  residue  left  after  treatment  with  malt  extract  is  heated  with  two 
per  cent  hydrochloric  acid,  converting  the  gums  and  pentosans  into  reducing 
sugars  to  be  determined  by  Fehlings  solution  and  considered  as  xylose. 

5.  The  residue  undecomposed  by  the  dilute  hydrochloric  acid  is  heated  with 
sodium  hydrate  of  1.25  per  cent,  dissolving  certain  bodies  of  an  obscure  com- 
position. 

6.  The  residue  is  washed,  dried  and  weighed,  then  ignited  and  the  residue  of 
mineral  matter  found.    The  difference  between  the  two  weighings  is  put  down 
as  crude  fiber. 

For  the  analysis  of  vegetable  matter,  Parsons  J  applies  various  solvents  in 
the  following  order  — 
1.  Benzene  dissolves    alkaloids,  glucosides,  free  organic  acids,  chlorophyll, 


*  Analyst,  1893—338. 

t  Prin.  &  Practice  of  Brewing. 

J  Pharm.  Journ.  10—793. 


THE    CARBOHYDRATES.  451 

certain  resins,  fixed  oils,  fats  and  waxes,  camphors,  and  volatile  oils,  but  no 
mineral  matter. 

2.  Methyl  alcohol.  Tannin,   organic    acids,  alkaloids,   glucosides,    certain 
extractive  and  coloring  matters,  resins,  sugars,  and  mineral  matters. 

3.  Cold  water.  Albuminoids,  gums,  pectin  bodies,  salts  of  organic  acids, 
dextrinoid  bodies,  and  coloring  matters. 

4.  Dilute  sulf uric  acid.  Dextrin  and  maltose  from  starch,  also  albumenoids, 
and  certain  organic  acids  free  or  combined. 

5.  Dilute  sodium  hydrate  solution.  Albuminous  matters,  pectous   bodies, 
cutose,  humus,  and  products  of  decomposition. 

6.  Bromine  water  with  ammonia.  Lignin  and  coloring  matter. 

7.  The  residue  is  cellulose. 

Each  of  the  resulting  solutions  is  further  treated  to  separate  the  dissolved 
constituents. 

The  analysis  of  woody  fiber  may  be  carried  out  on  the  following  lines.  The 
powdered  and  dried  wood  is  successively  extracted  by 

1.  Water,  dissolving  various  extractive  matters. 

2.  Alcohol  and  ether,  removing  various  coloring  matters. 

3.  Cold  dilute  hydrochloric  acid  —  alkaline  pectates. 

4.  Hot  dilute  hydrochloric  acid  —  pectose. 

5.  Cold  sulfuric  acid  sp.  gr.  1.78  —  products  from  cellulose. 

6.  Hot  dilute  sodium  hydrate  solution  —  cutose . 

7.  The  residue  is  lignin  (C19H18O8). 

Lignin  may  be  directly  determined  by  applying  the  Zeisel  process  (page  316). 
The  fiber  is  boiled  with  hydriodic  acid  and  the  methyl  iodide  formed  is  washed 
in  a  special  apparatus  to  remove  accompanying  hydriodic  acid;  it  is  then 
passed  into  an  alcoholic  solution  of  silver  nitrate  and  the  silver  iodide  deter- 
mined as  usual.  A  current  of  carbon  dioxide  is  passed  continuously  through 
the  apparatus. 

Lignin  has  also  a  reducing  action  on  the  compounds  of  gold,  and  may  be 
determined  by  the  weight  of  metallic  gold  formed  on  digestion  with  a  solution 
of  auric  chloride.  The  method  is  applied  for  the  determination  of  '  mechan- 
ical '  wood  pulp  in  mixtures. 

A  number  of  schemes  for  the  analysis  of  mixed  animal  and  vegetable  fibers 
have  been  described,  applicable  to  textile  fabrics  and  waste  from  their  manu- 
facture, and  for  the  determination  of  make-weights  and  substitutes  in  silk.* 
All  aim  at  a  separation  by  treatment  with  a  succession  of  solvents.  One  of 
these  by  Remont  follows. 

1.  The  disintegrated  fibers  are  boiled  in  water  containing  three  per  cent  of 
hydrochloric  acid,  to  remove  coloring  matters,  size,  etc. 

2.  The  residue  is  immersed  in  a  hot  solution  of  zinc  oxychloride  which  dis- 
solves the  silk.    The  residue  is  washed,  dried  and  weighed. 

3.  The  residue  is  boiled  with  a  sodium  hydrate  solution,  sp.  gr.  1.02,  for  15 
minutes,  the  wool  dissolving.    The  residue  of  cotton  is  washed,  dried  and 
weighed. 

4 .  True  silk  is  distinguished  from  f  wild  silk  »  by  treatment  with  hot  concen- 
trated hydrochloric  acid,  the  former  dissolving  in  one -half  minute,  the  latter 
in  not  less  than  two  minutes.    Another  reagent  is  a  hot  solution  of  chromic 
acid  which  dissolves  true  silk  in  one  minute. 

5.  The  absolute  specific  gravity  of  cotton  fibers  is  1.50;  of  wool,  1.30;  and  of 
silk,  1.33;  weighted  silk  may  reach  as  high  as  2.01. 


Chem.  News,  1893-1—132. 


452  QUANTITATIVE    CHEMICAL    ANALYSIS. 


THE  OILS  AND  FATS. 

In  respect  to  origin,  the  oils  and  fats  may  be  classified  as  animal,  vegetable 
and  mineral.  Most  of  the  animal  and  a  few  of  the  vegetable  species  are  solid 
at  ordinary  temperatures,  while  most  of  the  mineral  and  vegetable  are  liquid. 

The  animal  and  vegetable  oils  and  fats  are  neutral  glycerides  of  one 
or  more  of  the  fatty  acids;  since  glycerol  is  a  tri-hydric  alcohol 

fOH 

(C3H5)  I  OH,  the  radical  C3H6  may  be  in  combination  with  one,  two,  or  three 
I  OH 

fOC4H70 

radicals  of  a  fatty  acid;  for  example,    (C3H5)  1  OC4H7O,  the  tri-glyceride  of 

I OC4H7O 

butyric  acid  (C4HrO.OH),  or  shortly  butyrin.  Only  the  tri-glycerides  are 
found  in  nature,  but  the  mono-  and  di-glycerides  are  produced  on  heating  a 
fatty  acid  with  glycerol  under  certain  conditions.  » 

A  few  of  the  principal  glycerides  are 

1.  Tri-olein  (commonly  called  olein)   (C^H^O^.Og. (C3H5),  is  a  colorless, 
tasteless  and  odorless  oil,  fluid  above  5  °  Cent,  and  has  a  specific  gravity  of 
.900.    It  is  immiscible  with  water  and  diluted  alcohol,  but  is    easily  soluble 
in  ether  and  absolute  alcohol.    Less  readily  hydrolyzed    than   palmitin  and 
stearin,  it  may  be  roughly  separated  from  these  by  fractional    saponification. 
Exposed  to  the  air  it  becomes  rancid  with  the  formation  of  various  organic 
acids  and  other  bodies. 

2.  Tri-stearin  (stearin),  (C18H35O)3.O3.(C3H5),  is  a  white  lustrous  solid  of 
.920  specific  gravity.    It  may  be  prepared  fairly  pure  from  certain    tallows  by 
repeated  crystallization  from  ether.    It  is  but  slightly  soluble  in  cold  alcohol 
and  ether.    Melting  point  about  70° . 

3.  Tri-palmitin  (palmitin),  (C16H31O)3.O3.(C3H6),  is  a  white  mass  of  pearly 
scales  melting  at  62  ° ,  but  slightly  soluble  in  hot  alcohol  separating  on  cool- 
ing, and  insoluble  in  water.     The  body  formerly  called  margarin  and  considered 
as  a  simple  glyceride  has  been  shown  to  be  a  mixture  of  palmitin  and  stearin. 

4.  Tri-butyrin  (butyrin)  (0^0)3. Os^CsHs),  is  a  neutral  oily  mass  of  pecu- 
liar odor  and  taste,  insoluble  in  water,  soluble  in  alcohol  and  ether:    specific 
gravity  1.054.    Occurs  in  butter. 

The  mineral  oils  of  commerce  are  the  distillates  of  petroleum,  said  to  be 
mainly  aliphatic  hydrocarbons  of  the  ethane  and  ethylene  series.  The  lighter 
distillates  are  largely  used  for  heating  purposes,  the  intermediate  fractions  for 
illumination,  and  the  heavier  for  lubrication.  Composed  essentially  of  hydro- 
carbons, they  have  none  of  the  chemical  characteristics  of  the  animal  or  vege- 
table oils,  and  of  course  cannot  be  saponified  by  an  alkali.  One  of  the  charac- 
teristics is  the  familiar  blue  fluorescence  due  to  ultra-violet  light  rays;  but  the 
sheen  can  easily  be  masked  by  the  incorporation  of  nitrobenzene  or  other 
bodies. 

The  destructive  distillation  of  rosin  yields  a  series  of  '  rosin  oils '  of  different 
specific  gravities  and  boiling  points.  Unsaponifiable  oils  are  also  produced 
when  menhaden  or  linseed  oil  is  distilled  under  pressure. 

The  waxes  are  a  class  of  solid  bodies  (a  few  are  liquid)  of  a  peculiar  con- 
sistency and  luster.  A  wax  is  chemically  an  ether,  a  union  of  fatty  acids  with 
alcohols  of  the  ethane  or  cetyl  series;  thus,  spermaceti  is  mainly  cetyl  palmitate 


THE    OILS    AND    FATS.  453 

(cetin)   which  yields  cetylic  alcohol   (ethal)   and  palmitic  acid  on  saponiflca- 
tion  — 

(cetin)  +  H2O  =  (CieHas)  OH  (ethal)  +H.Ci6H8lO2  (palmitic  acid). 


The  extraction  of  a  fat  or  oil  from  other  animal  or  vegetable  matter  may  be 
done  by  expression,  or  more  commonly  by  treating  the  finely  divided  substance 
with  gasoline,  ether,  or  carbon  disulflde,  rarely  alcohol,  in  a  Soxhlet  or 
similar  apparatus,  these  solvents  leaving  the  oil  on  distillation. 

A  fat  or  oil  in  solution  may  be  determined  by  simple  evaporation  of  the 
ether,  gasoline  or  other  solvent,  or  if  in  an  emulsion,  by  removing  the  water 
in  some  manner  and  weighing  the  dried  oil,  but  during  the  evaporation  some 
varieties  will  be  volatilized  to  a  considerable  extent  even  when  the  evapora- 
tion takes  place  at  ordinary  temperatures,  and  the  animal  and  vegetable  oils, 
especially  those  of  the  *  drying '  variety,  become  somewhat  oxidized.  For 
the  heavier  mineral  oils  however  the  process  is  unobjectionable. 

The  following  are  the  principal  physical  and  chemical  tests  applied  for  iden- 
tification and  determination.  Sometimes  a  single  test  will  give  the  information 
desired,  more  often  conclusions  must  be  drawn  from  the  results  of  several. 
Some  of  these  reactions,  almost  characteristic  for  crude  oils,  are  less  pro- 
nounced in  proportion  as  the  oil  has  been  refined,  leading  to  the  conclusion  that 
they  originate  with  some  impurity  eliminated  in  the  refining  processes. 

1.  The  colors  of  some  crude  oils  are  marked  and  peculiar,  but  the  mode  of 
extraction,  age,  etc.,  may  greatly  modify  them.    Refined  oils  are  of  every  shade 
to  colorless. 

Contact  with  solution  of  sodium  hydrate,  sulfuric  acid,  or  nitric  acid  of  cer- 
tain strengths  develops  with  some  oils  colors  ranging  from  yellow  to  brown, 
and  in  a  few  varieties  shades  of  green  or  purple.  Other  reagents  for  this  pur- 
pose are  zinc  and  tin  chlorides,  and  phosphoric  acid.  The  test  is  useful  as  cor- 
roborative evidence,  but  alone  is  liable  to  mislead. 

The  odor  is  often  indicative  of  the  origin  of  an  oil  or  fat;  fish  oils  have  a 
peculiar  offensive  smell,  and  rosin,  mineral,  linseed,  and  others,  can  often  be 
recognized  in  mixtures..  But  it  is  not  difficult  for  a  manufacturer  to  disguise 
or  remove  an  odor  unless  very  pronounced. 

The  appearance  under  the  microscope  of  certain  pure  oils  is  characteristic,  but 
in  mixtures  the  configuration  is  less  distinctive  than  the  proportions  of  the  con- 
stituent oils  would  indicate. 

2.  The  absorption  spectra  of  crude  vegetable  oils,  due  to  chlorophyll,  are  gen- 
erally well  defined ;  those  of  the  animal  oils  are  less  distinct  or  altogether 
wanting. 

3.  Specific  gravity.  This  constant  varies  greatly  for  the  different    varieties, 
ranging  from  .875  to  .970.    It  may  be  observed  by  means  of  a  delicate  hydrome- 
ter, the  Westphal  balance,  or  the  Sprengel  tube,  attending  closely  to  the  tem- 
perature and  exactness  of  weighing  where  accuracy  is  desired.    As  with  other 
constants,  the  results  must  not  be  considered  as  an  assurance  of  the  purity  or 
the  adulteration  of  the  oil  examined,  unless  corroborated  by  other  tests,  for  the 
age  of  the  oil,  method  of  preparation  and  storing,  contact  with  the  air,  etc., 
may  alter  the  accepted    constant  not  a  little.    Again  the  change  in  gravity  of 
a  pure  oil  by  the  admixture  of  an  adulterant  may  be  corrected  by  the  judicious, 
blending  of  a  third  variety,  bringing  the  gravity  back  to  the  original  figure. 

4.  Melting  and  congealing  points.  These  are  often  useful  as  a  means  of  recog- 
nition, but  no  exact  constants  can  be  determined  for  the  reason  that  an  oil  or  fat 
does  not  pass  sharply  from  the  liquid  to  the  solid  state  or  the  reverse,  and  on 


454  QUANTITATIVE    CHEMICAL   ANALYSIS. 

this  account  there  are  considerable  discrepancies  among  the  figures  of  different 
observers,  aggravated  at  times  by  the  fact  that  portions  of  a  fat  coming  from 
different  parts  of  the  same  animal  or  vegetable  have  not  a  uniform  melting 
point,  The  methods  of  determination  are  described  on  page  163. 

5.  The  refractive  index*  varies  from  1.44  to  1.50  at  60°  Cent,  where    water 
has  a  refraction  of  1.33;  it  has  been  shown  to  bear  no  relation  to  the  specific 
gravity,  viscosity,   or  clearness  or  turbidity  of  an  oil.    The  refractometer  is 
described  on  page  167. 

6.  Relations  toward  solvents.  All  oils  and  fats  are  practically  insoluble  in 
water,  but,  with  a  few  exceptions,  freely  soluble  in  ether,  carbon  disulfide  and 
gasoline;  the  essential  oils  dissolve  freely  in  alcohol,  but  only  a  few,  notably 
castor  oil,  of  the  animal  and  vegetable  oils.    In  acetic  acid  of  specific  gravity 
1.056  some  dissolve  readily,  others  on  heating,  and  a  few  are  only  incompletely 
dissolved  even  at  the  boiling  point.    As  a  means  of  differentiating  oils   of 
unequal  solubility,  first  any  associated  fatty  acids  are  removed,  then  a  certain 
weight  is  treated  with  a  limited  measured  volume  of  absolute  alcohol  or  glacial 
acetic  acid,  filtered,  and  an  aliquot  part  of  the  filtrate  evaporated  to  dryness 
and  the  residue  weighed,  f 

Acetone  dissolves  most  oils,  and  in  some  cases  is  preferable  to  any  of  the 
usual  solvents  for  certain  determinations. 

7.  Absorption  of  oxygen.  Long  exposure  of  a  fat  or  oil  to  the  air  results  in 
the  formation  of  greater  or  less  amounts  of  oxidation  products  that  communi  - 
cate  an  unpleasant  odor  and  taste.    As  the  rancidity  increases,  free  fatty  acids 
are  liberated,  though  there  does  not  appear  to  be  any  well-defined  relation 
between  the  degree  of  rancidity  and  the  acid-value  (14)  of  an  oil.    By  a  not 
well  understood  series  of  changes,  certain  *  drying '  or  { semi  -drying '  oils 
thicken  on  exposure  to  the  air  and  eventually  dry  to  a  resinous  or  leathery  skin 
characteristic  of  linolein  and  its  homologues.    This  siccative  property,  indis- 
pensable for  a  paint-oil,  unfits  it  of  course  for  lubrication  or  burning. 

The  proportion  of  oxygen  assimilated  by  a  drying  oil  in  a  given  time  is  an 
indication  of  its  purity,  that  is,  of  the  absence  of  a  non-drying  variety.  The 
rate  of  absorption  is  determined  by  bringing  the  oil  for  a  specified  period 
into  intimate  and  direct  contact  with  gaseous  oxygen,  or  by  mixing  with  a 
readily  reducible  oxide,  such  as  plumbic.  Bishop  would  incorporate  with  the 
oil  a  certain  proportion  of  manganese  binoxide  and  silica  and  allow  the  mix- 
ture to  stand  for  a  certain  time.  He  finds  that  under  these  conditions  linseed 
oil  absorbs  14  per  cent  of  oxygen  in  24  hours. 

8.  A  few  of  the  fixed  oils  have  the  property  of  reducing  salts  of  silver  and 
gold  to  the  metals.    The  test  known  as  BeccbTs  is  applied  mainly  for  the 
detection  of  cottonseed  oil  in  olive  oil  or  lard.    The  method  as  modified  by 
later  investigators,  consists  in  heating  the  suspected  oil  in  a  test-tube  with  a 
dilute  solution  of  silver  nitrate  in  alcohol  and  ether,  the  formation  of  a  shining 
deposit   of  metallic  silver  on  the  test-tube  indicating  the  presence  of  cotton- 
seed oil.    Wesson  states  that  even  perfectly  pure  lards  may  darken  under 
this  treatment,  due  to  certain  associated  bodies,  and  advises  their  previous 
removal  by  washing   the  lard  with  dilute    alkali    solution    and  nitric  acid. 
Milliau,  on  the  hypothesis  that  the  fatty  acids  of  the  oil  are  most  chemically 
active  directly  they  are  separated,  would  note  the  action  of  the  silver  solution 
on  the   freshly  prepared   mixed   fatty   acids  rather  than  on    the    oil   itself. 
HirchsohnJ  dissolves  the  suspected  oil   in  chloroform  and   compounds    the 


*  Journ.  Socy.  Chem.  Ind.  1898—102. 
t  Chem.  News,  1889-1—206. 
J  Chem.  Zelt.  12—341. 


THE    OILS    AND    FATS.  455 

solution  with    gold    chloride;    on  heating,  cottonseed    oil    produces    a    red 
color. 

9.  The  elaidin  test.  By  the  action  of  nitrogen  trioxide,  olein  is  converted  into 
an  isomer  elaidin,  and  oleic  acid  into  elaidic  acid,  both  of  these  solid  at  ordi- 
nary temperatures ;  and  according  to  the  proportion  of  olein  in  an  oil  is  the 
solidity  of  the  product.    Olive  oil,  largely  olein,  gives  a  characteristic  hard 
mass;  neatsfoot  oil,  one  of  a  buttery  consistence;  cottonseed  oil,  a  pasty;  and 
linseed  oil,  a  liquid  residue.    Other  oils  fall  into  one  of  these  classes. 

The  easiest  way  of  applying  the  test  is  by  dissolving  mercury  in  cold  nitric 
acid ;  the  solution,  retaining  for  a  time  much  of  the  nitrogen  trioxide  generated 
by  the  reaction,  is  incorporated  with  the  oil  to  be  tested,  and  the  consistence 
of  the  product  noted  after  standing  for  two  hours  with  frequent  agitation. 

10.  Non-drying  oils  on  treatment  with  sulfur  chloride  yield  products  soluble 
in  carbon  disulflde,  while    drying  oils  change    to    insoluble    solid    masses. 
According  to  Bruce  Warren,  the  reaction  is  chiefly  a  combination  of  chlorine 
with  hydrogen,  the  sulfur  combining  with  the  dehydrogenized  portion  of  the 
oil.    Five  grams  of  the  oil  or  mixture  is  weighed  in  a  porcelain  dish  and  mixed 
with  two  cubic  centimeters  of  carbon  disulflde,  then  with  two  cubic  centimeters 
of  the  reagent  (yellow  sulfur  chloride  in  carbon  disulflde).    The  mass  is  evap- 
orated on  the  water  bath  to  dryness  with  constant  stirring,  then  dried  to  con- 
stant weight.    The  residue  is  finely  powdered  and  extracted  by  carbon  disul- 
flde, the  percolate  evaporated  to  dryness  and  weighed.    The  process  is  criticised 
by  Lewkowitsch.* 

11.  Exothermic  reactions.  When  mixed  with  concentrated  sulf uric  acid,  oils 
evolve  a  certain  specific  amount  of  heat,  the  rise  in  temperature  being  a  func- 
tion of  the  chemical  action  taking  place,  chiefly  saponiflcation.    With  animal 
and  vegetable  oils  and  fats,  for  certain  specified  proportions  of  oil  and  acid  and 
strength  of  the  latter,  the  rise  is  from  37°  to  128°  Cent.,  while  the  mineral 
oils  show  only  from  3  °  to  10  °  . 

In  the  Maumene  test,  50  Cc.  of  the  dry  oil  at  about  15®  Cent,  is  treated  with 
10  Cc.  of  concentrated  sulf  uric  acid;  the  mixture  is  constantly  stirred  with  a 
thermometer  and  the  highest  thermal  point  observed.  Since  the  heat  generated 
varies  considerably  with  the  strength  of  acid,  efficiency  of  the  protection  against 
radiation,  and  other  factors,  it  is  best  to  make  a  parallel  test  on  pure  water  and 
express  the  result  on  the  oil  as  the  ratio  between  the  two  observations. 

The  combination  of  bromine  with  a  fat  or  oil  evolves  heat  approximately 
commensurate  with  the  ratio  of  iodine  absorbed  by  the  oil  (12).  Since  the  rise 
in  temperature  is  too  great  for  an  accurate  thermometric  measurement,  the  oil 
is  not  compounded  directly  with  bromine,  but  both  are  dissolved  in  chloroform 
or  other  solvent  before  mixing. 

12.  Halogen  absorption.  The  oils  and  fatty  acids  of  the  oleic  and  acrylic  groups 
form  additive   compounds   with  bromine    and  iodine  in  contradistinction  to 
those  of  the   (saturated)  stearic  and  acetic  series.     Huebl,f  the  originator, 
recommends  dissolving  a  fraction  of  a  gram  of  the  oil  in   chloroform  and 
mixing  with  an  excess  of  a  standard  solution  of  iodine  in  alcohol  containing 
mercuric  chloride  (which  acts  to  hasten  the  assimilation  of  the  iodine  by  the 
oil).    After  standing  for  two  to  four  hours  in  a  dark  place  at  the  ordinary 
temperature  the  mixture  is  diluted  with  a  weak  solution  of  potassium   iodide 
(water  alone  might  precipitate  mercuric  iodide),  and  the  unabsorbed  iodine 


*  Benedlkt-Lewkowitsch,  Oils,   Fats  and  Waxes,  228;  Chem.    News.  1888-1—113   and 
190—2-5  etc. 
t  Chem.  Zelt.  13—1375. 


456  QUANTITATIVE    CHEMICAL    ANALYSIS. 

titrated  by  sodium  thiosulfate  that  has  been  standardized  against  iodine.    A 
blank  determination  is  carried  along  with  the  test. 

Several  modifications  of  the  reagent  and  the  manner  of  applying  it  have  been 
described.  Wijs  commends  a  solution  of  iodine  chloride  in  glacial  acetic 
acid  as  preferable  to  t  he  original  Huebl  in  being  more  stable  and  requiring  a 
shorter  time  for  the  reaction  to  be  completed. 

The  method  applied  to  a  mixture  of  two  oils  whose  absorbent  capacities 
differ  considerably,  is  capable  of  giving  an  approximate  determination  of 
their  proportions,  but  usually  is  only  qualitatively  applied  as  a  test  of  purity. 
For  example,  genuine  olive  oil  absorbs  from  81  to  85  per  cent  of  its  weight 
of  iodine,  while  cottonseed,  rape  and  sesam6  oils,  its  most  common  adulter- 
ants, absorb  from  97  to  108  per  cent  when  unrefined,  somewhat  less  when 
highly  refined.  The  ratio  of  iodine  absorbed  is  higher  for  fats  and  oils  than 
for  their  respective  fatty  acids. 

The  absorption  of  bromine  is  determined  by  Hehner*  by  dissolving  a  gram  or 
more  of  the  oil  in  chloroform  in  a  tared  flask;  bromine  is  added  drop  by  drop 
to  slight  excess,  then  heated  until  the  excess  of  bromine  and  the  chloroform  are 
driven  off.  The  gain  in  weight  of  the  oil  is  claimed  to  represent  the  bromine 
absorbed. 

Mcllhiney  f  points  out  that  bromine  may  be  fixed  in  two  ways,  (1)  by  replacing 
hydrogen,  one  atom  of  bromine  displacing  one  atom  of  hydrogen  which  com- 
bines with  another  atom  of  bromine  forming  a  molecule  of  hydrobromic  acid ; 
and  (2)  by  direct  addition  to  the  unsaturated  groups  of  the  oil,  not  forming 
hydrobromic  acid.  The  parts  of  bromine  absorbed  by  100  parts  of  oil  in  (1)  he 
calls  the  "  bromine  substitution  figure  ". 

The  method  recommended  is  to  dissolve  the  oil  or  resin  in  carbon  tetrachlo- 
ride  and  add  an  excess  of  third-normal  bromine  in  the  same  solvent.  The  bot- 
tle is  stoppered  and  kept  in  the  dark  for  eighteen  hours,  then  water  added 
to  dissolve  the  hydrobromic  acid  formed,  with  precautions  to  prevent  its  escape 
in  gaseous  form.  Excess  of  potassium  iodide  is  added  and  the  iodine  (liberated 
by  the  excess  of  bromine)  titrated  by  thiosulfate  and  starch;  the  result  is  cal- 
culated to  bromine,  and  the  difference  between  this  and  the  weight  of  bromine 
originally  added  is  the  total  bromine  absorbed. 

The  liquid  is  filtered  through  linen,  and  the  aqueous  solution,  containing 
free  hydrobromic  acid,  titrated  to  neutrality  by  standard  alkali  and  methyl 
orange,  giving  the  bromine  substitution  figure.  The  total  bromine  absorbed 
minus  twice  the  bromine  substitution  figure  equals  the  "bromine  addition 
figure."  For  example,  for  rosin  oil  the  total  bromine  figure  (percentage)  is 
116.2;  the  bromine  substitution  figure  58.1,  and  the  addition  figure  zero; 
while  with  linseed  oil  the  corresponding  numbers  are  102.9,  zero,  and  102.9. 

13.  The  acetyl- value. J  Certain  fatty  acids  that  contain  an  alcoholic  hydroxyl 
group  react  to  form  an  acetyl -fatty-acid  on  heating  with  acetic  anhydride ;  thus 
the  ricinic  acid  from  castor  oil, 

(1).  C18H34O3  (ricinic  acid)  -f  (C2H3O)2O  (acetic  anhydride)  = 
CisHssOs^HsO  (acetyl-ricinic  acid)  -{-  HC2H302  (acetic  acid). 

On  treatment  of  the  acetyl-ricinic  acid  with  standard  potassium  hydrate  an 
atom  of  potassium  replaces  one  of  hydrogen  with  the  formation  of  water  — 

(2).  Ci8H3303.C2H3O  (acetyl-ricinic  acid)  -f  KOH 
(potassium  acetyl -ricinate)  -f-  HOH. 


*  Analyst,  1895—50. 

t  Journ.  Amer.  Chem.  Socy.  1894—275  and  1899—1084. 

}  Analyst,  1899—319. 


THE    OILS    AND   FATS.  457 

But  on  heating  the  potassium  acetyl-ricinate  with  an  excess  of  potassium 
hydrate  in  alcohol,  hydrolysis  takes  place  with  the  formation  of  potassium 
ricinate  and  acetate  — 

(3).  Ci8H32KO3.C2H3O  (potassium  acetyl-ricinate)  -f  KOH  = 

KCisHsaOs  (potassium  ricinate)  -{-  KC2H302*( potassium  acetate). 

The  weight  of  KOH  required  in  equation  (3)  for  one  gram  of  acetyl-ricinic 
acid  is  called  the  "  acetyl  value."  Oleic,  stearic,  palmitic,  etc.,  acids  contain- 
ing no  hydroxyl  groups  give  (theoretically)  no  acetyl  value  as  the  reaction  of 
equation  (3)  does  not  take  place. 

The  process  is  as  follows :  A  large  quantity  of  the  oil  is  hydrolyzed  and  the 
fatty  acids  washed  and  dried  and  boiled  with  acetic  anhydride.  After  dilution 
with  water  the  floating  layer  of  acetylated  acids  is  re  moved  and  thoroughly 
washed  with  hot  water.  A  weighed  portion  of  the  dried  product  is  dissolved 
in  neutral  alcohol  and  titrated  by  standard  potassium  hydrate  and  phenol- 
phthalein.  As  soon  as  the  red  color  appears  a  measured  excess  of  alcoholic 
standard  potash  is  run  in,  and  after  boiling,  the  excess  titrated  by  standard 
acid,  and  from  the  volume  of  acid  is  calculated  the  weight  of  potash  required 
for  equation  (3)  ;  or  the  liquid  may  be  acidified  by  sulfuric  acid,  and  the  freed 
acetic  acid  (from  the  potassium  acetate)  distilled  and  determined  in  the 
distillate. 

14.  The  acid  value.  Nearly  all  commercial  samples  of  the  animal  and  vegetable 
oils  contain  from  a  fraction  of  one  per  cent  up  to  several  per  cents  of  free  fatty 
or  organic  acids,  either  as  normal  constituents  or  resulting  from  decomposition 
by  age  or  exposure  to  the  air.    For  illumination  and  lubrication  a  neutral  oil 
is  always  preferred,  as  acids  tend  to  char    amp  wicks  and  corrode  metallic 
bearings.    The  determination  is  made  by  heating  the  oil  with  neutral  alcohol 
in  which  the  fatty  acids  are  easily  soluble,  and  titrating  by  weak  standard 
alkali  and  an  indicator  like  turmeric  or  litmus.    The  mixture  must  be  well 
stirred  before  titration  to  emulsify  the  oil  and  bring  the  alcohol  and  acids  into 
contact. 

15.  The  ether  value.  The  animal  and  vegetable  oils  are  made  up  of  neutral 
glycerides  of  various  fatty  acids,  and  when  distilled  in  superheated  steam  are 
hydrolyzed  into  free  fatty  acids  and  glycerol;  thus 

C3H6O3.R3  (neutral  fat)  +3H2O  =R3.(OH)3  +  C3H8O3 (glycerol), 
where  R  is  the  radical  of  any  fatty  acid.    Similarly  when  saponified  by  a  caustic 
alkali, 

C3H5O3.R3  +  3NaOH  =  R3.  (ONa)3  +  C3H8O3. 

Other  reagents  for  saponification  are  concentrated  sulfuric  acid,  sodium  alco- 
holate,  bromine,  etc.,  sometimes  used  for  special  material. 

The  ether  value  is  the  number  of  milligrams  of  potassium  hydroxide  (KOH) 
required  to  saponify  one  gram  of  a  neutral  oil  o'r  fat,  and  is  determined  by 
emulsifying  the  oil  with  alcohol,  neutralizing  any  free  fatty  acids  by  potassium 
hydrate,  heating  the  liquid  with  a  known  volume  of  standard  alcoholic  solution 
of  potassium  hydrate,  and  determining  the  excess  of  alkali  by  titrating  back  by 
standard  acid. 

With  the  exception  of  butyric  and  a  few  associates  found  in  butter  and 
cocoanut  oil,  the  fatty  acids  are  not  volatilized  in  a  current  of  steam  at  atmos  - 
pheric  pressure.  The  '  Eeichert  testt'  an  important  feature  in  butter  analysis, 
withdraws  and  determines  the  volatile  members  from  the  mixed  fatty  acids ; 
the  process  is  essentially  a  distillation  of  the  mixed  acids  with  water  and 
titration  of  the  acids  in  the  distillate  by  standard  alkali.  Scala  supports  the 
plan  of  accepting  the  proportion  of  volatile  fatty  acids  in  fats  other  than  butter 
as  a  criterion  of  the  degree  of  rancidity. 


458  QUANTITATIVE    CHEMICAL,   ANALYSIS. 

16.  Saponification.  In  the  laboratory,  saponiflcation  is  easiest  accomplished 
by  heating  the  oil  or  fat  with  a  solution  of  potassium  or  sodium  hydrate. 
Since  the  rapidity  of  the  reaction  is  the  greater  the  more  intimate  the  contact 
between  the  oil  and  alkali,  an  alcoholic  solution  of  the  latter  is  generally 
preferred  to  an  aqueous  one,  though  concentrated  lyes  of  caustic  soda 
or  a  mixture  of  caustic  soda  and  potash  at  boiling  heat  act  energet- 
ically. An  excellent  means  of  hastening  the  decomposition  of  the  less 
easily  saponified  bodies,  such  as  the  waxes,  was  devised  by  Henriques,* 
namely,  by  dissolving  the  oil  or  wax  in  ether  or  gasoline  before 
the  addition  of  the  alcoholic  alkali  solution;  saponiflcation  is  complete  at  ordi- 
nary temperatures  within  a  few  hours.  By  conducting  the  operation  without 
application  of  heat,  no  ethers  of  the  volatile  fatty  acids  are  formed,  as  occurs 
to  a  slight  extent  under  the  higher  temperatures  of  the  ordinary  process. 

As  saponification  proceeds,  the  fatty  acids  combine  at  once  with  the  alkali  to 
form  soaps ;  thus  the  transformation  of  lard  by  potassium  hydrate  — 

C3H5(C18H3502)3  +  3KOH  =  C3H5(OH)3  +  SK^S^) 

Stearin  Glycerol  Potassium  stearate. 

C3H5(C18H3302)3  +  3KOH  =  C3H5(OH)3  +  3K(Ci8Ha302) 

Olein  Potassium  oleate. 

C3H5(C16H3102)3  +  3KOH  =  C3H5(OH)3  +  3K(C16H31O2) 

Palmatin  Potassium  palmitate. 

Saponiflcation  is  employed  for  the  following  purposes : 

(1)  To  obtain  the  fatty  acids  for  a  physical  or  chemical  examination.    On 
acidifying  the  hot  solution  of  the  soaps  obtained  by  the  action  of  an  alkali  on  an 
oil,  the  fatty  acids  are  liberated  and  float  as  an  oily  layer  on  the  surface.    Thus 
with  lard  soap  — 

SKCCisH^)  -f  3HC1  =  3KC1  +  3HC18H3502  (stearic  acid). 

3K(C18H3302)  -f  3HC1  =  3KC1  +  3HC18H33O2  (oleic  acid). 

3K(C16H31O2)  +  3HC1  =  3KC1  -f-  3HC16H31O2  (palmitic  acid), 
and  the  mixture  may  be  separated  from  the  solution  of  potassium  chloride  and 
glycerol  by  decantation  or  filtration. 

The  alcoholic  liquid  is  evaporated  to  dryness  on  the  water  bath,  taken  up  with 
hot  water,  acidified,  and  filtered  through  a  close  paper,  the  aqueous  liquid  pass- 
ing through;  or  the  liquid  may  be  cooled  and  the  solidified  cake  of  fatty  acids 
separated  by  pouring  out  the  water  solution.  The  reason  for  changing  the 
alcoholic  solution  to  an  aqueous  one  before  acidulation  is  that  fatty  acids  are 
somewhat  soluble  in  alcohol.  The  fatty  acids  are  washed  with  water,  then 
dried  at  a  temperature  not  much  exceeding  100  o . 

Lactones,  like  the  glycerides,  yield  soaps  on  saponiflcation ;  on  acidification 
the  lactones  are  reprecipitated. 

(2)  To  determine  the  ether  value.  The  ether  value  is  the  number  of  milli- 
grams of  potassium  hydrate  required  to  saponify  one  gram  of  a  neutral  oil. 
The  process  requires  an  accurately  standardized  alcoholic  solution  of  potas- 
sium hydrate  of  which  a  measured  volume,  largely  in  excess  of  that  needed  for 
saponiflcation,  is  heated  with  a  weighed  amount  of  the  previously  neutralized 
oil.     When  the  oil  has  been  fully  decomposed,  as  evidenced  by  the  absence  of 
oily  globules,  the  excess  of  alkali  is  titrated  by  standard  hydrochloric  acid,  or 
by  standard  sulfuric  acid  after  dilution  with  water  to  prevent  the  separation  of 
potassium  sulf  ate  during  the  titration.    With  dark  colored  oils,  especially  when 
an  old  and  brown  solution  of  alkali  is  used,  the  change  of  the  indicator  is  ob- 
scured and  it  has  been  recommended  to  distill  the  solution  with  an  excess  of 


Analyst,  1896-67  and  192. 


THE    OILS    AND    FATS.  459 

ammonium  chloride  and  titrate  the  free  ammonia  in  the  distillate  correspond- 
ing to  the  excess  of  potassium  hydrate  —  KOH  -f  NH4C1  =  KC1  -f  NH4OH. 

The  saponiftcation  value  is  simply  a  convenient  technical  expression  for  the 
sum  of  the  acid  and  ether  values;  and  may  be  stated  either  as  Roettstorfer1  s 
number,  the  number  of  milligrams  of  potassium  hydrate  saponifying  one  gram 
of  an  unneutralized  oil  or  fat,  or  as  the  saponification  equivalent,  the  number  of 
grams  of  an  oil  saponified  by  one  liter  of  strictly  normal  potassium  or  sodium 
hydrate.  The  former  is  the  quotient  of  56112  (that  is,  the  molecular  weight  of 
potassium  hydrate  times  1000)  divided  by  the  latter.  Obviously,  with  a  neutral 
oil  the  Koettstorfer  number  is  also  the  ether  value ;  and  on  the  other  hand,  the 
Koettstorfer  number  of  a  pure  free  fatty  acid  equals  the  acid  value,  and  the 
ether  value  is  zero. 

(3)  To  identify  an  animal  or  vegetable  oil.  As  will  be  seen  from  the 
equations  on  page  458,  three  molecules  of  KOH  (168.354)  decompose  respec- 
tively one  molecule  (890.88)  ol  stearin,  one  molecule  (884.82)  of  olein,  or 
one  molecule  (806.79)  of  palmitin.  For  a  mixture  of  these  fats  in  a  fairly 
constant  ratio,  such  as  lard,  the  weight  of  alkali  saponifying  one  gram  is  a 
constant  differing  more  or  less  from  those  of  other  fats  or  oils.  A  greater 
variation  is  where  butyrin,  laurin,  and  similar  glycerides  are  constituents. 

The  physical  and  chemical  constants  of  a  few  of  the  common  oils  are  tabu- 
lated below.  They  are  averages  of  results,  differing  considerably  in  some 
cases,  obtained  by  several  observers. 


Species 

Specific 
gravity. 

Congeals  Maumeng 
o  Cent.       test. 

Koetts. 
number. 

Iodine 
number 

Acetyl 
.  value. 

Refractive 
index 
(Jean's). 

Olive 

.9165 

2 

41 

191 

82 

4.7 

1.5 

Sesame* 

.9210 

—  5 

65 

190 

107 

11.5 

18. 

Cottonseed 

.9220 

—  2 

75 

193 

109 

16.6 

18. 

Linseed 

.9350 

—  27.5 

105 

193 

172 

8.5 

50. 

Castor 

.9655 

—  17 

46 

179 

83 

153.4 

40. 

Almond 

.9180 

—  10 

52 

191 

97 

5.8 

6.5 

Rape 

.9140 

—  3 

60 

175 

101 

6.3 

17.5 

Lard  oil 

.9145 

7 

45 

194 

79 

3.1 

5.5 

Descriptions  of  the  many  special  tests  for  the  detection  and  determination  of 
adulterants  and  substitutes  will  be  found  in  the  treatises  on  oils  and  oil 
analysis. 


The  analysis  of  a  mixture  of  oils  is  often  a  difficult  problem  and  the  outcome 
not  infrequently  open  to  doubt.  The  separation  of  a  saponiflable  from  an  un- 
saponifiable  oil  offers  no  particular  difficulties,  but  mixtures  of  either  class 
seldom  admit  of  direct  separation.  Attributive  methods  can  often  be  applied 
to  two  or  possibly  three  mixed  oils,  the  data  derived  from  their  specific  gravi- 
ties, iodine  absorption,  sulfur  chloride  reaction,  etc.  And  in  a  binary  mixture, 
if  the  species  of  one  member  can  be  learned,  that  of  the  other  may  be  opined 
by  a  comparison  of  a  constant  of  the  former  and  that  of  the  mixture  (page  156). 

A  scheme  for  the  general  course  of  analysis  is  outlined  below,  but  it  is  seldom 
that  any  detailed  directions  can  be  followed  without  considerable  modification. 

1.  The  color,  odor,  taste,  and  general  appearance  will  often  be  a  clue  to  one 
or  more  of  the  constituents.     A  specific  gravity  below  .900  indicates  mineral  or 
light  rosin  oils  entirely  or  chiefly,  and  above  .975,  heavy  rosin  or  tar  oils.    The 
great  majority  of  pure  oils  and  mixtures  have  a  gravity  between  .900  and  .975. 

2.  The  sample  is  treated  with  carbon  disulflde.    In  this  menstruum  all  oils 
are  soluble,  but  not  the  soaps.    If  a  residue  remains,  the  solution  is  filtered, 


460  QUANTITATIVE    CHEMICAL    ANALYSIS. 

washed  by  carbon  disulflde,  the  residue  weighed  and  decomposed  by  a  dilute 
mineral  acid,  and  the  resulting  fatty  acid  and  salt  of  the  base  examined  quali- 
tatively. 

The  carbon  disulflde  is  evaporated  from  the  filtrate,  and  the  remaining  oils 
treated  with  warm  alcohol;  free  fatty  acids  dissolve,  together  with  castor, 
croton,  olive-kernel  and  rosin  oils,  while  other  oils  remain.  If  a  clear  solution 
results,  only  the  above  mentioned  may  be  present;  if  not,  the  liquid  is  filtered 
and  the  alcohol  evaporated  from  the  filtrate  to  see  if  any  oil  has  been  taken  up 
by  it. 

3.  The  sample  is  moderately  heated  in  a  copper  retort  connected  to  an 
efficient  condenser ;  the  distillate  will  consist  of  the  more  volatile  hydrocar- 
bons, rosin  oils,  turpentine,  etc.,  which  might  escape  detection  in  (4).    Water, 
an  occasional  constituent,  is  found  at  the  bottom  of  the  receiver,  and  may  be 
tapped  off  in  a  separatory  funnel  and  measured  or  weighed.*    A  more  accurate 
method  for  determining  water  is  to  stir  into  the  original  oil,  diluted  if  neces- 
sary with  dried  gasoline,  a  weighed  quantity  of  recently  ignited  calcium  sul- 
fate.    The  liquid  is  filtered,  washed  with  gasoline,  and  the  increase  in  weight 
of  the  calcium  sulfate  noted. 

4.  For  the  detection  of  animal  and  vegetable  oils  and  their  separation  trom 
mineral  or  rosin  oils,  the  sample  is  boiled  with  an  alcoholic  solution  of  potash 
under  a  reflux  condenser.    The  alcohol  is  evaporated  off  at  a  low  heat,  the 
residue  taken  up  with  hot  water,  and  some  sodium  bicarbonate  stirred  in.    Two 
cases  may  result. 

A.  The  solution  is  clear  and  free  from  oily  drops,  indicating  that  the  sample 
is  entirely  animal  or  vegetable,  possibly  containing  a  rosin  or  wax  or  bearing 
a  small  proportion  of  mineral  or  rosin  oil  which  remains  in  solution. 

B.  On  the   surface  floats  oily  matter.    The  sample  is  then  either  entirely 
mineral  or  rosin  oil  or  a  mixture  with  a  saponiflable  oil.    Of  the  two  layers 
the  upper  is  the  mineral  or  rosin  oil,  the  lower  a  solution  of  potash  soaps  of 
the  fatty  acids  with  glycerol  and  the  excess  of  alkali  as  carbonate.    The  object 
of  obtaining  an  aqueous  solution  is  to  avoid  the  loss  of  small  quantities  of 
unsaponifiable  oils  through  their  solubility  in  alcohol. 

In  either  case  the  liquid  is  transferred  to  a  separatory  funnel  and  the  un- 
saponiflable  oils  extracted  by  ether  or  gasoline;  this  plan  is  preferable  to 
simply  tapping  out  the  aqueous  solution.  The  extraction  is  continued  until 
a  few  drops  of  the  ether  leave  no  residue  on  evaporation.  The  separation  is 
not  quite  complete,  however,  traces  of  the  unsaponifiable  matter  remaining  in 
the  alkaline  solution,  while  a  little  soap  enters  the  gasoline.  The  ether  solu- 
tion is  washed  with  water  to  remove  traces  of  dissolved  soap,  and  the  water 
washed  in  turn  by  ether  to  recover  any  unsaponifiable  matter  taKen  up  by  it 

The  ethereal  solution  is  evaporated  to  dryness  and  the  residue  weighed.  It 
may  consist  of  mineral  or  rosin  oils  or  a  mixture  of  the  two,  and  may  be  exam- 
ined as  on  page  473.  Possibly  also  there  are  higher  alcohols  of  the  ethane 
series  (rutic,  cetylic,  etc.),  that  may  be  determined  from  cheir  acetyl  value  or 
the  volume  of  hydrogen  evolved  on  heating  with  potash-lime,  page  399.  If  the 
residue  appears  to  be  homogeneous  (consist  of  but  one  body)  an  ultimate  anal- 
ysis or  appropriate  physical  tests  may  show  its  nature,  and  if  composed  of  two 
or  more  analogous  bodies  they  may  be  separated  by  fractional  solution,  t rac  • 
tional  crystallization  from  ether,  or  like  means. 

The  unsaponifiable  matter  may  be  a  mixture  of  mineral  and  rosin  oils.  A 
rough  separation  of  the  two  can  be  accomplished  by  treating  the  mixture 


*  Hopkins'  Oil-chemists'  Handbook,  30, 


THE    OILS    AND    FATS.  461 

with  alcohol,  rosin  oils  being  freely  soluble  therein,  while  mineral  oils  are  but 
sparingly  soluble.  To  recover  as  far  as  possible  what  mineral  oil  has  gone  into 
solution,  the  alcoholic  nitrate  is  evaporated  to  dryness  and  the  residue 
extracted  by  alcohol,  when  much  of  that  originally  dissolved  will  be  left. 

(The  process  may  be  made  somewhat  more  exact  by  repeating  the  above 
extraction  with  a  smaller  volume  of  alcohol  than  the  first,  measuring  the 
volumes  of  alcohol  used  and  weighing  the  respective  residues.  Obviously  the 
difference  between  the  volumes  of  alcohol  is  the  volume  that  would  dissolve  a 
weight  of  mineral  oil  equal  to  the  difference  in  the  weights  of  the  residues ; 
and  knowing  the  rate  of  solubility  of  the  mineral  oil  in  alcohol,  the  amount  in 
the  last  residue  may  be  easily  calculated,  and  from  this  the  weight  of  rosin  oil 
by  difference.  No  account  is  here  taken  of  the  solvent  effect  of  the  rosin  oil  in 
the  alcohol.) 

The  aqueous  solution,  plus  the  traces  of  soap  recovered  from  the  gasoline,  is 
acidified  by  hydrochloric  acid  and  heated  until  a  clear  layer  of  the  melted  fatty 
acids  floats  on  the  solution  of  potassium  chloride  and  glycerol.  After  filtra- 
tion, the  fatty  acids  may  be  dried  and  weighed,  then  washed  with  acetone  to 
dissolve  out  small  amounts  of  unsaponifiable  matter  retained  In  the  aqueous 
solution.  The  combining  power  or  mean  molecular  weight  of  the  mixed  acids 
can  be  determined  by  dissolving  in  an  excess  of  standard  potassium  hydrate 
solution,  and  titrating  back  the  excess  by  standard  acid.  The  majority  of  the 
fixed  oils  yield  95  to  96  per  cent  of  fatty  acids  and  about  10  per  cent  of  glycerol 
(the  extra  5  per  cent  coming  from  the  assimilation  of  water). 

A  short  approximate  method  for  the  determination  of  the  animal  or  vegetable 
oil  in  admixture  with  mineral  or  rosin  oil  (as  are  many  of  the  lubricating  and 
burning  oils  of  commerce)  consists  in  finding  the  saponification  value  of  the 
sample  in  the  usual  way.  If  the  species  of  the  saponiflable  oil  is  known  the 
proportion  in  the  mixture  will  follow  from  the  ratio  of  the  saponiflcation  value 
of  the  sample  to  the  value  established  for  the  pure  oil,  or  if  the  species  is  un- 
known, the  mean  of  the  values  of  the  oils  in  common  use  for  the  purpose  in- 
tended for  the  sample.  A  closer  result  may  be  had  by  syphoning  off  a  part  of 
the  floating  mineral  oil,  determining  its  specific  gravity,  and  modifying  the 
calculation  accordingly. 

Technical  analysis.  The  oils  and  fats  are  used  mainly  for  illumination,  lubri- 
cation, food,  the  manufacture  of  soaps  and  candles  and  paints  and  varnishes, 
and  the  preparation  of  cloths  and  skins.  The  technical  analyst  has  both  to 
determine  the  intrinsic  quality  for  a  given  purpose  of  the  material  he  is  to 
examine,  and,  as  far  as  he  is  able,  to  learn  if  adulteration  or  substitution  has 
been  practiced.  A  comprehensive  scheme  for  the  analysis  of  mixed  oils  cannot 
be  formulated,  much  depending  on  the  judgment  and  experience  of  the  analyst, 
who  must  keep  clearly  in  mind  the  objects  for  which  the  analysis  is  made. 


Illuminating  oils.  At  the  present  time  these  are  largely  the  intermediate  dis- 
tillates of  petroleum,  the  ordinary  kerosene  of  45  °  Baume,  and  the  c  mineral 
seal  oil' of  39°  Baume.  Their  cheapness  precludes  any  adulteration,  and  if 
of  the  proper  gravity  and  clearness,  the  analyst  has  but  to  examine  for  their 
safety  for  burning  in  ordinary  lamps,  and  occasionally  determine  their  con- 
gealing temperatures. 

The  flash  and  burning  temperatures.  The  *  flash-point '  *  is  the  temperature 


*  Chem.  News,  1893—1-291. 


462 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


whereat  a  liquid  gives  off  vapor  so  freely  that  the  mixture  of  vapor  and  air 
above  the  surface  will  burn  for  an  instant  when  brought  in  contact  with  a  flame. 
If  the  liquid  is  a  mixture  of  two  or  more  of  different  boiling  points,  the  flash- 
point is  usually  higher  than  that  of  the  most  volatile  constituent.  The  c  burn- 
ing-point »  is  the  temperature  at  which  the  liquid  itself  will  take  fire  and  burn 
continuously  when  touched  with  a  flame  (very  small,  to  prevent  local  heating). 
Certain  minimum  flash  and  burning  points  indicate  the  safety  of  an  oil  for 
illuminating  or  heating  purposes.  For  kerosene  the  legal  minimum  in  many 
States  is  a  flash-point  of  110 o  Fahr.  (closed  cup),  and  a  burning-point  of 
150°  ;  for  the  next  heavier  distillate  of  petroleum,  known  variously  as  '  min- 
eral seal ',  *  Iowa  oil »,  etc.,  they  are  respectively  235°  and  300  o  Fahr. 

In  all  the  various  forms  of  apparatus  for  determining  these  points,  the  oil  is 
contained  in  a  metal  cup  arranged  to  be  heated  slowly  and  uniformly  to  the 
required  temperatures.  The  instrument  authorized  by  the 
New  York  State  Board  of  Health  is  shown  in  section  in 
Fig.  179.  For  testing  kerosene  the  cup  A  is  filled  nearly  up 
to  the  flange  B,  and  a  glass  plate  C  holding  a  cork  and  ther- 
'  "FUL  ~43T B  mometer  D  is  laid  on  tne  upper  flange  E.  The  outer  cup  F 
acts  as  a  water-bath  and  is  nearly  filled  with  cold  water  and 
heated  by  a  small  flame  of  a  Bunsen  burner  beneath,  so 
adjusted  that  the  temperature  of  the  oil  shall  not  rise  at  a 
greater  rate  than  two  degrees  Fahr.  per  minute.  At  each 
accession  of  two  degrees,  a  minute  flame  is  passed  through 
a  small  hole  in  the  glass  plate,  and  at  the  moment  when  a 
flash  of  light  is  observed  in  the  empty  space,  the  temper- 
ature of  the  oil  is  read,  the  flash-point. 

Where  the  flash   or  burning  point  is  above  100°,  the 
outer  cup  is  filled  with  oil  or  paraffin.    For  very  high  tem- 
peratures a  sand  bath  is  used. 
Apparatus  of  this  description  wherein  the  vapor  is  largely 


Fig.  179 


prevented  from  diffusing  into  the  air  are  known  as  tf  closed-cup,"  as  dis- 
tinguished from  those  without  the  glass  plate,  called  "  open-cup."  The 
former  afford  readings  considerably  lower  than  the  latter  and  are  generally 
regarded  as  more  reliable. 

To  determine  the  burning-point,  the  glass  cover  is  removed  and  the  ther- 
mometer bulb  hung  in  the  oil.  The  heat  is  slowly  increased  until  the  oil 
catches  fire  and  burns  quietly  when  a  minute  flame  is  passed  rapidly  over  the 
surface  about  an  inch  above  it. 

Distillation.  The  lighter  mineral  and  rosin  oils  distill  at  moderate  tempera- 
tures and  can  be  separated  in  this  way  from  less  volatile  bodies.  Fractional 
distillation  effects  the  separation  of  the  various  homologues  of  crude  and 
partially  refined  petroleum  or  rosin  oils,  previously  removing  any  water  or  sedi- 
mentary matter  by  allowing  the  oil  to  stand  in  a  tall  jar  for  a  day  or  longer. 
The  distillation  is  conducted  in  a  porcelain  or  metal  apparatus  in  the  usual 
manner.  The  receiver  is  changed  at  periods  indicated  by  the  temperature  of 
the  vapor  in  the  still;  if  a  technical  analysis,  at  certain  arbitrary  points  con- 
forming to  the  commercial  classification  of  the  products.  For  example*  a 
petroleum  from  Ohio  gave  on  distillation  — 

Specific  gravity.  Yield  per  cent. 

Gasoline,  .630  2. 

Naptha,  .700  15. 

Kerosene,  .800  55. 

Paraffin  oil,  .875  20. 

Coke  left  in  retort,  .....  8. 


THE   OILS   AND    FATS.  463 

The  further  examination  of  a  mineral  oil  may  include  the  following, 

1.  The  colors  of  kerosene  are  expressed  in  trade  parlance  as  water-white, 
superfine  white,  prime  white,  standard  white,  and  good  merchantable.     For 
comparison  of  a  sample  with  the  standards  a  Wilson's,  Stammer's,  or  Red- 
wood's colorimeter  may  be  used. 

2.  Calcium  and  magnesium  salts  in  an  oil  diminish  the  illuminating  power. 
A  volume  of  100  to  200  cubic  centimeters  is  evaporated  to  dryness,  the  residue 
ignited,  dissolved  in  hydrochloric  acid  and  the  bases  determined  by  the  usual 
methods, 

3.  Earthy  and  alkali  soaps.  The  metaphosphates  of  these  bases  are  insoluble 
in  absolute  alcohol  or  a  mixure  of  absolute  alcohol  and  gasoline.    The  soaps 
are  looked  for  by  dissolving  a  few  drops  of  the  suspected  oil  in  a  little  gasoline 
and  compounding  with  a  saturated  solution  of  metaphosphoric  acid  in  absolute 
alcohol.    The  formation  of  a  flocculent  precipitate  on  standing  for  a  time  shows 
the  presence  of  a  soap.    The  base  may  be  identified  by  the  usual  qualitative 
tests  for  the  earthy  and  alkali  phosphates. 

4.  Matter  volatile  below  65  o  Cent.  About  .2  gram  of  the  oil  is  imbibed  in  a 
disk  of  filter  paper,  this  heated  for  eight  hours  on  the  water  bath  at  60  °  to  65  °  c 
The  loss  in  weight  is  that  of  volatile  matter  and  water  if  present. 

5.  Rosin  as  an  adulterant  can  be  determined  by  heating  the  oil  with  nitric  acid 
and  extracting  the  mineral  oil  by  ether.    The  products  of  the  action  of  the  acid 
on  the  rosin  remain  insoluble.     The  mineral  oil  loses  about  ten  per  cent  in 
weight. 

60  Sulfur  is  usually  determined  by  burning  the  oil  in  an  apparatus  arranged  to 
oxidize  and  retain  the  combustion  products  of  the  sulfur  compounds.  Sulfuric 
acid  (from  refining  processes)  can  be  washed  from  the  oil  by  water;  it  is  all 
retained,  as  well  as  some  sulfur  compounds  of  the  oil,  when  the  latter  is  dis- 
tilled with  sodium  bicarbonate. 

Lubricating  oils.  The  value  of  an  oil  as  a  lubricant  depends  on  a  combination 
of  several  qualities  of  which  the  chief  is  that  the  oil  shall  possess  sufiicient 
adhesiveness  to  prevent  the  contact  of  the  metallic  surfaces,  and  that  its  co- 
hesion or  viscosity  shall  approach  the  limit  where  internal  friction  becomes  a 
detriment.  For  example,  a  mineral  oil  is  suited  to  a  slowly  moving  pinion 
bearing  a  moderate  load ;  under  a  heavy  load  the  adhesion  of  a  mineral  oil 
would  be  overcome,  unlike  some  vegetable  oils,  such  as  castor,  whose  adhe  - 
sion  persists  even  under  heavy  pressure. 

The  viscosity  of  an  oil  may  be  determined  in  several  ways,  the  most  com- 
mon that  of  observing  the  period  of  time  consumed  for  a  given  volume  to  flow 
through  an  orifice  of  standard  area  as  compared  with  that  for  water  or  a  cer- 
tain oil  taken  as  a  standard.  The  viscosities  of  the  oils  differ  greatly,  that  of 
raw  linseed,  for  example,  being  only  one -half  that  of  olive.  The  temperature 
has  a  marked  influence  on  the  rate  of  flow;  thus,  a  specimen  of  mineral  oil  re- 
quiring 1030  minutes  to  pass  a  certain  orifice  at  50  o  Fahr.  took  only  680  min- 
utes when  at  60  o ,  and  40  minutes  at  212  °  . 

A  simple  '  viscosimeter '  is  a  pipette  holding  100  Cc.  between  two  marks, 
one  above,  the  other  below  the  bulb.  The  orifice  at  the  bottom  is  of  such  an 
area  that  100  Cc.  of  water  at  100  o  Fahr.  will  run  out  in  34  seconds.  The  oil 
to  be  examined  is  heated  to  100  o,  or  higher  if  very  viscid,  and  the  pipette 
filled  by  suction;  the  number  of  seconds  required  for  the  outflow  of  the  volume 
of  oil  held  between  the  marks  is  called  the  'viscosity  number*  of  the 
oil. 

Obviously,  the  temperature  of  the  oil  is  continually  falling  during  the  out- 
flow, and  the  hydrostatic  pressure  diminishing.  To  eliminate  these  objection- 


464 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


able  features,  a  number  of  instruments  have 
been  invented  wherein  the  pipette  or  its  equiv- 
alent is  surrounded  by  a  bath  of  some  liquid 
heated  to  a  given  constant  temperature  and  kept 
in  motion  by  a  stirrer  to  avoid  local  overheating. 
And  instead  of  measuring  the  outflow  by  the 
volume  discharged,  the  oil-reservoir  is  kept  full 
and  the  oil  drawn  into  a  measuring  cylinder. 
For  a  detailed  description  refer  to  the  Journ, 
Anal,  and  Applied  Chem.  V  — 314  et  seq.* 

Other  apparatus  are  based  on  a  different  prin- 
ciple, namely  the  resistance  opposed  to  the 
movement  of  a  solid  body  in  the  oil;  though  it 
has  been  questioned  whether  this  retardation 
really  measures  viscosity.  Cockrell's  device  is 
a  small  paddle  revolved  in  the  oil  by  means  of 
clockwork,  the  coefficient  of  retardation  in 
speed  as  compared  to  that  in  water  being  con- 
sidered a  measure  of  the  viscosity.  Doolittle's 
torsion  viscosimeter  f  is  a  metal  cylinder,  Fig. 
180,  2  inches  high  by  1.5  in  diameter,  hung  ver- 
tically in  the  oil  by  a  long  steel  wire  bearing  a 
pointer  traversing  a  horizontal  graduated  circle. 
The  amplitude  of  the  arc  described  by  alternate 
rotations  of  the  cylinder  in  air  is  diminished 
when  the  cylinder  is  held  in  a  liquid,  and  is  re- 
ferred to  pure  water  as  a  standard. 

Kunkler  contends  that  the  viscosimeter  gives 
data  of  practical  value  with  mineral  oils  only, 
since  the  adhesion  of  the  other  oils  to  metal 
bearings  does  not  increase  with  the  viscidity. 

In  the  technical  examination  of  lubricating  oils,  the  so-called  "cold-test" 
is  observed  by  half-filling  a  long  narrow  bottle  with  the  oil  and  immersing  it  in 
a  freezing  mixture  until  the  oil  is  solidified.  The  bottle  is  removed  to  a  warm 
place  and  the  oil  constantly  stirred  with  a  thermometer  until  it  just  begins  to 
flow,  or,  as  others  direct,  until  it  will  just  flow  from  one  end  of  the  bottle  to 
the  other. 

The  flash  and  burning  temperatures  are  useful  tests  of  a  lubricant  and  may 
detect  the  presence  of  a  light  mineral  or  rosin  oil  in  a  supposedly  simple  animal 
or  vegetable  one. 

Lard  oil  has  a  large  consumption  as  an  illuminant  either  alone  or  in  admix- 
ture with  a  mineral  oil,  and  for  this  purpose  should  not  contain  more  than  one  or 
two  per  cent  of  free  fatty  acids,  and  be  free  from  the  cheaper,  though  less  suit- 
able, cottonseed  and  tallow  oils. 

In  paints  and  varnishes  linseed  oil  is  an  important  constituent.  The  drying 
quality  is  greatly  increased  by  boiling  for  a  certain  time,  and  addition  of 
*  dryers'  (litharge,  etc.).  The  examination  for  adulterants  should  be  mainly 
in  the  direction  of  the  cheaper  corn,  cottonseed  or  rosin  oils,  not  overlooking 
the  possible  incorporation  of  mineral  oil.  A  test  of  the  rapidity  of  drying, 
sometimes  of  importance,  can  be  made  by  spreading  a  thin  film  on  a  sheet  of 


*  Stevens  Inst.  of  Tech.  Indicator,  1899—251. 

t  Journ.  Amer.  Chem.  Socy.  1893— 173 ',  and  Chem.  News,  1893—1—168  and  226. 


THE    OILS    AND    FATS.  465 

glass  and  noting  the  number  of  hours  required  for  the  film  to  set  and  to 
harden. 

Paints  are  generally  put  on  the  market  in  paste  form,  that  is  ground  up  with 
ten  to  twenty  per  cent  of  boiled  linseed  oil,  or  '  ready  mixed  '  containing  a  much 
larger  proportion  of  oil,  and  usually  certain  dryers.  To  separate  the  oil  from 
the  pigment,  the  paste  may  be  boiled  with  ether  or  other  organic  solvent  and 
filtered  through  a  very  closely-woven  paper,  and  the  pigment  washed  with  ether. 
The  oil  is  obtained  for  examination  by  evaporating  off  the  ether  at  a  low  heat ; 
certain  gums  and  resins  may  also  be  contained  if  the  paint  was  a  *  gloss  '  or 
*  varnish '  paint. 

The  common  organic  solvents,  however,  do  not  dissolve  the  oil  of  a  paint  to 
a  perfectly  homogeneous  liquid,  the  solution  being  somewhat  gelatinous.  Han- 
ney*  finds  that  methylated  ether  (ordinary  ether  plus  ten  per  cent  of  methylic 
ether)  when  used  in  the  proportion  of  100  Cc.  to  one  gram  of  paste  yields  a 
perfect  solution  of  the  oil  and  allows  decantation  from  a  heavy  pigment.  When 
a  sample  of  paint  is  already  mixed  with  the  proper  proportion  of  oil  and  dryers 
for  immediate  application,  simply  diluting  it  with  gasoline  and  whirling  in  the 
centrifuge  will  allow  the  liquid  to  be  decanted  off  clear. 

Eepeated  evaporations  with  concentrated  nitric  acid  will  sometimes  destroy 
the  oil  completely  and  leave  the  pigment,  this  usually  somewhat  altered  from 
the  original  composition,  except  in  the  case  of  barite,  silica,  etc. 

Varnishes,  from  the  nature  of  their  ingredients  and  some  obscure  changes  in 
chemical  composition  during  or  after  manufacture,  present  a  difficult  problem 
to  the  analyst  —  in  fact,  a  practical  test  of  the  flowing  qualities,  covering  power, 
gloss,  and  resistance  of  the  coat  to  abrasion  and  the  weather  is  likely  to  be  of 
more  practical  value  than  an  analysis. 

TURPENTINE. 

Turpentine  is  an  essential  oil  distilled  by  steam  from  the  resinous  exudation 
of  the  yellow  or  Georgia  pine  tree,  and  is  largely  used  in  the  manufacture  of 
paints  and  varnishes.  It  is  a  nearly  colorless,  limpid,  inflammable  liquid,  com- 
posed of  several  polymers  (terpenes)  of  the  formula  C10H16.  Exposed  to  light 
and  air  certain  changes  take  place,  culminating  in  the  formation  of  a  resinous 
crystalline  body  of  the  formula  C10H18O2. 

Turpentine  is  sometimes  adulterated  with  a  small  proportion  of  petroleum 
spirit,  rosin  oil,  paraffin  oil,  or  shale  spirit.  Their  detection  and  determination 
is  difficult,  especially  when  they  are  in  but  small  proportions. 

1.  The  specific  gravity  of  commercial  turpentine  ranges  from  .860  to  .875,  the 
average  about  .867.    Should  the  gravity  of  a  sample  be  below  .860  there  is  indi- 
cated rosin  oil  (.856  to  .860),  or  petroleum  distillates  or  shale  uaptha  (.700  to 
.830)  ;  if  above  .875,  paraffin  oil  (.890),  or  a  fat  oil  (.912  to  937). 

2.  The  molecular  weight  is  about  138.6  to  147.7.    The  vapor  density  is  from 
4.80  to  5.11.    Light  petroleum  distillates  give  lower  values  than  these.    It  has 
been  recommended  to  fractionally  distill  the  sample  and  determine  these  con- 
stants in  the  first  fraction. 

3.  The  flash-point  is  quite  constant  at  32  o  to  33  o  ;  the  boiling  point  at  155  ° 
to  158°  .    If  below  these  temperatures  there  is  indicated  an  admixture  of  light 
petroleum  oils;  if  above  them,  paraffin  or  fat  oils.    However,  ordinary  petro- 
leum distillates  used  for  burning  purposes  have  approximately  the  same  flash 
or  boiling  point  as  turpentine. 


*  Chem.  News,  1893-1—268  and  301. 

30 


466  QUANTITATIVE    CHEMICAL   ANALYSIS. 

4.  On  evaporation  in  a  current  of  steam,  fresh  turpentine  should  not  leave 
over  .2  to  .5  per  cent  of  residue,  old  samples  from  .5  to  2  per  cent.  The  resin  - 
ous  matter  left  on  evaporating  old  turpentine  is  dissolved  by  nitric  acid,  a 
distinction  from  heavy  petroleum  or  fat  oils. 

Hinsdale  *  directs  to  put  ten  drops  of  a  suspected  sample  on  a  watch  glass, 
weigh,  and  float  the  glass  on  water  kept  at  170  °  Fahr.  If  pure  it  will  have 
evaporated  in  about  seven  minutes;  any  considerable  residue  is  considered 
proof  of  adulteration  with  petroleum.  A  parallel  test  is  made  with  pure  tur- 
pentine, and  when  this  has  evaporated  the  glass  containing  the  sample  is 
weighed.  Five  per  cent  of  petroleum  is  said  to  be  detectible  by  this  process. 

6.  Distillation  of  turpentine  begins  at  about  156  °  ;  about  85  per  cent  passes 
Over  below  163°,  and  nearly  all  below  185  o  to  190°.  The  usual  adulterants 
begin  to  distill  below  160  ° ,  and  the  final  temperature  is  above  190  ° ,  fat  oils 
remaining.  It  is  best  to  attach  a  dephlagmator  to  the  distilling  flask.  Phil- 
lipsf  in  separating  turpentine  from  ready-mixed  paints,  distills  the  paint  at 
225°  in  a  current  of  coal-gas  to  prevent  oxidation  of  the  residue. 

6.  Polymerization.  By  cautious  treatment  with  concentrated  sulfuric  acid, 
moderating  the  action  by  cooling  the  flask,  turpentine  is  polymerized  into  bod- 
ies soluble  in  concentrated  sulfuric  acid.    Armstrong  proposes  to  treat  500  Cc. 
of  the  sample  with  150  Cc.  of  sulfuric  acid  (two  volumes  of  acid  to  one  of 
water),  pour  the  mixture  into  a  separatory  funnel,  and  tap  off  the  solution  of 
the  polymers.    The  residual  liquid  is  distilled  in  steam  and  the  distillate  treated 
with  stronger  acid,  when  the  residue  should  not  exceed  25  Cc.    If  of  greater 
volume  it  is  mixed  with  several  volumes  of  very  concentrated  acid  and  heated 
to  122  °  Fahr.,  when  the  residue  should  not  exceed  2.5  Cc.    Petroleum  products 
and  paraffin  oil  are  practically  unaffected  by  sulfuric  acid,  but  rosin  spirit  is 
acted  on  to  a  considerable  extent. 

A  simple  test  is  made  by  mixing  30  Cc.  of  concentrated  sulfuric  acid  with  six 
Cc,  of  the  turpentine  in  a  graduated  tube,  uniting  the  two  slowly  and  cooling 
the  tube  in  water  to  prevent  any  great  rise  in  temperature.  After  thorough 
agitation  the  mixture  is  allowed  to  stand  for  a  few  hours,  when  the  floating 
layer  should  not  exceed  .3  Cc.  in  volume. 

A  process  similar  to  that  of  Armstrong  is  described  by  Burton.  On  mixing 
turpentine  with  fuming  nitric  acid  there  are  formed  oxidation  products  soluble 
in  water.  After  washing  out  soluble  matter  with  hot  water  the  residue  should 
not  exceed  two  per  cent  by  volume  of  the  original. 

7.  Turpentine  is  dextro-rotatory  to  polarized  light,  different  samples  show- 
ing 8°  to   16°  for  American  turpentine  4    Armstrong   considers    American 
products  to  contain  two  terpenes,  one  dextro-  and  one  laevo- rotatory. 


BBESWAX. 

The  most  familiar  of  the  wax  family  is  the  well-known  beeswax,  a  mixture 
of  myricyl  palmitate,  cerotic  acid,  certain  hydrocarbons,  and  small  quantities 
of  allied  acids  and  alcohols.  The  natural  color  is  pale  yellow,  changed  to 
pure  white  by  bleaching  in  sunlight  or  by  chemical  agents.  The  most  common 
of  the  adulterants  are  yellow  paraffin  or  ceresin  for  beeswax,  and  white  paraffin 
or  ceresin  for  white  wax. 

The  specific  gravity  of  pure  wax  ranges  from  .956  to  .964,  usually  lying 


*  Chem.  News,  1891—1-161. 

t  Idem,  1891—1-275. 

J  Journ.  Anal.  Appl.  Chem.  1892-1  and  1893—99. 


THE   OILS    AND    FATS.  467 

between  .958  and  .961.  The  gravity  of  ceresin  is  from  .915  to  .925,  while  that 
of  paraffin  is  from  .865  to  .908.  A  gravity  of  above  .964  points  to  adulteration 
with  stearic  acid,  rosin,  etc.,  and  below  .956  to  paraffin,  ceresin,  or  tallow. 

The  specific  gravity  is  conveniently  observed  according  to  Liverage  by  cut- 
ting a  small  cube  from  the  interior  of  a  cake  and  weighing  by  the  Westphal 
balance,  supporting  the  cube  by  inserting  the  pointed  end  of  the  plummet. 
Allen  prefers  to  determine  the  gravity  of  the  melted  wax  at  100  o  ,  it  ranging  at 
this  temperature  from  .818  to  .827. 

The  microscopic  appearance  of  pure  wax  crystallized  from  chloroform  is 
characteristically  crystalline;  paraffin  in  amounts  of  over  20  per  cent  discom- 
posing the  crystals. 

The  melting  point  is  from  61°  to  64°,  the  solidifying  point  about  the  same. 
Paraffin  melts  at  from  35  °  to  58  °  ;  ceresin  from  68  °  to  89  °  ;  and  spermaceti 
at  about  45  °  . 

The  refractive  index  as  registered  by  the  Zeiss  ref ractometer  varies  from 
42.60  to  45.4®;  paraffin,  22.5°;  ceresin,  41.0®;  and  tallow,  48.5°.  All 
these  figures  were  calculated  to  40  °  Cent,  from  the  readings  at  66  o  to  72  o . 

The  examination  of  a  commercial  wax  may  be  conducted  about  as 
follows. 

Water  is  determined  by  melting  the  wax  and  allowing  it  to  cool  slowly  to 
solidification,  when  most  of  the  water  will  collect  at  the  bottom  of  the  dish 
and  may  be  poured  out  and  weighed.  The  solidified  wax  is  then  dried  at  100  o 
to  constant  weight.  Mineral  make-weights  or  colors,  such  as  ochre,  barite, 
clay  or  gypsum,  separate  from  the  melted  wax  with  the  water  and  may  be 
recognjzed  by  examination  with  a  lens  or  by  qualitative  tests. 

The  acid  number.  Five  grams  of  the  wax  is  melted  with  20  Cc.  of  neutral 
strong  alcohol,  and  the  mixture  titrated  by  half -normal  potassium  hydrate  and 
phenol -phthalein.  The  number  of  milligrams  of  potassium  hydrate  required 
to  saturate  the  cerotic  acid  of  one  gram  of  wax  is  called  its  acid  number;  it, 
is  about  19  to  21  —  high  as  compared  with  those  of  the  oils  and  fats. 

The  ether  number.  The  neutralized  solution  of  the  wax  obtained  as  above 
is  heated  with  a  measured  volume  of  standard  potassium  hydrate  in  alcohol  to 
saponify  the  myricyl  palmitate ;  the  ether  number  (milligrams  of  potassium 
hydrate  per  gram  of  wax)  is  found  by  a  reverse  titration  by  standard  acid. 
It  ranges  from  73  to  76,  and  the  saponification  number  (acid  number  plus 
ether  number),  from  92  to  97.  Paraffin  and  ceresin  lower  the  numbers  propor- 
tionally. 

As  the  saponification  of  a  wax,  especially  in  presence  of  much  unsaponifiable 
matter,  is  slow  and  difficult,  It  is  well  to  mix  the  alcoholic  solution  with  an 
equal  volume  of  ether  which  has  the  property  of  hastening  the  action  of  the 
alkali.  Some  prefer  to  saponify  a  large  weight  of  the  wax  by  a  concentrated 
aqueous  solution  of  caustic  alkali,  precipitate  the  mixed  constituent  acids  and 
unsaponifiable  matter  by  acidification  with  a  mineral  acid,  filter  and  dry  the 
mixture;  then  weigh  a  suitable  portion  of  this  "decomposed  wax,"  dissolve 
in  neutral  alcohol,  and  titrate  the  acid  by  standard  alkali. 

In  determining  the  Hehner  number  it  must  be  remembered  that  the  fatty  acids 
are  mixed  with  considerable  unsaponiflable  matter  which  must  first  be  extracted 
by  ether. 

The  iodine  number  is  low,  about  11  to  12;  after  bleaching  by  permanganate  or 
bichromate,  only  5  to  8, 

For  the  determination  of  paraffin  or  ceresin  in  commercial  wax  the  following 
have  been  proposed. 

1.  The  wax  is  saponified  by  a  concentrated  alcoholic  solution  of  caustic  soda 


468  QUANTITATIVE    CHEMICAL    ANALYSIS. 

and  the  solution  evaporated  to  dryness,  The  residue  is  extracted  by  chloro- 
form in  a  Soxhlet  apparatus,  dissolving  the  paraffin  or  ceresin  and  the  myricyl 
alcohol.  The  chloroformic  solution  is  evaporated  to  dryness  and  the  residue 
boiled  with  acetic  anhydride  which  forms  a  soluble  ester  with  the  myricyl 
alcohol,  while  the  paraffin  floats  on  the  surface  and  may  be  separated  by  filtra- 
tion and  washing  with  acetic  anhydride  and  water. 

2.  Another  method  is  based  on  the  insolubility  of  paraffin  in  a  mixture  of 
ethyl  and  amyl  alcohols  at  a  low  temperature,  while  wax  is  held  in  solution. 
The  wax  is  dissolved  in  a  small  quantity  of  amyl  alcohol  and  the  solution 
mixed  with  an  equal  volume  of  75  per  cent  ethyl  alcohol.    After  standing  for 
some  hours  at  4  <=>  or  below,  the  liquid  is  filtered  through  a  dry  paper  and 
the  residue  washed  with  a  mixture  of  the  alcohols,  the  temperature  being 
kept  as  low  as  possible.    The  residue  is  dissolved  in  ether  or  gasoline,  evap- 
orated to  dryness,  heated  to  125°  and  weighed. 

3.  Following  the  Buisenes,  the  wax  is  heated  with  solid  potash -lime  to 
250  °  whereupon  the  alcohols  of  the  wax  are  decomposed  with  formation  of 
potash  soaps  —  e.  g.,  C^H^OH  (ceryl  alcohol)  +  KOH  =  KC27H5302  (potassium 
cerotate)  -\-  2H2;  one  gram  of  wax  should  yield  from  53.5  to  57.5  Cc.  of  hydro- 
gen corresponding  to  52.5  to  56.5  per  cent  of  myricyl  alcohol.    The  potash  soaps 
are  insoluble  in  gasoline.    The  residue  is  extracted  by  gasoline,  and  the  solution 
evaporated  leaving  a  mixture  of  the  hydrocarbons  and  the  paraffin  or  ceresin. 
From  the  weight  of  the  residue  can  be  approximately  calculated  the  proportion 

(X)  of  paraffin  or  ceresin  from  the  formula  X=  100  —  ^— ,  where  a  is  the  per- 
centage of  hydrocarbon  in  the  adulterant ;  b,  the  percentage  of  hydrocarbons 
in  pure  wax;  and  d,  the  percentage  of  the  residue  from  the  evaporation. 
Where  the  adulterant  is  ceresin  or  paraffin,  a  is  taken  to  equal  100,  and  b 
about  13.5. 

Stearic  acid  separates  when  the  wax  is  dissolved  in  ten  times  its  weight  of 
hot  80  per  cent  alcohol,  the  solution  cooled  and  filtered  from  the  cerotic  acid, 
then  diluted  with  a  large  proportion  of  water. 


SOAPS.  469 


SOAPS. 

A  soap  may  be  described  as  a  compound  of  a  metal  with  one  or  more  fatty  or 
resin  acids,  but  in  popular  use  the  term  without  qualification  has  reference  to 
combinations  of  the  fixed  alkalies  with  stearic,  oleic,  palmitic,  or  resin  acids. 

The  soaps  of  commerce  are  hydrous  compounds  of  soda  with  the  above 
named  acids  that  do  not  retain,  or  but  partially,  the  glycerol  formed  during  the 
saponiflcation.  As  associates  may  be  found  free  sodium  carbonate  or  hydrate, 
unsaponifled  fat,  and  fatty  acids,  and  in  toilet  soaps,  odoriferous  principles 
in  minute  amounts.  Other  substances  of  the  most  varied  nature  may  have  been 
incorporated  during  the  manufacture  to  confer  some  detergent  or  medicinal 
quality,  or  as  adulterants  or  make- weights.  The  most  common  are  the  alkali 
carbonates,  powdered  sand  or  kieselguhr,  borax,  whiting,  sugar,  sulfur,  etc. 

According  to  the  special  fats  used  for  stock  will  the  characteristics  of  the 
resulting  soap  be  modified.  Tallow  furnishes  a  very  hard  product  requiring 
but  little  salt  for  precipitation;  palm  oil  is  easily  saponified,  gives  a  good  hard 
soap,  and  is  much  used  in  admixture  with  tallow;  cottonseed  oil  requires  a 
weak  lye  for  saponification,but  is  difficult  to  salt  out  and  gives  a  comparatively 
soft  soap,  hence  it  is  always  mixed  with  other  stock;  the  cheaper  grades  of  olive 
oil  are  the  best  basis  for  textile  soaps,  the  curd  being  medium  hard  and  the 
solution  remaining  liquid  at  ordinary  temperatures  even  when  very  concen- 
trated ;  cocoanut  oil  requires  a  concentrated  lye  but  readily  saponifies  in  the 
cold,  though  from  the  liability  of  the  product  to  become  rancid,  other  stock  is 
mixed  with  it;  low-grade  linseed  oil  gives  a  soft  inferior  soap  and  is  seldom 
used;  oleic  acid  (a  waste  product),  is  usually  boiled  with  sodium  carbonate 
instead  of  the  more  costly  hydrate. 

The  common  soaps,  when  pure,  are  readily  and  completely  soluble  in  hot 
alcohol,  though  but  slightly  soluble  in  ether,  potassium  oleate  more  readily 
than  the  stearate.  Soap  dissolves  in  cold  water  but  slowly,  but  in  a  moderate 
quantity  of  hot  water  passes  to  a  clear  solution  that  becomes  a  soft  mass  on 
cooling.  On  diluting  an  aqueous  solution  with  a  large  proportion  of  water 
hydrolysis  ensues,  the  soap  partially  decomposing  to  acid  and  alkaline  com- 
pounds that  may  be  separated  by  dialysis,  the  latter  permeating  the  membrane. 
It  is  said  that  the  cleansing  properties  of  soap  are  due  in  part  to  this  decom- 
position. 

The  specific  gravity  is  above  that  of  water  though  so  near  to  it  that  a  *  float- 
ing soap'  is  easily  made  by  incorporating  air  or  carbonic  acid  gas  during 
manufacture. 


A  technical  analysis  should  comprise  the  water,  fatty  acids  and  alkali  both 
free  and  combined,  unsaponifled  oil,  adulterants  and  fillers,  coloring  matter, 
and  medicaments  if  any.  The  amount  of  perfume  oils  is  usually  too  small  for 
determination. 

Water.  The  loss  in  weight  on  drying  is  due  to  water  plus  any  alcohol  or 
other  volatile  matter.  Thin  shavings  cut  from  the  interior  of  a  bar  are 
heated,  at  first  gently  to  avoid  melting,  then  at  105°  to  constant  weight  — 


470  QUANTITATIVE    CHEMICAL   ANALYSIS. 

several  hours  may  be  required  for  complete  desiccation.  The  results  are 
always  a  little  low  from  the  tendency  of  the  surface  of  the  soap  to  dry  to  a 
horny  skin  impenetrable  by  the  moisture  of  the  interior. 

A  better  method  is  to  melt  the  sample  in  a  tared  dish  and  compound  with 
double  the  weight  of  alcohol;  when  the  mass  becomes  homogeneous  an 
equal  weight  of  dry  sand  is  stirred  in  and  the  mixture  evaporated  to  dryness 
on  the  water  bath.  The  disintegrated  residue  is  moistened  with  absolute 
alcohol,  evaporated,  and  dried  at  100  °  to  105  ° . 

A  more  rapid  scheme  is  to  tare  a  porcelain  dish  and  short  glass  rod  with 
flattened  end;  the  soap  shavings  are  introduced  with  some  gasoline  and 
triturated  to  a  paste,  this  heated  to  105  °  to  constant  weight. 

Probably  the  most  accurate  method  is  to  dry  the  shavings  at  110°  to 
115®  in  a  current  of  some  dry  gas  and  absorb  the  water  in  weighed  cal- 
cium chloride  tubes. 

Unsaponifted  fat  is  extracted  from  the  dried  soap  by  dry  ether  or  gasoline 
in  a  Soxhlet's  apparatus  (page  53).  The  fats,  with  any  other  matter  soluble  in 
ether,  is  left  on  evaporation  of  the  solution  and  should  be  examined  quali- 
tatively. 

Insoluble  matter  remains  when  the  residue  from  the  ether  extraction  is  treated 
with  hot  water  and  filtered.  A  residue  will  likely  be  sand,  silica,  pumice,  talc, 
etc.,  and  may  be  further  separated  if  needful,  though  its  nature  will  generally 
be  disclosed  by  inspection  with  a  lens. 

Fatty  acids.  The  filtrate  is  acidified  by  addition  of  a  known  volume  of  stand- 
ard sulfuric  acid.  If  the  fatty  acids  are  not  to  be  further  examined  filtration 
may  be  dispensed  with  by  the  addition  of  a  weighed  lump  of  ceresin  or  stearic 
acid  before  acidulation;  on  cooling  the  acidified  liquid  the  solidified  disk  of 
fatty  acids  and  ceresin  may  be  lifted  out,  rinsed  and  weighed.  Otherwise  the 
liquid  is  heated  until  the  layers  are  clear,  filtered  through  a  wet  paper,  and  the 
filter  washed  with  hot  water. 

A.  In  the  filtrate  are  sodium  sulfate,  the  excess  of  sulfuric  acid,  any  glycerol 
that  may  have  been  in  the  soap,  and  possibly  a  small  amount  of  dissolved  fatty 
acids  assumed  to  be  caprylic.    It  is  titrated  back  by  decinormal  alkali  and 
methyl-orange  to  find  the  excess  of  sulfuric  acid.    From  the  difference  is  cal- 
culated the  total  alkali  of  the  soap.    Phenol- phthalein  is  now  added  and  the 
soluble  fatty  acids  neutralized  by  titrating  with  decinormal  alkali. 

The  liquid  is  evaporated  at  a  gentle  heat  to  a  small  bulk  and  tested  for  gly- 
cerol; if  found,  a  large  weight  of  soap  is  dissolved  in  water,  acidified,  the  lib- 
erated fatty  acids  filtered  off,  and  the  glycerol  determined  in  the  filtrate. 

B.  The  fatty  and  perhaps  rosin  acids  on  the  filter  are  dried  and  weighed, 
then  dissolved  In  hot  neutral  alcohol  and  titrated  by  decinormal  alkali  and 
phenol -phthalein.     A  residue  insoluble  in  alcohol  may  be  silicic  acid  coming 
from  water-glass  in  the  soap.    The  solution  of  soap  is  now  ready  for  a  deter- 
mination of  resin  acids  if  contained  (page  473) . 

Since  the  neutralizing  power  of  the  mixed  fatty  acids  varies  with  their 
respective  proportions,  the  empirical  titre  of  the  standard  alkali  is  best  found 
against  a  suitable  quantity  of  the  dried  fatty  or  resin  acids  prepared  by  decom- 
posing a  large  weight  of  the  aqueous  solution  of  the  soap  by  acid  and  filtering. 
But  if  the  nature  of  the  soap  stock  used  in  the  manufacture  of  the  brand  under 
analysis  is  known,  the  titre  may  fairly  be  calculated  from  the  alkalimetric 
strength. 

A  rapid  method  for  a  determination  of  the  fatty  acids  only,  useful  for  check- 
ing the  manufacture  of  *  run  soaps  '  is  due  to  Walsh.  He  dissolves  the  soap 
in  hot  water  and  pours  the  solution  into. a  separatory  funnel.  After  decom- 


SOAPS.  471 

posing  with  a  fair  excess  of  sulfuric  acid,  the  funnel  is  cooled  under  the  water- 
tap  and  ether  added  to  dissolve  the  liberated  fatty  acids.  The  subnatant  liquid 
is  run  off  and  the  ether  washed  with  saturated  brine  until  the  excess  of  sul- 
furic acid  is  gone.  The  ethereal  solution  is  then  diluted  with  alcohol  and 
titrated  by  standard  alkali  and  phenol -phthalein.  In  a  modification  of  this 
scheme  the  ether  solution  is  evaporated  and  the  residue  weighed,  it  consisting 
of  fatty  acids  and  unsaponified  fat. 

The  nature  of  the  soap-stock,  if  unknown,  may  be  deduced  from  the  con- 
stants of  the  fatty  acids,  sometimes  with  reasonable  assurance.  The  color, 
consistency,  odor  (in  an  unscented  soap),  etc.,  may  also  afford  clues. 

The  direct  separation  of  the  mixed  fatty  acids  is  a  matter  of  difficulty  and 
cannot  be  relied  on  as  giving  more  than  approximate  results.* 

For  the  determination  in  the  mixed  acids  of  those  liquid  at  ordinary  tempera- 
tures (oleic,  linoleic  and  their  homologues)  and  solid  (stearic,  palmitic,  etc.), 
the  Muter  and  DeKoninck  process  applies  the  solubility  in  ether  of  the  lead 
compounds  of  the  liquid  acids  as  against  the  comparative  insolubility  of  those 
of  the  solid.  The  separation  is  made  by  precipitating  the  hot  neutral  aqueous 
solution  of  their  potash  soaps  by  lead  acetate ;  the  lead  soaps  are  washed  with 
liot  water,  dried,  and  extracted  by  hot  ether.  The  ethereal  solution  of  the 
liquid  acid  soaps  (here  usually  lead  oleate)  is  treated  with  hydrochloric  acid 
to  decompose  the  lead  oleate,  forming  lead  chloride  which  passes  into  the 
aqueous  solution,  and  free  oleic  acid  which  remains  in  the  ethereal  layer.  The 
latter  is  decanted,  washed  with  water  to  remove  any  dissolved  hydrochloric 
acid,  then  titrated  by  standard  alkali ;  or  the  weight  of  iodine  absorbed  (page 
455)  may  be  found.  The  lead  compounds  of  the  solid  fatty  acids,  left  after  the 
ether  extraction,  are  decomposed  by  hydrochloric  acid,  and  the  liberated  fatty 
acids  determined  as  above.  A  special  apparatus  is  recommended  for  the 
analysis.  The  results  are  only  fair  at  best,  and  should  the  ratio  of  oleic  to 
stearic  acid  be  small,  the  lead  compound  of  the  former  is  not  completely 
extracted  by  ether. 

Roese,  following  Varrentrapp,  treats  the  mixed  fatty  acids  with  ether  and  lead 
protoxide,  and  allows  the  solution  to  stand  for  a  day  or  two  with  occasional 
shaking  up,  then  dilutes  to  a  definite  volume  and  filters.  An  aliquot  part  of  the 
filtrate  is  evaporated,  and  the  residual  lead  compounds  of  the  liquid  acids  dried 
in  a  current  of  carbon  dioxide  and  weighed,  then  decomposed  by  sulfuric  acid 
and  alcohol,  the  fatty  acids  passing  into  solution,  the  lead  sulfate  weighed  and 
calculated  to  lead  oxide ;  the  difference  in  weight  is  the  weight  of  the  anhydrides 
of  the  liquid  fatty  acids.  The  molecular  weights  and  the  approximate  percen- 
tage of  each  fatty  acid  can  be  calculated  —  since  only  normal  compounds  are 
formed  —  from  the  combining  ratio  with  lead  oxide  and  a  determination  of 
another  constant,  according  to  the  formulae  on  page  157. 

The  residue  of  the  lead  compounds  of  the  solid  fatty  acids  plus  the  excess  of 
litharge  is  dried  and  weighed.  The  lead  oxide  in  the  residue  is  determined  as 
before  and  the  difference  in  weight  is  that  of  the  solid  fatty  acids. 

Other  modifications  are  by  Fahrsteiner  who  separates  the  lead  compounds  by 
cold  benzene,  and  by  Halphen  who  obtains  the  zinc  salts  of  the  mixed  acids  and 
separates  by  carbon  disulflde. 

Hazura  states  that  when  treated  in  dilute  solution  with  alkaline  potassium  per- 
manganate, the  hydroxy -group  is  added  to  the  unsaturated  fatty  acids  only;  thus, 
oleic  acid  (C18H34O2)  is  converted  to  dihydroxystearic  acid  (C18H84O2.(OH)2) 
insoluble  in  water  and  but  sparingly  soluble  in  ether,  but  readily  in  hot  alcohol. 


Brannt,  Oils  and  Fats,  78;  Analyst,  1898—285;  Journ.  Amer.  Chem.  Socy.  1895—289. 


472  QUANTITATIVE    CHEMICAL   ANALYSIS. 

The  soaps  in  dilute  solution  are  digested  for  a  short  time  with  permanganate 
solution,  filtered  from  precipitated  manganic  oxide  and  the  filtrate  acidified  by 
hydrochloric  acid.  The  precipitated  fatty  acids  are  extracted  by  ether  which 
leaves  the  hydroxylated  acids  behind.  If  the  oxidation  be  carried  too  far,  pro- 
ducts other  than  the  hydroxy-acids  are  formed. 

According  to  Twitchell  unsaturated  fatty  acids  react  with  concentrated  sul- 
furic  acid  to  form  additive  compounds  that  are  insoluble  in  petroleum  ether. 
Saturated  acids  do  not  combine  with  sulf  uric  acid. 

Other  methods  are  by  fractional  fusion  and  expression,  fractional  precipi- 
tation by  magnesium  acetate  from  an  alcoholic  solution,  extraction  of  the 
barium  compounds,  etc. 

Free  alkali'  A  portion  of  the  soap  is  dried  in  air  free  from  carbon  dioxide 
and  the  unsaponified  matter  washed  out  by  ether ;  the  residue  is  dissolved  in 
hot  neutral  strong  alcohol,  and  titrated  by  deci-normal  sulf  uric  acid.  Insoluble 
in  alcohol  are  sodium  carbonate,  silicate  and  borate,  sugar,  starch  and  other 
matters,  to  be  further  examined. 

The  determination  of  free  alkali  is  important  in  a  toilet  or  medicinal  soap. 
Those  made  by  the  cold  process  (saponification  below  a  boiling  temperature) 
are  apt  to  contain  both  free  alkali  and  unsaponifled  fat,  and  on  dissolving  in 
alcohol  or  water  the  two  may  unite  to  some  extent.  As  it  is  always  safer  to 
assume  that  both  are  contained  in  a  sample,  the  method  adopted  must  provide 
for  the  removal  of  one  or  the  other  before  the  soap  is  dissolved ;  in  the  above 
the  fat  is  extracted  by  ether. 

If  the  absence  of  free  fat  in  the  soap  is  assured,  simpler  methods  can  be  used. 
Wilson  calls  free  alkali  the  difference  between  the  total  alkali  and  that  calcu- 
lated to  neutralize  the  fatty  acids,  both  determined  by  analysis.  Hope  applies 
the  insolubility  of  sodium  carbonate  in  strong  alcohol;  he  dissolves  the  dried 
soap  in  hot  absolute  alcohol,  adds  phenol-phthalein,  and  passes  a  few  bubbles 
of  carbon  dioxide  until  the  solution  becomes  colorless  (to  convert  the  caustic 
alkali  to  carbonate),  then  filters  and  washes  with  hot  strong  alcohol.  In  the 
residue  is  all  the  free  alkali  as  carbonate  with  other  insoluble  matter,  and  may 
be  titrated  directly  by  weak  standard  acid  and  methyl  orange. 

A  separation  based  on  the  insolubility  of  soap  in  strong  brine  is  possible. 
The  concentrated  aqueous  soap  solution  is  precipitated  by  saturation  with 
sodium  chloride,  filtered  and  washed  with  saturated  brine ;  the  precipitate  is 
redissolved  in  water  and  the  precipitation  repeated.  The  united  filtrates  con- 
tain the  free  alkali  and  are  to  be  titrated  by  standard  acid ;  the  free  fatty  acids 
are  in  the  precipitate  and  may  be  extracted  by  alcohol  and  titrated  by  standard 
alkali  and  phenol-phthalein. 

Low  determines  sodium  carbonate  by  dissolving  the  soap  in  hot  alcohol, 
running  in  more  than  enough  standard  hydrochloric  acid  to  decompose  the 
carbonate,  boiling  to  expel  carbonic  acid,  and  titrating  back  by  equivalent 
sodium  hydrate  and  phenol-phthalein.  The  difference  in  volume  is  that  of  the 
hydrochloric  acid  corresponding  to  the  carbonate  of  sodium.  There  is  now  in 
solution  a  neutral  soap  plus  sodium  chloride,  etc.  The  combined  alkali  is 
titrated  by  standard  hydrochloric  acid  and  lacmoid;  the  liquid  remains  clear 
since  fatty  acids  are  soluble  in  alcohol. 

If  a  determination  of  the  fatty  acids  is  wanted,  they  are  neutralized  by 
sodium  hydrate,  the  soap  evaporated  to  dryness,  dissolved  in  water,  acidified 
and  filtered.  The  fatty  acids  are  dissolved  in  alcohol,  neutralized  by  titration 
with  sodium  hydrate  (free  from  potassium  hydrate),  evaporated  to  dryness,  and 
the  soap  weighed.  The  amount  of  sodium  oxide  therein  is  calculated  from  the 
titration ;  the  difference  is  fatty  acids. 


SOAPS.  473 

The  combined  alkali  is  understood  to  be  the  difference  between  the  total  alkali 
and  that  in  the  free  state  or  as  carbonate,  plus  any  that  is  combined  with  acids 
other  than  fatty  or  resin  acids.  If  there  is  no  free  fatty  acid  or  fat  in  a  soap 
and  the  composition  of  the  original  fat  is  known,  the  combined  alkali  can  be 
calculated  from  the  weight  of  fatty  acids  found.  A  direct  determination  can  be 
made  by  dissolving  the  soap  in  absolute  alcohol,  filtering,  evaporating  the 
filtrate  to  dryness,  and  weighing  the  residue ;  then  igniting  to  burn  the  fatty 
acids,  and  titrating  the  residue  of  carbonate  of  sodium  by  a  standard  acid.  The 
difference  between  the  alkali  as  calculated  from  the  titration  and  the  weight  of 
the  residue  from  evaporation  is  the  fatty  anhydrides,  which  multiplied  by  1.03 
gives  the  corresponding  fatty  acids. 

Of  the  more  common  admixtures  and  adulterations,  sugar  may  be  deter- 
mined by  dissolving  the  soap  in  water,  liberating  the  fatty  acids  by  heating 
with  dilute  hydrochloric  acid,  filtering  and  determining  the  invertose  in  the 
filtrate,  but  here  the  optical  activity  of  other  soluble  matters  in  the 
soap  may  increase  the  reading.  Wilson  precipitates  the  fatty  acids  from  an 
aqueous  solution  of  soap  by  a  saturated  solution  of  magnesium  sulfate,  fil- 
ters, evaporates  the  faintly  alkaline  filtrate,  acidifies  and  clarifies  by  lead 
acetate  and  alumina  cream,  and  polarizes. 

Another  plan  is  to  treat  the  aqueous  solution  of  the  soap  with  slaked 
lime  and  a  quantity  of  sand  and  evaporate  to  dryness.  A  pasty  grease  re- 
mains on  cooling  containing  the  sugar  as  calcium  saccharate,  which  is  to 
be  treated  with  a  mixture  of  alcohol  and  ether  to  dissolve  out  the  glycerol. 
The  residue  is  boiled  with  dilute  sulfuric  acid  to  decompose  the  saccharate 
and  invert  the  sugar;  the  latter  is  then  determined  by  Fehlings  solution 
or  the  saccharimeter. 

Sand  and  like  insoluble  bodies  are  left  when  the  soap  is  dissolved  in  hot 
water  and  may  be  dried  and  weighed ;  amorphous  silica  can  be  dissolved  out 
by  a  solution  of  caustic  alkali.  Sodium  silicate  remains  when  a  soap  is  dis- 
solved in  alcohol,  and  may  be  decomposed  by  hydrochloric  acid  and  the  silrca 
determined  as  usual.  It  is  said  that  sodium  silicate  does  not  react  alkaline  to 
phenol -phthalein  in  a  soap  containing  less  than  15  per  cent  of  this  compound.* 

The  analysis  of  metallic-fatty-acid  compounds  (other  than  of  the  alkali 
metals),  known  as  plasters,  greases,  etc.,  proceeds  along  the  same  lines  as  that 
for  an  ordinary  soap,  but  being  insoluble  in  water,  the  fatty  acids  are  liberated 
by  boiling  at  once  with  dilute  hydrochloric  acid.  After  filtering,  the  fatty  acids 
are  neutralized,  and  any  saponiflable  oils  or  fats  hydrolyzed  by  potassium  hy- 
drate, and  the  unsaponifiable  matter  extracted  from  the  soap  by  ether.  The 
determination  of  the  metallic  base  of  the  soap  usually  presents  no  difficulties. 

Resin  anhydrides.  The  resins  are  a  class  of  bodies  whose  exact  composition 
has  not  as  yet  been  fully  established.  Common  colophony,  or  shortly  rosin,  is 
the  residue  left  when  the  turpentine  has  been  distilled  from  the  exudation  of  the 
Georgia  pine,  and  consists  mainly  of  an  acid  (abietic,  CigH23O2  ?)  or  mixture 
of  acids  with  certain  hydrocarbons.  When  the  acid  is  neutralized  by  sodium 
hydrate  a  rosin  soap  is  formed  which  is  soluble  in  hot  water  and  in  alcohol  but 
insoluble  in  ether,  and  presents  many  of  the  characteristics  of  a  fatty  acid  soap. 

In  the  analysis  of  a  soap  containing  rosin  acids  the  problem  is  to  separate 
them  from  the  fatty  acids  in  the  mixture  obtained  on  decomposition  of  the  soap 
with  a  mineral  acid.  Four  methods  will  be  outlined. 

1.  Twichell's  f  Of  the  four  this  is  generally  acknowledged  the  superior.    It 


*  Analyst,  1896—72. 

t  Journ.  Anal.  Appl.  Chem.  1891—379. 


474  QUANTITATIVE    CHEMICAL    ANALYSIS. 

is  based  on  the  principle  that  the  fatty  acids  are  converted  into  ethylic  esters 
when  treated  with  hydrochloric  acid  in  alcohol  solution,  while  under  these 
circumstances  the  resin  acids  are  unaffected  and  separate  as  such. 

If  a  commercial  soap  is  under  examination,  it  is  dissolved  in  water  and  acidi- 
fied, and  the  mixture  of  fatty  and  rosin  acids  separated  from  the  solution  and 
dried.  A  few  grams  of  the  mixture  is  weighed  and  dissolved  in  absolute  alcohol 
and  a  current  of  dry  hydrochloric  acid  passed  through  the  cold  solution  to 
saturation.  On  diluting  with  water  and  boiling,  the  ethers  and  resin  acids 
float,  and  after  cooling  the  solution,  may  be  dissolved  in  ether.  The  aqueous 
solution  is  drawn  off  from  the  ethereal  in  a  separatory  funnel  and  the  latter 
washed  with  water. 

The  ether  solution  is  now  diluted  with  alcohol  and  the  resin  acids  titrated 
by  standard  sodium  hydrate,  the  ethers  not  interfering.  The  mean  combin- 
ing weight  of  the  acids  of  rosin  may  be  taken  as  346.  The  percentage  of 
fatty  acids  is  found  from  the  difference  on  titrating  a  portion  of  the  mixed 
acids  by  alkali. 

Or  the  resin  acids  may  be  determined  gravimetrically  by  dissolving  the 
ethers  and  resin  acids  in  gasoline  and  separating  from  the  aqueous  solution. 
On  treating  the  former  with  water  and  potassium  hydrate  the  acids  are 
neutralized  and  pass  into  the  aqueous  solution  while  the  ethers  remain  in  the 
gasoline.  The  soap  solution  is  decanted,  decomposed  by  acid,  and  the  resin 
acids  filtered  and  weighed. 

2.  Gladding's.*  The  silver  salts   of  the   resin    acids  are  soluble  in  ether 
while  the  silver  salts  of  the  fatty  acids  are  insoluble.    The  mixture  of  fatty 
and   resin  acids  is  dissolved  in  a  small  quantity   of  alcohol,  the   solution 
neutralized  by  potassium  hydrate,  and  diluted  to  a  known  volume  with  ether. 
Finely  powdered  silver  nitrate  is  stirred  in  and   the  mixture  agitated,  when 
the  silver  salts  of  the  fatty  acids  precipitate  in  flocks.    One-half  of  the  clear 
liquid  is  drawn  off  and  the  silver  resinate  decomposed  by  hydrochloric  acid, 
chloride  of  silver  precipitating.    From  the  filtrate  the  resin  acids  are  recovered 
by  evaporation.    The  silver  salts  of  the  fatty  acids  may  be  suspended  in  water 
and  decomposed  by  hydrochloric  acid  and  the  fatty  acids  determined. 

Bouley  f  dispenses  with  the  isolation  of  the  mixed  acids  by  compounding 
the  aqueous  solution  of  the  soap  with  silver  nitrate,  evaporating  to  dryness, 
and  dissolving  out  the  silver  resinate  by  extraction  with  ether  in  a  continuous 
percolation  apparatus. 

3.  Barfoed's.    The  sodium  resinates  are  soluble  in  a  mixture  of  anhydrous 
alcohol  and  ether  while  the  fatty  acid  soaps  are  insoluble.    The  mixed  acids 
are  dried  and  weighed,  neutralized  by  sodium  hydrate,  the  solution  evaporated 
to  dryness  and  the  residue  powdered,  then  heated  with  absolute  alcohol ;  on 
the  addition  of  ether  the  fatty  acids  are  precipitated  and  the  resin  acids  go 
into  solution.    The  liquid  is  made  up  to  a  definite  volume  and  an  aliquot 
part  of  the  clear  supernatant  liquid  withdrawn,  evaporated  to  dryness,  the 
residue  dissolved  in  water  and  the  resin  acids  precipitated  by  a  mineral  acid. 
Subtracting  their  weight  from  that  of  the  mixture  gives  the  weight  of  the  fatty 
acids.    The  separation  is  not  as  sharp  as  has  been  claimed,  especially  where 
one  of  the  fatty  acids  is  oleic. 

A  similar  approximate  separation  is  by  the  process  of  Jean-Remont.  The 
mixed  acids  are  combined  with  barium  and  the  baryta  soaps  extracted  by  hot 
85  per  cent  alcohol  which  dissolves  only  the  baryta  resinates. 


*  Journ.  Socy.  Dpers  &  Col.  1895—86. 
t  Journ.  Socy.  Dyers  &  Ool.  1892-82. 


SOAPS.  475 

4.  Cornette's  method  depends  on  the  insolubility  of  the  sodium  compounds 
of  the  higher  fatty  acids  in  a  concentrated  solution  of  sodium  chloride,  the  cor- 
responding resinates  being  freely  soluble.  To  the  solution  of  the  fatty  acids 
in  a  little  alcohol  are  added  sodium  hydrate  to  neutralization  and  after  cooling, 
strong  brine,  and  the  precipitate  of  the  soaps  of  the  fatty  acids  filtered  and 
washed  with  salt  solution.  From  the  filtrate  the  resin  acids  are  precipitated 
by  a  mineral  acid;  the  fatty  acid  soaps  may  also  be  decomposed  in  the  same 


*  Chem   Zeit.  1897—25. 


476  QUANTITATIVE   CHEMICAL   ANALYSIS. 


Total 

solids. 

Fat. 

Casein. 

Milk- 
sugar. 

Ash. 

Albumin. 

11.81 

3.78 

1.00 

6.22 

.31 

.56 

12.77 

3.66 

3.02 

4.85 

.71 

.53 

13.52 

4.34 

2.53 

3.78 

.65 

... 

16.60 

6.05 

5.73 

3.96 

.68 

... 

10.99 

1.85 

3.57 

5.05 

,  .  . 

9.55 

1.31 

2.53 

5.42 

.29 

... 

19.36 

8.45 

4.25 

4.51 

.85 

... 

13.66 

2.90 

3.67 

5.78 

.66 

18.20 

6.00 

5.30 

6.07 

.83 

... 

33.30 

22.07 

3.21 

7.39 

.63 

MILK  —  BUTTER. 

The  lacteal  secretion  of  mammals  is  a  white  liquid  composed  of  globules  of 
fat  suspended  in  a  fluid  known  as  milk -plasma,  serum,  or  whey. 
The  following  are  analyses  of  the  milks  of  various  animals. 

Water. 

Human 88.19 

Cow 87.23 

Goat 86.85 

Ewe 83.30 

Ass 89.01 

Mare 90.45 

Buffalo 80.64 

Camel 86.34 

Sow 81.80 

Elephant 66.70 

Cows  milk  is  essentially  a  solution  of  milk-sugara  casein,  albumin,  mineral 
matter  and  small  quantities  of  various  organic  bodies,  holding  in  suspension 
innumerable  minute  globules  of  a  fat.  The  quantitative  composition  varies 
considerably,  marked  differences  resulting  from  the  breed  of  the  cows,  kind  and 
amount  of  feed,  the  season  of  the  year,  time  and  manner  of  milking,  etc.,  hence 
legal  limits  as  to  the  percentage  of  the  different  constituents  in  salable  milk 
are  only  to  be  established  from  data  furnished  by  a  large  number  of  samples 
known  to  be  unsophisticated. 

Richmond  *  has  published  the  following  figures  as  the  averages  of  172,000 
samples  of  new  cows-milk  received  at  the  Aylesbury  Dairies  in  England : 
specific  gravity  at  60°  Fahr.,  1.03215;  total  solids,  12.86  percent,  consisting 
of  fat,  4.02,  and  of  solids- not-fat,  8.84.  In  fourteen  years  the  yearly  averages 
showed  extreme  differences  in  specific  gravity  of  .0008;  of  total  solids,  .40; 
of  fat,  .38;  and  of  solids-not-fat,  .20  per  cent.  Of  the  milk  believed  to  be 
genuine,  that  with  less  than  8.3  per  cent  of  solids-not-fat  was  only  .059  per 
cent  of  the  total  receipts,  and  under  8.1,  only  .01  per  cent.  Faber  states  that 
the  results  of  about  50,000  analyses  show  the  solids-not-fat  to  be  an  almost 
constant  number,  viz.,  8.7  to  8.8  per  cent. 

The  legal  minimum  in  many  of  the  United  States  is  a  specific  gravity  of 
1.029;  total  solids,  not  less  than  12  per  cent;  and  fat  not  less  than  3  per  cent. 
But  Van  Slyke  believes  that  genuine  milk  not  infrequently  falls  below  the  two 
latter  requirements. 

The  proximate  analysis  of  a  milk  for  the  major  constituents  is  not  dinlcult. 
The  number  of  constituents  to  be  determined  depends  on  the  object  for  which 
the  analysis  is  prosecuted ;  usually  the  investigation  is  carried  only  so  far  as 
will  suflBce  to  assure  the  chemist  that  the  sample  has  or  has  not  been  sophisti- 


*  Analyst,  1897-93  and  1898—90. 


MILK BUTTER.  477 

cated.  Some  authorities  urge  that  the  chemical  analysis  be  always  followed 
by  a  microscopical  examination,  and  a  search  for  disease  germs  when  warranted 
by  an  unhealthy  condition  of  the  cow. 

Where  an  immediate  examination  of  a  sample  of  milk  is  not  practicable, 
decomposition  can  be  prevented  for  some  time  by  the  addition  of  a  trace  of  a 
preservative  or  germicide.  Special  precautions  are  required  in  the  analysis  of 
a  milk  altered  by  fermentation. 

On  account  of  the  considerable  difference  between  the  specific  gravities  of 
the  fat  and  whey,  the  portions  for  analysis  should  be  drawn  out  only  after 
thorough  agitation  of  the  original  quantity.  The  portions  for  the  various 
determinations  may  be  weighed;  or  more  conveniently  measured  from  a  pipette, 
and  the  weight  calculated  from  the  specific  gravity  of  the  sample.  Pipettes  are 
on  sale  graduated  to  deliver  certain  weights  based  on  the  average  specific 
gravity  of  milk. 

Water.  The  average  percentage  is  about  88.75.  The  determination  is  most 
accurately  made  by  evaporating  in  vacuo  a  small  volume  of  the  milk  and  ab- 
sorbing the  distillate  in  a  tared  chloride  of  calcium  tube,  but  the  practical  value 
of  the  determination  does  not  justify  so  elaborate  a  process.  With  few  excep- 
tions, the  content  of  water  is  found  (1),  by  the  loss  in  weight  on  evaporation 
of  the  milk  to  dryness;  (2),  calculated  from  the  specific  gravity  of  the  milk;  or 
(3),  in  a  complete  analysis,  by  difference. 

The  usual  method  is  to  tare  a  flat-bottomed  platinum  dish,  introduce  five 
grams  or  five  cubic  centimeters  of  milk,  evaporate  to  dryness  on  the  water- 
bath  and  heat  the  residual  solids  in  an  air  bath  to  100°  ;  the  loss  in  weight  is 
water.  The  residue  can  be  used  for  the  determination  of  fat  or  other  constitu- 
ents if  desired. 

Practically  milk  is  adulterated  only  by  watering,  skimming  off  the  cream,  or 
both.  It  is  somewhat  difficult  to  distinguish  between  a  genuine  milk  low  in 
fat  and  one  from  which  part  of  the  fat  has  been  abstracted.  Richmond  calcu- 
lates the  water  of  dilution  by  uniting  the  percentage  of  fat  found  by  analysis 
to  the  specific  gravity  (water  =  1000),  and  deducting  1000  from  the  sum.  The 
remainder  in  the  case  of  normal  milk  is,  on  an  average,  36,  but  taking  34.5  as 
a  safer  basis,  the  proportion  stands  34.5  :  remainder  :  :  100  :  percentage  of 
genuine  milk  in  the  sample.  Thus  a  milk  having  a  specific  gravity  of  1029.2 
and  containing  3.27  per  cent  of  fat,  would  appear  to  contain  only  94.1  per  cent 
of  genuine  milk  plus  5.9  of  added  water.  This  on  the  assumption  that  the 
addition  of  water  reduces  the  specific  gravity  and  percentage  of  fat  in  the 
same  proportion. 

(1)  When  the  sample  has  simply  been  diluted  with  water  the  proportion 
of  fat  and  solids-not-fat  are  correspondingly  reduced,  and  the  percentage  of 

added  water  is  calculated  from  the  formula  X=^-JT  —  w,  where  X is  the 

o 

added  water;  W,  the  percentage  of  water  in  the  sample;  S,  the  solids  not 
fat;  and  W  and  S't  the  minimum  averages  of  water  and  solids-not-fat  in 
genuine  milk.  An  allowance  must  be  made  for  the  solid  constituents  of  the 
added  water  if  it  be  very  hard. 

(2)  For  skim  milk  or  separated  milk  the  amount  of  fat  abstracted,  Tt  is 

Of  f 

obtained  from  the  formula  Y=f  —  _±  where  /  is  the  percentage  of  fat  in 

the  sample,  and  S  of  the  solids-not-fat;  and  /'  and  S't  the  minimum  average 
of  fat  and  solids-not-fat  in  genuine  milk.  The  fat  removed  times  1.54  is 
approximately  the  percentage  of  cream  removed. 


478  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Should  the  water  of  dilution  contain  nitrates,  absent  from  genuine  milk, 
one  of  the  colorimetric  methods  used  in  water  analysis  may  be  applied  to  the 
coagulated  and  filtered  sample. 

Total  solids,  The  total  solid  matter  may  be  calculated  with  fair  accuracy 
from  the  specific  gravity.  This  is  a  resultant  of  two  factors,  the  proportion 
of  fat,  which  is  lighter  than  water,  and  of  the  solids -not-fat,  which  are  heavier ; 
and  therefore  as  the  fat  is  greater  or  the  solids-not-fat  less,  the  lower  the 
specific  gravity,  and  vice  versa.  Hence  the  possibility  of  removing  part  of 
the  fat  by  skimming,  then  lowering  the  specific  gravity  again  to  the  normal  by 
pure  Water  —  a  common  practice  of  dairymen  in  former  times  when  the  gravity 
alone  was  the  criterion  of  unadulterated  milk. 

The  density  of  normal  milks  at  60  o  Fahr.  ranges  from  1.020  to  1.035,  though 
only  a  small  proportion  fall  outside  of  the  limits  of  1.028  to  1.034,  and  when  the 
milks  of  several  cows  are  mixed,  is  fairly  constant  at  1.030  to  1.032;  the  legal 
minimum  in  some  States  is  1.029.  Any  method  of  observation  may  be  em- 
ployed, the  simplest  being  the  lactometer,  a  specially  graduated  hydrometer 
with  a  thermometer  inclosed.  As  read  by  this  instrument,  the  mean  density  of 
several  thousand  samples  was  1.032.  The  Quevenne  lactometer  is  graduated 
in  degrees  of  specific  gravity,  the  integer  (1)  and  first  decimal  (0)  being 
omitted  on  the  scale  for  want  of  space.  It  is  usually  graduated  from  20  to  40. 

Although  combined  skimming  and  watering  may  fail  of  detection  by  a 
density  determination  alone,  if  the  fat  also  is  found  the  total  solids  may 
be  calculated  with  fair  certainty  by  the  formula  of  Hehner  and  Richmond,* 

T=  —       D  ~ — -  +1.2  F;  where  T  represents  the  total  solids  D,  the  specific 

gravity  (water  =  1)  and  F,  the  fat. 

The  direct  determination  of  the  total  solids  is  done  by  evaporating  a  small 
volume  of  the  milk  in  a  flat-bottomed  dish  on  the  water  bath,  and  drying  the 
residue  at  100  °  to  constant  weight.  Davenport  f  directs  five  grams  in  a  dish 
of  2.5  inches  diameter  with  a  flat  bottom  rounding  up  into  the  sides,  as  in  this 
form  the  residue  is  left  in  a  uniformly  thin  layer  readily  parting  with  the  last 
traces  of  water.  BlythJ  finds  that  evaporation  in  glass  or  porcelain  dishes 
invariably  yields  slightly  higher  weights  of  residue  than  in  platinum,  due  per- 
haps to  the  shape  of  the  vessels  and  consequent  variations  in  the  thickness  of 
the  film,  or  to  the  lower  temperature  of  drying  from  the  inferior  conductivity 
of  glass  for  heat.  The  water  passes  off  more  quickly  when  the  evaporation 
is  done  in  vacuo. 

Instead  of  evaporating  the  milk  in  a  dish,  some  prefer  to  soak  it  up  in  sand 
or  asbestos,  thereby  extending  the  surface  and  facilitating  the  removal  of  water. 
Babcock  proposes  a  perforated  metal  vase  filled  with  flocculent  asbestos,  the 
residue  being  left  in  a  form  suitable  for  the  extraction  of  the  fat. 

The  solids-not-fat  are  milk-sugar,  proteids  (principally  casein),  and  inorganic 
saltSc  Traces  of  citric  acid  (about  .1  per  cent)  and  various  organic  bodies  are 
also  contained.  The  solids-not-fat  rarely  fall  below  8.5  in  a  normal  milk,  and 
are  practically  constant  at  8.76. 

Ash.  The  ash  of  genuine  milk  is  composed  principally  of  the  alkalies  and 
calcium  united  with  phosphoric  acid  and  chlorine,  but  owing  to  changes  induced 
on  combustion  of  the  organic  matter  of  the  residue  from  evaporation,  the  ash 


*  Analyst,  1898—2. 

t  Journ.  Anal.  Chem.  1889—309. 

}  Blyth,  Foods,  268. 


MILK BUTTER.  479 

does  not  represent  exactly  the  salts  as  they  existed  in  the  milk.  About  30  per 
cent  of  the  ash  is  soluble  in  water,  the  solution  reacting  neutral,  and  the 
remainder  soluble  in  dilute  hydrochloric  acid.  Carbonates  and  borates  are 
absent. 

However  low  in  solids-not-fat  a  sample  of  genuine  milk  may  be,  it  yields 
never  less  than  .7  per  cent  of  ash,  while  the  majority  of  abnormal  samples  leave 
an  unusually  high  ash  with  an  unduly  large  proportion  of  chlorine.  Watering 
a  milk  reduces  the  ash  almost  proportionally  if  the  water  be  fairly  pure.  Rich- 
mond regards  watering  as  practically  proved  if  the  weight  of  solids-not-fat  is 
exceptionally  low,  and  the  ash,  though  of  normal  weight,  has  an  alkaline  reac- 
tion and  contains  sulfates,  or  of  which  more  than  30  per  cent  is  soluble  in  water. 

The  determination  is  made  by  calcining  the  residue  from  evaporation  in  a 
platinum  dish  until  the  inorganic  matter  is  fairly  white.  Owing  to  the  ready 
fusibility  of  some  of  the  constituents,  the  dish  is  never  allowed  to  become 
heated  above  dull  redness.  The  volume  of  milk  evaporated  for  the  ash  deter- 
mination should  be  moderate  —  say  25  Cc.  —  as  with  a  vessel  of  the  usual  size 
so  much  carbon  separates  from  a  bulky  residue  on  ignition  that  its  combustion 
increases  the  heat  to  above  the  melting  point  of  the  ash.  To  prevent  loss  of 
sulfur  and  phosphorus  in  organic  combinations,  the  milk  may  be  acidified  by 
nitric  acid  before  evaporation,  this  however  converting  sodium  chloride  to  the 
nitrate. 

Acidity.  Immediately  after  leaving  the  cow,  the  reaction  of  milk  is  ampho- 
genous,  said  to  be  due  to  calcium  phosphate.  But  shortly  the  reaction  turns 
decidedly  acid  from  the  conversion  of  milk-sugar  to  lactic  acid  —  C12H24O12  = 
4HCSH603. 

For  a  determination,  the  carbon  dioxide  is  removed  from  the  milk  by  dilu- 
tion with  hot  water  and  boiling,  then  the  liquid  titrated  by  decinormal  alkali 
and  phenol-phthalein.  The  result  is  expressed  in  terms  of  lactic  acid. 

Lactic  acid  may  be  determined  gravimetrically  by  combining  it  with  lead 
to  form  lead  lactate,  this  salt  soluble  in  acid  and  neutral  solutions,  but  precip- 
itated from  an  ammoniacal  solution  containing  alcohol.  The  lactic  acid  and 
fat  are  extracted  by  ether  from  the  residue  left  on  evaporating  the  milk.  The 
ether  solution  is  mixed  with  water  and  the  ether  driven  off  by  heating,  when 
the  fat  separates  and  may  be  filtered  off.  The  filtrate  is  treated  with  lead 
acetate,  filtered  from  any  precipitate  of  carbonate,  etc.,  and  mixed  with  alco- 
hol and  ammonia.  The  precipitate  is  washed  with  alcohol,  dried  and  weighed 
as  3PbO.2(C3H6O3).  If  desired  the  precipitate  may  be  suspended  in  water,  the 
lead  thrown  down  by  hydrogen  sulfide,  and  the  lactic  acid  determined  in  the 
filtrate. 

In  a  modification  of  the  above,  the  milk  is  evaporated  to  dryness  and  the 
fat  extracted  by  carbon  disulfide.  The  residue  is  treated  with  oxalic  acid  and 
water,  then  with  lead  hydroxide,  and  filtered.  The  filtrate  contains  lead  lac- 
tate which  may  be  precipitated  and  weighed,  or  converted  to  zinc  lactate  by 
digestion  with  zinc  oxide  and  the  product  crystallized. 

Another  method  applies  the  property  of  ether  to  extract  from  milk  both  fat 
and  lactic  acid,  and  of  carbon  disulflde  to  extract  fat  only.  Equal  volumes 
of  milk  are  extracted  by  these  reagents,  the  two  solutions  evaporated,  and 
the  residues  weighed ;  the  difference  between  the  weights  is  lactic  acid. 

Fat.  The  fat  of  cows  milk  is  a  glyceride  yielding  fatty  acids  and  glycerol  on 
saponification,  but  is  distinguished  from  other  animal  fats  by  containing  a 
notable  proportion  of  butyrin  and  its  analogues. 

The  fat  is  obviously  the  constituent  of  most  importance  to  the  consumer, 


480  QUANTITATIVE    CHEMICAL    ANALYSIS. 

and  many  methods  have  been  contrived  for  its  determination.      They  may  be 
divide  d  into  four  classes  : 

1.  Wherein  the  fat  globules  are  segregated  or  collected  to  a  liquid  floating 
on  the  whey,  the  volume  measured,  and  the  weight  calculated. 

2.  Wherein  the  fat  is  extracted  directly  from  the  milk  by  an  organic  solvent, 
the  solution  evaporated,  and  the  residual  fat  weighed  or  its  weight  computed 
from  data  given  by  measuring  some  physical  property  of  the  solution. 

3.  Wherein  the  water  is  removed  from  the  milk  by  evaporation  or  absorp- 
tion and  the  fat  extracted  from  the  total  solids  by  an  organic  solvent,  yield- 
leg  it  on  evaporation. 

4.  Approximately,  by  some  physical  characteristic  of  the  milk. 

1.  The  milk  is  allowed  to  stand  for  24  hours  in  a  tall  graduated  jar  until 
the  globules  have  risen  to  the  surface,  when  the  volume  of  the  cream  is 
read.  The  *  creamometer  '  holds  100  volumes  of  milk  and  is  graduated 
from  zero  at  the  top  down  to  20  volumes,  the  cream  rising  to  compass 
ordinarily  from  11  to  13  divisions.  The  method  is  practically  obsolete,  on 
account  of  its  slowness,  that  the  fats  of  milks  from  different  breeds  and  in- 
dividual cows  do  not  separate  uniformly,  and  that  the  temperature  and 
time  of  repose  largely  affects  the  extent  of  the  separation. 

Marchand's  lactobutyrometer  is  a  specially  graduated  test-tube  about 
twelve  inches  long  by  one-half  inch  diameter,  divided  from  20  to  30  Cc. 
into  tenths  of  a  Cc.  Ten  Cc.  of  milk  is  introduced  with  enough  potassium 
hydrate  to  dissolve  the  casein;  then  ten  Cc.  of  ether  is  added,  which  on 
shaking  extracts  the  fat.  Finally  strong  alcohol  is  poured  in  and  the  mixture 
well  shaken.  The  alcohol  throws  out  the  fat  from  the  ethereal  solution, 
and  on  standing,  the  fat  collects  above  the  layers  of  serum  and  alcohol  - 
ether,  where  its  volume  may  be  read  by  the  graduations  on  the  tube,  one 
Cc.  corresponding  to  .233  gram  of  fat.  A  correction  is  made  for  the  con- 
stant quantity  of  fat  retained  in  solution  by  the  ether-alcohol. 

As  the  adulteration  of  the  milk  has  been  so  extensively  carried  on,  sys- 
tematic examination  becomes  a  necessity  for  the  protection  of  the  public, 
Affording  a  means  for  the  rapid  division  of  a  number  of  samples  into  two 
^_^  classes,  the  probably  genuine  and  the  doubtful  or  probably  adul- 
terated, the  processes  described  below  have  made  possible  an  effec- 
tive control  of  the  quality  of  the  supply  to  cities  and  dairies.  The 
processes  are  very  rapid  and  a  large  number  of  samples  can  be 
carried  through  in  one  period.  The  results  are  accurate  enough 
for  the  purpose  of  differentiation  and  are  as  a  rule  reliable,  yet  it 
is  advised  that  any  abnormal  showing  on  which  much  depends  be 


\ 


checked  by  a  gravimetric  method. 

The  separation  and  measuring  of  the  fat  is  done  in  a  small  cyl- 
indrical flask,  Fig.  181,  with  a  long  narrow  neck  graduated  into 
cubic  centimeters  or  directly  into  percentages  and  tenths  of 
fat.  A  measured  volume  of  the  milk  is  run  into  the  flask  followed 
by  the  reagent.  The  flask  containing  the  hot  mixture  is  laid  in  a 
centrifuge  (page  86)  and  whirled  for  a  few  minutes,  lying  mean- 
time in  a  horizontal  radial  position  mouth  inwards.  The  flask  is 
then  removed,  the  liquid  made  up  to  the  highest  division  with  hot 
water,  and  again  centrifuged  for  a  minute.  The  entire  mass  of 
Fig.  181.  the  fat  will  now  have  risen  to  lie  between  some  two  of  the  divisions 

where  its  volume  may  be  read. 
A  number  of  modifications  of  the  flask,  reagent,  and  details  of  operating 


MILK  —  BUTTER. 


481 


Fig.  182. 


have  been  proposed.    The  original  device  of  Laval  known  as  the  lactocrite  has 

been  largely  superseded  by  the 

apparatus  of  Babcock,   Leff- 

man    and  Beam,   Gruber  and 

others. 

Babcock  directs  the  use  of 

17.6  Cc.  of  milk,  equal  practi- 
cally to  18  grams,  with  an  equal 

volume  of  sulfuric  acid.    The 

mixture  becomes  dark  in  color 

and  very  hot,  and  the  casein 

is  nearly  or   completely  dis-jf 

solved.     The  flasks  and  centri  - 

fugal    machine  are  shown  in 

Fig.  1 82 .    Leff man  and  Beam's 

flasks    are   graduated    on  the 

neck  directly  to  tenths  of  one 

per  cent  V/V  of  the  fat.    Their  formula  for  the  mixture  is  15  Cc.  of  milk  with  9 

Cc.  of  concentrated  sulfuric  acid,  1.5  Cc.  of  concentrated  hydrochloric  acid,  and 

1.5  Cc.  of  amyl  alcohol,  the  two  latter  aiding  to  a  sharp  separation  of  the  fat. 

Afterward  the  mixture  is  diluted  with  a  hot  mixture  of  one  volume  of  sulfuric 

acid  with  two  volumes  of  water,  then  whirled  for  two  minutes. 
Gruber's  directions  call  for  ten  Cc.  of  milk,  ten  Cc.  of  sulfuric  acid,  and  one 

of  amyl  alcohol ;  that  of  Stokes  is  but  slightly  different.    Patrick  heats  with  a 

mixture  of  nine  parts  of  concentrated  acetic  acid,  five  of  sulfuric  acid,  and  two 

of  hydrochloric.  Leze  heats  one  part  of  milk  with  two  of  concentrated  hydro- 
0  chloric  acid  until  the  liquor  turns  brown,  then  dilute  ammonia 
until  clear,  and  finally  dilutes  with  hot  water.  He  finds  the 
specific  gravity  of  the  fat  liberated  in  this  way  to  be  nearly  .900. 
All  the  above  modifications  act  or  are  supposed  to  act  to  advan- 
tage for  the  segregation  of  all  the  fat  to  a  clear  homogeneous 
liquid. 

Instead  of  collecting  and  measuring  the  fat  itself,  some  would 
operate  on  the  fatty  acids  therefrom.  In  the  methods  of  Short 
and  Thoerner,  the  milk-fat  is  saponified  by  potassium  and  so- 
dium hydrates,  which  on  long  heating  also  dissolve  the  casein 
and  albumin.  The  soap  is  decomposed  by  sulfuric,  or  sulfuric 
and  acetic  acids,  and  the  fatty  acids  floated  by  a  centrifuge  and 
measured,  the  volume  being  to  that  of  fat  as  100  to  87. 

2.  In  the  areometric  method  of  Soxhlet  the  fat  is  extracted  by 
ether  from  the  milk  made  alkaline  by  potash.  As  the  fat  is  in 
the  form  of  an  emulsion  the  extraction  is  complete  in  one 
operation.  Two  hundred  Cc.  of  the  milk  is  made  alkaline,  a 
known  volume  of  water-saturated  ether  added,  the  mixture 
shaken  for  a  few  minutes,  and  the  specific  gravity  of  the  ether 
observed ;  from  the  increase  in  density  of  the  ether  is  calculated 
the  proportion  of  fat  taken  up.  The  operation  is  conducted  in 
an  apparatus  shown  in  Fig.  183.  In  the  bottle  A  is  fitted  a  cork 
through  which  pass  the  tube  B  ending  just  below  it,  and  C, 
which  can  be  raised  or  lowered  as  desired.  The  upper  end  of 
C  is  enlarged  to  a  cylinder  D  containing  a  delicate  areometer  F 
with  thermometer  inclosed,  and  surrounded  by  a  water-jacket 

holding  water  at  17.5°  to  keep  the  temperature  of  D  constant.     When  the 

31 


Fig.  183. 


482  QUANTITATIVE    CHEMICAL    ANALYSIS. 

ethereal  solution  of  the  fat  has  risen  to  the  surface,  the  tube  C  is  lowered  until 
the  lower  orifice  is  just  above  the  surface  of  the  aqueous  solution.  If  now  air 
be  blown  into  B  by  compressing  the  rubber  bulb  G,  the  ethereal  solution  is 
forced  up  into  D  where  its  gravity  may  be  read  by  the  areometer.  The  increase 
in  gravity  is  referred  to  a  table  drawn  up  from  gravimetric  determinations. 
Wiley  *  secures  a  prompt  separation  of  the  ether  and  water  solutions  by  the 
aid  of  a  centrifuge.  The  method  fails  for  certain  milks. 

Cronander  proposes  to  evaporate  the  ether  extract  and  measure  the  volume 
of  the  molten  residual  fat. 

In  the  method  of  Werner  Schmid,f  ten  grams  of  milk  in  a  graduated  test- 
tube  of  50  Cc.  capacity  is  heated  to  100°  with  ten  Cc.  of  concentrated 
hydrochloric  acid,  and  after  cooling,  the  fat  taken  up  by  30  Cc.  of  ether. 
The  volume  of  the  ethereal  layer  is  noted,  and  ten  Cc.  drawn  out  with  a 
pipette,  evaporated  to  dryness,  and  the  fat  weighed.  A  safer  plan  is  to  extract 
all  the  fat  by  four  smaller  portions  of  ether,  evaporate  the  whole  and  weigh. 
The  method  can  also  be  applied  to  the  residue  of  total  solids  from  evaporation 
of  the  milk,  by  heating  it  with  six  to  eight  Cc.  of  hydrochloric  acid,  and  trans- 
ferring the  paste  to  the  graduated  tube. 

Failyer  and  Willard  substitute  gasoline  for  ether  in  the  above,  while  Gottleib 
prefers  a  mixture  of  ammonia,  alcohol,  ether  and  gasoline. 

Liebermann  and  Szekely  emulsify  50  Cc.  of  milk  with  five  Cc.  of  potassium 
hydrate  solution  sp.  gr.  .663.  To  the  emulsion  is  added  50  Cc.  of  96  per  cent 
alcohol,  and  the  mixture  shaken  for  several  minutes.  When  the  gasoline  has 
separated,  20  Cc.  is  withdrawn,  evaporated  in  a  tared  flask,  and  the  residual 
fat  weighed. 

In  a  method  due  to  Wollny,  the  proportion  of  fat  is  estimated  from  the  refrac- 
tive index  of  an  ethereal  solution.  The  milk  is  acidified  by  acetic  acid  and 
treated  with  an  alkaline  glycerol  solution  of  copper  oxide,  and  one-fifth  the  volume 
of  water- saturated  ether.  After  brisk  agitation,  a  little  of  the  ether  solution  is 
withdrawn  and  examined  in  a  special  form  of  refractometer.  It  is  claimed  that 
the  results  are  accurate  within  .1  of  one  per  cent  of  gravimetric  methods. 

3.  Of  the  methods  wherein  the  fat  is  dissolved  from  the  total  solids  that  of 
Bell  is  well  known,  and  up  to  recent  times  has  been  extensively  employed.  Five 
to  ten  grams  of  milk  in  a  flat-bottomed  dish  is  evaporated  to  dryness  on  the 
water  bath.  The  fat  is  dissolved  from  the  residue  by  hot  ether  or  like  solvent, 
filtered,  the  filtrate  evaporated  to  dryness,  and  the  fat  weighed.  Bell  lays 
stress  on  the  condition  of  the  residue  of  total  solids,  advising  that  it  be 
neither  too  moist  nor  too  dry,  as  in  either  case  a  little  fat  remains  undissolved. 
For  sour  milks,  carbon  disulfide  has  the  advantage  over  ether  of  not  dissolving 
lactic  acid. 

Instead  of  dissolving  out  the  fat  by  ether,  Davenport  fills  the  platinum  cap- 
sule with  naptha,  allows  to  boil  down  to  one -half,  and  decants  carefully,  re- 
peating the  operation  three  or  four  times.  The  residue  is  dried  and  reweighed, 
the  fat  being  the  difference.  The  residue  may  be  used  for  an  ash  determina- 
tion. 

But  it  appears  to  be  conceded  by  the  majority  of  chemists  that  the  attenuation 
of  the  film  of  total  solids  as  secured  in  the  methods  of  Bell  and  his  follow- 
ers is  not  sufficient  to  insure  the  perfect  extraction  of  the  fat,  the  casein  and 
salts  enveloping  and  protecting  it  from  contact  with  ether.  Recognizing  this, 
KummerJ  would  drop  about  one  gram  of  milk  on  a  tared  flat  glass  plate  about 


*  Journ.  Anal.  Chem.  1887—124. 
t  Zeits.  anal.  27—464. 
{  Chem.  News,  1893—2—1. 


MILK  —  BUTTER .  483 

three  inches  In  diameter,  dry  and  weigh ;  then  remove  the  scale  to  a  small  thick 
flask  and  digest  with  ether  at  100°  to  120°  Fahr.  The  flask  is  cooled, 
weighed,  most  of  the  clear  ether  solution  poured  out  and  the  flask  reweighed. 
The  solution  is  then  evaporated  to  dry  ness  on  a  tared  watch-glass  and  the  fat 
weighed. 

To  further  increase  the  surface  exposure  of  the  total  solids,  the  milk  may  be 
absorbed  by  some  porous  solid  or  powder  insoluble  in  ether.  Many  substances 
have  been  named  for  the  purpose  — sand,  pumice,  infusorial  earth,  precipitated 
silica,  glass-wool  or  powder,  asbestos,  sponge,  wood  fiber  or  pulp,  lint,  paper, 
cloth,  etc.  Obviously,  refractory  inorganic  bodies  have  an  advantage  in  that 
they  may  be  deprived  of  ether-soluble  organic  matter  by  simple  ignition,  also 
that  they  are  less  hygroscopic  and  that  the  residue  from  the  fat  extraction  may 
be  used  fora  determination  of  ash;  however,  they  are  much  less  used  than 
organic  absorbents. 

In  Adams'  method1,  official  in  England,  a  strip  of  filter  paper,  about  22  inches 
long  by  2.5  inches  broad,  is  rolled  into  a  tight  coil.  On  one  end  of  the  coil  is 
dropped  from  a  pipette  five  cubic  centimeters  of  milk.  It  is  then  dried  at  steam 
heat  and  the  fat  extracted  by  ether  in  a  Soxhlet  or  like  apparatus.  The  coil 
must  have  been  previously  freed  from  matter  soluble  in  ether ;  paper  is  now  on 
sale  that  has  been  so  purified  by  the  manufacturer.  The  ether  used  for  the 
extraction  should  be  anhydrous  and  the  time  of  extraction  ample. 

Thompson,  to  shorten  the  time  required  for  drying,  would  stretch  the  strip  of 
paper  horizontally,  drop  the  milk  upon  it  uniformly  from  end  to  end,  and  dry 
by  radiated  heat  before  coiling  up.  It  is  said  that  where  blotting  paper  is  the 
absorbent,  most  of  the  fat  remains  on  or  near  the  surface  and  is  easily  ex- 
tracted. 

To  dispense  altogether  with  evaporation  of  the  water  it  has  been  proposed  to 
soak  up  the  milk  in  recently  ignited  calcium  sulfate  or  anhydrous  copper  sul- 
fate.  After  grinding  up  the  mixture  with  sand  the  fat  may  be  extracted  by 
ether  or  gasoline. 

Other  processes  direct  to  compound  the  milk  with  certain  precipitants, 
such  as  copper  sulfate  or  calcium  phosphate  in  acetic  acid,  which  throw 
down  the  proteids  inclosing  the  fat  mechanically.  The  precipitate  is  filtered 
off,  dried,  and  extracted  by  ether.  Greater  accuracy  is  claimed  for  this  class 
of  methods. 

The  ethereal  or  gasoline  solution  of  the  fat  is  evaporated  in  a  tared  dish,  the 
residue  dried  at  100 <=>  and  weighed;  or  the  solvent  may  be  partly  or  entirely 
evaporated  and  the  residual  fat  saponified  by  a  known  weight  of  potassium 
hydrate  in  alcohol,  then  the  excess  of  alkali  determined  by  titration  with 
standard  acid,  and  the  weight  of  fat  calculated  from  the  saponification 
equivalent. 

4.  The  quality  of  milk  may  be  approximated  by  various  instruments  which 
measure  the  color  or  opacity  —  the  opacity,  however,  is  said  to  be  due  princi- 
pally to  colloidal  casein  rather  than  to  fat.  The  "pioscope  "  of  Heeren  is  a 
black  rubber  disk  in  the  form  of  a  shallow  circular  well  holding  a  thin 
layer  of  milk;  for  comparison  there  are  painted  six  sectors  of  different  shades 
radiating  from  the  well,  grading  from  the  white,  which  matches  cream  iu 
color,  to  the  darkest  marked  'very  poor.'  But  the  indications  are  not  very 
reliable  at  best. 

The  viscosity  of  milk  is  said  to  bear  a  direct  relation  to  the  percentage  of 
fat  contained. 

Hehner  and  Richmond  state    that  the  percentage  of  fat  in  a  milk  bears  a 


484  QUANTITATIVE    CHEMICAL    ANALYSIS. 

constant  ratio  to  the  specific  gravity  and  total  solids,  viz.:  T—  .2540  = 
1.164  Ft  where  T  is  the  percentage  of  total  solids;  #,  the  specific  gravity 
at  15  °  ;  and  F,  the  fat. 

Proteids.  The  chief  nitrogenous  constituent  of  milk  is  casein,  with  a 
smaller  amount  of  a  form  of  albumin  designated  as  lactalbumin.  Minute 
amounts  of  several  other  nitrogenous  bodies  are  believed  to  accompany 
these,  but  the  difficulty  of  isolating  them  makes  their  identity  somewhat 
uncertain. 

Casein  in  normal  milk  varies  from  1.8  to  3.5  per  cent  or  more  with  an 
average  of  about  2.5.  The  density  in  solution  is  nearly  1.34;  it  is  supposed 
to  be  in  a  colloidal  combination  with  mineral  matters,  released  on  the  addi- 
tion of  an  acid  or  acid  salt.  The  proportion  of  nitrogen  contained  is  about 
16  per  cent.  Albumin  ranges  from  .55  to  .86  per  cent.  It  is  said  that  a 
milk  with  less  than  1.3  parts  of  fat  to  one  of  casein  is  probably  watered. 

The  relation  of  the  proteids  of  milk  to  the  other  constituents  is  expressed 
in  Richmond's  formula  — 


P=2.8  T+  2.5A  —  3.33P—.7  10QO  ^-,  where  P  is  the  percentage  of 

proteids;  T,  of  the  total  solids;  A,  of  the  ash;  F,  of  the  fat;  and  D3  the 
density  at  15.5/15.50  (water  =1). 

The  direct  determination  of  the  total  nitrogen  of  a  milk  includes  that  of  the 
proteids  and  other  nitrogenous  bodies.  It  is  most  conveniently  made  by  the 
Kjeldahl  method,  which  according  to  Kruessler  gives  figures  corresponding 
to  the  Dumas'  combustion  method,  while  the  Will-Varrentrapp  results 
are  always  slightly  lower.  In  genuine  milk  the  nitrogen  seldom  falls  below 
.55  per  cent. 

In  an  approximate  method  the  residue  of  total  solids  is  lixiviated,  first  by 
ether  to  remove  fat,  then  by  hot  water  containing  a  little  acetic  acid  to  remove 
sugar  and  salts.  The  residue  of  proteids  is  dried  and  weighed. 

Casein  is  precipitated  from  milk  by  the  mineral  acids  and  some  of  the 
organic  acids  and  a  large  number  of  inorganic  salts,  but  never  pure,  always 
retaining  some  fat,  calcium  phosphate,  etc.  The  precipitation  by  acetic 
acid  is  assisted  by  heating  or  by  passing  a  current  of  carbonic  acid.  Of 
the  reagents  proposed  for  the  determination  are  zinc  sulfate,  which  precipi- 
tates all  proteids  except  peptones;  magnesic  sulfate,  which  leaves  most  of  the 
nuclein  in  solution;  calcium  phosphate  in  acetic  acid,  that  on  neutralization 
carries  down  casein  mechanically;  potassium  mercuric  iodide  in  acetic  acid; 
mercuric  nitrate  ;  lead  acetate;  lactic  acid;  phospho-tnngstic  acid  ;  alum;  etc. 

For  the  precipitation  by  acetic  acid,  10  Co.  of  the  milk  is  diluted  with  90  Cc. 
of  water  at  40  °  ,  then  acidified  by  1.5  Cc.  of  acetic  acid  and  allowed  to  stand 
for  five  minutes.  The  precipitate  is  filtered  and  washed  by  decantation,  the 
nitrogen  determined  and  the  result  multiplied  by  the  factor  6.25  giving  the 
casein.  Van  Slyke  finds  that  a  definite  proportion  of  acetic  acid  is  advisable 
and  that  the  method  is  uncertain  for  milk  that  has  undergone  a  noticeable 
change  by  age.  A  shorter  scheme  is  to  filter  the  precipitate  on  a  tared  paper 
and  extract  the  fat  by  ether,  then  weigh  the  residue  of  proteid  plus  mineral 
matter  ;  on  ignition  in  an  open  crucible  the  casein  burns  and  is  determined  by 
the  loss  in  weight. 

According  to  Palm,  tannin  precipitates  all  the  albuminoids  of  milk,  but  the 
composition  of  the  precipitate  is  not  constant.  The  tannin  may  be  entirely 
washed  out  of  the  precipitate  by  ether-alcohol,  or  the  "protein  tannate  "  de- 
composed by  lead  acetate  and  the  solution  of  proteids  freed  from  the  excess  of 
lead  acetate  by  hydrogen  sulflde. 

Copper  sulfate  added  to  highly  diluted  milk,  followed  by  sufficient  alkali  to 


MILK BUTTER .  485 

precipitate  the  copper,  throws  down  casein  and  albumin  as  a  copper  compound 
of  fairly  constant  composition.  A  nitrogen  determination  is  preferable  to  direct 
weighing.  In  the  filtrate  the  albumoses  may  be  determined. 

For  the  separate  determination  of  casein  and  albumin,  the  former  is  precip- 
itated by  the  acetic  method,  and  the  albumin  coagulated  in  the  filtrate  by  boil- 
ing, or  precipitated  by  saturating  the  solution  with  zinc  sulfate.  Or  the  casein 
may  be  precipitated  by  mixing  the  milk  with  twice  its  volume  of  saturated  solu- 
tion of  magnesic  sulfate,  then  saturating  the  mixture  with  the  solid  salt ;  the 
precipitate  is  washed  with  a  saturated  solution  of  the  reagent.  In  the  filtrate 
is  the  albumin,  that  may  be  thrown  down  by  phosphotungstic  acid  or  tannin. 

Van  Slyke*  makes  three  nitrogen  determinations  by  the  Kjeldahl  process,  the 
first  on  the  milk  directly,  giving  the  total  nitrogen ;  the  second  on  the  casein 
precipitated  by  acetic  acid ;  and  the  third  on  the  albumin  precipitated  in  the 
filtrate  from  the  casein.  The  nitrogen  of  the  other  nitrogenous  bodies  is  the 
difference  between  the  first  and  the  sum  of  the  second  and  third.  The  multi- 
plier 6.25  converts  the  result  for  nitrogen  to  that  for  proteids. 

The  ratio  of  the  volumes  of  a  solution  of  albuminoid  matter  required  to 
decolorize  a  given  volume  of  a  weak  standard  solution  of  potassium  perman- 
gate  acidified  by  sulfuric  acid,  (1)  in  the  cold,  and,  (2)  at  a  boiling  heat,  is 
termed  by  Smith  "  the  expression  of  oxidation  capacity  ".  He  finds  that  for 
cow's  milk  the  ratio  is  about  1.2  to  .5. 

Milk-sugar  or  lactine,  CigH^Oia.HgO,  forms  hard  white  crystals.  The  percent- 
age in  milk  ranges  from  4.25  to  5.20.  It  is  soluble  in  water  and  dilute  alcohol, 
and  may  be  separated  from  casein  and  inorganic  salts  by  lixiviating  the  resi- 
due left  on  evaporation  of  the  milk,  though  never  quite  completely.  An 
approximate  method  is  to  extract  the  fat  by  ether  from  the  solids  left  on  evapo- 
rating the  milk,  weigh  the  residue  of  milk-sugar,  casein  and  salts,  and  calcine 
it.  Subtracting  the  weight  of  the  ash  and  casein  from  that  of  the  residue  gives 
the  sugar  by  difference. 

The  usual  methods,  however,  are  (1)  by  the  reduction  of  a  copper  salt,  and 
(2)  by  the  polariscope. 

(1)  One  molecule  of  milk  sugar  reduces  Fehlings  solution  with  the  forma- 
tion of  about  seven  molecules  of  cuprous  oxide.  Muter  insists  on  the  dilu- 
tion of  the  milk  to  a  degree  where  there  is  no  sensible  action  of  the  alkali  on 
the  sugar.  In  all  cases  it  is  best  to  make  a  parallel  determination  on  pure 
milk  sugar  and  calculate  the  results  accordingly. 

The  milk  is  prepared  for  the  test  by  removing  the  fat  and  proteids.  Boiling 
with  acid  and  filtering  will  remove  both,  but  to  obtain  a  clear  filtrate  is  often 
a  tedious  operation.  It  is  likely  that  the  figures  for  milk  sugar  thus  obtained 
are  too  high,  since  lacto-peptone,  hemialbumose,  and  coloring  pigments  left  in 
solution  by  the  acid  also  react. 

Ritthausenf  recommends  copper  sulfate  as  a  precipitant  followed  by  neutral- 
ization by  potassium  hydrate;  filtration  may  be  avoided  by  making  up  the 
mixture  to  a  definite  volume  and  drawing  out  a  portion.  The  filtrate  should 
contain  some  copper  sulfate  and  not  have  an  alkaline  reaction.  Gill  heats  the 
diluted  milk  with  an  emulsion  of  aluminum  hydrate  and  filters  by  decantation 
through  a  ribbed  paper,  obtaining  a  clear  filtrate.  Palm  would  evaporate  the 
milk  to  dryness,  extract  the  fat  by  gasoline,  treat  the  residue  with  lead 
oxide  and  water,  and  evaporate  to  dryness,  then  lixiviate  the  residue  by  water 
and  filter;  the  filtrate  contains  the  milk  sugar  plus  a  little  lead  oxide. 


*  Journ.  Amer.  Chem.  Socy.  1894—714. 
t  Zelts.  angew.  1896-46. 


486  QUANTITATIVE    CHEMICAL    ANALYSIS. 

(2)  According  to  Deniges,  the  rotatory  power  of  anhydrous  milk  sugar  in 
aqueous  solution  at  a  concentration  of  from  four  to  thirty-six  per  cent  is  +  55.3 
at  20  °  Cent.  Both  casein  and  albumin  are  laevo- rotatory. 

For  clarification,  lead  acetate  is  stirred  into  the  milk,  the  mixture  made  up 
to  a  definite  volume,  and  filtered  through  a  dry  paper.  Wiley  prefers  acid 
mercuric  nitrate  or  potassium  mercuric  iodide  in  acetic  acid,  while  Blyth 
would  use  copper  sulfate  or  acetic  acid,  the  settling  of  the  precipitate  aided  by 
the  centrifuge. 

Various  other  constituents  of  milk  are  found  in  minute  quantities,  but  their 
quantitative  determination,  when  practicable,  is  not  often  called  for.  Such  are 
minor  proteids,  extractive  and  coloring  matters,  citric  and  other  organic  acids, 
urea,  alcohol,  various  metals,  etc. 

The  adulteration  of  milk  outside  of  skimming  and  watering  is  but  rarely 
practiced.  The  use  of  preservatives,  however,  appears  to  be  on  the  increase. 
The  most  common  of  these  are  boracic  and  salicylic  acids  and  formaldehyde. 
Boracic  acid  is  said  to  increase  the  acidity  to  four  times  the  extent  of  the  same 
quantity  added  to  water,  so  that  milk  which  neither  tastes  nor  smells  sour  yet 
contains  over  .3  per  cent  of  acid  expressed  as  lactic,  is  probably  adulterated 
with  some  preservative,  such  as  boracic  acid. 

Since  so  small  a  quantity  of  a  preservative  is  needed  for  the  purpose,  only  a 
qualitative  test  is  practicable  in  most  cases.  Dialysis  may  sometimes  be  of 
service  in  separating  the  greater  part  and  allow  of  a  fair  colorimetric  or  other 
determination. 

For  cream  the  methods  of  analysis  are  practically  the  same  as  for  milk,  pro- 
portionally smaller  amounts  being  taken  for  the  determinations. 

BUTTER. 

In  churning  cream,  the  violent  agitation  ruptures  the  membrane  enveloping 
each  fat  globule,  whereupon  the  fat  coalesces  to  a  soft  mass  inclosing  a  part 
of  the  casein  and  traces  of  sugar  and  the  salts  of  the  cream.  The  butter  is 
pressed  to  remove  some  of  the  water,  and  a  small  proportion  of  salt  incorpo  - 
rated  as  a  preservative.  Genuine  butter  is  mainly  fat,  with  a  variable  quantity 
of  water,  small  amounts  of  casein,  mineral  matter  and  sugar,  and  traces  of 
lecithin,  cholesterol,  phytosterol,  and  coloring  matter. 

The  composition  of  butter  varies  quite  considerably.  The  fat  ranges  between 
75  and  90  percent;  the  water  from  6  to  18;  salt  from  .5  to  6;  casein  from  .5  to 
3;  and  sugar  from  .1  to  1  percent.  Of  960  samples  90  per  cent  held  between  10 
and  15  per  cent  of  water,  and  37.5  per  cent  between  13  and  14  per  cent.  The 
melting  point  is  between  29.4  °  and  34.7  °  Cent.,  congealing  at  18  to  21  ° . 

It  is  yet  a  disputed  point  as  to  whether  the  fat  globules  of  milk  are  inclosed 
in  a  film  of  proteld.*  Storch,  from  an  extended  investigation  of  the  subject, 
concludes  that  there  is  an  envelope  of  mucoid  substance  which  forms  over  60 
per  cent  of  the  proteids  of  butter.  The  fat  of  butter  is  composed  of  a  mix- 
ture of  the  glycerides  of  the  fatty  acids  oleic,  palmitic  and  stearic  (?),  common 
to  animal  fats,  together  with  a  certain  proportion  of  the  glycerides  of  butyric 
acid  and  its  associates,  the  latter  absent  from  other  animal  fats.  According  to 
Bell,  the  molecule  of  butter-fat,  unlike  those  of  other  fats  (page  240),  is  made 
up  of  the  glycerol  radical  combined  with  different  acid  radicals,  thus  — 

O.C4H7O  (butyric) 
.C16H31O  (palmitic) 
(oleic). 


fO. 

jHJo. 
(O. 


*  Richmond,  Dairy  Chemistry,  1  et  seq. 


MILK BUTTER.  487 

On  undisturbed  cooling  butter-fat  separates  into  about  45.5  per  cent  of  oil 
and  54.5  per  cent  of  fat  solid  at  ordinary  temperatures.  Bell  states  the  compo- 
sition of  butter-fat  as  butyrin  7.01  percent;  caproin,  caprylin,  and  caprin,  2.28; 
olein,  37.73;  and  palmitin,  stearin,  etc.,  52.98. 

A  theory  of  Johnstone*  asserts  that:  "  Butter-fat  then  becomes  a  mixture  of 
iso-oleo-palmito-capriate  of  glycerin,  and  tri-nondecatoic  of  glycerin  in  varying 
proportions,  a  compound,  complicated  triglyceride.  *  *  *  Furthermore,  genuine 
butter  fats  yielding  insoluble  fatty  acids  above  85.81  per  cent  do  not  contain 
stearic  acid  (?)  as  is  generally  supposed,  but  non-decatoic  acid,  the  next 
higher  acid  of  the  series  as  a  glyceride." 

Butyrin,  CsH5  (O.C4H7O)3,  hasaspeciflc  gravity  of  1.052  at  22°  Cent,  and  dis- 
tills unchanged  at  285  ° .  When  boiled  with  an  alcoholic  solution  of  an  alkali  in 
quantity  insufficient  for  complete  saponification,  ethyl  butyrate,  exhaling  its 
characteristic  odor,  is  yielded.  Butyric  acid,  CH3.CH3.CH2.COOH,  is  a  color- 
less liquid  boiling  at  162  ° ,  and  of  a  specific  gravity  of  .958  at  14  ° .  It  mixes 
with  water,  alcohol,  and  ether,  and  readily  distills  in  steam. 


The  methods  of  analysis  follow  to  a  great  extent  those  for  milk.  Since  the 
water  in  butter  may  be  distributed  more  or  less  ununiformly,  care  must  be 
taken  that  the  sample  is  a  representative  of  the  original.  As  many  of  the  tests 
for  adulteration  are  made  on  the  fat  alone,  a  large  quantity  of  the  butter  is 
melted  at  a  low  temperature  and  filtered  from  water  and  curd.  It  must  not 
be  forgotten  that  in  undisturbed  cooling  from  the  melted  state  considerable 
segregation  takes  place. 

Acidity.  According  to  Duclaux  fresh  butter  contains  from  .005  to  .010  gram 
of  butyric  acid  per  1000  grams  of  butter.  With  age  the  proportion  increases 
rapidly,  until  at  .030  gram  per  1000  a  rancid  taste  is  perceptible. f  The  deter- 
mination is  made  in  the  usual  way  for  oils  and  fats  —  stirring  the  butter  in  hot 
neutral  alcohol  or  a  mixture  of  alcohol  and  ether,  and  titrating  by  weak  stand- 
ard alkali  and  phenol-phthalein.  The  limit  for  a  salable  article  is  about  eight 
Cc.  of  decinormal  alkali  for  100  grams  of  butter. 

Water.  A  small  beaker  covered  with  a  watch-glass  is  weighed  and  about  25 
grams  of  butter  introduced.  The  beaker  is  heated  in  an  air-bath  to  105  °  with 
occasional  stirring  until  no  more  globules  of  water  can  be  seen  —  generally 
about  an  hour  will  suffice.  If  a  second  heating  results  in  no  material  decrease 
in  weight,  the  loss  is  put  down  as  water. 

Of  1120  English  and  Continental  butters,  81  per  cent  contained  from  11  to  14 
per  cent  of  water,  and  only  .8  per  cent  of  the  butters  contained  above  16  per 
cent.  Bell's  conclusion  that  a  greater  amount  than  12  per  cent  is  unnecessary, 
and  over  16  per  cent  injurious  to  keeping  qualities,  is  generally  concurred  in. 

Fat.  The  beaker  is  again  heated  until  the  fat  melts,  then  the  liquid  filtered 
through  a  dry  paper  into  a  small  tared  dish.  When  all  the  fat  has  passed 
through,  the  beaker  is  rinsed  and  the  paper  washed  with  ether  or  gasoline  until 
the  washings  leave  no  traces  of  fat  on  evaporation.  The  ether  is  distilled  from 
the  fat,  the  dish  heated  to  100  °  for  a  few  minutes,  and  the  fat  weighed. 

Casein.  The  filter  is  washed  a  few  times  with  hot  water  containing  a  few 
drops  of  acetic  acid,  receiving  the  washings  in  a  small  tared  beaker.  The  filter 
and  casein  are  dried  at  100°  for  an  hour  and  weighed. 


*  Chem.  News,  1891-1-56. 

t  Journ.  Amer.  Chem.  Socy.  1899—980. 


488  QUANTITATIVE    CHEMICAL   ANALYSIS. 

Koenig  states  that  in  302  samples  of  butter  the  casein  ranged  from  .19  to 
4.78  per  cent. 

Salt.  The  washings  in  the  beaker  are  evaporated  to  dryness  in  the  water- 
bath  and  weighed  as  salt  plus  milk  sugar.  The  residue  is  dissolved  in  hot 
water,  filtered  if  not  quite  clear,  acidified  by  a  few  drops  of  nitric  acid,  and  the 
chlorine  precipitated  by  silver  nitrate.  From  the  weight  of  silver  chloride  is 
calculated  that  of  the  salt. 

Another  plan  is  to  boil  a  portion  of  the  butter  with  water  and  filter  from 
the  fat  and  casein.  The  filtrate  is  titrated  by  silver  nitrate  with  potassium 
chromate  as  indicator. 

In  113  butters  the  percentage  of  salt  ranged  from  A  to  9.2,  the  majority 
within  2  to  7  per  cent. 

Sugar.  The  weight  of  the  sugar  is  found  by  subtracting  the  weight  of  the 
salt  from  that  of  the  salt  plus  sugar.  If  a  more  accurate  determination 
is  desired,  a  large  weight  of  butter  is  boiled  with  water,  and  the  aqueous 
solution  filtrated  from  the  fat  and  tested  by  Fehlings  solution  or  other 
method. 

Adulterations.  In  America  the  principal  adulterant  or  substitute  for  butter  is 
margarine,  less  frequently  lard.  The  detection  of  small  quantities  of  mar- 
garine is  a  matter  of  some  difficulty,  though  when  it  forms  the  whole  or  the 
greater  part  of  a  mixture,  as  in  commercial  butterine,  less  trouble  is  experi- 
enced. No  single  one  of  the  following  tests  can  be  relied  on  to  establish  the 
fact  of  adulteration,  since  it  is  not  difficult  to  prepare  mixtures  that  will  ap- 
proximate any  one  constant  of  pure  butter. 

Oleomargarine  or  margarine  is  a  commercial  product  made  by  depriving 
beef  or  other  fat  of  a  part  of  its  stearin.  The  composition  of  commercial  mar- 
garine is  stated  by  Blyth  as  follows. 

Water 12.01 

Palmitin...., 18.31 

Stearin 38.50 

Olein 24.95 

Other  fats •. 26 

Casein 74 

Salts 5.23 

The  so-called  "  renovated"  butter  is  made  by  melting  the  stale  article,  draw- 
ing off  the  water  containing  most  of  the  butyric  acid  and  other  offensive  prod- 
ucts, and  separating  the  curd,  then  chilling  the  fat  by  ice,  or  rechurning. 

The  various  tests  for  purity  are  briefly  noticed  below.  It  would  appear  that 
many  of  these  could  be  made  quantitative  by  applying  the  formula  for  mix- 
tures (page  16),  but  from  the  wide  variation  from  the  average  for  any  one  con- 
stant, both  in  the  butter  and  adulterant,  the  results  are  but  approximate  at 
best,  and  often  unworthy  of  confidence. 

1.  As  a  rule,  genuine  butter  when  carefully  melted  yields  a  practically  clear 
fat,  while  straight  oleos  and  badly  adulterated  samples  appear  turbid.* 

2.  The  refractive  index  of  butter- fat  at  25®  is  1.459  to  1.462,  that  of  marga- 
rine is  1.465  to  1.470.    Either  the  Zeiss  or  Amagat-Jean  refractometer  is  suited 
for  the  observation;  on  the  latter  instrument  butter  shows  —29  to  —31 ;  mar- 
garine, —13  to  —18;  lard,  —8  to  —14;  cottonseed  oil,  17  to  23. 

3.  The  unequal  solubility  in  gasoline,  absolute  alcohol,  toluene,  phenol,  amyl 
alcohol,  etc.,  has  been  proposed  as  a  means  of  distinguishing  butter  from  other 


*  Analyst,  1892—100. 


MILK  —  BUTTER.  489 

fats,  and  is  suited  for  dividing  butters  from  margarines.  But  the  behavior  of 
a  mixture  of  fats  can  but  rarely  be  predicted  from  the  solubilities  of  the  several 
constituents. 

Valenta's  test  of  the  temperature  of  saturation  has  come  into  some  use.  A 
mixture  of  equal  volumes  of  melted  fat  and  glacial  acetic  acid  is  heated  until 
clear,  then  allowed  to  cool  spontaneously  while  stirring  with  a  thermometer. 
The  temperature  of  incipient  turbidity  ranges  for  margarine  from  95  °  to  100  ° , 
while  for  butter  there  is  a  wider  variation,  generally  stated  to  be  between  53  ° 
and  63°. 

Jean  prefers  to  mix  equal  volumes  of  the  butter-fat  and  acid  in  a  graduated 
tube  and  read  the  volume  of  the  upper  layer  which  is  the  excess  of  acid  over 
that  required  for  solution  of  the  fat. 

Crismer  modifies  Valenta's  test,  sealing  up  the  fat  with  a  slightly  greater 
volume  of  alcohol  in  a  narrow  tube  and  heating  until  the  plane  of  separation 
of  the  liquids  becomes  flattened.  Then  the  tube  is  slowly  cooled  with  constant 
agitation  until  a  marked  turbidity  appears;  this  temperature  he  calls  "the  criti- 
cal temperature  of  dissolution".  For  genuine  butter  the  average  is  100°, 
for  margarine  125  ° ,  for  cotton  oil  116  °  .* 

4.  Most  animal  fats  have  a  Koettstorfer  number  of  about  197,  while  that  of  but- 
ter is  higher,  from  220  to  233.    The  determination  is  made  in  the  usual  way  — 
heating  a  weighed  quantity  of  the  sample  with  a  measured  volume,  an  excess,  of 
potassium  hydrate  in  alcohol,  then  titrating  back  by  standard  acid  and  calcu- 
lating the  reacting  alkali.    An  approximation  to  the  percentage  of  butter  in  a 
mixture  may  be  derived  from  the  formula  on  page  16,  assuming  a  to  be  227, 
and  6,  196. 

Heated  with  a  solution  of  a  caustic  alkali  in  alcohol,  in  quantity  insufficient 
for  complete  saponiflcation,  gives  rise  to  an  an  ester  commonly  known  as  buty- 
ric ether  — H(O.C4H7O)  (butyric  acid)+C2H5OH  (alcohol)  =  C2H6.C4H7O.O 
(ethyl  butyrate)  -}-  H2<3.  The  fragrant  odor  of  this  compound  distinguishes 
butter  from  other  fats  free  from  butyrin. 

After  saponification  the  fatty  acids  may  be  combined  with  barium,  the  barium 
determined  by  converting  it  into  the  sulfate  and  weighing,  and  the  combining 
proportion  of  barium  calculated. 

Following  the  determination  of  the  saponification  equivalent,  the  neutral 
solution  may  be  made  slightly  alkaline  and  evaporated  for  a  further  examina  - 
tion  of  the  fatty  acids  as  below. 

5.  Insoluble  fatty  acids.  The  mixed  fatty  acids  derived  from  most  animal  and 
vegetable  fats  are  practically  wholly  insoluble  in  water,  while  of  those  derived 
from  butter,  containing  around  8.5  per  cent  of  butyrin  and  allied  fats,  a  part 
is  soluble  in  water.    We  may  therefore  distinguish  butter  by  the  lower  pro- 
portion of  insoluble  acids  or  by  its  containing  soluble  acids.    In  genuine  butter 
the  proportion  of  insoluble  fatty  acids  (the  Hehner  number)  is  from  86.5  to 
89.5  with  a  mean  of  about  87.5.    In  most  other  fats  the  proportion  is  higher, 
from  95  to  96  per  cent.    Hence  if  a  sample  shows  above  90  per  cent,  adultera- 
tion  is   very   probable,    and     over   88   per  cent   is   suspicious.      Roughly 

d  —  b 
X  =  100    ,  where  Xis  the  percentage  of  foreign  fat  in  a  sample  of  butter; 

a  is  95.5;  6  is  87.5,  and  d  is  the  percentage  of  insoluble  fatty  acids  found  on 
analysis. 

A  weighed  quantity  of  the  butter  fat  is  saponified,  the  alcohol  removed  by 
evaporation,  and  the  soap  dissolved  in  water.  From  this  point  the  test  may 


Blyth,  Foods,  350. 


490  QUANTITATIVE    CHEMICAL    ANALYSIS. 

be  carried  out  in  several  ways;  the  simplest  is  by  acidulation,  filtration  of  the 
insoluble  fatty  acids,  drying  and  weighing,  either  with  or  without  a  purifica- 
tion by  solution  in  alcohol  and  filtration. 

Leonard*  finds  that  there  is  a  definite  relation  between  the  percentage  of  in- 
soluble fatty  acids  and  their  specific  gravity,  expressed  by  the  formula  Y=K 
(1  —  X),  where  T  is  the  percentage;  2T,  the  specific  gravity;  and  K a  con- 
stant =  951  ±1.6. 

6.  Volatile  fatty  acids.  The  fatty  acids  volatile  in  steam  comprise  practically 
those  soluble  in  water.  The  Reichert-Meissl  Number  is  the  volume  in  cubic 
centimeters  of  decinormal  alkali  required  to  saturate  the  volatile  fatty  acids 
from  five  grams  of  butter-fat.  It  is  a  somewhat  variable  constant  in  genuine 
butter,  differences  arising  from  the  food  of  the  ccw,  period  of  lactation,  season 
of  the  year,  etc.  The  extreme  range  may  be  put  down  as  from  20  to  83 ;  as  a 
rule,  however,  the  number  will  lie  between  23  and  29.  The  average  may  be 
taken  as  about  27.  The  values  of  other  fats  likely  to  be  used  as  adulterants 
are  margarine  from  .5  to  1;  lard,  1  or  less;  cottonseed  oil,  1  to  2. 

While  admittedly  imperfect,  the  fact  that  the  number  is  derived  from  a  dis- 
tinguishing property  of  butter,  one  impossible  to  counterfeit  by  admixtures  of 
the  common  butter  adulterants  and  only  to  be  imitated  by  fats  not  easily  pro- 
cured or  unsuitable  for  other  reasons,  explains  the  esteem  in  which  it  is  held 
among  food  chemists.  Its  weakest  point  is,  of  course,  the  considerable  range 
of  the  values  for  genuine  butter. 

In  the  original  process  of  Beichert,  2.5  grams  of  the  filtered  fat  was  to  be 
mixed  with  one  gram  of  potassium  hydrate  and  20  Cc.  of  alcohol  of  89  per  cent. 
After  complete  saponiflcation  water  was  added,  the  alcohol  boiled  off,  then 
acidified  by  sulfuric  acid  to  liberate  the  fatty  acids.  Fifty  Cc.  was  distilled, 
filtered,  and  titrated  by  decinormal  sodium  hydrate  and  litmus.  Meissl  modified 
the  above  by  doubling  the  quantity  of  fat  operated  on,  hence  the  original 
numbers  are  only  about  one-half  as  great  as  Meissl's. 

Wollnyf  criticizes  the  directions  of  Reichert  and  alleges  several  inherent 
sources  of  error,  namely  the  absorption  of  carbon  dioxide  by  the  alkali  and  its 
transference  to  the  distillate:  formation  of  butyric  ethers ;  mechanical  carry- 
ing over  of  insoluble  acids ;  retention  of  volatile  acids  by  the  insoluble  acids; 
and  variations  in  the  size  and  shape  of  distilling  vessels  and  rapidity  of  distilla- 
tion. He  proposes  to  reduce  the  errors  by  certain  modifications  of  the  details. 

To  prevent  losses  through  etheriflcation,  a  very  concentrated  aqueous  solu- 
tion of  potassium  hydrate  may  be  substituted  for  the  usual  alcoholic  lye. 
Henriques'  process  of  cold  saponiflcation  (page  458)  is  said  to  give  slightly 
higher  results  than  the  ordinary  mode,  for  the  reason  that  in  this  case  there  are 
formed  no  ethers  of  the  volatile  fatty  acids. 

Obviously,  in  one  distillation  the  volatile  acids  are  not  wholly  carried  over 
into  the  receiver,  as  much  as  25  per  cent  of  the  total  being  left  in  the  flask. 
Hence  the  necessity  of  following  the  details  of  the  method  regarding  the  ratio 
of  the  volume  of  the  distillate  to  that  originally  in  the  flask.  Waller  modifies 
the  usual  mode  of  distillation  by  first  collecting  50  Cc.  and  titrating,  then 
adding  50  Cc.  of  water  to  the  flask  and  again  distilling  50  Cc.  and  titrating ; 
and  proceeding  in  this  manner  until  the  distillate  is  practically  neutral. 

Planchon  saponifies  the  butter-fat  as  usual  by  a  measured  volume  of  standard 
sodium  hydrate,  then  adds  a  volume  of  standard  sulfuric  acid  exactly  equiva- 
lent to  the  alkali  —  this  found  by  a  previous  experiment.  The  solution  now 
contains  only  sodium  sulfate,  glycerol,  and  soluble  free  fatty  acids,  the  insol- 


*  Analyst,  1898— 282. 

t  Chem.  News,  1889—1—20. 


MILK  —  BUTTER.  49 1 

uble  fatty  acids  floating  on  the  surface  of  the  liquid.  The  insoluble  acids  are 
removed  by  filtration  and  may  be  determined  by  weighing  or  otherwise ;  the 
filtrate  is  titrated  by  standard  alkali. 

Uniting  the  weight  of  the  soluble  fatty  acids  calculated  as  butyric  to  that  of 
the  insoluble  acids  should  give  a  sum  of  not  less  than  94  per  cent  of  the  weight 
of  the  fat. 

7.  The  Iodine  Number  of  butter-fat  lies  between  26  and  40,  an  average  of  56 
samples  showing  33.32.    The  Number  for  lard  is  from  55  to  65;  for  cotton  oil 
from  100  to  115;  and  for  margarine  from  62  to  75;  all  showing  variations  within 
a  wide  range. 

8.  The  heat  of  combustion  of  butter  fat  is  about  9.3  calories;  oleo  from  9.57 
to  9.79;  and  lard  about  9.60.* 

9.  Viscosity.  The  viscosity  of  margarine  is  greater  than  that  of  butter  fat. 
Killing  observes  the  viscosity  at  40  °  Cent,  against  that  of  water  at  20  °  taken 
for  a  standard  as  100.    The  result  is  the  "  viscosity  number  "  or  "  viscosity 
ratio."    Wunder  determines  the  viscosity  of  a  solution  of  given  concentration 
in  chloroform  at  20°  Cent.,  under  which  conditions  the  value  of  butter -fat  is 
344.3,  that  of  margarine  373.2. 

10.  The  specific  gravity  of  butter-fat  is  higher  than  those  of  the  majority  of 
other  fats.    For  several  reasons  it  is  best  observed  at  a  temperature  of  about 
35  ° ,  compared  with  water  at  the  same  temperature  or  at  15  o .    At  37.8/37.8  ° 
the  gravity  ranges  from  .910  to  .914,  rarely  falling  below  the  first  figure;  at  the 
same  temperature  oleo  is  from  .901  to  .906;  beef  fat  averages  about  .904,  and 
lard  .905.    At   100/100°,  butter-  fat  registers   .867  to  .870;  oleo,  from  .858  to 
.863.    But  some  possible  adulterants  have  as  high  a  gravity  as  butter-fat  or 
higher. 

Casamajor  proposed  that  a  mixture  of  alcohol  and  water  be  prepared  of 
such  a  strength  that  at  15  °  Cent.,  drops  of  the  fat  neither  sink  nor  float  therein; 
then  the  gravity  of  the  spirit  is  ascertained.  He  states  that  for  butter-fat  the 
gravity  should  be  .926,  and  for  oleo,  .915. 

11.  Under  the  microscope  the  absence  of  crystals,  and  with  polarized  light  an 
uncolored  field,  is  indicative  of  pure  fresh  butter;  the  appearance  is  well 
shown  in  a  photo-micrograph.    When  crystallized  from  amyl  alcohol  and  mag- 
nified 100  diameters,  pure  butter-fat  shows  large  disks  with  acicular  edges ; 
margarine  shows  smaller  disks  with  smooth  edges.    But  mixtures,  especially 
if  butter  largely  predominates,  do  not  always  give  conclusive  appearances. 

A  photo-micrograph  of  a  slide  of  butter  between  crossed  Nicols  shows  only 
a  dull  amorphous  patch,  while  one  of  margarine  has  a  distinctive  semi-crystal- 
line appearance. 

Opinions  differ  as  to  the  value  of  a  microscopic  examination  for  proof  of 
adulteration,  many  believing  that  the  only  sure  conclusion  that  can  be  drawn  is 
that  the  fat  under  test  has  (margarine,  lard)  or  has  not  (butter)  undergone 
fusion. 

12.  Cryoscopic  test.    The  molecular  weight  of  butter-fat  as  determined  by  the 
method  of  Raoult  is  stated  at  from  696  to  716  as  determined  in  benzene  solution  — 
Bl yth  makes  it  much  lower,  about  580,  in  paraxylene  solution  —  while  margarine  is 
from  780  to  883.    Determinations  are  made  in  a  special  apparatus,  first  observing 
the  congealing  point  of  a  given  volume  of  the  solvent,  then  the  congealing  point  of 
the  same  volume  in  which  has  been  dissolved  a  known  weight  of  butter-fat.  The 

molecular  weight  M is  calculated  by  the  equation  M—    P'K    where    P  is  the 


*  Journ.  Amer.  Chem.  Socy.  1896—174. 


492  QUANTITATIVE    CHEMICAL    ANALYSIS. 

percentage  of  butter  in  the  solvent;  t,  the  depression  in  temperature  (the  differ- 
ence in  the  two  experiments) ;  and  K,  a  constant  for  the  special  solvent  used. 

13.  Substitutes  for  the  natural  coloring  matter  of  butter  are  extensively  sold 
under  various  fanciful  trade -names.  The  bases  of  the  most  common  are 
anatto,  curcumine,  and  oil-yellow.  Used  in  such  small  quantities,  the  isolation 
of  the  chromogen  is  somewhat  difficult,  but  can  usually  be  done  by  extraction 
with  a  suitable  organic  solvent,  or  better  by  a  mixture  of  two  or  more.  Or 
the  color  may  be  withdrawn  into  an  absorbent  solid,  such  as  Fullers  earth, 
with  subsequent  extraction  by  a  proper  solvent.* 


*  Journ.  Amer.  Ohem.  Socy.  1898—112;  Wiley  Agrlc.  Chem,  Anal.  3—522. 


URINALYSIS.  493 


URINALYSIS. 

Normal  human  urine  is  a  clear  or  nearly  clear  liquid  of  an  amber  color, 
peculiar  aromatic  odor,  and,  except  shortly  after  meals,  an  acid  reaction.  It 
is  essentially  an  aqueous  solution  of  urea,  sodium  chloride,  and  earthy  phos- 
phates and  sulfates,  and  small  amounts  of  a  great  number  of  organic  and 
inorganic  bodies.  The  average  composition  is  in  grams  per  liter.* 

Total  solids 45.0to65.0       Sulfur  trioxide 1.5to3.0 

Urea 20.0  to  50.0       Potassium  oxide 2.5  to  3.5 

Uric  acid 3to     .8       Sodium  oxide 4.0  to  G.O 

Creatinin 4  to  1.3       Ammonia..... 5  to  .8 

Hippuric  acid 4  to  1.0       Calcium  oxide 2  to   .4 

Chlorine 5.0tolO.O       Magnesium  oxide 3  to   .5 

Phosphorus  pentoxide 2.0to  3.5       Iron OOlto.010 

Sulfur  dioxide  in  ethereal  sulfates 090  to  .500 

Oxalic  acid 020  to  .030 

Glycero-phosphoric  acid 010  to  .020 

Propionic,  valeric,  caproic,  and  butyric  acids , 008  to  .080 

Indoxyl-sulfuric  acid  (calculated  as  indigo) 006  to  .019 

Thiocyanic  acid 001  to  .008 

Paraoxyphenylacetic,  paraoxyphenylpropionic,  dioxyphenylacetic  and  para- 

oxyphenylgly collie  acids 010  to  .030 

Silicic  acid,  carbonic  acid,  hydrogen  peroxide,  nitrates,  nitrites,  and  metals, 

e.  g.,  manganese  and  copper traces 

Xanthin,  sarcin,  etc OOlto.010 

Phenol,  cresol,  etc 005  to  .020 

Bile  salts OOOto.010 

Urobilin,  urochrome,  etc 080  to  .140 

Carbohydrates 014  to  .075 

Sarco -lactic,  snccinic,  glycuronic,  and  oxaluric  acids,  acetone,  inosite,  cystin, 
taurin,  urorubinogen,  urorubin,  pigment  of  Giacosa,  skatoxylsulfuric  acid 
(often  in  considerable  amount) ,  skatoxylglycuronic  acid;  nephrozymase, 
pepsin,  and  other  ferments;  pseudo-xanthin,  para-xanthin,  hetero- 
xanthin,  guanin,  adenin,  etc.;  pyrocatechin,  quinol,  protocatechuic  acid, 

etc traces 

Carbon  dioxide  (cubic  centimeters  per  liter  of  urine) 15.957 

Oxygen  "  .658 

Nitrogen  "  7.775 

The  habits  of  life  exercise  an  influence  on  the  composition  of  the  urine,  and 
marked  temporary  changes  may  follow  bodily  or  mental  fatigue,  excesses,  the 
use  of  certain  articles  of  food,  stimulants,  etc.  Diseases,  especially  febrile, 
tend  to  raise  or  lower  the  percentage  of  some  constituents,  and  certain  grave 
lesions  of  the  urinary  system  introduce  notable  quantities  of  decomposition 
products  normally  absent  or  present  only  in  insignificant  amounts.  A  test  of 
the  urine  may  give  the  first  warning  of  an  incipient  ailment  and  be  of  the  high- 
est importance  in  the  diagnosis,  and  in  the  prognosis  as  well  by  increase  or 
diminution  as  the  disease  runs  its  course. 

For  pathological  purposes,  only  a  few  of  the  constituents  need  be  determined. 
Generally  a  urinoscopic  examination  is  limited  to  the  total  solid  contents  and  a 
search  for  albumen  and  sugar,  perhaps  urea  and  uric  acid  also,  except  where  a 
specific  disease  is  suspected  that  is  manifested  by  other  than  these. 


*  Platt,  Journ.  Amer.  Chem.  Socy.  1897—382. 


494  QUANTITATIVE    CHEMICAL    ANALYSIS. 

1.  Color.*  The  coloring  matters  of  human  urine  are  principally  the  com- 
pounds known  as  urobilin,  a  dark- brown  resinous  body,  C32H40N4O7,  and  urox- 
anthin  or  indican ;  other  pigments  are  uroerythrin,  uroglaucin,  uromelanin,  uro- 
phaein,  urrhodin,  etc.  In  health  the  color  ranges  from  light  to  full  yellow, 
while  in  certain  diseases  the  urine  may  be  on  the  one  hand  almost  colorless,  and 
on  the  other  nearly  opaque  brown.  Vogel's  scale  comprises  nine  shades 
ranging  from  No.  1,  pale  yellow,  to  No.  9,  brownish-black. 

Smith  adopts  a  scale  of  50  colors  corresponding  to  standard  solutions  of 
iodine  in  water.  Into  a  medium-sized  test-tube  is  measured  five  Cc.  of  the 
urine  to  be  examined,  and  into  another  of  equal  diameter,  five  Cc.  of  an  aqueous 
solution  of  potassium  iodide.  Into  the  latter  is  run  from  a  graduated  pipette  a 
.01  per  cent  solution  of  potassium  permanganate  acidified  by  sulfuric  acid, 
until  the  color  matches  that  of  the  urine.  Iodine  is  liberated  from  the  iodide 
according  to  the  equation  10KI  +  K2Mn2O8  -f  8H2SO4  =  5I2  +  6K2SO4  + 
2MnS04  +  8H2O. 

2.  Healthy  urine  is  nearly  or  quite  transparent,  although  on  standing  for 
some  hours  a  haze  of  mucus  appears.  Turbidity  in  urine  may  be  due  to  sus- 
pended matter  of  one  or  more  of  the  following  varieties :  uric  acid,  urates,  cal- 
cium oxalate,  earthy  phosphates,  calcium  carbonate,  calcium  sulfate,  leucin 
and  tyrosin,  cystin,  mucus,  pus,  ephithelium,  blood,  tube  casts,  spermatozoids, 
fungi,  infusoria,  elements  of  morbid  growth,  urocyanogen,  and  entozoa.  Cloud- 
iness is  frequently  due  to  an  alkaline  reaction  of  the  urine  causing  a  separation 
of  earthy  phosphates,  the  urine  clearing  up  on  acidification. 

Cloudy  urine  is  examined  by  filling  a  heavy  test  tube  and  whirling  in  a  cen- 
trifuge until  the  suspended  matter  has  collected  at  the  bottom  of  the  tube. 
The  clear  liquid  may  then  be  poured  off  and  the  deposit  inspected  under  the 
microscope  with  an  objective  of  moderate  power.  If  the  nature  of  the  deposit 
cannot  be 'readily  recognized,  various  solvents  and  staining  materials  may  as- 
sist. 

3.  The  specific  gravity  of  normal  urine  ranges  from  1015  (water  at  1000)  to 
1025,  averaging  about  1019.    In  disease  it  may  fall  as  low  as  1002  or  rise  as 
high  as  1040  or  more.    The  determination  is  made  by  a  Westphal  balance  or 
picnometer ;   or  with  sufiicient  accuracy  for  clinical  purposes,  by  the  urinometer, 
a  small  hydrometer  graduated  in  specific  gravities  between  the  limits  found  in 
urine. 

4.  Total  solids.  Five  or  ten  cubic  centimeters  of  urine  is  evaporated  in  a  small 
platinum  capsule  on  the  water-bath  and  the  residue  weighed.    Since  urea  is  in 
part  decomposed  mto  ammonia  and  carbon  dioxide  during  the  evaporation,  the 
dish  is  inclosed  in  such  a  way  that  the  steam  passes  through  a  measured  quan- 
tity of  a  standard  acid ;  the  ammonia  absorbed  is  determined  by  titration  by 
standard  alkali,  and  calculated  back  to  urea. 

A  more  accurate  determination  of  total  solids  is  by  evaporating  five  Cc.  of 
the  urine  at  ordinary  temperatures  over  sulfuric  acid  in  vacuo. 

By  multiplying  the  specific  gravity  less  1000  by  2.33  (Haesser's  number)  the 
product  will  be  nearly  the  weight  of  total  solids  in  1000  parts  of  urine;  it  is 
claimed,  however,  that  the  formula  fails  for  certain  morbid  urines. 

5.  The  inorganic  solids  or  ash  is  left  on  carefully  burning  the  residue  from  the 
above  determination.    To  avoid  .fusion  of  the  chlorides  the  heat  is  kept  as  low 
as  possible,  and  a  safer  plan  is  to  lixiviate  the  char  (page  105).    Wanklyn 
remarks  that  a  certain  normal  ratio,  about  1  to  1.65,  holds  between  the  inor- 
ganic and  organic  constituents  of  healthy  urine. 


*  Tyson,  Guide  to  the  Pract.  Exam,  of  Urine,  Frontispiece. 


URINALTSIS.  495 

6.  Inorganic  bases.  Calcium  is  precipitated  by  ammonium  oxalate  from  the 
urine  made  slightly  acid  by  acetic  acid.  Notwithstanding  the  acid  reaction, 
the  precipitate  of  calcium  oxalate  is  always  impurifled  by  small  amounts  of 
coprecipitated  bodies,  and  is  best  determined  by  dissolving,  after  well  wash- 
ing, in  dilute  sulfuric  acid,  and  titrating  hot  by  standard  permanganate. 

The  magnesium  in  the  filtrate  from  the  above  is  thrown  down  by  ammonium 
phosphate  and  ammonia,  and  the  precipitate  calcined  to  pyrophosphate. 

Potassium  and  sodium.  The  urine  is  freed  from  phosphoric  acid,  the  earths, 
and  sulfuric  acid  by  precipitation  by  a  slight  excess  of  barium  hydroxide. 
One-half  of  the  filtrate  is  evaporated  to  dryness  and  ignited  gently  to  destroy 
organic  matter,  the  residue  taken  up  by  ammonium  carbonate  dissolved  in 
dilute  ammonia,  filtered,  and  the  filtrate  acidified  by  hydrochloric  acid.  The 
solution  of  the  alkali  chlorides  is  evaporated  to  dryness,  ignited  gently  to 
drive  off  ammonium  chloride,  and  weighed.  The  alkalies  may  now  be  sepa- 
rated by  platinic  chloride  (page  387). 

The  ammonia  and  its  compounds  come  Jor  the  most  part  from  the  decom- 
position of  urea.  If  the  urine  shows  an  alkaline  reaction  the  free  ammonia 
may  be  titrated  directly  by  weak  standard  sulfuric  acid,  with  litmus  paper 
as  an  indicator.  The  total  ammonia  is  then  determined  in  the  same  liquid  by 
boiling  with  a  measured  volume  of  standard  potassium  hydrate  in  large 
excess,  until  all  the  freed  ammonia  is  dissipated.  The  residual  alkali  is  then 
titrated  back  by  standard  acid;  the  loss  in  alkalinity  equals  the  combined 
and  free  ammonia  of  the  urine.  Urea  is  decomposed  by  the  alkali  — 
CO(NH2)2-f  2KOH  =  K2CO3  +  2NH3— but  without  affecting  the  result  of  the 
determination,  for  the  reason  that  potassium  carbonate  has  an  equal  alkalinity 
with  the  hydrate  when  using  an  indicator  not  affected  by  carbon  dioxide. 

Or  all  the  ammonia  may  be  directly  precipitated  from  the  urine  by  platinic 
chloride,  hydrochloric  acid  and  alcohol,  and  the  (impure)  precipitate  of  am- 
monic  platinic  chloride  distilled  with  sodium  hydrate ;  the  ammonia  is  passed 
into  standard  acid  and  determined  by  a  residual  titration. 

Schloessing's  method,  though  tedious,  is  free  from  certain  objectionable 
features  of  others.  The  urine  is  mixed  with  slaked  lime  in  a  shallow  dish 
which  is  then  placed  under  a  bell-jar  in  close  proximity  to  a  dish  containing 
a  measured  volume  of  standard  sulfuric  acid.  The  ammonia  liberated  by  the 
calcium  hydrate  is  absorbed  by  the  acid,  the  operation  requiring  two  or  three 
days  for  completion.  The  remaining  free  sulfuric  acid  is  found  by  titration 
by  alkali. 

Moerner  and  Sjoquist*  remove  the  phosphates,  sulfates,  etc.,  from  five  Cc. 
of  the  urine  by  barium  chloride  containing  barium  hydrate  dissolved  in  a  large 
volume  of  alcohol  and  ether.  After  standing  for  some  hours  in  a  closed  flask 
the  liquid  is  filtered  and  the  filtrate  distilled,  finally  with  the  addition  of  mag- 
nesia and  water.  The  ammonia  (set  free  by  barium  hydrate)  In  the  distillate  is 
determined  as  usual.  The  residue  from  the  distillation  is  used  for  the  deter- 
mination of  urea. 

The  examination  of  urine  for  heavy  metals  such  as  lead,  mercury,  arsenict 
antimony,  etc.,  administered  as  medicine  or  otherwise  ingested,  is  done  by  the 
usual  methods  for  small  amounts  in  presence  of  organic  matter.  The  organic 
constituents  can  be  destroyed  by  any  of  the  strong  oxidizers,  perhaps  easiest 
by  hydrochloric  acid  and  potassium  chlorate.  Electrolytic  precipitation  is 
suitable  for  some  metals,  but  Frankel  f  calls  attention  to  the  fact  that  lead 


*  Zeits.  Anal.  30-388. 
t  Chem.  News,  1893-2-5. 


496  QUANTITATIVE    CHEMICAL    ANALYSIS. 

after  passing  through  the  system  is  in  a  combination  that  resists  direct  elec- 
trolytic decomposition. 

7.  The  acid  radicals  in  quantity  are  chlorine,  phosphoric  and  sulfuric.  Free 
acid,  mainly  phosphoric  with  perhaps  some  uric  or  lactic,  may  be  directly 
titrated  by  decinormal  alkali  or  alkali  carbonate  in  a  large  volume  of  urine. 
Should  the  urine  be  dark  colored,  litmus  paper  is  used  to  show  the  end-point. 
Oxalic  acid.  A  large  volume  of  urine  is  made  ammonical,  then  distinctly  acid 
by  acetic  acid.  On  addition  of  calcium  chloride  there  falls  calcium  oxalate 
accompanied  by  some  uric  acid.  The  precipitate  is  filtered  and  treated  with 
dilute  hydrochloric  acid  which  dissolves  the  calcium  oxalate  but  leaves  the 
uric  acid.  The  oxalate  is  precipitated  by  ammonia  and  ammonium  oxalate 
and  either  ignited  and  weighed  as  calcium  oxide,  or  the  oxalic  radical  deter- 
mined by  decomposing  the  precipitate  by  sulfuric  acid  and  titrating  by  potas- 
sium permanganate. 

Salkowski*  evaporates  200  to  500  Cc.  of  urine  to  one-third,  acidifies  by 
hydrochloric  acid,  and  extracts  the  oxalic  acid  by  several  treatments  with 
ether-alcohol.  The  extract  is  filtered  and  evaporated  to  dryness;  the  residue 
is  dissolved  in  water,  evaporated  somewhat,  and  filtered  from  certain  bodies 
soluble  in  ether  but  insoluble  in  water.  Finally  the  oxalic  acid  is  precipitated 
by  calcium  chloride  in  a  faintly  acid  (acetic)  solution,  and  the  precipitate 
dealt  with  as  above. 

Chlorine  is  determined  gravimetrically  or  volu metrically  by  precipitation 
with  silver  nitrate.  The  silver  chloride  is  always  impure  when  directly  thrown 
out  of  urine,  and  a  previous  oxidation  of  the  organic  matter  is  always  ad- 
visable. 

For  the  volumetric  determination  Mohr  recommends  to  evaporate  the  urine 
with  ammonium  nitrate,  heat  the  residue  to  low  redness,  dissolve  in  water,  and 
acidify  by  acetic  acid,  then  neutralize  by  calcium  carbonate.  The  titration  by 
silver  nitrate  may  then  be  proceeded  with,  using  potassium  chromate  as  an 
indicator.  Pibram  prefers  to  oxidize  the  organic  matter  by  boiling  with  potas- 
sium permanganate,  filtering  off  the  precipitate  of  manganic  hydrate  formed. 

Instead  of  the  potassium  chromate  indicator,  there  may  be  substituted  a  drop 
of  red  ferric  sulfocyanide  made  by  mixing  solutions  of  ferric  sulfate  and  potas- 
sium sulfocyanide,  this  reacting  with  silver  nitrate  to  form  insoluble  silver 
sulfocyanide;  bleaching  and  turbidity  give  a  double  indication  of  the  end- 
point.  Volhard  would  add  an  excess  of  silver  nitrate  in  known  weight,  make 
up  to  a  definite  volume  with  water,  draw  off  an  aliquot  part  of  the  supernatant 
liquid,  and  titrate  by  ammonium  sulfocyanide  with  ferric  chloride  as 
indicator. 

Liebig  proposed  to  titrate  the  neutralized  urine  by  mercuric  nitrate,  the  re- 
action with  sodium  chloride  being  an  interchange  of  radicals  giving  mercuric 
chloride  and  sodium  nitrate,  the  solution  remaining  clear.  The  end-point  is 
shown  by  a  clouding  due  to  a  reaction  between  mercuric  nitrate  and  the  urea  of 
the  urine,  this  taking  place  only  after  all  the  chlorine  has  combined  with  mer- 
cury (page  500).  Ostwald  explains  the  reaction  by  pointing  out  that  as  long 
as  there  are  chlorine  ions  present  to  form  non-ionized  mercuric  chloride,  no 
precipitation  takes  place. 

The  greater  part  of  the  phosphoric  acid  in  urine  is  combined  as  phosphates  of 
the  earths  and  alkalies,  but  there  is  also  a  small  amount  of  glycero-phosphoric 
acid. 

Total  phosphoric  acid  is  determined  by  first  destroying  the  organic  matter  by 


Analyst,  1899—249. 


URINALYSIS.  497 

boiling  the  urine  with  nitric  acid  or  evaporating  with  sulfuric  acid,  which 
treatment  also  decomposes  the  glycero-phosphoric  acid  that  may  be  contained. 
On  now  adding  ammonia  and  magnesia  mixture  there  falls  ammonium  magnesium 
phosphate,  somewhat  impure  however.  The  precipitate  is  dissolved  in  acetic 
acid,  and  the  phosphoric  acid  determined  in  one  of  two  ways :  one  the  direct 
titration  by  standard  uranium  acetate  with  potassium  ferrocyanide  as  indicator 
(page  385)  ;  the  other  by  precipitation  with  uranic  acid  as  ammonium  uranium 
phosphate,  which  is  filtered,  washed  with  a  hot  solution  of  ammonium  chlo- 
ride, and  dissolved  in  dilute  sulfuric  acid.  The  uranic  oxide  is  reduced  to 
uranous  oxide  by  metallic  zinc,  and  the  latter  compound  reoxidized  by  titra- 
tion by  standard  permanganate.  The  phosphoric  acid  is  calculated  from  the 
permanganate  required  for  oxidation. 

The  phosphoric  acid  combined  with  the  earths  and  alkalies  is  determined  by 
treating  the  urine  with  magnesium  chloride  solution  containing  ammonium 
chloride  and  free  ammonia;  the  impure  magnesium  ammonium  phosphate  is 
filtered  off,  dissolved  in  acid,  and  the  solution  titrated  as  above.  The  phos- 
phoric acid  in  organic  combinations  remains  in  the  original  solution,  and  is 
found  by  difference. 

Sulfur  exists  in  urine  in  several  forms,  mainly  combined  as  sulfates  of  the 
inorganic  bases,  but  partly  also  in  ester  and  other  organic  combinations. 

A.  The  total  sulfur  of  the  urine  is  found  by  evaporating  a  measured  quantity 
with  sodium  carbonate  and  nitrate,  fusing  the  residue,  lixiviating  with  water, 
acidifying  by  hydrochloric  acid,  and  precipitating  the  sulfuric  radical  by  barium 
chloride  as  usual.     Another  plan  is  to  destroy  the  organic  matter  of  the  urine 
by  boiling  with  hydrochloric  acid  and  potassium  chlorate  until  colorless,  pre- 
cipitate by  barium  chloride,  and  wash  the  barium  sulfate  with  water  and  a  hot 
five  per  cent  solution  of  ammonic  chloride. 

B.  To  determine  the  inorganic  sulfates  the  urine  is  acidified  by  hydrochloric 
acid  and  directly  precipitated  by  barium  chloride,  and  the  barium  sulfate  puri- 
fied and  weighed. 

C.  The  difference  between  the  quantities  of  sulfur  found  in  A  and  B  is  the 
sulfur  combined  as  organic  compounds. 

Organic  constituents. 

8.  Oxidizable  substances  other  than  urea.  As  stated  by  Smith,  there  is  a  definite 
ratio  between  the  weights  of  potassium   permanganate  reduced  by  cold  and 
boiling  urine,  in  healthy  urine  about  1  to  3.5,  which  may  rise  to  1  to  12  in  diabetic 
urine.    His  process  is  to  dilute  the  urine  with  four  parts  of  water  and  add  it  to 
five  Cc.  of  dilute  sulfuric  acid  containing  .001  gram  of  permanganate.  The  addi- 
tion is  continued  to  decolorization.    The  above  is  repeated  with  the  diluted 
urine  at  a  temperature  of  nearly  100°  . 

9.  Uric  acid.  This  is  a  white  crystalline  powder  of  specific  gravity  1.86,  and 
has  the  formula  CsI^N^e.    It  is  nearly  insoluble  in  water  and  dilute  hydro- 
chloric acid,  and  easily  fermented  with  the  formation  of  ammonium  carbonate 
and  other  bodies.    In  urine  it  is  usually  combined  with  sodium  or  ammonium 
as  urates,  sometimes  in  part  free. 

An  old  but  inaccurate  method  depends  on  the  insolubility  of  uric  acid  in  dilute 
hydrochloric  acid.  Urates  are  decomposed  by  hydrochloric  acid,  e.0.,Na2C5H2N4O3 
-f  2HCI  =  2NaCl  -f  H2C6H2N4Oe.  The  urine  is  acidified  and  allowed  to  stand  for 
some  days,  the  uric  acid  slowly  separating  as  a  crystalline  powder  that  may  be 
dried  and  weighed.  A  more  satisfactory  plan  is  to  evaporate  the  urine  to  dry- 
ness,  dissolve  out  urea,  etc.,  from  the  residue  by  dilute  hydrochloric  acid  and 


498  QUANTITATIVE    CHEMICAL    ANALYSIS. 

alcohol,  and  submit  the  residual  impure  uric  acid  to  decomposition  by  sodium 
hypobromite,  measuring  the  nitrogen  evolved  (page  246). 

Insoluble  ammonium  biurate  is  formed  when  urine  is  saturated  with  ammo  - 
nium  chloride.  The  precipitate  is  filtered  and  washed  with  a  saturated  solution 
of  ammonium  chloride,  and  may  be  treated  in  several  ways  to  determine  the 
acid  present.  The  simplest  is  that  of  decomposition  by  hydrochloric  acid, 
drying  and  weighing  the  residue  of  uric  acid;  or  it  may  be  suspended  in  water 
and  titrated  by  a  standard  solution  of  the  organic  base  piperidine  with  phenol  - 
phthalein.  The  titrand  is  standardized  against  hydrochloric  acid;  one  mol- 
ecule of  uric  acid  combines  with  one  molecule  of  piperidine. 

Uric  acid  in  sulfuric  acid  solution  is  oxidized  by  potassium  permanganate. 
The  first  appearance  of  a  faint  pink  color,  permanent  for  a  few  moments,  is 
regarded  as  the  end-point.  The  precipitate  of  ammonium  biurate  may  be  dis- 
solved in  a  solution  of  sodium  carbonate  or  simply  suspended  in  water,  strongly 
acidified  by  sulfuric  acid,  then  at  once  titrated  by  N/20  permanganate,  of  which 
one  cubic  centimeter  oxidizes  .00375  gram  of  uric  acid.  An  objection  to  these 
methods  is  the  difficulty  of  filtering  the  gelatinous  ammonium  biurate. 

Argentic  urate  and  argentic  magnesium  urate  are  both  practically  insoluble 
in  water,  ammonia  and  acetic  acid,  but  readily  soluble  in  nitric  acid.  Hay- 
craft  precipitates  the  uric  acid  from  urine  by  silver  nitrate  with  the  addition 
of  a  little  ammonia  and  sodium  bicarbonate.  The  precipitate  is  dissolved  in 
nitric  acid  and  the  silver  titrated  by  ammonium  sulfocyanide.  Deroide  avers 
that  the  compound  of  silver  and  uric  acid  is  of  constant  composition,  and  that 
if  xanthic  bodies  are  eliminated,  this  process  is  the  most  accurate  of  any 
known. 

In  several  proposed  methods  the  acid  is  precipitated  as  silver  magnesium 
urate  by  silver  nitrate  in  conjunction  with  magnesia  mixture.  Chapek 
measures  the  standard  silver  nitrate  solution  added,  dilutes  the  liquid  to  a 
definite  volume,  and  determines  the  excess  of  silver  in  an  aliquot  part  of  the 
filtrate  by  standard  sodium  hydrogen  sulfide;  the  chlorine  of  the  urine  does 
not  interfere  since  silver  chloride  is  soluble  in  the  ammonia  of  the  magnesia 
mixture.  Bartley,*  after  the  addition  of  magnesia  mixture  and  ammonia, 
titrates  directly  by  N/50  silver  nitrate,  finding  the  end-point  by  filtering  a  few 
drops  and  testing  with  sodium  sulfide.  Ludwig  decomposes  the  precipitate 
by  sodium  sulfide,  then  filters  off  the  silver  sulfide,  concentrates  the  filtrate 
and  decomposes  by  hydrochloric  acid;  the  residual  uric  acid  is  filtered  off 
and  washed  by  carbon  disulflde  to  remove  sulfur,  dried  and  weighed.  Cam- 
merer  determines  the  nitrogen  in  the  precipitate  by  the  method  of  Kjeldahl 
and  calculates  the  uric  acid  therefrom. 

Budisch  and  Boroschek  f  object  on  several  accounts  to  the  use  of  ammonia 
in  the  precipitation  cf  magnesium  silver  urate.  They  prefer  to  employ  a 
solution  of  silver  chloride  in  an  aqueous  solution  of  sodium  sulflte,  and  to 
make  the  urine  strongly  alkaline  by  sodium  carbonate.  The  precipitate  is 
AgC5H3N4O3. 

Various  other  bases  have  been  proposed  as  precipitants,  but  no  method  yet 
devised  has  proved  entirely  satisfactory  4 

10.  Urea  or  carbamide  is  the  chief  form  in  which  the  nitrogen  of  food  is 
eliminated  from  the  system.  An  average  of  about  30  grams  is  daily  excreted 
by  an  adult;  a  continuous  excess  points  to  abnormal  tissue  waste,  while  a  de- 


*  Hartley's  Medical  Chemistry,  641. 
t  Journ.  Amer.  Chem.  Socy.  1902—562. 
\  Journ.  Amer.  Chem.  Socy.  1897—235. 


URINALYSIS.  499 

flciency  indicates  diminished  metabolism  or  retention  from  enfeebling  or  chronic 
diseases. 

Urea  has  the  formula  CO(NH2)2  and  is  the  type  or  nucleus  of  a  series  of 
allied  bodies  formed  by  replacement  of  one  or  both  of  the  amidogen  groups  by 
organic  radicals.  It  crystallizes  in  long  prisms  colorless  and  odorless.  Boiled 
with  water  it  is  partially  transposed  to  its  isomer  ammonium  cyanate,  and 
when  heated  to  160°  is  converted  to  biuret.  Heated  with  potassium  hydrate 
it  is  entirely  converted  to  carbon  dioxide  and  ammonia,  and  with  potash  and 
potassium  permanganate  it  is  decomposed,  but  yields  no  ammonia  if  perfectly 
pure  (Wanklyn).  It  is  decomposed  to  nitrogen,  carbon  dioxide  and  water  by 
nitric  acid  containing  nitrous  acid.  In  urine  it  is  associated  with  a  ferment 
(  Torula  ureae)  that  by  assimilation  of  water  speedily  converts  it  into  ammonium 
carbonate. 

Basic  in  character,  salts  are  formed  with  the  stronger  acids.  The  solutions 
are  not  precipitated  by  tannin,  lead  acetate,  or  other  precipitants  of  the  alka- 
loids, nor  do  they  reduce  Fehlings  solution. 

Bunsen's  method.  The  principle  is  that  of  decomposing  the  urea  by  heating 
the  urine  with  barium  hydrate, — the  carbonic  acid  formed  being  fixed  by 
barium  —  CO(NH2)2  -f-  Ba(OH)2  =  BaCOs +  2NH3.  In  a  modification  due  to 
Bunge,  30  Cc.  of  the  urine  is  treated  with  10  Cc.  of  a  cold  saturated  solution 
of  barium  chloride  containing  some  free  ammonia.  After  filtering  from  barium 
sulfate,  carbonate,  etc.,  through  a  dry  paper,  20  Cc.  of  the  filtrate  is  introduced 
into  a  stout  glass  tube  containing  3  grams  of  solid  barium  chloride.  The  tube 
is  sealed  and  heated  to  160  o  for  four  hours.  The  barium  carbonate  is  filtered 
off,  converted  to  barium  sulfate  and  weighed,  and  the  carbon  dioxide  calculated. 
One  molecule  of  carbon  dioxide  comes  from  one  molecule  of  urea. 

Wanklyn  mixes  the  urine  with  a  strong  solution  of  caustic  potash,  heats  in  a 
retort  to  150  ° ,  and  collects  the  distillate  which  contains  the  free  ammonia  from 
the  decomposition  of  the  urea  —  CO(NH2)2  +  2KOH  +  2H2O =2NH4OH  +  K2CO3. 
The  ammonia  is  determined  by  Nesslers  test  (page  376). 

Cross  and  Bevan  *  find  that  urea  is  completely  decomposed  to  nitrogen  and 
carbon  dioxide  by  a  mixture  of  chromic  and  sulfuric  acids  containing  a  little 
nitric  acid.  The  nitrogen  is  collected  in  a  gas-measuring  tube  standing  over 
soda-lye,  the  alkali  absorbing  the  carbon  dioxide.  The  volume  of  nitrogen  is 
measured,  reduced  to  normal  conditions  of  temperature  and  pressure,  and  its 
weight  calculated.  Very  accurate  results  were  obtained  on  pure  urea  by  this 
process. 

By  fermentation.  Musculus  proposes  to  collect  the  peculiar  ferment  of  de- 
composing urine,  by  passing  the  urine  through  filter  paper,  cutting  the  paper 
into  strips,  and  drying  at  a  low  temperature,  when  the  entangled  ferment  re- 
mains active  for  some  days.  The  urine  to  be  tested  is  neutralized  and  a  few 
strips  of  paper  introduced.  After  standing  for  six  hours  at  about  40°  C.  in  a 
closed  vessel,  the  free  ammonia  formed  by  the  decomposition  of  the  urea  is 
titrated  by  standard  acid. 

Urine  mixed  with  diastasiferous  broth  containing  urophagous  bacilli  is  also 
decomposed,  though  if  the  urea  be  above  ten  per  cent  the  solution  is  poison- 
ous to  the  active  bacilli.  Miguel  mixes  equal  volumes  of  the  broth  and  urine 
with  enough  ammonium  carbonate  to  make  the  whole  slightly  alkaline ;  in  a 
part  of  the  mixture  ammonia  is  at  once  determined  by  alkalimetry;  another 
part  is  heated  in  a  full  closed  bottle  to  50°  for  two  hours  and  the  ammonia 
determined.  The  difference  is  the  ammonia  from  urea. 


*  Chem.  News,  1889-2-13. 


500 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


Liebig's  method.  (1)  With  mercuric  nitrate,  urea  forms  an  insoluble  white 
compound  —  2CO(NH2)2  +  3H2O  -f  4Hg(NO3)2  =  2CO(NH2)2.Hg(rNO3)2.3HgO  + 
6HNOs.  (2)  Sodium  carbonate  precipitates  yellow  mercuric  oxide  from  mer- 
curic solutions.  With  these  reactions  as  a  basis,  urea  can  be  titrated  by 
mercuric  nitrate  (best  standardized  against  pure  urea),  the  end-point  being 
observed  as  the  yellow  precipitate  struck  with  sodium  carbonate. 

The  urine  is  prepared  for  the  titration  by  coagulating  any  albumen  by  boiling, 
and  precipitating  the  sulfuric,  phosphoric  and  carbonic  acids  by  baryta 
water  containing  barium  nitrate.  After  filtration  the  urine  is  neutralized  by 
nitric  acid  and  titrated,  the  end-point  observed  by  bringing  a  drop  in  contact 
with  a  paste  of  sodium  bicarbonate  and  water.  The  solution  must  be  con- 
tinuously neutralized  during  the  titration  (as  by  suspended  calcium  carbonate) 
or  at  least  at  the  close,  since  free  nitric  acid  alters  the  composition  of  the  pre- 
cipitate from  the  normal  formula;  a  correction  is  to  be  made  for  the  volume  of 
titrand  needed  to  convert  sodium  chloride  into  sodium  nitrate.  The  method 
calls  for  many  precautions  and  has  largely  fallen  into  disuse.* 

Knop's  methods.  For  clinical  demonstrations,  this  method  is  eminently 
adapted  by  reason  of  its  simplicity,  rapidity  and  fair  accuracy.  The  basis  is  the 
decomposition  suffered  by  nreawhen  brought  in  contact  with  an  alkaline  solu- 
tion of  a  hypobromite,  one  of  the  educts  being  nitrogen;  thus,  NET2.CO,NH2 
-j-  3NaOBr  -f  2NaHO  =  N2  -f  Na2COs  +  3H2O  -f  3NaBr.  The  nitrogen  is 
collected  and  measured  and  the  urea  calculated  by  the  formula  of  Hufner  — 
— ft') 

*      354.3 


where  h is  the  weight  of  urea;  6,  the  height  of 


760  (1+  .003660 

the  barometer,  and  b'  the  tension  of  aqueous  vapor  at  the  time  of  the  experiment ; 
t  °  ,  the  temperature,  and  F,  the  observed  volume  of  moist  nitrogen.  The  result 
includes  whatever  free  ammonia  may  be  in  the  urine.  Practically,  one  gram 
of  urea  furnishes  354.3  Cc.  of  nitrogen. 

Bat  it  is  well  recognized  that  the  full  theoretical  yield  of  ni- 
trogen is  not  obtained  under  these  conditions,  since  for  a  given 
weight  of  pure  urea  added  to  urine,  nitrogen  correspond- 
ing to  only  about  92  per  cent  of  urea  is  evolved.  In  practice  the 
loss  is  variously  stated  at  from  three  to  ten  per  cent.  Luther  ex- 
plains the  discrepancy  on  the  ground  that  three  to  four  per  cent  is 
oxidized  to  nitric  acid,  and  one  to  two  per  cent  to  a  compound 
yielding  ammonia  on  distillation  with  sodium  hydrate.  Fenton 
ascribes  it  to  the  formation  of  ammonium  cyanate.  An  ad- 
dition of  sucrose  or  glucose  brings  the  yield  nearly  up  to  the  theo- 
retical. Allen  to  prevent  the  reversion  to  the  isomeric  ammonium 
cyanate,  makes  an  addition  of  potassium  cyanate  to  the  urine 
before  the  test;  then  adds  sodium  hydrate,  finally  bromine. 

The  analysis  is  made  in  a  glass  apparatus,  one  of  the  many 
forms  shown  in  Fig.  184.  The  lower  bulb  A  holds  exactly 
five  cubic  centimeters,  and  is  filled  with  urine  up  to  the  stop- 
cock B,  which  is  then  closed  and  the  tube  G  filled  with 
the  mixture  of  bromine  and  sodium  hydrate  solution,  often 
referred  to  as  the  "bromized  soda  solution."  A  gas -measur- 
ing tube  D  is  filled  with  water  and  inverted  over  the  orifice  of  (7,  and  B 
opened.  The  heavier  reagent  flows  down  into  the  urine  in  A,  and  the  nitrogen 
evolved  rises  into  D;  when  the  evolution  has  ceased  D  is  transferred  to  a 
jar  of  water,  and  the  volume  of  moist  gas  read,  corrected  for  temperature  and 


Fig.  184. 


*  Journ.  Amer.  Chem.  Socy.  1901—632. 


URINALYSIS. 


501 


Fig.  185. 


pressure,  and  calculated  to  urea.  Under  ordinary  conditions  the  volume  of  the 
gas  is  increased  by  the  tension  of  the  aqueous  vapor  to  an  extent  nearly  com- 
pensating for  the  volume  of  that  part  of  the  nitrogen  not  evolved  in  the 
reaction,  so  that  the  uncorrected  volume  may  for  practical  purposes  be  con- 
sidered as  the  absolute  volume. 

The  proposal  of  Davy  to  substitute  a  solution  of  calcium 
hypochlorite  (bleaching  powder)  or  sodium  hypochlorite  for 
the  hypobromite  has  the  sanction  of  many  chemists. 

A  simpler  though  less  exact  apparatus  for  the  use  of  physi- 
cians is  known  as  the  te  ureameter  "  and  shown  in  Fig.  185. 
It  is  inverted  and  the  graduated  tube  A  filled  with  either 
the  bromized  solution  or  one  of  chlorinated  soda.  When 
the  apparatus  is  returned  to  an  upright  position  the  solu- 
tion is  held  up  by  atmospheric  pressure.  One  cable  centi- 
meter of  urine  is  passed  into  A  from  B,  and  the  volume  of 
nitrogen  rising  in  A  read  on  the  graduations ;  these  show 
the  content  of  urea  directly  and  save  any  calculation. 
Hamburger,  on  the  assumption  that  one  molecule  of  urea  yields  two  atoms 
of  nitrogen  through  the  reduction  of  three  molecules  of  sodium  hypobromite, 
adds  to  the  urine  an  excess  of  standard  alkaline  hypobromite,  reduces  the 
excess  by  standard  arsenious  acid,  then  titrates  the  excess  of  the  latter  by 
iodine  solution  and  starch.  The  method  is  pronounced  unreliable  by  Pflueger. 
Of  other  methods  that  have  been  published  may  be  mentioned  the  direct 
precipitations  by  oxalic  acid,  phosphotungstic  acid,  and  orthonitrobenzaldehyd, 
and  the  decomposition  to  nitrogen  by  Millon's  reagent,  heating  with  phos- 
phoric anhydride,  etc.  It  has  been  proposed  to  calculate  the  weight  of  urea 
from  specific  gravity  or  refractive  index  determinations  before  and  after 
decomposition  by  hypobromite.  An  approximate  clinical  determination  can 
be  made  on  the  basis  of  the  rise  in  temperature  from  the  reaction  between 
urea  and  alkaline  sodium  hypochlorite. 

Nitrogen.  From  85  to  90  per  cent  of  the  total  nitrogen  of  urine  is  in  the  form 
of  urea.  The  simplest  method  for  total  nitrogen  is  that  of  Kjeldahl,  evap- 
orating the  urine  with  concentrated  sulfuric  acid,  heating  until  nearly  color- 
less, making  alkaline  with  sodium  hydrate  and  distilling  the  ammonia, 
which  may  be  determined  in  any  suitable  manner.  Caseneuve  and  Hugouneng 
prefer  to  mix  the  urine  with  an  equal  weight  of  plaster  of  Paris  and  a  little 
oxalic  acid,  dry  in  an  air-bath,  mix  with  oxide  of  copper,  and  burn  by 
the  method  of  Dumas  (page  304),  somewhat  modified.  When  using  the  soda- 
lime  method,  five  cubic  centimeters  of  urine  may  be  poured  Into  the  filled 
tube,  dispensing  with  any  preliminary  evaporation. 

Albumen.  Absent  from  healthy  urine  or  exhibited  only  under  temporary 
lesions  of  the  system,  in  certain  diseases  it  is  a  constant  symptom,  and  the  de- 
tection is  therefore  an  important  aid  in  diagnosis,  and  the  quantity  an  index  to 
the  progress  of  the  disease.  For  qualitative  tests  many  rsagents  have  been 
proposed  such  as  uranium  acetate,  potassium  ferrocyanide  with  acetic  acid, 
picric  acid,  salicyl-sulfonic  acid,  mercuric  potassium  iodide,  mercuric 
succinimid,  etc. 

The  usual  test  is  founded  on  the  property  of  albumen  in  solution  to  coagulate 
at  a  moderate  heat.  The  urine  is  filtered,  faintly  acidulated  by  acetic  acid  to 
prevent  precipitation  of  earthy  phosphates,  and  boiled  for  a  few  minutes.  If 
there  be  albumen  in  the  urine,  a  cloud  appears,  and  shortly  the  albumen 
separates  in  flocks  if  present  in  considerable  quantity. 
The  albumen  may  be  filtered,  washed  with  alcohol  and  ether,  dried  and 


502  QUANTITATIVE    CHEMICAL    ANALYSIS. 

weighed.  It  has  been  recommended  to  filter  through  a  plug  of  purified  cotton 
in  a  glass  filtering- tube  by  the  aid  of  a  vacuum,  and  wash  until  no  reaction 
appears  with  silver  nitrate.  The  tube  is  dried,  first  at  100  o  then  at  110  o  to 
constant  weight,  assisting  the  removal  of  water  by  passing  dry  air  through  the 
tube  during  the  desiccation.  Should  the  weight  of  albumen  exceed  .2  gram,  a 
second  test  is  made  with  a  smaller  volume  of  urine  diluted  with  water.  From 
the  weight  is  deducted  that  of  the  traces  of  earthy  phosphates  always  left 
when  the  albumen  is  burned.* 

The  volume  of  the  albumen  as  deposited  after  boiling  or  precipitation  may 
be  measured  in  a  graduated  tube.  In  Purdy's  modification  of  Esbach's  tube, 
Tig.  75,  the  lower  portion  is  drawn  to  a  cone  for  more  accurate  measuring.  A 
fixed  volume  of  urine  is  placed  in  the  tube,  the  precipitating  reagent  added,  and 
the  tube  whirled  in  a  centrifuge  until  the  albumen  has  compacted  in  the  bottom. 
The  "  bulk  percentage  "  is  read  by  the  graduations  and  converted  to  percen- 
tage by  weight  by  a  table  furnished  with  the  apparatus:  over  two  per  cent  by 
weight  is  unusual.  The  conditions  of  time,  velocity  of  the  centrifuge,  and  dis- 
tance of  the  end  of  the  tube  from  the  axis  of  the  machine  must  be  the  same  in 
all  experiments.  Esbach's  reagent  is  a  mixture  of  picric  acid  to  coagulate  the 
albumen,  with  citric  acid  to  hold  up  phosphates:  Purdy's  reagent  is  a  mixture 
of  acetic  acid  and  potassium  ferricyanide.f 

Precipitation  by  phenol  or  tannin  is  the  basis  of  several  methods.  Van  Nuys 
and  Lyons  determine  the  total  nitrogen  in  five  Cc.  of  the  urine ;  then  to  ten  Cc. 
add  an  equal  volume  of  tannin  in  alcoholic  solution  and  some  acetic  acid,  and 
filter  through  a  dry  paper.  In  one-half  of  the  filtrate  is  determined  the  nitrogen ; 
the  loss  from  the  preceding  determination  is  the  nitrogen  of  the  albumen,  and 
this  times  6.24  is  the  weight  of  the  albumen,  including  also  the  globulin.  Mehu 
mixes  100  Cc.  of  the  filtered  urine  with  two  Cc.  of  nitric  acid  and  ten  Cc.  of  a 
mixture  of  one  part  of  phenol,  one  of  acetic  acid,  and  two  of  alcohol.  After  fil- 
tering, the  precipitate  is  washed  by  a  cold  four  per  cent  solution  of  phenol  in 
water,  and  dried  and  weighed,  or  the  nitrogen  determined  by  the  method  of 
Kjeldahl. 

Girgensohn  mixes  ten  Cc.  of  urine  with  five  Cc.  of  a  20  per  cent  solution  of 
sodium  chloride,  then  adds  an  excess  of  tannin.  After  filtration,  the  precipitate 
is  washed  with  water  to  remove  salt  solution,  then  with  alcohol  to  remove 
tannin.  The  residue  is  said  to  be  pure  albumen. 

Potassium  ferrocyanide  precipitates  albumen  as  a  compound  containing  211 
parts  of  ferrocyanide  to  1612  parts  of  albumen.  Following  Boedeker,  the  urine 
is  mixed  with  an  equal  volume  of  acetic  acid  and  titrated  by  a  standard  solution 
of  the  ferrocyanide  of  which  one  cubic  centimeter  precipitates  ten  milligrams 
of  albumen.  The  indication  of  complete  precipitation  of  the  albumen  is  the 
yellow  color  of  the  filtrate  due  to  a  slight  excess  of  ferrocyanide.  The  process 
is  tentative,  successive  trials  being  made  with  different  ratios  of  urine  to  ferro- 
cyanide until  with  a  certain  volume  of  urine  a  clear  yellow  filtrate  is  obtained  giv- 
ing a  precipitate  with  a  drop  of  urine,  while  the  filtrate  from  a  slightly  greater 
volume  of  urine  is  nearly  colorless  and  gives  no  precipitate  with  urine ;  the 
mean  of  the  two  volumes  is  assumed  to  be  the  equivalent  of  the  fixed  vol- 
ume of  ferrocyanide.  For  example,  ten  Cc.  of  ferrocyanide  solution  tested 
10  Cc.  (yellow  filtrate)  — 20  Cc.  (colorless)  — 15  Cc.  (yellow)  — 17.5  Cc.  (yel- 
low) — 18  Cc.  (colorless)  of  urine,  indicating  an  equivalent  volume  of  about 
17.75  Cc.  of  urine. 


*  Zeits.  anal.  27—635. 

t  Journ.  Amer.  Medical  Assn.  1899—763. 


URINALYSIS.  503 

Other  reagents  that  have  been  proposed  for  the  precipitation  of  albumen  are 
nitric,  metaphosphoric,  and  trichloracetic  acids,  mercuric  potassium  Iodide, 
etc. 

The  loss  in  specific  gravity  and  the  difference  in  refractive  index  before  and 
after  precipitation  of  the  albumen  by  heat  furnish  approximate  tests  for  urines 
fairly  high  in  albumen;  and  for  moderate  amounts  in  urine  that  can  be  made 
perfectly  clear  and  is  not  too  dark  in  color,  a  polarimetric  examination  is  at 
once  rapid  and  fairly  accurate.  Using  a  200  Mm.  tube,  in  the  Soleil-Ventske 
polarimeter,  each  division  of  left-handed  rotation  corresponds  to  one  gram  of 
albumen  per  100  Cc.  nearly. 

It  is  known  that  the  so-called  albumen  of  urine  is  really  but  a  comprehen- 
sive term  for  an  indefinite  group  of  proteids.  Attempts  have  been  made  with 
some  success  to  differentiate  these  but  the  methods  are  hardly  suitable  for 
technical  analysis. 

13.  Sugar.  Dextrose  in  more  than  traces  is  an  abnormal  constituent  of  urine 
and  an  indication  of  the  disease  known  as  diabetes  mellitus.  Usually  diabetic 
urine  is  of  higher  density  and  paler  in  color  than  when  normal. 

The  carbon  dioxide  evolved  on  fermentation  by  yeast  may  be  determined  by 
any  of  the  usual  methods,  but  the  process  is  subject  to  the  uncertainties  at- 
tending any  estimation  of  sugar  in  a  complex  mixture  by  fermentation  unless 
corrected  by  a  parallel  test,  and  moreover  is  interfered  with  by  antiseptics  that 
may  be  in  the  urine,  derived  from  medicines  exhibited  or  otherwise.  To  re- 
move interfering  bodies  Bishop  advises  to  shake  up  the  urine  with  animal 
charcoal,  and  when  colorless  to  add  baryta  water  and  boil.  After  cooling  and 
filtering,  copper  sulfate  is  added  to  the  filtrate  in  moderate  excess,  and  after 
standing  for  an  hour  in  a  covered  vessel  is  decanted  and  filtered.  The  copper 
is  removed  from  the  filtrate  by  hydrogen  sulfide,  and  the  excess  of  hydrogen 
sulfide  by  heating.  The  liquid  is  now  ready  for  a  determination  which  may  be 
made  in  the  usual  way  (page  429).* 

A  simple  apparatus,  Fig.  186,  was  devised  by  Einhorn  for  clinical  use.  The 
tube  A  is  inverted  and  filled  with  urine,  then  turned  upright 
and  a  little  yeast  introduced.  The  apparatus  is  kept  in  a 
warm  place  until  the  evolution  of  gas  ceases,  and  the  vol- 
ume of  carbon  dioxide  read  by  the  graduations  which  are 
spaced  to  show  percentages  of  sugar  directly,  allowing  for 
the  retention  of  carbon  dioxide  in  the  urine  and  other  fac- 
tors. 

The  specific  gravity  of  diabetic  urine  is  lowered  through 
fermentation  of  the  saccharine  matter,  and  the  reduction  Is 
a  fair  index  of  the  sugar  content. 

The  rotation  of  the  plane  of  polarized  light  by  dextrose 
Fig.  186.  (+52°)>  as  observed  in  the  polarimeter,  is  an  easy  test, 

and  unde'r  certain  conditions  may  be  quite  as  accurate  as  any 
other  method.  However,  albumen  and  some  other  constituents  of  urine  also 
rotate  the  plane.  Such  interfering  constituents  can  be  removed  by  the  usual 
reagents,  but  clarification  by  lead  acetate  is  unsatisfactory  for  the  small  quan- 
tities of  sugar  contained  in  urine,  and  even  for  exceptionally  high  percentages; 
a  better  medium  is  talc  powder. 

Phenylhydrazin  yields  a  precipitate  of  glucosazon  with  the  sugar  of  dia- 
betic urine.  Von  Jaksch  removes  any  albumen  by  boiling,  filters,  adds  to  the 
filtrate  sodium  acetate  and  phenylhydrazin  hydrochloride,  and  heats  the  liquid 


*  Amer.  Journ.  Pharm.  1898 — i50 


504  QUANTITATIVE    CHEMICAL    ANALYSIS. 

to  100°  for  a  half  hour.  If  the  precipitate  falls  in  an  amorphous  condition  it 
is  filtered  off,  dissolved  in  hot  alcohol,  diluted  with  water,  and  the  alcohol 
boiled  off,  when  the  precipitate  will  reappear  on  cooling  as  yellow  prisms. 

As  a  qualitative  reagent  this  compound  has  distinct  advantages  over  others 
but  is  inferior  for  quantitative  work.  Coriat*  finds  that  substances  interfering 
with  Fehlings  and  Nylanders  tests  do  not  modify  the  reaction  with  phenylhy- 
drazin,  nor  is  it  necessary  to  remove  albumen.  The  limit  of  delicacy  as  a 
qualitative  test  is  said  to  be  one  part  in  10000  of  liquid. 

A  colorimetric  method  due  to  Johnson  is  that  of  boiling  the  urine  with  addi- 
tion of  picric  acid  and  potassium  hydrate  which  gives  with  sugar  a  dark  mahog- 
any-red color.  For  a  standard  a  solution  of  pure  grape  sugar  is  treated  in  the 
same  manner,  or  more  comparably  a  normal  urine  compounded  with  a  weight 
of  grape-sugar  approximately  equal  to  that  in  the  urine  under  examination. 

More  prominent  than  any  other  method  is  the  volumetric  determination  by 
titration  with  Fehlings  solution.  But  certain  other  constituents  of  urine  inter- 
fere somewhat,  notably  creatinin  and  uric  acid,  also  xanthin  and  its  ana- 
logues that  give  a  precipitate  containing  a  cuprous- xanthin  compound  instead 
of  pure  cuprous  oxide.  It  is  said  that  the  reducing  power  of  normal  urine  is  on 
an  average  equal  to  .3  per  cent  of  glucose. 

Albumen,  if  present  in  the  urine  is  coagulated  by  acidification  and  boiling, 
the  filtrate  is  made  alkaline  and  filtered  from  phosphates,  and  xanthin  bodies 
removed  by  precipitation  by  copper  sulfate,  A  measured  quantity  of  Feh- 
lings solution  is  then  titrated  by  the  filtrate.  Pavys  ammoniacal  copper  solution 
is  to  be  preferred  to  the  original  Fehling  since  any  free  ammonia  in  the  urine 
would  dissolve  some  of  the  precipitate  of  cuprous  oxide;  the  titration  is 
conducted  under  a  layer  of  melted  paraffin,  the  burette  tip  immersed  in  the 
urine.  For  routine  work  the  solution  may  be  made  up  of  such  a  strength 
that  one  cubic  centimeter  represents  one  milligram  of  dextrose,  the  sugar 
solution  being  approximately  of  a  given  concentration. 

Knapps  mercuric  solution  (page  433) ,  is  preferred  by  some  authorities  to 
that  of  Fehling  or  its  modifications. 

14.  Creatinin.  The  method  of  Liebig  f  is  to  exactly  neutralize  the  urine  by 
milk  of  lime,  and  add  calcium  chloride  as  long  as  phosphoric  acid  is  precipi  - 
tated.  The  filtrate  is  evaporated  to  a  small  bulk,  decanted  from  crystals  of 
sodium  chloride,  etc,  and  treated  with  zinc  chloride  and  left  for  several  days. 
Creatinin  zinc  chloride  separates  in  nodules,  to  be  filtered  and  washed  with 
a  little  cold  water,  then  with  alcohol.  The  compound  is  decomposed  by  lead 
hydroxide  to  creatinin,  zinc  hydrate,  and  lead  chloride,  filtered  and  concen- 
trated, and  the  creatinin  extracted  from  creatin  by  absolute  alcohol.  Johnson 
regards  the  creatinin  obtained  by  the  above  process  as  having  undergone  some 
change,  for  the  reason  that  its  reactions  differ  somewhat  from  those  given  by 
creatinin  extracted  without  application  of  heat. 

Neubauer  separates  the  phosphoric,  sulfuric  and  carbonic  acids  of  the  urine 
by  calcium  oxide  and  chloride,  or  by  barium  hydrate  and  nitrate,  evaporates  the 
filtrate  to  dryness,  extracts  the  residue  with  alcohol,  and  precipitates  the  crea- 
tinin by  zinc  chloride  in  alcoholic  solution.  The  precipitate  has  the  formula 
(C4H7N30)2.ZnCl2. 

Gautrelot  and  Viellard  determine  creatinin  indirectly,  calculating  its  weight 
from  three  determinations  of  nitrogen —  (1)  on  the  original  urine;  (2)  on  the 
filtrate  from  the  urine  precipitated  by  basic  lead  acetate;  and  (3;,  on  the  fll- 


*  Joarn.  Amer.  Chem.  Socy.  1900—19. 
t  Allen,  Coml.  Org.  Anal.  3—3—288. 


UUINALYSIS.  505 

trate  from  the  urine  precipitated  by  lead  acetate  and  zinc  chloride.  la  the 
first  determination  is  found  the  total  nitrogen  of  all  nitrogenous  bodies;  in  the 
second,  that  of  creatinin  and  nitrogenous  bodies  other  than  urea;  and  in  the 
third,  that  of  nitrogenous  bodies  other  than  urea  and  creatinin. 

15.  Acetone.  A  method  due  to  Engel  and  Deveto  is  as  follows.    From  the 
urine  passed  during  24  hours  is  drawn  from  30  to  100  Cc.  which  is  diluted  with 
an  equal  volume  of  water.    A  little  acetic  acid  is  added  to  retain  phenol,  and 
the  liquid  distilled  to  one-tenth.    The  acetone  passes  into  the  distillate,  and 
after  qualitatively  testing  the  residue  in  the  retort   to  insure   that  all  has 
passed  over,  the  distillate  is  redistilled  with  the  addition  of  a  little  sulf  uric 
acid  to  retain  ammonia.    To  this  distillate  is  added  an  excess  of  decinormal 
iodine  solution  and  sodium  hydrate  which  react  with  the  acetone  — 

CH3.CO.CH3  +  3I2  +  NaOH  =  CHI3  -f  CH3.COONa  -f-  3NaI  -f  3H2O. 
The  excess  of  iodine  is  then  titrated  back  by  decinormal  sodium  thiosul  - 
fate  and  the  weight  of  acetone  calculated  from  the  above  equation. 

16.  Xanthin  bases.  According  to  Salkowski  these  are  not  true  xanthin  but 
more  nearly  resemble  hypoxanthin.  He  directs  to  first  precipitate  the  phosphates 
from  a  liter  of  urine  by  magnesia  mixture;  then  after  filtering,  the  xanthin, 
uric  acid,  etc.,  by  ammonium  silver  nitrate.    The  precipitate  is  suspended  in 
water  and  decomposed  by  hydrogen  sulflde  and  the  filtrate  evaporated  to  dry- 
ness.    The  xanthin  is  extracted  from  the  residue  by  weak  sulfuric  acid,  leav- 
ing the  uric  acid;  the  filtrate  is  again  precipitated  as  before.    The  silver  in 
the  precipitate  is  converted  to  chloride  and  weighed,  and  the  xanthin  cal- 
culated.   Normal  urine  is  said  to  contain  from  .0027  to  .0030  gram  of  xanthin 
bodies  per  100  Cc.  of  urine. 

Krueger  and  Woolf  free  the  sample  of  urine  from  albumen,  if  present,  by 
boiling.  The  xanthin  bodies  are  then  to  be  precipitated  by  a  mixture  of  sodium 
bisulfite  and  copper  sulf  ate  with  a  little  barium  chloride  to  promote  settling ; 
in  a  few  hours  the  precipitate  is  filtered  and  washed  with  air-free  water.  The 
nitrogen  in  the  precipitate  is  determined  by  Kjeldahl's  process,  and  the  differ- 
ence between  the  result  and  nitrogen  previously  found  to  exist  as  uric  acid  is 
the  nitrogen  of  the  xanthin  and  allied  bodies. 

Hoffmeister  removes  xanthin  from  urine  by  hydrochloric  and  phospho- 
tungstic  acids,  the  reagents  employed  by  Von  Pohl  for  the  precipitation  of 
leucomaines  for  their  quantitative  determination. 

For  the  determination  of  various  other  normal  and  abnormal  constituents 
of  urine  the  student  is  referred  to  the  numerous  monographs  on  the  subject. 


506  QUANTITATIVE    CHEMICAL    ANALYSIS. 


THE  ORGANIC  DYE-STUFFS. 

The  phenomenon  of  the  fixing  of  a  dye-stuff  by  animal  or  vegetable  fibers  is 
variously  interpreted  by  different  writers  on  the  subject.  The  theories  ad- 
vanced are  mechanical  or  chemical  or  combinations  of  the  two.  The  adher- 
ents oi  the  mechanical  theory  "  assume  that  the  coloured  particles  gradually 
leave  the  dye-bath  to  fix  themselves  between  the  molecules  of  the  fibre  without 
any  chemical  action".  The  chemical  theory  "presumes  that  a  chemical 
combination  takes  place  between  the  fibre  and  colouring  matter,  e.  g.,  the  salt 
of  a  coloured  base  is  dissociated  by  the  fibre  ",  and  it  is  asserted  that  all  the 
phenomena  of  dyeing  premise  two  essential  conditions  —  the  presence  of  acid 
or  basic  functions  in  the  fiber  and  also  in  the  coloring  matter.  The  only 
exceptions  are  the  tetrazo-dyes.* 

A  number  of  attempts  have  been  made  to  classify  dye-stuffs  according  to 
their  chemical  composition.  That  of  De  Eonstanecki  is  given  below, f  the 
dyes  arranged  according  to  the  nature  and  number  of  their  chromophores  all 
of  which  contain  double  bonds. 

A.  Coloring  matters  containing  a  single  chromophore. 
C  :  C.            Diphenylene-ethane. 

C     O.  Oxyketones,  oxycoumarines,  oxyanthones,  oxyflavones. 

C     N.  Auramine,  thioflavine,  quinoline  yellow. 

O     N  :  O.    Nltro- coloring  matters. 
N     N.  Azo- colors. 

N     N  :  O.    Azoxy-colors. 

B.  Coloring  matters  containing  several  chromophores. 
1    Strep tostatic  chromophores  (ketone  type). 

C     C  with  C  :  O.    Unsaturated  oxyketones,  indogenides,  oxyindogenides,  indigo. 
C     O  with  C  :  O.    Oxydiketones,  oxydlanthones. 
C     N  with  C  :  N.    Hydrazone  coloring  matters. 
N     NwlthN  :  N.    Dlazo  colors. 

2.  Cyclostatic  chromophores  (qulnon  type). 

C  O  with  C  :  C.    Aurines,  benzeines,  phthaleins. 

C  O  with  C  :  O.    Oxyquinones. 

O  N  with  C  :  C.    Basic  coloring  matters  of  the  triphenylmethane  group,  pyronines. 

C  N  with  C  :  O.    Indophenols,  nitrosophenols. 

C  N  with  C  :  N.    Indamlnes,  azines,  safranines,  indullnes. 

C  N:OwithC:O.    Resazurine. 

3.  Streptostatic  and  cyclostatic  chromophores. 

This  group  comprises  several  complicated  coloring  matters  such  as  alizarin  blue,  sty- 
rogallol,  etc. 

Natural  products  of  interest  to  the  dyer  are  indigo,  madder,  logwood,  saffron, 
bar  wood,  and  others  of  less  importance.  Their  quality  varies  greatly  with  the 
locality  of  growth,  time  and  manner  of  collection,  age,  and  care  in  preservation. 
They  are  found  in  commerce,  in  bulk  or  powder,  or  as  liquid  or  solid  extracts, 
the  latter,  however,  offering  greater  opportunities  for  successful  adulteration.! 

The  enormous  development  in  recent  years  of  the  manufacture  of  artificial 
dyes  has  resulted  in  the  practical  retirement  of  many  of  the  natural  wares 
formerly  held  in  high  esteem,  until  at  present  there  remain  but  few  that  possess 
more  than  a  historical  interest. 


*  Journ.  Socy.  Dyers  &  Col.  1893—44  and  1897—62. 
t  7dm,  1897— 27.  ' 
J  Idem,  1892—9. 


THE    OKGAN1C    DYE-STUFFS.  507 

In  determining  the  value  of  a  commercial  article  it  is  to  be  remembered  that 
from  an  assay  of  the  leading  constituent  or  constituents  there  may  be  drawn 
conclusions  quite  unlike  those  from  a  test  along  the  lines  of  the  practical  em- 
ployment of  the  substance,  and  this  rule  is  eminently  true  of  the  natural  dye- 
stuffs,  complex  bodies  whose  impurities  exercise  a  great  influence  on  the 
behavior  of  the  dye-bath;  and  also  in  some  degree  applies  to  the  artificial 
dyes. 

1.  The  dye-test.  The  oldest  and  best  known  scheme  for  finding  the  quality  of 
a  dye-ware  is  a  miniature  dyeing  test  made  under  the  same  conditions  as  apply 
in  practice.    Opinions  differ  widely  as  to  the  value  of  this  form  of  assay,  some 
holding  that  when  properly  performed  the  results  can  be  accepted  with  assur- 
ance of  confirmation  when  working  on  a  large  scale,  while  others  deny  it  to  be 
more  than  a  rough  approximation,  for  the  reasons  that  it  is  impossible  to  follow 
closely  the  practice  of  the  dye  house  on  a  small  scale,  and  that  in  judging  the 
results  considerable  latitude  is  to  be  expected  among  different  observers  and 
at  different  times  by  one  observer.    But  conceding  that  the  test  often  proves 
neither  exact  nor  reliable  in  the  hands  of  the  inexperienced,  it  must  be  allowed 
that  an  expert,  well  acquainted  with  the  dyeing  processes  employing  the  ware 
examined,  can  deduce  reliable  estimations  as  to  the  quality  of  a  sample.* 

Essentially  an  empirical  process  modified  to  conform  with  the  practice  of  the 
dye  house,  no  details  for  the  test  can  be  formulated  that  may  be  followed  with- 
out modification.  The  tests  are  made  on  cloth  or  yarn  of  clean  cotton,  wool  or 
silk,  mordanted  or  not  to  conform  to  the  kind  of  fiber  and  nature  of  the  dye. 
In  general  a  weighed  amount  of  the  dye-stuff  is  brought  into  solution  in  a  fixed 
volume  of  solvent,  the  prepared  fiber  digested  for  a  given  time  and  at  a  given 
temperature,  withdrawn,  washed,  dried,  and  the  color,  shade  and  luster  com- 
pared with  a  standard.  It  is  always  advisable  to  make  a  parallel  test  upon 
another  sample  of  the  given  dye  that  has  yielded  uniformly  good  results  in 
practice,  with  all  the  conditions  practically  identical.  And  in  comparing  two 
samples  it  is  well  to  make  several  trials,  diluting  the  stronger  solution  until 
the  shades  or  tints  of  the  dyed  hanks' are  the  same,  using  equal  volumes  for  the 
baths.  The  relative  strengths  of  the  dyes  are  then  directly  proportional  to 
the  dilutions. 

As  an  example,  Kratzf  in  testing  extract  of  logwood  proceeds  in  this  way. 
White  woolen  yarn  is  washed,  dried,  and  divided  into  ten- gram  hanks.  The 
mordant  solutions  are  for  one  hank  two  per  cent  bichromate  with  two  per  cent 
bisulfite;  for  two  other  hanks,  two  per  cent  bichromate  with  two  per  cent  tar- 
tar. One  of  the  latter  two  is  then  passed  through  a  hot  two  per  cent  solution 
of  soda.  After  mordanting  all  hanks,  they  are  washed.  Exactly  eight  grams 
of  each  dye  to  be  tested  is  washed  into  a  liter  flask  with  boiling  water,  stirred 
well,  allowed  to  cool  and  made  up  to  the  mark.  For  each  sample,  three  hanks 
prepared  as  above  are  dyed  in  100  Cc.  of  the  dye  solution,  boiling  for  one  hour. 
The  baths  are  then  allowed  to  cool  for  a  half  hour,  and  the  hanks  washed. 
The  hank  showing  the  best  dye  signifies  the  best  extract. 

Von  Cochenhausen  directs  to  use  as  mordants  (1),  a  strong  oxidizer,  as  bi- 
chromate; (2)  one  not  so  strong,  as  bichromate  with  tartar;  (3)  a  non- 
oxidizer,  as  alum  with  tartar.  It  is  recognized  that  the  best  logwood  extract 
loses  its  value  if  an  expensive  and  complicated  mordanting  process  must  be 
adopted. 

2.  Colorimetric  methods  can  be  applied  to,  most  dyes.    The  standards  for 


*  Journ.  Socy.  Dyers  &  Col    1896—82. 
t  Idem,  1892-49. 


508  QUANTITATIVE    CHEMICAL    ANALYSIS. 

comparison  may  be  either  the  dye  itself  or  its  chromofer  in  a  state  of  chemical 
purity,  or  another  compound  possessing  the  same  color.  Many  of  the  coal- 
tar  dyes  come  on  the  market  almost  chemically  pure  or  can  be  made  so  by  a 
simple  purification,  and  are  thus  fitted  as  standards  for  the  same  or  analogous 
commercial  dyes  or  for  vegetable  dyes  of  corresponding  colors  —  thus  6, 
2.5  per  cent  solution  of  thioflavin  is  said  to  exactly  match  a  .1  per  cent  solu- 
tion of  curcumine  (the  basis  of  turmeric).  Some  inorganic  salts  can  be  used 
for  the  same  purpose,  e.g.,  potassium  bichromate  for  saffron.  And  often 
the  standard  is  a  particular  sample  of  the  commercial  dye-stuff  known  from 
its  history  or  by  practical  use  to  be  of  a  desirable  quality. 

For  extract  of  logwood  Mafat*  compares  the  diluted  extract  with  pure 
haematin  as  a  standard;  since  a  common  impurity  of  the  commercial  extract 
is  a  mixture  of  treacle  and  chestnut-extract  in  the  ratio  of  two  parts  of  the 
former  to  one  of  the  latter,  he  prepares  a  series  of  ten  tubes  of  100  cubic 
centimeters  capacity,  the  first  of  ten  grams  of  the  pure  extract;  the  second 
of  9.5  grams  extract,  .33  gram  of  treacle,  and  .17  gram  of  chestnut- 
extract;  the  third  of  9  grams  of  extract,  .67  gram  of  treacle,  and  .33  gram  of 
chestnut-extract;  and  so  on. 

The  distrust  of  colorimetric  methods  for  dye-wares  that  has  been  expressed 
in  some  quarters  is  not  without  foundation,  for  it  must  be  considered  that  in  a 
solution  of  a  dye  of  given  concentration  the  depth  of  color  is  one  thing  and  the 
dyeing  capacity  another,  and  it  will  not  do  to  assume  that  the  two  are  identical 
or  even  strictly  comparable.  Moreover  the  bodies  associated  with  the  chro- 
mogen  may  modify  the  color,  many  dyes  are  more  or  less  dichroistic,  and  the 
color  of  even  a  largely  diluted  solution  is  still  so  intense  that  a  slight  variation 
in  tint  is  impossible  to  observe. 

Schoopf  recommends  the  spectroscope  as  a  means  of  quantitative  determina- 
tion for  coal  tar  colors. 

3.  By  a  direct  determination  of  the  chromogen  or  an  auxichrome  or  some 
associated  body  that  bears  a  definite  proportion  to  it.  The  most  direct  and  sat- 
isfactory method  is  the  isolation  of  the  chromogen  in  the  pure  state,  but  this 
is  seldom  practicable.  The  active  principles  of  some  of  the  vegetable  and 
animal  and  many  of  the  coal  tar  dyes  are  acid  or  basic  in  character,  or  form 
definite  compounds  with  reagents  and  so  admit  of  a  gravimetric  or  volu- 
metric determination.  For  example,  all  of  the  basic  coal-tar  derivatives  are 
precipitated  by  tannic  acid  and  many  by  various  inorganic  salts,  notably  lead 
acetate,  aluminum  sulfate,  alum,  and  barium  chloride;  the  tripheny- methane 
colors  by  virtue  of  the  amido-  and  hydroxyl  groups  they  contain,  have  the 
property  of  taking  up  one  or  more  atoms  of  bromine  in  ortho-  or  para-posi- 
tion; the  phenylated  derivatives  of  rosaniline  show  a  similar  behavior;  etc. 

Some  of  the  coal-tar  colors  are  decolorized  by  sodium  hyposulfite  at  100° 
Cent.,  and  this  is  made  the  basis  fora  volumetric  determination;  in  many  cases 
the  end-point  is  sharp  and  the  titration  easy.  One  method  is  to  measure  two 
equal  volumes  of  the  hyposulflte  solution  into  flasks  and  cover  them  with 
layers  of  kerosene.  The  two  dyes  to  be  compared  are  dissolved  in  water  to 
suitable  volumes  and  run  in  from  burettes  until  the  colors  show  faintly.  It 
is  said  that  one  molecule  of  magenta,  Hoffman's  violet,  Paris  violet,  etc., 
requires  the  same  volume  of  hyposulfite  solution  for  decolorization  as  do  two 
molecules  of  ammonium  cupric  sulfate,  affording  an  easy  means  of  standard- 
ization. 


*  Journ.  Socy.  Dyers  &  Col.  1392—66. 
t  Idem,  1886—71. 


THE    ORGANIC    DYE-STUFFS.  509 

Rawson*  applies  the  reaction  between  night-blue  and  napthol  yellow  S 
(anilin  dyes),  the  two  combining  to  form  an  insoluble  compound  in  the  ratio 
of  two  molecules  of  the  former  to  one  of  the  latter.  He  proceeds  by  dissolv- 
ing ten  grams  of  pure  night-blue  in  glacial  acetic  acid  and  diluting  to  one  liter, 
also  one  gram  of  the  sample  of  napthol  yellow  to  be  tested  in  a  liter  of 
water.  Into  ten  Cc.  of  the  former  is  run  about  30  Cc.  of  the  latter  and  the 
mixture  filtered.  If  the  filtrate  is  blue  or  yellow,  other  tests  are  made,  in- 
creasing or  reducing  the  volume  of  the  napthol  yellow  until  a  proportion  is 
reached  where  the  filtrate  is  but  faintly  yellow.  Of  several  samples  the  dye- 
ing values  are  inversely  as  the  volumes  required. 

The  decomposition  of  the  chromogen  may  furnish  products  that  can  be  de- 
termined; thus  the  yellow  chromophyl  of  saffron  is  a  glucoside  yielding  on 
treatment  with  sulfuric  acid,  glucose,  crocine  (C16H18O)  and  an  essential  oil 
C10H140;  curcurmine,  the  chromogen  of  turmeric,  yields  oxalic  acid  when 
treated  with  nitric  acid,  and  pyrocatechuic  acid  on  fusion  with  caustic  potash. 

4.  By  absorption  of  the  chromogen  in  a  porous  solid.    Some  of  the  dyes  are 
completely  withdrawn  from  a  solution  by  vegetable  or  animal  fibers  and  by 
certain  inorganic    compounds.    The  amount    absorbed    may    be  determined 
directly  by  the  increased  weight  of  the  fiber  or  by  the  diminished  weight  of 
the  residue  left  on  evaporation  of  the  solution.    And  generally  the  dye  may  be 
recovered  by  boiling  the  fiber  with  certain  solvents. 

Thus  the  coloring  matter  of  logwood  (haematoxylin  and  haematin)  resembles 
tannin  in  being  absorbed  by  hide  powder.  A  weight  of  the  extract  is  diluted, 
an  aliquot  part  evaporated  to  dryness,  and  the  residue  weighed.  An  equal 
volume  is  also  evaporated  after  percolation  through  a  column  of  purified  hide 
powder.  The  difference  in  the  weights  represents  the  coloring  matter  taken 
up  by  the  hide.  Obviously  if  the  extract  contained  any  tannin  as  a  constituent 
of  an  adulterant,  this  would  also  be  absorbed  and  count  as  coloring  matter. 

5.  By  determination  of  some  associated  constituent  normally  present  in  the 
dye-stuff  and  not  contained  in  the  usual  adulterants.     Some  of  the  extracts  of 
the  vegetable  dyes  and  coal  -tar  colors  retain  certain  matters  originally  in  the 
crude  article  or  that  have  been  formed  in  or  introduced  during  the  processes  of 
manufacture.    A  determination  of  this  kind  is  always  open  to  criticism,  and 
should  only  be  resorted  to  under  exceptional  circumstances  and  never  relied 
on  in  important  cases. 

For  example,  the  most  common  adulterant  of  saffron  is  an  amido-azo  com- 
pound known  as  feminelle,  which  differs  from  saffron  in  containing  no  cro- 
cetine  (the  coloring  matter), .and  in  having  a  much  larger  amount  (7.44  per 
cent)  of  chlorine  than  has  saffron  (.23  per  cent)/f 

6.  A  microscopic  examination,  besides  detecting   impurities,  may  possibly 
allow  a  fair  estimate  to  be  formed  of  the  proportion  of  pure  dye  in  a  mixture. 


Many  of  the  commercial  dyes  and  extracts  are  sent  into  the  market  in  the 
form  of  a  paste  or  in  suspension  in  water;  others  dry,  in  admixture  with  some 
cheap  inert  body.  The  object  of  retaining  some  dyes  in  moist  condition  is  that 
evaporation  or  drying  would  impair  their  ready  solubility  in  the  dye -bath. 
The  use  of  '  fillers '  is  for  the  purpose  of  preventing  deliquescent  colors  from 
setting  to  a  solid  mass,  to  allow  the  weighing  out  by  workmen  of  a  charge  for 
a  dye -bath  on  ordinary  scales,  or,  the  filler  being  white  and  in  a  definite  ratio 


*  Journ.  Socy.  Dyers  &  Col.  1888—82 
t  Idem,  1898-236. 


510  QUANTITATIVE    CHEMICAL    ANALYSIS. 

to  the  dye,  to  lessen  the  intensity  of  the  color  and  make  easier  an  estimation 
of  the  proper  amount  to  be  used.  These  fillers  are  commonly  salt,  sugar, 
gypsum,  sodium  sulfate,  magnesium  sulfate,  etc.,  and  are  not  to  be  considered 
as  adulterations  except  where  the  proportion  exceeds  that  specified  at  the  time 
of  purchase.  Usually  the  proportion  is  50,  75,  80  or  90  per  cent  of  the  mix- 
ture. 

The  analysis  of  an  extract  or  dye  is  about  as  follows.* 

Moisture,  carbon  dioxide,  and  matter  insoluble  in  cold  water  are  determined  by 
the  usual  methods  for  these  bodies  and  need  no  detailed  mention. 

In  the  matter  insoluble  in  cold  water  or  alcohol  will  be  found  any  starch  that 
may  be  in  the  dye.  Where  the  remainder  of  the  insoluble  matter  is  wholly 
inorganic,  the  loss  on  ignition  of  the  residue  is  taken  to  be  the  weight  of  the 
starch,  but  if  partly  organic,  the  starch  is  extracted  by  hot  dilute  hydrochloric 
acid  and  determined  by  the  usual  methods. 

Sulfates  of  calcium,  magnesium  and  sodium,  and  sodium  chloride.  The  bases 
of  these  compounds  will  be  found  in  the  ash  when  a  quantity  of  the  sample  is 
burned  in  a  large  platinum  crucible  or  dish,  and  may  be  separated  and  deter- 
mined. 

Sulfuric  acid,  combined  with  alkalies  or  earths,  is  determined  by  lixiviating 
the  dye  with  hot  dilute  hydrochloric  acid  until  no  more  is  extracted,  then  pre  - 
cipitated  by  barium  chloride.  Any  adhering  barium  sulfonate  (Ba(RSO3» 
can  be  separated  by  digesting  the  precipitate  with  ammonium  carbonate  solution 
which  reacts  with  the  sulfonate  but  not  with  the  sulfate.  After  filtering  and 
washing,  the  barium  carbonate  is  dissolved  from  the  precipitate  by  dilute  hy- 
drochloric acid,  and  the  barium  sulfate  weighed.  . 

Chlorine  may  be  combined  in  the  dye  itself,  or  in  the  form  of  sodium  chlo- 
ride. If  only  as  the  latter  the  usual  course  of  precipitation  as  silver  chloride 
is  followed,  but  if  in  both  forms,  the  sample  is  first  incinerated  at  a  low  heat 
and  the  sodium  chloride  extracted  from  the  ash  by  hot  water;  then  precipi- 
tated by  silver  nitrate.  The  total  chlorine  is  determined  by  the  conventional 
methods  for  this  element  in  organic  bodies. 

Dextrin  and  sugar.  Dextrin  may  be  determined  by  treating  the  sample  with  a 
little  water,  then  precipitating  by  addition  of  alcohol.  On  filtering  and  weigh- 
ing the  residue  and  deducting  the  weight  of  the  matter  insoluble  in  water, 
the  difference  may  be  set  down  as  dextrin.  The  result,  however,  is  apt  to 
be  too  high  since  other  bodies  soluble  in  water  may  be  precipitated  by 
alcohol. 

Sugar  may  be  polarized  after  extracting  the  dye  by  absolute  alcohol  or  some 
other  organic  solvent  in  which  sugar  is  insoluble ;  or  the  coloring  matter  may 
be  precipitated  from  the  aqueous  solution  by  basic  lead  acetate,  and  the  filtrate 
polarized,  then  the  liquid  heated  with  hydrochloric  acid  which  precipitates  the 
lead  and  inverts  the  sugar,  and  the  determination  made  by  Fehlings  solution. 

Arsenic  is  determined  by  fluxing  the  dye  with  a  mixture  of  sodium  carbon- 
ate and  nitrate  which  destroys  the  organic  matter  and  converts  the  arsenic  to 
sodium  arseniate.  The  melt  is  lixiviated  with  water,  and  the  arsenic  precipi- 
tated by  magnesia  mixture  and  ammonia,  and  weighed  as  magnesium  pyroar- 
seniate.  The  precipitate  is  redissolved  and  examined  for  magnesium  pyro- 
phosphate  that  may  have  been  precipitated  at  the  same  time. 

For  various  reasons  the  common  adulterants  of  a  given  dye  -stuff  may  be 
restricted  to  one  class  of  bodies  and  so  be  more  readily  detected  and  deter- 
mined than  if  more  varied. 


*  Journ.  Anal.  Appl.  Chem.  1892—368. 


THE    ORGANIC    DYE-STUFFS.  511 

For  example  the  vegetable  extracts  are  frequently  fortified  by  the  cheaper 
anilin  dyes  of  corresponding  color;  as  cudbear  extract  by  magenta.  In  this 
case  the  orceine  (the  chromofer  of  cudbear)  can  be  completely  precipitated 
from  a  dilute  alcoholic  solution  by  basic  lead  acetate  followed  by  ammonia, 
magenta  remaining  entirely  in  solution.  The  filtrate  may  be  acidified  by  acetic 
acid  and  compared  colorimetrically  with  standard  solutions  of  magenta  acidi- 
fied by  acetic  acid.  Presumably  no  other  adulterant  will  accompany  the 
magenta. 

ALIZARIN. 

Madder  is  the  root  of  the  Rubia  tinctorium.  It  contains  but  little  already 
formed  coloring  constituents,  but  in  it  are  several  glucosides  that  yield  aliza- 
rin (dihydroxy-anthraquinon,  C14H8O4)  on  fermentation.  Alizarin  is  the  essen- 
tial dyeing  principle,  and  is  now  made  artificially  at  so  Iowa  cost  that  it  bids 
fair  to  supplant  that  derived  from  madder.  When  perfectly  pure  the  two 
alizarins  are  identical,  but  the  madder-alizarin  of  commerce  always  contains 
purpurin  (hydroxy-anthraquinon,  C14H8O5),  and  the  artificial  product  usually 
anthraquinon  (C14H8O2)  an  intermediate  product  in  the  manufacture.  On 
moderate  heating,  the  first  sublimate  from  madder- alizarin  will  be  pure 
alizarin  in  yellow  needles;  in  that  from  the  artificial  variety  anthraquinon 
may  be  distinguished. 

The  basis  of  the  manufacture  of  artificial  alizarin  is  anthracen,  obtained 
very  impure  as  the  third  fraction  of  the  distillation  of  coal-tar.*  After 
partial  purification  it  is  sent  into  the  market  from  40  to  60  per  cent  pure, 
containing  varying  amounts  of  pyrene,  napthalene,  crysene,  methyl -anthra- 
cen, carbazol,  etc.  Some  of  these  impurities  closely  resemble  anthracen 
in  chemical  and  physical  properties  but  have  less  or  no  value  for  the  pur- 
pose intended. 

The  manufacture  of  alizarin  comprises  four  stages. 

1 .  On  heating  anthracen  with  a  strong  oxidizer  it  is  converted  Into  anthra- 
quinon—  CwHio  +  30  =  Ci4HsO2  +  H2O.    For  cheapness   a    mixture    of  potas- 
sium bichromate  and  sulfuric  acid  furnishes  the  oxygen.    The  anthraquinon 
is  separated  by  a  centrifugal  machine  and  purified   by  dissolving  in  sulfuric 
acid   at    110  °  ;    on    cooling,  part  of  the  anthraquinon  separates,  the  rest  on 
dilution  with  water.    Now  about  90  per  cent  pure,  it  is  in  the  form  of  a  gray 
or   yellowish  powder  that  may  be  still  further  purified  by  boiling  with  soda, 

2.  On    heating   anthraquinon   with    concentrated  sulfuric    acid    there   are 
formed   three  products,  their    proportion  varying   with  the  temperature  and 
time  of  digestion.    If  the  heat  be  kept  below  160  °   there  is  formed  anthra- 
quinon-monosulfonic  acid  CeH^CO^CeHs.HSOs;  from    160°   to  170°beta-an- 
thraquinon-disulfonic  acid;  and  if  the  heat  rises  to  180 «  to  185 °,  alpha- 
anthraquiuon-disulfonic  acid;  both  the  latter  compounds  having  the  formula 
S03H.C6H3(CO)2C6H3.HS03. 

3.  The  mixture  is  melted  with  caustic  soda.    The  anthraquinon-monosulfonic 
acid   yields    sodium   alizarate  —  C14H7O2.HSO3 -f  4NaOH  =  C14H6O2(NaO)2-f- 
Na2SO3  -f-  H2  -f-  2H2O ;  the  reducing  action  of  the  nascent  hydrogen  being  pre- 
vented by  the  addition  of  a  little  potassium  chlorate.    The  alpha-anthraquinon- 
sulfonic  acid  yields  principally  flava-purpurin,   and  the  beta-acid,    anthra- 
purpurin    though    some    sodium    alizarate    also  — C14H8(S02)2O4  +  6NaOH  = 
C14H6Na204  +  2K2S04  +  4H2O. 

4.  On  acidifying  the  lixiviation  of  the  melt  there  precipitates  '  colorin  »,  a 


*  Journ.  Socy.  Dyers  &Col.  1897—81. 


512  QUANTITATIVE    CHEMICAL    ANALYSIS. 

mixture  of  alizarin  CC14H6Na2O4  -f-  HC1  =  C14H8O4  -+-  2NaCl),  flavapurpurin,  and 
anthrapurpurin ;  on  the  relative  proportions  of  these  depends  the  shade  of 
the  dye.  4 

For  the  assay  of  commercial  anthracen,  an  old  method  is  that  of  washing  out 
the  impurities  by  alcohol  in  which  anthracen  is  insoluble,  but  the  separation  is 
never  more  than  approximate.  The  method  due  to  Luck,  variously  modified, 
is  in  common  use;  it  is  based  on  the  oxidation  of  the  impurities  (acenapthen, 
fluoren,  phenthren,  carbazol,  fluoranthren,  etc.)  to  acids  soluble  in  water,  or 
to  their  entire  decomposition  by  the  action  of  a  strong  solution  of  chromic  acid 
in  glacial  acetic  acid,  while  anthracen  is  converted  to  anthraquinon.  It  is  said 
that  the  addition  of  nitric  acid  to  the  reagent  gives  a  more  thorough 
purification  and  also  insures  a  final  product  of  anthraquinon  of  a 
clear  yellow  color  indicating  its  purity.*  After  digestion  with  this  powerful 
oxidizer  for  several  hours  and  dilution  with  water,  anthraquinon  separates 
in  crystals  that  may  be  collected,  washed  with  water  and  weak  lye,  dried  and 
weighed.  Still  somewhat  impure  however,  treatment  with  fuming  sulfuric  acid 
sulfonates  any  remaining  impurities,  and  dilution  with  water  leaves  the  anthra- 
quinon almost  perfectly  pure  with  the  exception  of  some  mineral  matter  that 
may  be  deducted  after  a  determination  of  the  ash.  Should  paraffin  be  suspected 
in  the  sample  a  preliminary  heating  with  fuming  sulfuric  acid  will  leave  the 
paraffin  unchanged. 

Crude  anthraquinon  contains,  among  other  impurities,  some  undecomposed 
anthracen,  and  when  treated  with  concentrated  sulfuric  acid  all  these  are  sul- 
fonated,  the  anthraquinon  dissolving  unchanged  if  the  temperature  be  restricted 
to  130  ° .  On  lixiviating  the  mass  with  hot  water  the  impurities  dissolve,  leav- 
ing the  anthraquinon,  still  somewhat  impure,  however.  The  purification  must 
be  restricted,  since  if  carried  to  complete  conversion  of  the  anthracen,  certain 
of  the  impurities  become  oxidized  to  forms  not  readily  soluble  in  water.  The 
residual  anthraquinon  is  washed  and  dried,  then  further  purified  by  fuming 
sulfuric  acid. 

The  conversion  of  anthraquinon  should  be  conducted  according  to  the  pro- 
cess adopted  in  the  factory  —  thus  for  a  scarlet  shade  a  larger  weight  of 
sulfuric  acid  and  longer  heating  is  required  than  for  a  blue  shade ;  in  the  latter 
the  products  are  roughly  55  per  cent  of  the  monosulfonic  acid,  15  per  cent  of 
the  alpha-  and  beta-disulf  onic  acids,  and  30  per  cent  of  unattacked  anthraquinon ; 
but  in  the  former  very  little  unattacked  anthraquinon  is  left. 

After  the  digestion  with  sulfuric  acid  the  residue  is  diluted  with  hot  water 
and  the  residual  anthraquinon  filtered  off  and  analyzed  by  drying  and  dissolv- 
ing in  chromic  and  acetic  acids  as  above;  but  the  subsequent  purification  by 
sulfuric  acid  is  unnecessary  and  omitted. 

Should  the  relative  proportions  of  the  two  disulfonic  acids  be  desired,  the 
solution  is  neutralized  by  sodium  carbonate  and  evaporated  until  crystallization 
begins;  on  cooling,  the  sodium  salt  of  the  alpha- sulf onic  acid  remains  fairly 
insoluble  in  water;  and  on  drying,  oxidation  with  nitric  acid,  and  incineration 
there  is  left  sodium  sulfate  from  which  the  organic  compound  may  be  calcu  - 
lated.  The  difference  is  the  sodium  beta-disuifonate  in  the  filtrate. 

Otherwise,  the  mixture  of  the  acid  is  heated  under  pressure  at  a  fixed  tem- 
perature with  concentrated  soda -lye  containing  some  potassium  chlorate,  the 
latter  to  prevent  any  reduction  to  anthraquinon.  At  stated  intervals  during 
the  number  of  hours  required  for  conversion,  samples  are  withdrawn,  dissolved 
in  a  measured  volume  of  water,  and  the  alizarin  precipitated  by  acid  and 


*  Chem.  News,  1896-1—118. 


THE    ORGANIC    DYE-STUFFS.  513 

weighed;  a  sufficiently  close  approximation  may  be  had  from  the  apparent 
volume  of  the  precipitate  or  the  color  of  the  filtrate. 

The  anthraquinon-monosulfate  of  sodium  found  in  commerce  is  generally  so 
pure  that  only  a  moisture  determination  is  required. 

Alizarin  is  insoluble  in  cold,  slightly  soluble  in  hot  water,  insoluble  in  dilute 
acids,  but  readily  in  concentrated  sulfuric.  It  is  found  in  commerce  in  the  form 
of  a  20  per  cent  paste  of  either  a  blue  or  yellow  shade.  The  blue  shade 
consists  chiefly  of  alizarin,  while  the  yellow  is  mainly  anthra-purpurin  and 
flava-purpurin  (trioxyanthraquinons)  with  some  alizarin. 

The  quality  is  often  judged  by  a  dyeing  test  made  along  with  a  standard 
alizarin,  about  as  follows:  the  cloth  is  in  the  form  of  strips  of  cotton  about 
ten  inches  long  properly  mordanted;  the  dyeing  vats  preferably  thin  glass 
beakers  holding  6CO  Cc.  and  heated  side  by  side  in  a  water -bath.  Five  grams 
of  the  dye-paste  is  suspended  in  one  liter  of  water,  and  from  this  50  to  70 
Cc.  withdrawn  and  added  to  500  Cc.  of  water  containing  a  minute  amount  of 
calcium  acetate.  The  temperature  is  raised  to  80° ,  the  strips  immersed  for  an 
hour,  then  passed  through  two  soaping  baths,  dried  and  examined. 

For  the  valuation  of  commercial  alizarins,  there  is  determined  the  moisture 
(at  not  over  100°)  and  the  ash.  A  weighed  quantity  is  dissolved  in  sodium 
carbonate  solution  and  filtered  from  the  undissolved  anthraquinon  and 
oxyanthraquinon  —  these  are  separable  by  caustic  alkali  or  lime.  The  filtrate 
is  acidified  by  hydrochloric  acid  and  the  precipitate  boiled  with  milk  of  lime  to 
remove  anthraflavic  and  isoflavic  acids.  The  undissolved  lime-lakes  are  made 
into  a  paste  with  water,  decomposed  by  hydrochloric  acid,  and  the  residue  —  a 
mixture  of  alizarin,  flavapurpurin  and  anthrapurpurin  —  washed  and  dried. 

The  separation  of  the  three  compounds  is  difficult.  An  approximate  isola- 
tion of  the  flavapurpurin  follows  boiling  with  benzol  in  which  it  is  soluble. 
Schunck  and  Roemer  propose  a  qualitative  test  by  placing  a  small  quantity  of 
the  mixture,  dried  at  100  ° ,  on  a  glass  plate  on  which  stands  a  lead  ring  a  few 
millimeters  high;  the  ring  supports  another  glass  plate.  On  heating  the  lower 
plate  to  140O -150°  the  alizarin  volatilizes  and  escapes;  if  now  the  tempera- 
ture is  raised  to  170°  a  sublimate  of  a  mixture  of  flava-purpurin  and  anthra- 
purpurin is  obtained.  The  two  are  distinguishable  under  the  microscope. 

In  commercial  alizarins  any  oxyanthraquinons  as  impurities  can  be  deter- 
mined by  boiling  with  calcium  hydrate  and  filtering.  They  communicate  a 
brown  color  to  the  filtrate,  and  on  addition  of  an  acid  are  thrown  down.  A 
fair  assay  of  natural  alizarin  can  be  made  according  to  Schunck  by  prolonged 
sublimation  at  140  o ,  this  temperature  being  higher  than  the  point  of  vapor- 
ization of  alizarin,  but  lower  than  that  of  the  common  impurities. 


INDIGO. 

Commercial  indigo  is  prepared  by  a  peculiar  process  from  the  various  species 
of  indigofera.  The  plant  containing  the  basis  indogen  or  indoxyl  C8H7NO,  is 
fermented  with  water  and  a  little  lime,  during  which  indigo  is  deposited  as  a 
powder;  this  is  strained  off,  dried,  and  cut  into  cakes.  As  sold  in  the  market 
it  is  in  the  form  of  lumps  of  a  dark  blue  color,  developing  a  bronze  reflection 
on  rubbing  with  a  hard  material.  As  a  rule  the  softer  a  specimen  of  indigo  the 
better  the  quality,  but  contrary  to  the  opinion  of  some,  the  content  of  indirubin 
is  said  to  bear  no  relation  to  the  color. 

Indigo  is  a  complex  substance,  the  principal  constituents  being  indigotin  or 

indigo    blue  C16H10N2O2  or  C6H4  {  £H  }  C  :  C  {  NH  }  C«H4;  indirubin  or  indiS° 


514  QUANTITATIVE    CHEMICAL   ANALYSIS. 

red;  indiglucin  C6H10O6,  a  colorless  or  light  yellow  sweet  compound;  indican, 
CggH^NOjy,  a  light  brown  syrupy  glucoside  by  whose  decomposition  indigotin, 
indirubin,  and  a  sugar  are  formed;  and  indiretin  or  indigo  brown,  C18H17NO5,  a 
dark  brown  resinous  compound. 
The  composition  of  commercial  indigo  varies  greatly;*  thus: 

Indigotin 20  to  80  per  cent. 

Indirubin 2  to  10       " 

Indigo  brown 1  to    6        " 

Indigo  gluten 2  to    5       " 

Ash 3  to  20       " 

Moisture 5  to  15        " 

The  most  important  of  the  constituents  is  indigotin,  a  dark-blue  crystalline 
powder,  subliming  at  290°  and  condensing  to  prismatic  crystals.  It  is  insol- 
uble in  most  of  the  common  reagents  except  concentrated  sulfuric  acid 
which  converts  it  to  sulflndigotic  acid  (sulflndylic  acid,  C16H8N2O2(HSO3)2) 
believed  by  some  to  be  a  mixture  of  two  acids. 

Indirubin  is  isomeric  with  indigotin  and  may  be  extracted  from  indigo  by 
hot  alcohol  in  which  indigotin  is  Insoluble.  When  indigo  is  extracted  by 
napthalene  both  indigotin  and  indirubin  are  dissolved;  the  latter  may  be 
extracted  from  the  former  by  ether.  Under  certain  circumstances  indirubin  is 
converted  to  indigotin. 

Of  the  various  constituents  of  indigo  only  the  indigotin  and  indirubin  are 
believed  to  be  of  value  to  the  dyer,  and  so  little  is  known  of  the  advantage  or 
detriment  of  the  others  that  an  indigo  test  usually  comprises  only  a  determina- 
tion of  the  two.  The  percentage  of  indigotin  alone  is  not  a  fair  criterion  of 
the  dyeing  quality  of  the  sample  .f 

Moisture  and  ash  are  determined  in  the  usual  way.  Indigo,  both  in  lumps  and 
powder,  is  peculiarly  subject  to  hygroscopic  changes  of  the  atmosphere ;  thus, 
a  sample  of  Kurpah  indigo  containing  originally  11.25  per  cent  of  moisture, 
showed  only  5.15  per  cent  after  standing  in  a  laboratory  for  seven  days  at 
about  75  o  Fahr. 

Many  methods  have  been  proposed   for  the  determination  of  indigotin  and 
indirubin.    The  best  known  are  summarized  as  follows. 
1.  By  loss  on  ignition.    In  a  rapid  approximate  method  there  is  determined 

The  process  of  Heumann  for  the  manufacture  of  artificial  indigo  is  said  to  be  based  on 
the  following  reactions,  i 

1.  Napthaline  is  oxidized  by  concentrated  sulfuric  to  phthalic,  sulfurous,  and  carbonic 

acids  — 

Ci0H8  +  9H2SO4  =C6H4(COOH)2  +9H2SO3  +  2CO2  +  H2O. 

2.  Phthalic  acid  is  converted  by  ammonia  to  phthalimide  — 

C6H4(COOH)2  +  NH3  =  C6H4(CO)2NH  +  2H2O. 

8.  Phthalimide  is  oxidized  to  anthranilic  acid  by  sodium  hypochlorite  — 
C6H4(CO)2NH  +  O  +  H26  =  U6H4.NH2.COOH  +  CO2. 

4.  Anthranillc  acid  is  converted  to  phenyl-glycocoll-orthocarboxylic  acid  by  the  action 
of  chloracetic  acid  — 

C6H4.NH2.COOH  +  CH2C1OOOH  =  C6H4(COOH)2.NH.CH2  +  HOI. 

5.  Phenyl-glycocoll-orthocarboxylle  aeld  yields  indoxyl  on  fusion  with  caustic  soda  — 

C6H4(COOH)2.NH.CH2  +  2NaOH  =  C6H4NH.CH2.OO  +  Na2CO3  +  2H2O. 

6.  Indoxyl  becomes  indigo  on  oxidation  by  the  air  in  presence  of  an  alkali  — 

2C6H4NH.CH2.CO  +  O2  =  d  6HL  0N2O2  +  2H2O. 


*  Journ.  Socy.  Dyers  &  Col.  1898. 
t  Journ.  Socy.  Chem.  Ind.  21—222. 
i  Journ.  Amer.  Chem.  Socy.  1901—911. 


THE    ORGANIC    DYE-STUFFS.  515 

the  indigotin  and  indirubin  by  the  loss  in  weight  when  these  are  volatilized 
at  a  moderate  heat.  On  a  flat  platinum  tray  having  an  area  of  14  sq.  cm.  is 
spread  .250  gram  of  the  finely  powdered  indigo;  the  tray  is  heated  on  an  iron 
plate  just  to  the  point  of  sublimation  of  the  indigotin,  covered  meanwhile  by 
an  arched  hood  of  sheet  iron  that  the  beat  may  be  kept  more  uniform. 
When  violet  fumes  cease  to  appear  the  capsule  is  reweighed,  the  loss  set 
down  as  indigotin.  Against  the  method  is  the  notable  decomposition  of  both 
indigotin  and  the  other  constituents  at  the  temperature  of  sublimation. 

2.  Extraction  of  indigotin  by  a  solvent.    Honig  mixes  the  powdered  indigo 
with  fragments  of  pumice-stone  and  extracts  in  a  Soxhlet's  apparatus  with  anhy- 
drous anil  in  or  nitrobenzol  until  the  syphonings  are  colorless.    The  extraction 
takes  two  or  three  hours.    The  tube  of  the  apparatus  is  washed  with  alcohol, 
and  the  anilin  distilled  leaving  the  indigotin,  which  is  purified  by  washing  on 
an  asbestos  filter  with  strong  alcohol,  dried  at  110°  and  weighed. 

Brylinski  criticises  the  use  of  anilin  as  a  solvent,  asserting  that  a  certain 
proportion  of  the  indigotin  is  destroyed,  and  that  as  the  indigotin  crystal- 
lizes it  incloses  some  anilin.  He  would  substitute  glacial  acetic  acid.  Brandt 
admits  the  destruction  of  indigotin  by  anilin,  finding  that  if  the  indigo  be 
mixed  with  powdered  garnets  the  action  is  lessened,  but  that  after  extraction 
for  an  hour  the  destruction  proceeds  rapidly.  Concordant  results  follow 
when  the  indigo  is  mixed  with  small  garnets,  sand,  etc.,  and  the  extraction 
conducted  as  rapidly  as  possible.  As  a  solvent  he  prefers  phenol ;  on  cool- 
ing the  extract  and  largely  diluting  with  water  containing  caustic  soda,  all 
the  indigotin  is  precipitated.  It  is  filtered  on  a  tared  paper,  washed  with 
hot  water  until  the  washings  become  nearly  neutral,  then  with  alcohol  until 
it  passes  nearly  colorless.  Finally  the  indigotin  is  dried  at  110  °  and  weighed. 

Other  solvents  that  have  been  proposed  are  animal  oils  and  napthalin,  the 
indigo  precipitated  from  the  extract  by  ether.  Schneider  ascertains  the  loss 
incurred  by  the  action  of  the  solvent  on  the  indigotin  by  subjecting  the  weighed 
indigotin  to  the  process  of  extraction  under  identical  conditions  with  the 
original  and  corrects  the  weight  accordingly. 

3.  Conversion  to  indigo  white.    Various  reducing  agents  convert  indigotin 
to  a  soluble  compound  known  as  indigogen  or  indigo  white ;  thus,  ferrous 
sulf ate  with  calcium  hydrate  — 

C16H10N202  (indigotin)    +    2FeSO4  +  2Ca(OH)2=C16H12N2O2  (indigogen)  + 
Fe2  (OH)6  4.  2CaSO4. 

The  conversion  takes  place  with  indigotin  suspended  in  water  or  with  sulfin- 
digotic  acid  in  solution.  Since  the  indigo  white  is  readily  oxidized,  the 
reduction  must  be  conducted  out  of  contact  with  the  air,  usually  accomplished 
by  passing  a  current  of  a  reducing  gas  through  the  flask. 

On  contact  with  air,  indigo  white  is  reoxidized  to  indigotin  —  C16H12N2O2 
-[-  O2  =  C16H10N2O2  -f-  H2O2  —  which,  being  insoluble,  separates  as  a  fine  powder. 

A  weighed  quantity  of  the  indigo  in  a  flask  fitted  with  gas-transmission 
tubes  is  heated  with  a  measured  volume  of  a  solution  of  the  reducer ;  then  an 
aliquot  part  is  withdrawn  by  a  syphon  into  a  smaller  flask.  Through  this  is 
passed  a  current  of  air  until  the  regenerated  indigotin  is  precipitated ;  the  liquid 
is  acidified  by  hydrochloric  acid,  filtered,  and  the  precipitate  dried  and  weighed. 

Other  reducing  agents  that  have  been  proposed  are  sodium  bisulfite,  sodium 
hyposulfite,  grape  sugar,  metallic  aluminum,  etc.,  all  in  connection  with  an 
alkali  or  earth ;  Norton  recommends  zinc  dust  with  lime,  and  determines  the 
indigotin  by  drawing  the  reduced  solution  into  a  solution  of  ferric  sulfate, 
then  titrates,  by  standard  potassium  bichromate,  the  ferrous  sulfate  formed  by 
the  reaction  between  the  ferric  sulfate  and  indigogen. 


516  QUANTITATIVE    CHEMICAL    ANALYSIS. 

For  the  analysis  of  textile  fabrics  dyed  by  indigo,  Renard  directs  to  treat  ten 
grams  of  the  disintegrated  fibers  directly  with  sodium  bisulfite  and  lime. 

4.  By  solution  in  sulfuric  acid  and  titration  by  an  oxidizer.    Indigo  dissolves 
readily  in  moderately  hot,  concentrated  sulfuric  acid.    According  to  Rawson  * 
the  best  plan  for  the  dissolution  of  a  sample  is  to  mix  one  gram  of  the  fine 
powder  with  an  equal  weight  of  ground  glass  and  project  the  mixture,  by  small 
portions  and  with  constant  stirring,  into  20  Cc.  of  sulfuric  acid  of  1.84  sp.gr. 
held  in  a  porcelain  crucible.    The  mortar  is  rinsed  with  a  little  powdered  glass, 
and  the  crucible  heated  in  a  water-oven  for  an  hour  to  90  ° .    The  solution  is 
cooled,  and  diluted  with  water  to  one  liter. 

For  the  titration  with  permanganate,  the  solution  is  filtered  and  50  Cc. 
drawn  into  a  porcelain  dish  and  diluted  with  250  Cc.  of  water.  A  standard  solu- 
tion of  permanganate  is  run  in  until  the  color  of  the  titrate  passes  from 
greenish  to  light  yellow  denoting  the  complete  oxidation  of  the  sulfindigotic 
and  sulfindirubic  acids  to  sulflsatic  acid  — 

5C16H8N202(HSO3)2  +  2K2Mn208  -f  6H2S04  =  5C16H8N2O4(HSO3)2  +  2K2SO4  -f 
4MnS04  +  6H20. 

The  standardization  of  the  permanganate  is  made  on  pure  indigotin  dissolved 
and  diluted  as  above. 

Since  permanganate  is  also  reduced  by  other  constituents  of  the  indigo  solu- 
ble in  sulfuric  acid,  it  is  recommended  that  before  titration  these  be  removed 
by  salting  out  the  sultindigotic  and  sulflndirubic  acids,  which  after  filtering 
are  dissolved  and  titrated.  Barium  chloride  has  also  been  recommended  for 
purification.  Donald  and  Strasse  would  purify  the  indigo  before  sulf onation  by 
successive  extractions  with  water,  hydrochloric  acid,  alcohol,  and  ether. 

In  lieu  of  permanganate  the  oxidizer  may  be  potassium  ferricyanide  which  in 
alkaline  solution  forms  isatin  or  indigotic  acid,  or  potassium  bichromate  in 
acid  solution,  but  neither  of  these  has  any  marked  advantages  over  perman- 
ganate. 

5.  By  reduction.  Strong  reducing  agents  convert  sulflndigotic  acid  to  the 
nearly  colorless  compound  disulfo-leukindigotic  acid.    The  determination  is 
made  volumetrically  by  sodium  hyposulfite  (NaHSO2)  in  a  current  of  some 
non-oxidizing  gas.    The  titrand  is  standardized  against  pure  indigotin  dis- 
solved in  sulfuric  acid  and  diluted.    Mueller  claims  that  indigo-red,  indigo- 
brown,  and  indigo-gluten  are  not  reduced  by  this  reagent.    This  reaction  is 
C16H8N2O2CHSO3)2  +  NaHSO2  +  H2O  =  C16H10N202(HSO8)2  +  NaHSO3. 

Indigotin  is  reduced  quantitatively  by  vanadyl  sulfate,  vanadium  dioxide 
(V2O2)  being  oxidized  to  the  pentoxide  (V2O6).  The  titrand  is  made  by  dis- 
solving ammonium  vanadate  in  sulfuric  acid,  diluting  the  solution  and  reducing 
by  zinc  powder.  The  titration  is  carried  out  in  a  current  of  carbon  dioxide, 
the  end -point  being  the  disappearance  of  the  blue  color  of  the  sulfindigotic 
acid. 

6.  Various  other  methods  have  been  proposed.    Gerland  removes  other  con- 
stituents by  converting  the  indigotin  and  indirubin  into  their  mowo-sulfonic 
acids  by  vitriol  of  1.67  sp.  gr. ;  on  dilution  with  water  these  are  completely  pre- 
cipitated.   After  filtration  the  precipitate  is  treated  with  acid  of  1.84  gravity 
which  produces  the  disulfonic  acids,  determined  by  titration.    Voeller  purifies 
indigo    by  successive   treatments  with  hydrochloric   acid,   sodium    hydrate, 
alcohol,  and  water,  then  determines  the  nitrogen  in  the  residue  by  the  method 
of  Kjeldahl;  the  indigotin  is  calculated  from  the  formula  C16H10N202. 


*  Journ.  Socy.  Dyers  &  Col.  1—75. 


THE    ORGANIC    DYE-STUFFS.  517 

7.  For  a  determination  of  indirubin,  Gardner  and  Denton  boil  .2  gram  of 
the   powdered    indigo  with  100  Cc.  of   acetone    under  a  reflux  condenser. 
The  liquid  is  then  made  up  to  200  Cc.  with  a  ten  per  cent  salt  solution,  this 
precipitating  any  traces  of  indigotin  that  may  have  dissolved.      After  filtra- 
tion through  asbestos,  the  filtrate  is  compared  colorimetrically  with  pure  indi- 
rubin treated  as  above.    The  residue  of  indigotin  is  washed  free  from  salt  by 
hot  water,  dried,  dissolved  in  sulfuric  acid,  and  determined  by  titration  with 
permanganate. 

8.  The  dye -test  can  be  made  by  grinding  one  gram  of  the  dried  indigo  with 
five  grams  of  glass,  then  extracting  successively  by  hot  dilute  hydrochloric 
acid,  hot  dilute  solution  of  sodium  hydrate,  and  hot  water.    The  dried  residue 
is  dissolved  at  100®  in  sulfuric  acid  of  1.85  sp.  gr.,  the  solution  diluted  with 
water,  and  made  up  to  one  liter.    A  special  form   of   apparatus  has  been 
described  by  Grossman.* 

As  to  the  relative  merits  of  the  various  processes  there  is  some  difference 
of  opinion.  The  conclusions  of  Rawsonf  are  given  below,  though  it  must  be 
recognized  that  his  views  on  several  points  are  not  concurred  in  by  other 
authorities.  $ 

te  2.  The  permanganate  method  affords  a  quick  and  ready  means  for  the 
approximate  valuation  of  indigoes,  but  as  substances  soluble  in  dilute  acids 
are,  at  the  same  time,  more  or  less  acted  upon,  the  results  obtained  are  some- 
what too  high. 

3.  If  the  solution  of  indigo  be  saturated  with  sodium  chloride,  the  colouring 
matter  is  thereby  precipitated.    When  the  precipitate  is  washed,  dissolved  in 
dilute  sulfuric  acid,  and  titrated  with  potassium  permanganate,  results  are 
obtained  that  for  all  practical  purposes  are  trustworthy  and  reliable.    Indigo- 
red  and  indigotin  are  simultaneously  estimated  by  this  modified  process. 

4.  Of  all  the  volumetric  methods  which  have  been  devised  for  estimating  the 
indigotin,  the  sodium  hyposulflte  process  is  capable  of  giving  at  the  same  time 
the  quickest  and  most  accurate  results ;  but  as  previously  stated,  considerable 
care  and  delicacy  are  required  in  its  manipulation.    If  the  solution  of  indigo 
to  be  titrated  with'hyposulfite  contain  iron  in  the  ferric  state,  then  the  result 
obtained  will  be  too  high. 

5.  Other  bodies  than  indigotin  which  are  present  in  indigoes  are  more  or  less 
affected  by  the  process  of  sublimation,  while  indigotin  itself  is  partly  decom  - 
posed  into  a  dark  brown  substance,  which  does  not  volatilize  without  complete 
destruction.    According  to  the  quality  of  indigo  the  results  obtained  by  this 
process  may  be  either  too  high  or  too  low. 

6.  The  gravimetric  reduction  processes  as  commonly  described  are  not  quite 
so  accurate  as  is  generally  supposed.  Perfectly  reliable  and  accurate  results  are, 
however,  obtained  by  the  use  of  sodium  hyposulflte    and   lime  water.    The 
reduction  is  complete  in  less  than  half  an  hour.    Where  an  exact   chemical 
analysis  is  required,  this  method  I  consider  gives  the  best  results  of  any  process 
which  has  hitherto  been  published." 

A  large  number  of  analytical  schemes  for  the  identification  of  dyes  in  dyed 
fabrics  have  been  described. § 


*  Journ .  Socy.  Dyers  &  Col.  1897—124. 
t  Chem.-News,51— 256. 

}  Jonrn.  Socy.  Dyers  &  Col.  1896—93,  and  1897—124 . 

§  Journ.  Anal.  Chem.  1—440;  Journ.  Socy.  Dyers  &  Col.  1898—210;  Prescott  Org.  Anal.  182; 
Analyst,  1899-41. 


PART  4. 


NOTES  ON  THE  METHODS  OF  ANALYSIS. 


NOTES    ON   THE   METHODS    OF   ANALYSIS.  521 


NOTES  ON  THE  METHODS  OF  ANALYSIS. 

The  development  of  the  art  of  quantitative  analysis  may  be  credited  on  the 
one  hand  to  the  extension  of  qualitative  analysis,  and  on  the  other  to  the  adapta- 
tion in  miniature  of  technical  processes. 

Probably  the  earliest  attempt  at  a  quantitative  analysis  was  of  the  nature  of 
a  fire-assay  of  the  ores  or  alloys  of  the  precious  metals ;  for  the  reproduction 
on  a  small  scale  of  the  crude  processes  of  extraction  and  refining  known  to  the 
ancients  would  be  readily  suggested  to  those  engaged  therein.  From  this  in- 
ception the  art  grew  by  small  accessions,  traceable  in  the  writings  of  the 
alchemists  and  their  followers,  and  extended  in  succession  to  other  useful 
minerals  and  ores,  the  compounds  of  the  common  metals,  the  inorganic  acids, 
the  ultimate  constituents  of  organic  bodies,  the  rarer  metals,  and  finally  to 
general  proximate  analysis.  Of  an  art  so  interwoven  with  others  and  so 
eminently  one  of  detail,  it  is  idle  to  speculate  as  to  whom  belongs  the  honor  of 
being  the  founder ;  of  the  three  or  four  that  have  been  named,  all  have  un- 
doubtedly made  most  important  contributions,  but  a  firm  foundation  had  been 
laid  prior  to  the  time  of  the  earliest. 

The  art  in  general  and  particular,  has  been  materially  advanced  at  various 
times  by  the  discovery  of  leading  principles  and  the  invention  of  appliances 
for  facilitating  the  practice.  Of  these  may  be  mentioned  the  principles  of 
volumetric  analysis,  gasometry  and  electrolysis,  the  application  of  the  polari- 
scope,  colorimeter  and  dephlagmator,  the  invention  of  the  deluminated  gas- 
burner  and  the  vacuum  filtering  apparatus,  the  construction  of  platinum 
utensils,  and  others  of  less  conspicuous  advantage. 

Another  factor  has  been  the  progress  toward  precision  in  the  valuation 
of  such  physical  constants  as  are  concerned  in  the  calculations  of  analysis. 
The  atomic  weights  on  which  depends  the  accuracy  of  nearly  every  chemical 
analysis  have  been  corrected  to  such  a  degree  of  exactness  that  the  errors 
introduced  are  inconsiderable  even  in  the  most  refined  investigations.  And 
reliable  determinations  of  constants  of  more  special  application  have  admitted 
several  purely  physical  methods  to  the  resources  of  the  analyst,  frequently  to 
supplant  chemical  methods  more  intricate  or  less  accurate. 

Again,  the  assistance  afforded  by  the  continuous  advance  in  the  way  of 
increasing  the  variety  and  improving  the  quality  of  the  appliances  for  chemical 
work  can  not  be  overestimated,  for  the  manufacturer  has  well  kept  pace  with 
the  ever  increasing  demand.  The  precision -balance  of  today  is  a  most  exact, 
reliable,  and  withal,  low-priced  instrument,  and  the  accompanying  weights 
remarkably  accurate ;  glass  and  porcelain  vessels  are  at  hand  convenient  in 
form  and  well  resisting  the  corrosive  action  of  chemicals,  platinum  wrought 
to  almost  any  desired  shape,  and  physical  instruments  and  volumetric  ware  of 
high  precision.  And  one  cannot  but  appreciate  the  skill  of  the  glass-blower  as 
he  fashions  the  most  intricate  forms  of  apparatus,  making  possible  many  chem- 
ical operations  that  otherwise  could  be  conducted  only  with  difficulty  if  at  all. 
In  the  equally  important  requisite  of  reagents,  chemicals  are  now  supplied  in 
large  variety,  of  guaranteed  purity,  and  at  a  reasonable  price.  Surely  the 
readiness  of  the  manufacturer  to  meet  or  anticipate  the  wants  of  the  chemist 
may  not  infrequently  be  accredited  to  a  loftier  motive  than  mere  business  thrift. 


522  QUANTITATIVE    CHEMICAL    ANALYSIS. 

But  as  a  whole  the  practice  has  been  built  up  mainly  by  accretions  of  minor 
importance,  the  outcome  of  patient  and  persistent  efforts  in  the  direction  of 
instituting  and  perfecting  particular  methods  of  analysis;  widening  their  scope 
when  possible  to  include  other  than  the  special  classes  of  bodies  for  which 
they  were  originally  designed,  or,  on  the  other  hand,  so  modifying  the  details  — 
here  by  the  omission  of  a  superfluous  feature,  there  by  the  introduction  of  a 
measure  tending  to  greater  simplicity,  accuracy,  or  dispatch,  or  a  precaution 
against  an  avoidable  source  of  error  —  as  to  adapt  them  for  special  work  call- 
ing for  exceptional  rapidity  or  many  analyses  in  a  limited  time.  To  these 
extensions  a  great  impetus  was  the  publication  of  the  works  of  Rose  and  Fre- 
senius,  the  first  monographs  of  methods  for  the  systematic  examination  of 
inorganic  bodies  and  practical  directions  for  manipulation. 

A  feature  that  has  had  a  marked  influence  on  the  advance  of  the  art  is  the 
unreserved  manner  in  which  analysts  in  general  have  made  public  the  dis- 
coveries and  inventions  they  have  considered  of  interest  to  their  fellows. 
Particularly  is  this  to  be  commended  of  specialists  whose  extended  study  in 
one  single  department  of  analysis  constrains  respectful  consideration  of  the 
views  they  advance.  A  few  are  more  reticent,  defending  their  secretiveness 
on  the  plea  of  fidelity  to  employers  or  clients,  or  that  the  publication  of  cer- 
tain methods  of  analysis  in  detail  might  give  information  to  those  who  would 
profit  by  it  for  illegitimate  ends,  as  for  adulteration  or  counterfeiting.  But 
the  mutual  benefit  to  be  derived  from  a  free  interchange  of  experiences  and 
opinions  is  so  generally  recognized  that  a  claim  to  the  possession  of  a  secret 
method  superior  to  any  that  are  common  property  may  well  be  regarded  with 
suspicion.  And  in  the  rare  instances  where  a  patent  has  been  secured  for 
an  analytical  method,  it  is  probable  that  protection  was  sought,  through 
establishing  priority,  for  any  technical  processes  that  might  be  founded 
thereon,  rather  than  the  right  to  control  its  analytical  use. 


Glancing  at  the  present  status  of  the  art,  how  far  and  along  what  lines  it 
has  been  developed,  it  may  be  premised,  first  that  the  accuracy  and  ease 
with  which  an  analysis  can  be  made  —  outside  of  the  physical  methods  —  are 
in  a  measure  correlative  with  the  chemical  activity  of  the  constituents  de- 
termined, for  the  more  positive  and  pronounced  the  relations  of  a  body  toward 
reagents,  the  more  likely  are  the  methods  for  its  determination  to  be  numer- 
ous and  satisfactory :  and,  second,  the  fact  that  the  determination  of  a  body 
will  further  some  practical  end  or  prove  a  financial  advantage  is  always  a 
stimulus  to  the  invention  and  perfection  of  methods  therefor.  For  these  and 
other  reasons,  the  elementary  bases  of  the  common  inorganic  compounds  and 
the  organic  tetrad  have  received  the  most  attention.  For  some  elements  and 
compounds  we  have  a  choica  of  a  number  of  methods  all  reasonably  accurate 
and  generally  applicable,  while  for  others  are  limited  to  a  few  or  but  one, 
often  tedious  and  unsatisf atory  at  best.  And  it  is  not  uncommon  that  a  method 
will  answer  the  purpose  when  prosecuted  under  the  conditions  detailed  by  the 
originator,  yet  be  unreliable  when  these  are  departed  from  even  but  slightly. 

Most  of  the  common  metals  can  be  determined  with  great  or  at  least  reason- 
able accuracy  and  by  a  number  of  methods,  and  their  separation  from  accom- 
panying bases  and  acid  radicals  presents  no  great  difficulties.  With  a  few  of 
the  more  chemically  indifferent  the  methods  are  complicated  and  not  over- 
exact.  Of  the  rarer  metals,  on  the  contrary,  but  comparatively  few  can  be 
determined  with  any  assurance  of  accurate  results,  owing  chiefly  to  the  diffi- 
culty of  separating  from  the  various  groups  the  individual  members  whose 
reactions  show  great  similarity. 


NOTES    ON   THE    METHODS    OF   ANALYSIS.  523 

Of  the  inorganic  acids,  the  mono -sulfur  oxides  and  the  halogen  radicals 
offer  no  particular  obstacles  to  an  accurate  determination ;  phosphoric  acid  is 
rather  more  troublesome  from  the  necessity  of  a  previous  separation  from 
most  bases;  while  the  determination  of  boracic  and  hydrofluoric  is  not  an 
easy  task,  and  nitric  still  lacks  one  simple  convenient  method.  Associated 
oxygen  compounds  of  nitrogen  and  of  the  halogens,  and  the  thionic  radicals  can 
only  be  determined  indirectly  or  through  somewhat  equivocal  oxidations  or 
perductions. 

Thanks  to  the  labors  of  Bunsen  and  his  followers,  the  proximate  analysis  of 
mixed  gases  has  been  brought  to  a  degree  of  accuracy  scarcely  inferior  to  a 
refined  gravimetric  analysis,  and  with  few  exceptions  the  simple  and  compound 
gases  can  be  determined  with  exactness,  or  with  great  rapidity  where  a  slight 
inaccuracy  is  not  unallowable. 

Ultimate  organic  analysis  has  been  developed  to  a  degree  that  leaves  little 
to  be  desired  in  point  of  exactness,  but  requires  complicated  apparatus  and  un- 
remitting attention  throughout  the  process  of  combustion,  and  only  after 
considerable  practice  in  the  routine  can  one  place  confidence  in  his  results. 

Although  proximate  organic  analysis  offers  what  is  perhaps  the  most  inter- 
esting and  profitable  field  to  the  analyst,  yet  as  compared  with  other  branches 
it  is  at  present  the  least  developed.  Among  the  analytical  difficulties  are  that 
many  of  the  multitude  of  known  organic  bodies  are  closely  related  in  ultimate 
composition,  though  differing  widely  in  habitus  and  physical  properties ;  that 
many  compounds  easily  degenerate  to  decomposition  products,  either  spon- 
taneously, on  exposure  to  the  air,  or  by  contact  with  the  reagents  during  the 
course  of  the  analysis;  that  in  mixtures  of  allied  complex  bodies  the  identity  of 
the  members  may  be  wholly  lost;  and  that  comparatively  few  form  combina- 
tions sufficiently  insoluble  to  be  utilized  for  a  separation.  The  determination 
of  the  various  organic  groups  composing  a  molecule,  necessary  for  the  identi- 
fication of  a  compound,  can  be  done  with  sufficient  accuracy  in  many  cases, 
but  often  the  normal  reactions  are  interfered  with  or  entirely  annulled  by  vari- 
ations in  the  configuration  of  the  molecule  or  the  modified  behavior  of  substi- 
tuted groups. 

The  strictly  quantitative  methods  that  are  available  for  organic  bodies  are 
few  in  comparison  to  those  for  inorganic  analysis,  since  for  the  latter  the 
usual  reagents  have  in  general  a  broader  application,  a  given  reagent  being 
suited,  as  a  rule,  to  most  or  all  the  combinations  in  which  the  element  or  radi- 
cal to  be  determined  may  enter,  though  for  practical  reasons  but  one  form  of 
combination  may  be  best  suited  for  the  purpose  of  analysis. 

Of  the  multitude  of  organic  bodies  comparatively  few  unite  with  reagents 
to  form  precipitates  sufficiently  insoluble  and  stable  to  admit  of  filtration  and 
drying,  and  their  direct  determination  is  therefore  limited  to  the  plan  of 
evaporating  their  solutions  to  dryness  and  weighing  the  residues,  a  scheme 
often  equivocal  in  view  of  the  ready  volatility  or  decomposibility  of  many 
varieties,  Sometimes  a  fair  result  can  be  arrived  at  through  an  indirect  or 
physical  method ;  if  of  a  pronounced  acid  or  basic  character  or  if  readily  oxidized 
or  reduced  or  otherwise  transformed,  a  volumetric  method  may  be  availed; 
and  a  number  form  measurable  decomposition  products  with  certain  inorganic 
bases,  ferments,  strong  oxidizers,  etc.  In  most  cases  the  difficulty  of  a  sepa- 
ration and  determination  is  greatly  increased  where  the  body  forms  but  a 
small  proportion  of  the  mixture  analyzed,  this  feature  more  evident  here  than 
in  inorganic  analysis. 

On  the  whole,  it  may  be  said  that  reasonably  accurate  results  are  possible 
with  only  a  minority ;  fair  results  are  obtainable  with  many,  and  approxima- 


524  QUANTITATIVE    CHEMICAL    ANALYSIS. 

tions  with  more;  while  not  a  few  can  only  be  estimated  by  difference,  and  in 
mixtures  of  allied  bodies  all  that  can  be  hoped  for  is  the  fair  isolation  of 
the  respective  groups,  falling  back  on  indirect  means  to  identify  and  determine 
as  far  as  may  be,  the  individual  members. 

Of  the  more  familiar  organic  bodies,  most  of  the  acids  form  precipitates 
with  the  earths  and  certain  metals,  the  insolubility  being  sufficient  for  a  fair 
determination;  those  that  are  volatile  at  moderate  temperatures  without 
decomposition  may  be  distilled  —  if  combined  with  a  base,  after  displacement 
by  a  non- volatile  inorganic  acid  —  and  determined  in  the  distillate  by  acidi- 
metry. 

Of  the  organic  bases,  the  vegetable  alkaloids  admit  in  most  cases  of  moder- 
ately close,  at  times  highly  accurate  determinations,  since  like  their  inorganic 
archetypes,  they  form  fairly  insoluble  compounds  with  a  number  of  reagents. 
But  when  only  a  minute  amount  is  mixed  with  a  large  proportion  of  other 
organic  matter  (e.  gr.,in  toxicological  examinations),  one  may  be  well  satisfied 
to  obtain  a  distinct  and  conclusive  qualitative  reaction.  The  same  is  true  of 
the  animal  bases  —  the  amines,  amido-and  imido-compounds,  xanthins,  py- 
ridins  and  urea  and  their  analogues  and  derivatives,  etc.,  which  have  for  the 
most  part  the  character  of  moderately  strong  bases,  a  few  reacting  as  a  base 
or  an  acid  according  to  circumstances. 

The  carbohydrates  show  relatively  little  chemical  activity  toward  reagents. 
For  the  separation  of  the  sugars,  differences  in  solubility  may  be  availed, 
though  the  determination  is  usually  made  by  a  physical  method,  decomposition 
by  a  metallic  salt,  or  fermentation.  The  starches  may  be  separated  from  ac- 
companying soluble  matter  by  cold  water,  but  a  determination  usually  follows 
the  conversion  of  the  starch  into  sugar  and  the  determination  of  the  latter. 
From  their  insolubility  and  resistance  to  most  reagents,  the  celluloses  form 
the  final  residue  in  an  analysis,  though  attempts  to  separate  retained  impuri- 
ties generally  result  in  the  loss  of  part  of  the  cellulose  as  well. 

The  alcohols  of  low  boiling  points  are  separable  from  less  volatile  asso- 
ciates by  distillation,  easiest  by  distillation  with  water;  usually  the  specific 
gravity  of  the  water-alcohol  distillate  is  a  sufficiently  accurate  means  of  de- 
termination. Oxidation  methods  are  complicated  by  commonly  occurring 
associates  difficult  of  separation,  and  by  the  relative  stability  of  members  of 
this  group.  The  higher  alcohols,  including  glycerol,  phenol,  etc.,  form  some 
definite  combinations  with  other  bodies  that  admit  of  a  fair  determination,  but 
physical  methods  are  generally  preferred. 

Similarly,  the  lighter  ethers  are  separable  from  fixed  bodies  by  distillation, 
and  may  usually  be  obtained  in  a  nearly  pure  state  and  weighed  or  measured. 
The  heavier  bodies  of  this  class  can  be  extracted  from  a  solution  by  a  solvent 
immiscible  therewith.  The  compound  ethers  admit  of  saponiflcation  and 
determination  by  the  weight  of  alkali  entering  the  decomposition  pro- 
ducts. 

For  the  fats  and  oils  hydrolysis  offers  not  only  a  means  of  direct  determina- 
tion and  separation  from  unsaponifiable  matters,  but  the  decomposition  prod- 
ucts possess  certain  physical  properties  that  may  be  taken  advantage  of  for 
identification  and  further  examination.  For  the  rest  one  must  rely  on  the  di- 
vergent values  of  physical  or  chemico- physical  constants  of  the  oils.  The 
indifference  of  the  numeral  and  rosin  oils  to  reagents  that  act  on  those  of 
animal  or  vegetable  origin  may  allow  their  isolation,  while  the  more  volatile 
members  can  be  distilled  as  a  whole  or  fractionally.  Many  of  the  essential 
oils  combine  wholly  or  in  part  with  reagents  to  form  determinable  decomposi- 
tion products;  like  the  lighter  mineral  oils,  they  are  separable  from  fixed 


NOTES    ON    THE    METHODS    OF    ANALrSIS.  525 

associates  by  distillation  with  water;  various  attributive  methods  are  fre- 
quently applicable  for  their  determination. 

For  the  tannins  have  been  proposed  a  very  large  number  of  methods  — 
"  several  hundred  "  (sic).  They  form  precipitates  with  a  number  of  metallic 
bases  but  of  such  indefinite  composition  and  so  readily  decomposed  that  the 
combining  weights  are  not  deducible  with  certainty.  Hence,  notwithstanding 
the  labor  of  many  investigators,  there  are  but  two,  perhaps  three,  possibly  four 
methods  that  have  gained  the  confidence  of  chemists.  The  members  of  the 
tannin  family  are  closely  allied  in  deportment  toward  reagents,  and  a  separa- 
tion is  impossible  in  most  cases.  None  of  the  methods  can  lay  claim  to 
determine  tannin  solus;  at  best  there  is  only  indicated  the  proportion  of  an 
indefinite  group  of  bodies  that  exhibit  a  peculiar  behavior  toward  the  albumins 
and  similar  bodies. 

The  organic  dye -wares  have  up  to  the  present  time  been  assayed  mainly  by 
colorimetric  methods  or  comparative  dye-tests,  and  even  the  anilin  dyes 
whose  composition  and  molecular  arrangement  is  so  well  understood  and 
whose  chemical  attributes  are  so  decided,  seem  to  have  been  neglected  in  the 
search  for  assay  methods. 

Several  schemes  afford  a  fair  separation  of  the  various  proteids  by  fractional 
precipitation. 

Finally  there  is  the  large  number  of  organic  bodies  for  which  no  satisfactory 
methods  of  separation  and  determination  are  known,  many  of  these  familiar 
from  their  use  in  the  arts  and  domestic  economy. 


The  qualities  that  most  commend  a  method  of  analysis  are  accuracy, 
simplicity  and  rapidity,  though  but  exceptionally  does  a  method  associate 
the  three,  and  many  can  claim  neither,  retained  only  for  want  of  others 
more  meritorious. 

The  relative  importance  of  these  qualities  toward  a  given  analysis  is  de- 
pendent on  the  object  for  which  the  analysis  is  made  and  the  deductions  to 
be  drawn  from  it.  In  scientific  investigations  and  in  certain  classes  of 
technical  analysis  the  highest  accuracy  is  paramount  to  all  other  considera- 
tions, and  methods  are  adopted  with  this  object  in  view.  On  the  other 
hand,  for  most  technical  analyses  usually  the  highest  accuracy  is  not  in- 
sisted on  and  may  with  advantage  be  subordinated  to  other  qualities. 

In  industrial  analyses  it  is  usually  left  to  the  discretion  of  the  chemist  as 
to  the  standard  of  accuracy  to  be  maintained  in  his  routine  examinations, 
and  in  establishing  such  standards  he  must  be  governed  by  several  con- 
siderations. Thus,  with  such  items  as  purchases  and  sales  on  a  guaranteed 
basis  of  purity  nothing  short  of  the  most  exact  results  attainable  should  be 
allowed  to  pass;  on  the  other  hand,  in  dealing  with  by-,  intermediate, 
and  waste  products  and  the  like,  a  much  lower  standard  may  be  adopted  with- 
out impairing  the  practical  value  of  the  results.  Again,  if  highly  or  even 
fairly  accurate  results  are  to  repay  the  necessary  time  and  care  bestowed, 
among  other  requisites  there  must  have  been  a  correspondingly  accurate  sam- 
pling of  the  material  analyzed,  and  the  analytical  results  must  be  capable  of 
being  interpreted  or  practically  applied  within  the  limits  of  error.  For 
it  seems  but  farcical  to  labor  for  refinements  in  the  analysis  of  a  sample  that 
may  be  far  from  a  representative  of  the  original;  nor  is  it  more  defensible  if 
the  same  conclusions  would  be  drawn  or  the  same  disposition  made  of  the  ma- 
terial though  the  results  differed  to  a  degree  many  times  greater  than  the  pre- 
sumed maximum  inaccuracy.  Finally,  if  circumstances  decree  that  the 


526  QUANTITATIVE    CHEMICAL   ANALYSIS. 

analytical  work  be  done  by  those  unacquainted  with  the  principles  of  chemistry 
and  without  training  in  general  analysis,  as  is  the  policy  in  some  industrial 
laboratories,  an  attempt  at  high  accuracy  is  futile  if  attainable  only  by  abstruse 
schemes  and  complicated  forms  of  apparatus. 


The  degree  of  accuracy  that  may  be  attained  in  a  given  determination  de- 
pends primarily  on  the  excellence  of  the  method  of  analysis.  A  method  for 
the  determination  of  an  element  or  compound  may  fall  into  one  of  three 
classes. 

1.  The  first  class  includes  those  which  combine  all  the  various  qualifications 
essential  to  exactness  and  are  suitable  for  the  most  refined  research  work. 
The  class  is  comparatively  small  and,  the  methods,  unless  based  on  specific 
reactions,  usually  complicated  and  tedious. 

Following  the  lines  of  quantitative  analyses  are  the  determinations  of  the 
atomic  masses,  a  task  calling  for  a  degree  of  care  and  nicety  beyond  that  of 
any  other  investigation.  The  element  itself  or  one  of  its  compounds  taken  as 
the  basis,  and  all  the  reagents  used  in  the  determination  require  most  thorough 
purification;  the  balance  and  weights  must  be  of  the  highest  precision,  and 
the  weighings  so  conducted  as  to  minimize  the  defects  inherent  to  even  the 
most  perfect  instruments ;  every  source  of  analytical  error  must  be  searched 
out  and  provided  against ;  and  the  final  product  proven  beyond  a  doubt  to  be 
exactly  of  the  assumed  composition.  And  as  all  these  precautions  may  be  sub- 
verted by  a  slight  error  in  manipulation,  the  simplest  methods  involving  the 
fewest  mechanical  operations  are  always  to  be  preferred. 

A  favorite  method  for  the  metals  is  to  weigh  a  suitable  quantity  after  elab- 
orate purification,  and  convert  it  into  the  most  stable  oxide  by  some  simple 
process,  as  by  heating  in  oxygen.  Another  combination  might  as  easily  be 
formed,  such  as  the  sulflde,  but  here  the  atomic  ratio  of  sulfur  to  the  basis  of 
the  system  (oxygen  or  hydrogen)  enters  the  calculation  with  whatever  uncer- 
tainty attaches.  Several  determinations,  if  possible  by  different  methods,  are 
always  made,  and  if  the  results  of  a  number  of  chemists  of  repute  agree  with 
reasonable  closeness,  the  average  of  all  is  probably  very  near  the  truth  — 
always  admitting  the  possibility  of  an  undiscovered  common  error. 

Methods  of  this  class  are  also  applied  for  the  analysis  of  chemical  com- 
pounds to  determine  or  corroborate  the  empirical  formulae.  The  formula  as 
deduced  from  an  analysis  is  the  more  defensible  the  nearer  the  results  harmon- 
ize throughout  with  the  composition  as  calculated  from  the  assumed  formula. 
This,  of  course,  on  the  presumption  that  the  compound  as  submitted  to 
analysis  was  in  a  perfectly  pure  condition,  or  at  least  practically  free  from 
other  bodies.  But  as  the  probable  errors  in  the  several  determinations 
usually  differ  more  or  less,  it  is  necessary  to  assume  an  approximate  probable 
error  in  the  determination  of  each  element  and  draw  conclusions  accordingly. 

2.  Comprised  in  the  second  and  by  far  the  largest  class,  are  those  associat- 
ing   reasonable  precision    with   general    applicability  and    ease  of  working. 
They  are  usually  qualified  for  the  analysis  of    well-known  natural  bodies  or 
staple  articles  of  commerce,  and  are  generally  planned  for  the  separation  and 
determination  of  the  constituents  normally  contained  therein,  not  providing 
for  any  that  are  exceptional,  accidental,  or  derived  from  foreign  sources. 

As  to  what  shall  constitute  the  limits  of  inaccuracy  conformable  with 
c  reasonable  precision  *  in  this  case  depends  as  much  (sometimes  more)  on 
the  skill  and  care  of  the  operator  as  on  the  method.  However  all  will  agree 
that  several  analyses  made  by  one  such  method  on  one  sample  by  one  chemist 


NOTES    ON   THE    METHODS    OF    ANALYSIS.  527 

should  yield  results  all  within  the  allowable  limits  of  inaccuracy  as  fixed  by 
analogy  and  common  experience. 

3.  Lastly,  there  is  a  class  of  highly  specialized  methods  planned  for  one 
material  only  with  conditions  of  the  practice  rigidly  fixed.  Like  those  of  the 
preceding  class  they  are  restricted  to  the  combinations  in  which  the  body 
determined  is  commonly  found,  and  are  planned  for  separation  from  only  the 
usual  concomitants;  moreover  as  a  rule  their  successful  working  is  contingent 
on  the  absence  of  certain  other  bodies,  this  presumed  from  the  nature  or  origin 
of  the  substance,  and  confirmed  if  any  doubt  exists,  by  qualitative  tests.  They 
are  characterized  by  simplicity  and  rapidity,  qualities  highly  appreciated  in 
industrial  analysis  for  which  they  have  been  chiefly  devised.  In  this  class 
fall  also  those  methods  that  are  simply  attempts  at  following  some  particular 
division  of  a  technical  process  along  lines  as  near  to  it  as  conditions  allow. 

Many  of  these  methods  have  been  adapted  from  others  more  accurate  by  so 
modifying  the  procedure  as  to  materially  reduce  the  time  required  for  their 
prosecution.  The  desired  rapidity  may  be  secured  by  hastening  the  operations 
of  a  gravimetric  analysis  regardless  of  small  losses  entailed;  e.  g.,  by  quickly 
boiling  down  a  solution  to  dryness  instead  of  evaporating  at  a  moderate  heat ; 
for  the  slower  process  of  the  filtration,  ignition  and  weighing  of  a  granular 
precipitate  may  be  substituted  the  (less  accurate)  one  of  measuring  its  volume. 
Broadly,  a  reaction  may  be  accelerated  by  so  arranging  matters  that  the  con- 
ditions for  the  exhibition  shall  be  most  favorable,  excluding  or  removing  what- 
ever tends  to  retard  its  incipience  or  completion. 

Volumetric  and  colorimetric  methods  of  this  kind  are,  as  a  rule,  much  more 
rapid  than  gravimetric,  and  may  be  still  further  shortened  by  various  simple 
expedients;  as  by  the  use  of  empirical  standard  solutions  of  such  concen- 
trations that,  upon  a  fixed  weight  of  sample,  the  percentage  of  the  constituent 
will  be  shown  directly  by  the  number  of  cubic  centimeters  used  in  titration ; 
colorimetric  comparisons  facilitated  by  series  of  standards  or  means  of  com- 
parison; etc. 

Physical  or  attributive  methods  can  be  applied  only  after  verifying  the  non- 
interference of  other  constituents  of  the  substance  normally  or  exceptionally 
present,  though  it  is  often  possible  to  deduce  and  apply  a  correction  for  the 
effect  of  an  associate  found  in  a  practically  constant  proportion  to  the  constit- 
uent determined.  Of  the  various  physical  characteristics  that  may  be  utilized, 
specific  gravity  is  most  often  available. 

In  defense  of  methods  of  this  class  it  may  be  said  that  their  rapidity  allows 
a  control  of  technical  and  manufacturing  processes  not  practicable  with 
methods  requiring  a  longer  time;  even  a  method  that  is  conceded  to  give 
only  approximate  results  may  be  of  great  practical  service  in  this  way  by  in- 
dicating the  suitable  subsequent  treatment  of  a  material  in  progress  of  con- 
version. Again,  in  the  case  of  some  organic  mixtures,  only  rapid  methods  can 
be  used  on  account  of  the  proneness  of  the  material  to  decomposition  by  oxi- 
dation or  fermentation.  Usually  the  maximum  error  likely  to  be  incurred  in 
the  ordinary  practice  of  such  a  method  can  be  predetermined  and  allowed  for 
in  drawing  deductions  and  applying  the  analyses,  and  in  designing  one  it  is 
often  possible  to  so  arrange  the  details  that  a  probable  plus  error  at  one 
stage  will  be  in  a  measure  counteracted  by  a  minus  one  at  another. 

Against  them  may  be  charged  that  although  inherent  errors  can  be  obviated 
or  corrected  for  by  observing  specific  directions,  yet  the  details  are  not 
always  easy  to  follow  without  undivided  attention  and  the  neglect  of  other 
analytical  work  carried  on  at  the  same  time;  that  while  in  technical  analysis  a 
result  indicating  that  the  particular  sample  under  analysis  is  of  abnormal  com- 


528  QUANTITATIVE    CHEMICAL    ANALYSIS. 

position  at  once  suggests  the  advisability  of  corroboration  by  a  more  accurate 
method,  yet  should  an  abnormal  material  be  returned  as  of  normal  composi- 
tion the  error  would  probably  pass  unnoticed ;  finally  it  cannot  be  denied  that 
some,  perhaps  many,  of  such  methods  are  liable  at  times  to  give  erratic  results 
without  apparent  reason.  In  any  event  caution  in  the  adoption  of  methods  of 
this  class  and  in  the  acceptance  of  their  results  is  the  part  of  prudence. 

Usually  the  saving  of  time  due  to  these  methods  is  most  manifest  where  a 
single  analysis  or  but  a  few  are  made  at  one  time,  and  may  quite  disappear 
where  a  considerable  number  of  determinations  are  begun  and  carried  along 
together 

At  times  one  may  be  at  a  loss  to  select  from  a  number  of  unfamiliar  methods 
the  one  that  will  answer  best  for  a  given  determination.  A  choice  will  be 
aided  by  a  consideration  of  the  object  for  which  the  analysis  is  to  be  made, 
this  determining  not  only  the  degree  of  accuracy  to  be  aimed  at,  but  helping  to 
decide  questions  in  regard  to  other  matters  that  may  arise  before  or  during 
the  analysis. 

The  chemist  may  feel  free  as  to  a  choice  of  methods  to  be  used  for  analyses 
made  only  for  his  personal  information  or  in  the  course  of  investigations 
of  a  specific  or  confidential  nature,  but  when  his  results  are  to  be  published, 
in  scientific  investigations,  or  in  the  valuation  of  merchandise,  or  when  acting 
as  an  expert  or  umpire,  he  will  be  wise  to  regard  not  only  his  own  convictions, 
however  positive  they  may  be,  as  to  the  best  method  to  be  adopted  or  what 
details  are  most  expedient,  but  the  consensus  of  opinion  of  his  fellow-workers 
as  well,  and  modify  his  practice  to  conform,  in  some  degree  at  least,  to  the 
prevailing  practice  of  his  professional  brethren.  Especially  is  this  concession 
appropriate  with  the  more  arbitrary  methods  where  details  are  so  much  in 
evidence.  . 

Out  of  the  host  of  methods  that  have  been  proposed,  some  have  come  into 
general  use,  others  dropped  into  comparative  obscurity.  The  survival  of  the 
fittest  will  in  part  account  for  this  election,  but  he  who  searches  chemical  lit- 
erature will  not  fail  to  discover  many  that  are  apparently  correct  in  theory 
and  highly  practical,  but  of  which  he  can  find  no  mention  in  the  works  on 
special  branches  of  analysis,  or  at  best  are  dismissed  with  but  a  brief  and 
equivocal  comment. 

A  method  devised  and  published  may  be  so  palpably  superior  to  others  in 
common  use  as  to  meet  with  immediate  favor  and  supplant  to  a  great  extent 
all  previously  in  use.  The  superiority  may  not  be  in  the  way  of  accuracy  alone, 
but  by  reason  of  comparative  rapidity  or  convenience  may  meet  with  favor 
and  take  precedence  over  those  perhaps  more  accurate  but  less  suited  to  the 
technical  chemist. 

On  the  other  hand,  a  method  may  offer  some  advantages  over  those  in  com- 
mon use  yet  not  sufficient  to  gain  general  adoption.  One  does  not  readily  turn 
from  a  well-tried  method  with  whose  details  he  is  familiar,  knowing  the  routine 
best  suited  for  his  purposes  and  most  practicable  with  his  facilities,  its  limi- 
tations and  weaknesses  where  caution  is  demanded,  its  adaptability  and  trust- 
worthiness, to  one  offering  but  little  in  exchange  for  the  risk  of  a  failure 
where  his  reputation  might  suffer.  Rather  would  he  defer  the  change  until 
the  high  opinion  of  the  deviser  was  supported  by  others  in  whose  impartiality 
and  judgment  he  could  place  confidence. 

Again,  many  methods  have  not  received  the  favor  they  deserve  solely  for  the 
reason  that  in  the  original  description  or  abstracts  some  detail  of  vital  im- 
portance has  not  been  sufficiently  emphasized,  and  the  first  trial  being  unsatis- 


NOTES    ON    THE    METHODS    OF    ANALYSIS.  529 

factory  on  this  account  has  led  to  a  depreciation  and  abandonment  of  the 
method. 

Another  and  common  reason  for  neglect  may  be  that  the  original  publication 
appeared  in  a  technical  or  trade  journal  of  very  limited  circulation  among 
chemists,  and  has  therefore  escaped  the  notice  of  both  the  majprity  of  chem- 
ists and  of  the  abstracters  for  chemical  journals. 

As  a  general  proposition,  the  most  serviceable  method  is  one  that  is  free  from 
any  considerable  inaccuracy  in  principle  or  practice,  and  does  not  demand 
many  or  delicate  operations  or  an  excessive  time  for  its  prosecution.  In 
most  departments  of  special  analysis  there  will  be  found  one  or  more  methods 
that  answer  these  requirements  at  least  fairly  well  and  have  been  generally 
accepted  by  specialists,  and  may  be  adopted  with  confidence  by  those  less 
familiar  with  the  special  subjects  concerned. 

Let  us  consider  to  what  standards  a  quantitative  method  may  reasonably  be 
expected  to  approach. 

GRAVIMETRIC    METHODS. 

A  gravimetric  determination  has  the  advantage  over  most  others  in  that  it  is 
direct  and  positive  in  character ;  at  the  same  time  there  is  afforded  the  oppor- 
tunity of  preserving  the  products  and  educts  for  examination  should  there  be 
suspected  an  imperfect  separation  or  contamination  from  other  sources  or  an 
error  in  weighing.  A  gravimetric  method  of  the  highest  order  should 

1.  Be  free  from  all  avoidable  sources  of  error. 

2.  Allow  a    considerable  departure    from  any  specific  directions  laid  down 
without  impairing  the  accuracy. 

3.  Be  equally  applicable  to  every  percentage,  small  or  great,  of  the  constit- 
uent determined  in  the  substance  analyzed. 

4.  Assume  no  manipulative  skill  beyond  that  of  the  average  chemist,  and 
call  for  but  few  reagents,  and  these  neither  rare,  expensive  or  difficult  of  puri- 
fication, and  not  require  forms  of  apparatus  that  are  complicated,  cumbersome, 
or  of  a  special  character. 

5.  Provide  a  means  of  separation  for  each  constituent  that  will  not  prejudice 
the  subsequent  determination  of  others  —  this,  of  course,  does  not  apply  to  an 
assay  only. 

6.  Provide  that  the  precipitate  or  residue  finally  weighed  be  of  a  perfectly 
definite  composition,  and  neither  unstable,  volatile,  hygroscopic,  efflorescent,  or 
liable  to  hold  occluded  or  adsorbed  matters,  and  preferably  containing  only  a 
small  proportion  of  the  body  determined. 

7.  Not  require  an  unreasonable  time  for  its  performance. 

Very  few  methods  will  be  found  that  comply  literally  with  all  these  require- 
ments, though  none  can  be  deemed  too  exacting. 

As  a  rule,  a  quantitative  method  is  designed  with  a  view  to  the  analysis  of 
some  distinct  substance  or  class  of  substances  wherein  normally  the  propor- 
tions of  the  constituents  do  not  vary  from  the  typical  composition  to  any  great 
extent,  and  must  be  modified  beyond  increasing  or  diminishing  the  amount  of 
substance  taken  for  the  analysis,  or  adjusting  the  quantities  of  the  reagents, 
when  dealing  with  those  deviating  beyond  certain  limits.  Often  it  will  be 
found  that  an  effective  separation  of  two  constituents  by  a  given  method  is  only 
successful  when  their  relative  proportion  is  within  a  certain  limited  range. 

Usually  a  method  specifies  the  order  in  which  the  constituents  are  to  be 
separated,  but  this  may  often  be  changed  with  advantage.  In  deciding  the 
most  advantageous  sequence  there  are  to  be  taken  into  consideration : 

1.  Their  relative  importance,  assigning  priority  to  any  one  on  whose  propor- 

34 


530  QUANTITATIVE    CHEMICAL    ANALYSIS. 

tion  the  value  or  utility  of  the  substance  under  examination  is  based,  as  being 
earliest  separated,  it  is  subject  to  fewer  operations  with  their  attendant  errors. 
And  where  a  delicate  or  tedious  process  must  begone  through  with,  it  is  better 
that  it  be  performed  as  early  in  the  analysis  as  possible,  for  at  that  period  is 
the  best  work  likely  to  be  done.  By  force  of  will  one  may  exercise  equal  care 
throughout  an  extended  analysis,  but  the  natural  tendency  is  to  become  less 
circumspect  as  the  work  draws  to  a  close  —  at  times  the  diminuendo  is  pain- 
fully in  evidence. 

Against  the  above  must  be  weighed,  of  course,  the  possibility  or  certainty  of 
interference  with  subsequent  separations. 

2.  The  generic  or  specific  type  of  the  reagents  used.    Group  reagents,  those 
reacting  with  all  the  members  of  a  certain  class  of  bodies,  are  less  often  em- 
ployed than  in  qualitative  analysis.     An   objection  is    that   the    precipitates 
formed  from  the  reactions  with  the  different  members  have  not  the  same  co- 
efficient of  solubility  in  the  supernatant  fluid  or  the  washing  medium,  and  the 
separation  from  the  members  of  another  class  will  seldom  be  as  complete  as  if 
specific  precipitants  were  applied  in  succession;  on  the  other  hand  the  latter 
plan  has  the  disadvantage  of  loading  the  liquid  with  the  excess  of  the  pre- 
cipitants and  soluble  products  of  the  reactions. 

Another  reason  for  preferring  a  specific  precipitant  is  that  the  product  of  the 
reaction  is  usually  left  in  a  form  that  allows  direct  weighing  or  measurement, 
while  a  group  reagent  implies  at  least  one  other  separation  of  the  conjointly 
precipitated  members. 

The  completeness  of  separation  of  a  pulverulent  mixture  by  a  given  solvent 
depends  primarily  on  the  ratio  of  the  solubilities  of  the  constituents  therein, 
but  other  influences  cannot  be  ignored.  These  may  be:  (1)  mechanical,  as 
where  the  particles  are  in  such  a  physical  condition  that  easy  permeation  is  not 
allowed ;  or  the  soluble  constituents  may  be  or  become  enveloped  by  the  insol- 
uble parts  and  shielded  from  contact  with  the  solvent,  and  this  however  well  the 
mixture  be  agitated  during  the  treatment,  or  the  digestion  prolonged.  If  the 
mixture  is  not  in  fine  subdivision,  the  soluble  constituent  should  not  be  in  less 
than  a  certain  proportion  in  the  mixture,  a  specific  ratio  determined  by  the 
physical  structure  and  density  of  the  components  and  other  considerations. 
(2)  A  soluble  and  an  insoluble  constituent  may  unite  on  contact  with  the  sol- 
vent to  a  form  not  decomposed  by  it  and  remain  insoluble.  (3)  A  solid  when 
pure  may  be  unaffected  by  a  given  solvent,  but  when  diluted  with  another  solid 
may  dissolve  perceptibly,  largely,  or  entirely.  (4)  In  the  separation  of  two 
liquids,  often  the  solubility  of  the  one  more  insoluble  is  increased  to  a  remark- 
able extent  by  the  presence  of  the  other  in  the  solvent,  aud  many  erroneous 
statements  as  to  the  purity  of  commercial  articles  may  be  traced  to  a  disregard 
of  this  peculiarity.  The  phenomenon  is  oftener  observed  in  a  separation  by  a 
group  reagent  than  when  specific  reagents  are  applied  in  succession. 

3.  The  liability  of  the  presence  of  the  residual  radical  of  the  body  separated 
or  the  excess  of  the  reagent  to  interfere  with  succeeding  separations.    For  this 
reason  many  organic  reagents  are  disqualified  for  inorganic  separations,  though 
otherwise  most  efficient  for  the  purpose. 

4.  The  proportion  of  the  constituents  to  each  other.     Two  bodies  of  similar 
chemical  attributes  may   often  be  parted  with  ease  when  the  ratio  of  their 
weights  is  a  small  number,  yet  difficulties  be  met  when  one  greatly  predomi- 
nates; a  familiar  example  is  that  of  the  alkali  salts  of  sea-water,  the  sodium 
chloride  exceeding  the  iodide  a  thousand  or  more  times.     Exceptions  to  this 
rule   are   found   in  some  organic  bodies  closely  allied  in  chemical  character 
(e.  g.,  catechol  and  pyrogallol),  where  the  separation  of  a  mixture  of  approxi- 


NOTES    ON   THE    METHODS    OF    ANALYSIS.  531 

mately  equal  proportions  is  more  difficult  than  when  one  greatly  preponder- 
ates. 

In  the  analysis  of  a  material  of  which  one  constituent  forms  nearly  the  whole, 
the  determination  of  the  others  will  be  much  facilitated  if  it  can  be  removed 
at  the  outset  by  a  specific  reagent,  some  physical  process  or  otherwise. 
Thus,  for  the  analysis  of  a  commercial  metal  there  may  be  found  a  specific 
solvent  that  will  dissolve  all  or  the  greater  part  of  the  major  constituent  leav- 
ing the  impurities;  the  latter  may  either  be  insoluble  in  the  menstruum  or  dis- 
solved only  after  all  the  major  constituent  has  passed  into  solution.  Less 
often  can  a  specific  solvent  be  used  to  extract  the  basis  of  a  commercial  salt, 
since  the  common  impurities  usually  show  a  similar  behavior  toward  solvents. 

Or  a  specific  precipitant  may  be  found  that  will  unite  with  the  chief  con- 
stituent to  an  insoluble  compound,  the  impurities  likely  to  be  associated  being 
freely  soluble  and  afterward  easily  separated  from  the  small  amount  of  the 
former  remaining  in  solution.  Instances  are  the  precipitation  of  lead  from 
the  commercial  sugar  of  lead  (the  crystallized  acetate)  by  sulfuric  acid, 
copper  from  the  raw  metal  by  a  sulfocyanide  and  sulfite,  tin  from  sodium 
stannate  by  evaporation  with  nitric  acid,  etc. 

Conversely,  there  may  be  applied  a  solvent  that  will  withdraw  the  impurities, 
at  least  in  great  part,  leaving  the  leading  constituent  insoluble.  But  for 
physical  reasons  this  method  is  inferior  to  the  former.  Similarly  a  precipitant 
(or  a  mixture  of  several)  may  sometimes  be  found  that  will  throw  down 
most  or  all  of  the  impurities  with  none  or  but  little  of  the  major  constituent. 

In  default  of  a  better  plan,  the  greater  part  of  the  principal  constituent  may 
be  isolated  by  fractional  solution,  distillation  or  crystallization,  by  congelation 
or  fusion,  etc.,  though  the  separation  is  never  better  than  approximate. 

In  the  determination  of  a  body  by  precipitation  it  is  presumed  that  the 
precipitate  can  be  freed  from  the  excess  of  the  precipitant  or  other  bodies 
present  in  the  solution  by  the  process  of  washing  with  a  suitable  liquid.  But 
however  thorough  the  washing,  a  precipitate  may  remain  impure  from  at  least 
four  causes. 

A.  Some  secondary  reaction  may  result  from  contact  of  the  supernatant  fluid 
with  the  air  or  laboratory  gases,  the  decomposition  of  organic  matter,  or  other 
cause,  with  the  formation  of  an  additional  precipitate  insoluble  in  the  washing 
fluid. 

B.  Matter  suspended  in  the  solution  is  entangled  and  carried  down  more  or 
less  completely  according  to  the  consistency  and  bulk  of  the  precipitate.    By 
a  glairy  or  gelatinous  precipitate  a  liquid  may  be  clarified  of  a  suspended 
powder  so  finely  divided  as  to  pass  through  the  pores  of  an  ordinary  filter 
paper. 

Quasi -soluble  or  colloidal  bodies  in  unstable  solution  maybe  influenced  to 
assume  an  insoluble  form  by  contact  with  a  precipitate,  perhaps  by  a  force 
analogous  to  the  promotion  of  crystallization  by  seeding. 

And  freely  soluble  bodies  may  be  retained  in  small  amount  —  be  it  through 
the  formation  of  a  double  salt,  be  it  in  a  mechanical  way  only,  nevertheless 
they  are  held  so  tenaciously  that  protracted  washing  will  not  remove  them. 
Gelatinous  metallic  hydrates,  for  example,  are  freed  with  difficulty  of  alkali 
salts  by  washing  with  water  (though  quite  easily  after  a  change  in  structure 
has  been  induced  by  drying  or  freezing),  and  considering  their  consistency 
such  a  tendency  is  not  surprising  —  witness  the  '  lakes.'  With  dense  granular 
precipitates  occlusion  would  not  be  expected,  yet  instances  are  not  uncom- 
mon; manganese  binoxide,  precipitated  from  a  hot  nitric  solution  has  a  marked 
attraction  for  the  nitrates  of  other  metals,  and  when  thrown  down  electrolyt- 


532  QUANTITATIVE    CHEMICAL    ANALYSIS. 

ically,  occludes  iron  compounds;  barium  chloride  in  aqueous  solution  is  not 
completely  decomposed  by  sulfuric  acid,  the  precipitate  always  containing  a 
small  amount  of  barium  chloride  even  when  the  sulfuric  acid  is  in  large  excess, 
the  solution  dilute,  and  the  mixture  heated  to  boiling;  etc.,  etc. 

That  this  phenomenon  is  not  always  simply  a  mechanical  inclosure  is  argued, 
among  other  evidence,  by  the  unequal  amounts  in  which  analogous  bodies  are 
retained.  Thus,  Schweitzer,*  in  determining  the  sulfuric  radical  combined 
with  various  metals,  precipitated  solutions  of  their  sulfates  by  barium  chloride 
and,  after  thorough  washing,  weighed  the  barium  sulfate;  on  potassium  sulfate 
he  obtained  99.32  per  cent  of  the  calculated  weight  of-  SOs,  and  on  sodium 
sulfate  100.18  per  cent,  showing  clearly  that  potassium  sulfate  is  occluded  in 
the  precipitate  to  a  greater  extent  than  sodium  sulfate.  Similarly,  barium  sul- 
fate precipitated  in  presence  of  ferric  chloride  always  contains  more  iron  than 
if  only  ferrous  chloride  is  present,  perhaps  from  the  formation  of  a  ferric 
barium  sulfate.  In  general  it  may  be  said  that  the  compound  predominating  in 
the  solution  during  the  precipitation  is  the  one  most  occluded,  hence  the  usual 
order  of  pouring  the  precipitant  into  the  solution  to  be  precipitated  may  some- 
times be  reversed  with  advantage. 

A  simple  and  effective  means  of  insuring  the  purity  of  a  precipitate  and  one 
ordinarily  available,  is  that  of  redissolving  it  after  filtration  and  washing  and 
repeating  the  precipitation;  here  the  soluble  associates,  largely  removed  in 
the  first  operation,  remain  in  so  comparatively  small  proportion  that  practi- 
cally none  are  carried  down.  Occluded  bodies  are  best  eliminated  before 
a  precipitate  is  weighed,  but  where  this  is  not  practicable  a  subsequent 
purification  should  be  attempted  only  by  a  method  of  the  simplest  nature; 
many  of  the  schemes  proposed  for  the  purification  of  ignited  precipitates  are 
likely  to  entail  losses  or  introduce  impurities  in  greater  weight  than  those 
sought  to  be  eliminated. 

It  is  plain  that  the  correctness  of  a  gravimetric  determination  made  in  the 
usual  manner  depends  primarily  on  a  fixed  and  invariable  relation  of  the  weight 
of  the  element  or  compound  determined  to  the  product  weighed.  In  inor- 
ganic analysis  generally,  the  relation  is  a  constant,  but  in  organic  determina- 
tions frequently  the  reaction  or  reactions  are  so  incomplete,  indefinite,  or 
complex,  intricated  by  secondary  reactions,  conditions  of  precipitation,  etc., 
that  it  varies  to  a  considerable  degree.  Nevertheless  a  method  based  on  such  a 
reaction  may  be  employed  with  fair  results  if  admitting  of  a  correction; 
thus,  (1)  if  the  reaction  be  incomplete  or  reversed  to  a  certain  definite 
extent,  a  simple  multiplication  by  a  coefficient  deduced  by  experi- 
ment; (2),  if  also  influenced  by  conditions  of  temperature,  duration 
of  contact  with  the  solution,  and  the  like,  the  analysis  is  conducted  under 
the  same  conditions  as  in  the  determination  of  the  coefficient;  (3),  if  the 
relation  varies  directly  or  inversely  with  the  weight  of  the  element  or  com- 
pound to  be  determined,  an  equation  of  the  form  P  =  aWJrbW2±:cW3.  .  .  . 
or  a  simpler  one  may  be  deduced  and  applied;  and  (4),  the  extent  of  some 
physical  property  of  the  filtrate  or  its  concentration  may  serve  to  fix  the  proper 
correction. 

Again,  the  rule  as  usually  stated  that  the  compound  finally  weighed  or  meas- 
ured shall  be  of  a  definite  chemical  composition,  includes  both  polymers  and 
metamers,  and  may  be  broadened  to  require  only  that  it  shall  contain  a  definite 
proportion  of  the  element  sought,  thus  covering  a  mixture  of  two  compounds, 
both  holding  the  same  proportion  of  a  common  element  or  compound  though 


*  Catalogue  State  Univ.  of  Missouri,  1876. 


NOTES    ON    THE    METHODS    OF    ANALYSIS.  533 

united  with  different  radicals.  Conventionally,  a  mixture  of  two  or  more 
analogous  compounds  too  small  in  weight  to  admit  or  to  justify  separation  is 
returned  as  of  the  composition  of  the  one  predominating;  the  same  applies 
in  technical  analyses  where  two  associated  bodies  are  equally  important  or 
commercially  valuable. 

Of  the  other  requisites,  a  precipitate  that  may  be  ignited  is  preferred,  as 
the  alternate  process  of  drying  at  or  below  the  temperature  of  100  o  is  tedious 
at  best  and  often  less  accurate.  Very  hygroscopic  bodies  need  so  careful 
shielding  from  the  air  and  must  be  weighed  so  rapidly  that  it  is  better  to 
change  the  form  to  one  less  hygroscopic  even  at  the  expense  of  an  extra 
precipitation.  With  solid  or  liquid  bodies  that  are  volatile  at  100°  or  below 
a  certain  loss  is  inevitable  on  drying  or  evaporation,  and  there  is  .often  no 
way  by  which  to  transform  them  to  non- volatile  compounds.  The  loss  may 
be  kept  within  bounds  by  arranging  for  their  solution  in  a  highly  volatile 
solvent  and  conducting  the  evaporation  at  a  lower  heat,  weighing  the  residue 
as  soon  as  the  solvent  has  gone. 

The  smaller  the  proportion  of  the  element  sought  in  the  compound  weighed, 
the  less  will  equal  errors  of  weighing,  etc.,  affect  the  result.  This  is  well 
illustrated  in  determinations  of  nitrogen  ("molecular  weight  28)  and  platinum 
(atomic  weight  194.89),  both  weighed  in  the  combination  of  ammonium  platinic 
chloride  (molecular  weight  443.654). 

VOLUMETRIC   METHODS. 

Contrasted  with  the  many  approved  methods  of  gravimetric  analysis,  but 
comparatively  few  volumetric  methods  are  available,  the  reasons  lying  in 
the  following  requirements. 

1.  A  single  reaction,  definite  and  preferably  instantaneous.    If  one  of  the 
reaction-products  is  insoluble,    the  precipitate    must    have    no  tendency  to 
occlude  the  yet  unprecipitated  active  constituent. 

Instead  of  a  single  reaction,  another  may  occur  simultaneously  or  asynchron- 
ously,  a  reversal  of  the  primary  or  set  up  by  the  products  of  the  volumetric 
reaction.  The  duality  may  of  course  be  disregarded  if  (1),  the  secondary 
reaction  is  inconsiderable  or  may  be  reduced  to  a  negligible  quantity,  as  by 
high  dilution  or  high  concentration  of  the  titrate,  addition  of  the  titrand  with  a 
certain  rapidity,  maintaining  the  titrate  at  a  specific  temperature,  or  the  pre- 
vious incorporation  of  some  product  of  the  primary  reaction ;  (2),  the  secondary 
reaction  begins  or  proceeds  so  slowly  that  there  is  110  perceptible  action  during 
the  ordinary  period  of  a  titration;  (3),  the  two  reactions  bear  a  definite  mutual 
relation  and  the  ratio,  found  by  experiment,  can  be  embodied  in  the  calculation 
of  the  result.  Otherwise  a  method  based  on  other  than  a  single  reaction  is  at 
best  doubtful  and  only  suited  for  approximate  determinations. 

It  does  not  always  follow  that  a  method  is  disqualified  because  the  reaction 
involved  is  somewhat  obscure.  Many  of  the  volumetric  methods  in  organic 
analysis  are  founded  on  reactions  imperfectly  understood,  yet  passable  results 
may  be  had  through  standardization  by  means  of  a  normal  sample  of  the  sub- 
stance under  examination  with  all  the  conditions  identical  to  those  of  the 
analysis,  or  by  a  second  reverse  titration. 

2.  The  titrand  to  be  perfectly  stable  for  at  least  a  few  hours,  and  its  strength 
ascertainable  with  accuracy  by  some  convenient  means. 

While  every  solution  should  be  tested  from  time  to  time  to  guard  against  a 
change  through  evaporation,  entrance  of  dust  or  fumes,  etc.,  it  is  nevertheless 
rather  irksome  to  be  obliged  to  standardize  before  every  titration  though  only 
a  day  or  so  apart,  and  therefore  the  technical  chemist  chooses  where  he  can  a 


534  QUANTITATIVE    CHEMICAL    ANALYSIS. 

solution  that  will  remain  unchanged  for  at  least  several  days  though  it  may  be 
somewhat  less  suitable  on  other  grounds. 

The  most  certain  method  of  standardizing  is  by  means  of  a  freely  soluble 
chemically  pure  salt  that  can  be  readily  prepared  to  correspond  to  the  formula, 
and  offers  no  difficulties  in  weighing.  If  no  such  compound  can  be  found,  a 
less  direct  process  may  answer  nearly  as  well.  For  titrations  where  the  actual 
reacting  value  is  of  no  consequence  by  reason  of  the  dubious  nature  of  the 
reactions,  the  titrand  is  standardized  against  the  substance  to  be  assayed  that 
may  either  be  of  the  best  obtainable  commercial  quality  or  specially  purified. 

It  has  been  claimed  for  many  crystallized  salts  chemically  pure  and  neither 
hygroscopic,  efflorescent,  or  volatile,  that  simply  dissolving  an  accurately  taken 
weight  in  water  and  making  up  to  an  accurately  measured  volume,  furnishes  at 
once  a  standard  solution  whose  titre  is  as  reliable  and  exact  as  is  given  by  any 
other  process.  No  objection  can  be  raised  against  the  principle  of  this  method, 
and  practically  it  must  be  conceded  that  it  is  hardly  logical  to  make  up  a  solu- 
tion with  great  care  from  a  salt  of  undoubted  purity,  then  proceed  to  stand- 
ardize it  by  a  compound  of  more  doubtful  composition  and  under  greater 
probable  errors  in  weighing  and  solution.  On  the  other  hand,  by  this  plan 
there  is  no  correction  provided  for  the  (fairly  constant)  errors  inherent  to 
every  titration,  which  to  a  greater  or  less  extent  are  compensated  when  the 
titre  is  established  by  a  titration,  under  like  conditions,  of  the  same  or  a 
similar  body  as  that  to  be  determined  in  practical  analysis.  Moreover,  pru- 
dence would  seem  to  point  to  the  desirability  of  some  check  on  the  weighing 
and  measuring  of  the  reagent  and  solvent,  and  possible  losses  or  gains  in  the 
preparation. 

3.  Reliable  tests  for  assuring  the  absence  from  the  titrate  of  reacting  bodies 
other  than  the  one  to  be  titrated,   and  if  present,  methods  for  their  removal 
and  also  for  other  constituents  that  would  interfere  by  their  color,  fluores- 
cence, turbidity,  obscuration  of  the  end- point,  etc. 

Volumetric  analysis,  unlike  gravimetric,  does  not,  as  a  rule,  require  the 
separation  of  other  substances  accompanying  the  one  to  be  determined, 
and  in  so  far  is  exempt  from  the  inaccuracies  attaching  to  this  operation. 
Yet  one  should  be  cautious  when  dealing  with  a  complex  titrate  as 
there  may  be  contained  certain  bodies  that  will  react  with  the  titrand  or 
interfere  with  the  exhibition  of  the  end-point,  and  of  whose  presence  there  is 
no  indication.  Again,  a  body,  in  solution  or  as  a  finely  divided  suspended 
solid,  acted  on  very  slowly  or  practically  not  at  all  by  the  titrand  when  alone, 
may  be  much  more  readily  attacked  during  the  titration  of  another  body  asso- 
ciated with  it. 

The  removal  of  interfering  associates  in  the  titrate  is  usually  accomplished 
by  the  ordinary  methods  of  gravimetric  analysis,  selecting  such  as  will  leave 
the  titrate  free  from  interfering  products  of  the  reactions  and  employ  no  re- 
agents whose  excess  would  be  detrimental  to  the  titration.  If  done  by  pre- 
cipitation, the  scheme  of  withdrawing  for  titration  a  small  aliquot  part  of  the 
supernatant  liquid  is  very  suitable  as  it  allows  easy  duplication. 

In  some  cases  a  titration  is  only  successful  in  the  presence  of  a  certain  ex- 
traneous compound  in  the  titrate,  that  though  having  no  direct  action  on  the 
titrand,  yet  causes  the  volumetric  reaction  to  proceed  more  rapidly  or  uni- 
formly, or  prevents  some  secondary  reaction.  Or  the  same  object  may  be 
attained  through  the  introduction  of  a  compound  that  unites  with  the  titrand  in 
a  definite  ratio,  correcting  for  the  equivalent  of  titrand  by  a  separate  deter- 
mination. 

4.  A  reasonably ,  wide  range  through  which  the  rate  of  titration  and  condi- 


NOTES  ON  THE  METHODS  OF  ANALYSIS.          535 

tions  of  dilution  of  the  titrate,  its  temperature,  degree  of  acidity  or  alkalinity, 
etc.,  may  be  varied  without  detriment  to  the  result. 

Certain  volumetric  reactions  proceed  normally  only  in  very  dilute  solutions, 
while  if  more  concentrated,  a  reverse  or  some  secondary  reaction  complicates 
the  principal.  On  the  other  hand  some  titrations  are  only  successful  when  the 
titrate  is  highly  concentrated,  and  although  it  is  a  simple  matter  to  dilute  a 
solution,  an  evaporation  means  at  least  the  drawback  of  an  expenditure  of 
time. 

In  the  great  majority  of  titration  methods  the  reaction  begins  promptly  and 
proceeds  regularly  within  the  limits  of  the  ordinary  temperatures  of  the  labora- 
tory. Exceptions  are  where  the  titrate  must  be  kept  near  or  at  the  boiling  point, 
either  because  the  reaction  is  only  induced  or  continues  normally  in  a  heated 
solution,  or  to  prevent  absorption  of  carbonic  acid  from  the  air,  to  expel  a 
gaseous  product  of  the  reaction,  to  cause  a  precipitate  to  clot  and  settle,  etc. 
Rarely  is  the  titrate  to  be  maintained  at  the  freezing  point  of  water,  this  to 
avoid  decomposition  of  a  sensitive  organic  compound,  to  retain  a  volatile  con- 
stituent, or  prevent  a  secondary  reaction. 

As  a  titration  proceeds,  the  concentration  of  the  titrate  as  regards  the  active 
body,  proportionally  diminishes.  In  some  cases  the  reaction  becomes  greatly 
retarded  before  the  end-point  is  reached ;  this  difficulty  is  easiest  overcome  by 
a  reverse  titration. 

Under  some  circumstances  a  titration  proceeds  normally  only  when  the 
titrand  is  run  in  very  slowly  and  the  titrate  so  continuously  stirred  that  diffu- 
sion takes  place  immediately ;  this  where  a  second  compound  in  the  titrate 
reacts  like  the  primary  though  less  promptly,  e.  gr.,  in  titrating  an  acid  by  a 
standard  alkali  in  presence  of  an  easily  saponiflable  compound.  But  in  gen- 
eral, a  practical  method  will  allow  the  rapidity  of  titration  to  be  varied  within 
wide  limits. 

Of  the  few  methods  that  require  a  definite  ratio  of  acidity  or  alkalinity  to 
the  volume  or  concentration  of  the  titrate,  none  have  come  into  general  prac- 
tical use. 

5.  A  sharp  and  unmistakeable  proof  of  the  consummation  of  the  reaction. 

The  most  satisfactory  evidence  is  a  decided  change  in  color  of  the  titrate, 
although  in  practiced  hands  a 'spot'  indication  is  but  little  inferior  except 
that  a  somewhat  longer  time  is  consumed  in  a  titration.  The  end-point  is 
shown  somewhat  earlier  or  later  according  to  the  relative  sizes  of  the  drops 
of  indicator  and  titrate,  the  time  allowed  for  the  color  to  be  developed,  the 
light  illuminating  the  drop,  and  other  variants.  Usually  these  are  not  sufficient, 
at  least  in  technical  work,  to  affect  the  results  to  any  great  extent.  Less  con- 
venient and  often  less  accurate  is  the  indication  observed  by  noting  the 
cessation  of  precipitation,  especially  where  the  precipitate  is  finely  divided 
and  filtered  portions  must  be  tested. 

In  any  case,  an  indication  that  is  not  instantaneous,  as  where  the  titrate 
passes  insensibly  through  one  color  to  another  and  a  certain  transition  shade 
must  be  accepted  as  the  end-point,  or  where  the  close  is  marked  by  the  com- 
plete solution  of  a  slowly  dissolving  suspended  solid,  is  always  liable  to  mislead 
the  inexperienced  and  not  infrequently  those  practiced  in  the  observation. 

In  the  numerous  titrations  based  on  a  change  in  valence  of  an  element  by  the 
reaction  with  the  titrand,  the  element  may  be  contained  in  the  body  to  be  ex- 
amined at  the  proper  valence  and  not  be  changed  during  the  process  of  solu- 
tion, or  if  at  another  valence  originally  or  after  solution,  can  be  brought  there- 
to by  some  simple  operation  as  by  boiling  or  evaporating  the  solution,  or  by 
precipitation  and  re-solution.  More  often  it  is  necessary  to  perduce  or  re- 


536  QUANTITATIVE    CHEMICAL    ANALYSIS. 

duce  the  element  by  the  action  of  some  reagent.  A  suitable  reagent  for  the 
purpose  is  not  always  easy  to  find,  since  in  most  cases  the  excess  of  the  reagent 
or  some  product  of  the  reaction  will  also  react  with  the  titrand  or  the  indicator 
used,  and  with  many  reagents  it  is  impossible  to  remove  the  excess  without 
again  transforming  the  element;  so  that  one  must  be  selected  whose  excess  can 
be  expelled  by  boiling  or  precipitation,  or  if  a  solid  by  solution  or  filtration. 

It  is  preferred  by  some  chemists  to  dispense  entirely  with  graduated  instru- 
ments by  weighing  a  titrand  instead  of  measuring  it,  the  advocates  of  this  sys- 
tem claiming  that  the  time  spent  at  the  balance  is  fully  compensated  by  the 
greater  accuracy  of  weighing  over  measuring  as  ordinarily  practiced, the  elimi- 
nation of  discrepancies  arising  from  variations  in  temperature,  and  minor  ad- 
vantages. On  the  other  hand,  the  chief  attraction  of  volumetry,  for  the  tech- 
nical chemist  at  least,  is  its  rapidity,  so  that  it  must  be  left  to  each  individual 
operator  to  decide  which  plan  is  best  suited  to  the  class  of  analyses  he  is 
called  on  to  perform. 

It  must  be  admitted  that  as  a  class  volumetric  methods  are  inferior  in  point 
of  accuracy  to  gravimetric.  That  an  opinion  to  the  contrary  is  held  by  many 
may  be  explained  by  the  fact  that  a  minority  of  the  methods  are  capable  of 
furnishing  results  of  unexceptionable  exactness  when  performed  with  proper 
care,  some  even  so  refined  as  to  be  admissible  for  the  determination  of  atomic 
weights.  The  publicity  attained  by  these  methods  has  no  doubt  created  the 
impression  that  this  standard  is  reached  or  approached  by  most  others.  Per- 
haps also  the  dictum  of  an  eminent  chemist  to  the  effect  that  the  highest  quali- 
fication of  a  volumetric  method  is  accuracy,  the  next,  accuracy,  and  the  third, 
accuracy  may  have  had  considerable  weight,  though  we  are  left  in  doubt  as  to 
his  reasons  for  exalting  this  particular  quality  —  highly  desirable  of  course  in 
any  class  of  methods,  it  may  be  and  often  is  sacrificed  to  other  considerations, 
and  in  volumetric  analysis,  peculiarly  fitted  by  rapidity  and  convenience  to  the 
needs  of  the  practical  chemist,  it  would  appear  that  the  waiver  is  the  more 
warrantable.  Of  the  several  items  that  make  to  impair  the  accuracy  of  titra- 
tion  methods,  two  at  least  are  often  in  evidence  —  the  sluggishness,  incom- 
pleteness, or  irregularity  of  the  manifestation  of  the  reaction,  and  the  difficulty 
of  expelling  or  neutralizing  the  effect  of  minor  reacting  or  interfering  associ- 
ates, especially  when  organic,  from  the  titrate. 

It  is  seldom  that  the  determination  of  all  the  members  of  a  complex  body  can 
be  performed  by  volumetric  methods  exclusively  either  as  accurately  or  con- 
veniently as  by  gravimetric  or  partly  by  both.  Of  the  great  number  of  volu- 
metric schemes  for  the  complete  analysis  of  commercial  articles  and  mixtures 
generally  that  are  to  be  found  in  monographs  on  volumetry,  few  have  come 
into  general  practice,  and  it  seems  well  settled  that  the  field  of  usefulness  for 
titration  methods  is  largely  confined  to  assays  only. 

COLORIMETRIC   METHODS. 

All  the  various  modifications  of  the  practice  of  colorimetry  have  as  a  basis 
the  principle  that  the  depth  of  color  given  to  a  solid  or  a  solution  by  a  chromo- 
genous  body  is  proportional  to  the  weight  entering  the  mixture  or  dissolved  in 
the  solution.  From  their  simplicity  and  ease  of  working  colorimetric  methods 
are  regarded  with  much  favor  by  technical  chemists,  but  in  all  cases,  uni- 
formity in  technic  is  demanded  to  a  greater  extent  than  in  any  other  class  of 
methods. 

Lovibond  *  summarizes  the  optical  phenomena  relating  to  colorimetric  deter- 
minations as  follows: 


*  Journ.  Socy.  Chem.  Ind.  1898—206. 


NOTES    ON   THE    METHODS    OF    ANALYSIS.  537 

"  1.  Normal  white  light  must  be  regarded  as  being  made  up  of  the  six  colour 
rays  —  red,  orange,  yellow,  green,  blue  and  violet  in  equal  proportion.  When 
the  rays  are  in  unequal  proportion  the  light  is  abnormal  and  coloured. 

2.  The  particular  colour  of  an  abnormal  beam  is  that  of  the  one  preponder- 
ating ray  if  the  colour  be  simple,  or  of  the  two  preponderating  rays   if  the 
colour  be  complex. 

3.  The  rays  of  a  direct  light  are  in  a  different  condition  to  the  same   rays 
after  diffusion  and  give  rise  to  a  different  set  of  colour  phenomena.     (Note  — 
for  the  present  this  limits  precise  colour  work  to  the  measurement  of  diffused 
daylight.) 

4.  The  vision  is  not  sensitive  at  the  same  time  to  more  than  two  colour  rays 
in  the  same  beam  of   light;  the  colours  of  any  other  abnormal    rays  being 
merged  in  the  general  luminosity  of  the  beam. 

5.  The  two  colour-rays  to  which  the  vision  is  simultaneously  sensitive  are 
always  adjacent    to    each  other  in  the  spectrum,    red  and    violet  being  con- 
sidered adjacent  for  this  purpose. 

6.  The  vision  is  unable  to  appreciate  colour  in  an  abnormal  beam  of  light 
outside  certain  limits,  either 

A.  Excess  of  luminosity  may  mask  the  colour,  or 

B.  The  luminous  intensity  may  be  too  low  for  the  preponderating  rays  to 
excite  a  definite  colour  sensation. 

7.  The  length  of  time  required  by  the  vision  to  appreciate  a  colour  is  longest 
for  the  red,  increasing  as  the  spectrum  is  ascended,  the  minimum  being  in  the 
violet. 

8.  The  colour  of  a  pure  substance  at  a  particular  density  is  constant. 

9.  Every  substance  has  its  own  specific  rate  of  light  absorption,  developing 
definite  colour  sensations  for  definite  increasing  densities." 

The  essential  features  of  a  practical  method  are 

1 .  The  color  of  the  solution  of  a  reasonable  weight  of  the  substance  to  be 
assayed  must  be  of  a  depth  suitable  for  a  close  comparison  with  the  standard. 

In  practice  the  best  results  are  secured  when  a  chromogen  forms  only  a  small 
percentage  of  the  substance  to  be  tested ;  otherwise,  since  within  limits,  a 
nuance  is  less  easily  perceived  in  proportion  as  the  color  is  deeper,  either  so 
small  an  amount  can  be  dissolved  for  a  test  or  the  solution  must  be  so  largely 
diluted  that  the  errors  introduced  become  excessive  and  the  results  only  ap- 
proximate. 

For  visual  reasons  some  operators  will  be  more  successful  than  others  from 
the  ability  to  more  sharply  distinguish  between  closely  related  depths  of  a  color. 
Again,  a  difference  in  shade  of  certain  colors  is  more  evident  to  the  average 
eye  than  of  others,  a  feature  familiar  to  those  accustomed  to  the  use  of  a 
common  form  of  polariscope. 

2.  Only  the  one  constituent  of  the  substance  assayed  to  impart  a  color  to  the 
solution.     An  exception  is  where  the  color  given  by  another  constituent  is  so 
faint  that  it  will  entirely  or  practically  vanish  on  moderate  dilution,  or  may  be 
effaced  by  some  means  not  interfering  with  the  primary  color;  or  as  occasion- 
ally happens,  where  a  minor  chromogen  bears  a  fairly  constant  proportion  to 
the  substance,  the  modification  of  the  primary  color  may  be  offset  by  a  like 
addition  to  the  standard,  if  not  already  present  therein. 

A  slight  extraneous  tint  from  some  associate  or  the  solvent  itself  may  be 
corrected  by  the  interposition  of  a  suitably  colored  translucent  screen,  or  by 
viewing  the  comparison  tubes  against  a  tinted  reflector. 

3.  The  color  not  to  be  transient,  at  least  within  the  period  of  comparison^ 
Should  the  chroma  develop  as  the  result  of  a  chemical  reaction  and  not  reach 


538  QUANTITATIVE    CHEMICAL    ANALYSIS. 

full  intensity  until  after  a  certain  time,  then  remaining  permanent  or  beginning 
slowly  to  lighten,  the  proper  interval  between  mixing  and  comparison  must  be 
learned  by  ample  experiments. 

4.  The  solutions  of  the  sample   and  standard  to  be  perfectly  clear  and  free 
from  air-bubbles  at  the  time  of  comparison,  and  of  a  uniform  moderate  tem- 
perature. 

A  cloud  or  haze  modifies  the  apparent  shade  of  any  color  by  reason  of  the 
opacity  of  the  solid  particles,  perhaps  also  by  reflection,  possibly  refraction 
of  light.  It  is  hardly  safe  to  assume  that  the  standard  and  sample  are  equally 
modified  when  the  turbidity  appears  of  the  same  density  in  both;  the  better 
plan  is  to  fine  the  solutions  by  suitable  means  or  to  choose  a  diluent  that  will 
act  as  a  solvent. 

Many  solutions  lose  their  transparency  on  dilution  with  water  though  retain- 
ing it  when  there  is  substituted  alcohol,  a  dilute  acid,  etc.,  according  to  the 
nature  of  the  solute  and  solvent.  When  a  solution  through  decomposition 
rapidly  clouds  and  no  preservative  or  retarder  can  be  introduced,  it  is  often 
possible  to  so  arrange  matters  that  a  comparison  need  occupy  but  a  few 
moments. 

Regarding  the  influence  of  temperature,  from  the  results  of  a  large  number 
of  experiments  on  metallic  salts  made  at  temperatures  ranging  from  20  °  to 
60° Cent.,  Vernon  finds  that  of  the  three  classes,  (1),  those  salts  whose 
solutions  increase  in  color  on  heating;  (2),  those  which  show  no  change;  and 
(3),  those  that  decrease  in  color,  nearly  all  of  the  salts  examined  belonged  to 
the  first  class,  a  few  to  the  second,  and  none  to  the  third,  and  that  the  incre- 
ments of  color  on  heating  show  general  relations  depending  on  (1),  the  nature 
of  the  base  of  the  salt,  ferric  solutions  being  most  affacted  by  variations  in 
temperature,  next  those  of  cobalt  and  uranium,  then  those  of  copper  and 
nickel;  (2"),  on  the  nature  of  the  acid  radical,  chlorides  being  the  most 
affected,  nitrates  next,  and  sulfates  least;  (3),  ou  the  degree  of  dilution  of  the 
solution,  sulfates  being  most  affected  in  their  decinormal  solutions  and  least 
in  their  centinormal,  while  chlorides  are  most  affected  in  their  normal  solu- 
tions and  least  in  their  centinormal.  He  is  of  the  opinion  that  *'  the  results 
obtained  for  the  effect  of  temperature  on  the  colours  of  salt  solutions  are  with- 
out exception,  in  favour  of  the  hydrate  theory  of  the  nature  of  solution  ". 

5.  A  comparison  standard  of  accurately  known  concentration  and  identical 
with  the  assay  in  color  and  tone  and  approximately  equal  in  color-depth. 

As  the  methods  of  colorimetry  are  essentially  relative,  it  becomes  all-im- 
portant that  there  be  prepared  or  selected  a  suitable  standard  against  which  to 
match  the  sample.  This  standard  may  be 

A.  The  identical  element  or  compound,  in  a  state  of  known  purity,  that 
forms  the  chromogen  of  the  sample  to  be  tested;  as  where  a  nitric  acid  solu- 
tion of  some  copper  alloy  is  matched  against  a  standard  of  a  nitric  acid  solution 
of  pure  copper,  or  a  sulfuric  solution  of  an  indigo  against  one  of  crystallized 
indigotin  and  indirubin.    This  is  the  most  direct  procedure  and  the  one  least 
liable  to  errors,  but  for  many  substances  is  open  to  the  objection  that  other 
constituents  of  the  sample,  absent  from  the  standard,  may  greatly  modify  the 
color  or  depth  of  the  solution. 

B.  A  specimen  of  the  substance  to  be  tested,  of  superior  or  average  quality, 
in  which  the  proportion  of  the  chromogen  has  been  determined  by  an  absolute 
method.    The  objection  to  (A)  does  not  apply  here,  but  on  the  other  hand 
the  result  is  vitiated  by  whatever  inaccuracy  attaches  to  the  analysis  of  the 
standard. 

C.  A  specimen  of  the  substance  to  be  tested  but  selected  as  being  of  a  prime 


NOTES    ON   THE    METHODS    OF    ANALYSIS.  539 

quality  and  of  undoubted  commercial  purity  —  this  when  the  chromogen  is  of 
such  a  nature  as  not  to  admit  of  an  absolute  determination.  The  vegetable 
dye-stuffs  are  examples.  As  the  selection  of  the  standard  is  but  an  arbitrary 
and  personal  matter,  the  results  obtained  against  different  standards  are 
hardly  comparable,  though  in  technical  analysis  this  difficulty  can  be  over- 
come by  the  establishment  of  an  authoritative  standard  by  the  co-operation  of 
the  trade  or  industry  interested. 

D.  A  more  permanent  duplicate  compounded  and  hermetically  sealed  up  in 
a  tube  of  standard  diameter  —  this  when  the  colors  of  the  standards  prepared 
as  in  (B)  and  (C)  are  fugitive,  fading  by  decomposition  of  the  solution  either 
spontaneously,  through  a  reduction  by  traces  of  organic  matter  impossible  to 
exclude,  or  by  exposure  to  sunlight  or  air.    Usually  the  foundation  is  an  inor- 
ganic salt  approaching  the  desired  color,  tinged  to  exact  correspondence  by 
other  salts. 

E.  Two  solutions  of  different  colors  or  tints  held  in  glass  wedges  are  super- 
imposed in  the  path  of  a  ray  of  white  light  and  adjusted  until  the  transmitted 
hue  matches  that  from  the  sample.    A  serious  objection  to  many  applications 
of  this  scheme  is  that  variations  result  from  the  dissimilar  ionic  decomposition 
in  the  three  solutions  causing  a  marked  difference  in  color  density.     As  an  ex- 
ample, the  color  transmitted  through  two  superimposed  solutions,  one  of 
potassium  chromate,  the  other  of  chromic  acid  in  molecular  ratio  to  the  first, 
is  said  to  be  only  about  one-fourth  as  deep  as  that  from  a  solution  of  potassium 
bichromate  of  equivalent  strength  (K2CrO4  -f  H2CrO4=  K2Cr2O7  +  H2O)  ;  indi- 
cating that  with  the  first  two  solutions  the  cathions  are  K2  and  H2  and  the 
anion  OO-i,  and  in  the  third  the  cathion  is  K2  and  the  anion  Cr2C>7. 

F.  A  solution,  usually  of  one  or  more  metallic  salts,  prepared  of  such  a  con- 
centration as  will  permit  comparison  with  all  ordinary  samples  of  certain 
liquids  or  solutions  of  certain  solids.    This  conventional  form  of  standard  is 
successfully  used  for  grading  complex  organic  coloring  matters.    It  is  clear 
that  no  information  as  to  the  actual  content  of  the  coloring  constituent  is 
afforded,  but  merely  an  expression  of  the  numerical  relation  of  the  respective 
tints,  allowing  a  more  or  less  arbitrary  differentiation;  and  that  the  concentra- 
tion of  the  standard  solution  that  corresponds  to  a  typical  quality  of  a  com- 
mercial article  or  to  the  maximum  color  of  a  food  or  drink  that  is  compatible 
with  wholesomeness  must  have  been  generally  accepted  by  the  analytical 
world  ere  an  expression  of  this  nature  can  serve  a  useful  purpose. 

G.  The  chromogen  of  the  standard  may  be  developed  by  the  tentative  addi- 
tion of  another  reagent.    For  example,  a  yellow  organic  solution  may  be  com- 
pared against  a  standard  solution  of  iodine  made  by  running  successive  small 
volumes  of  a  standard  solution  of  some  oxidizer  into  a  (colorless)  solution  of 
potassium   iodide;   each   addition  of  the  former  reacts  with  the  potassium 
iodide  to  set  free  an  equivalent  of  iodine  which  remains  in  solution.     Com- 
parisons of  this  kind  are  apt  to  be  tedious,  as  a  certain  time  must  be  allowed 
after  each  addition  of  the  reagent  to  insure  that  the  reaction  is  complete  and 
the  color  fully  developed. 

In  the  comparison  of  a  solution  with  a  standard  by  the  process  of  tentative 
dilution  it  is  assumed  that  the  depth  of  color  varies  harmoniously  with  the 
concentration  —  in  other  words,  that  the  shade  lightens  in  direct  ratio  as  the 
solution  is  diluted.  But  experiment  shows  that  this  assumption  is  seldom  if 
ever  substantiated,  the  shade  usually  lightening,  rarely  darkening,  to  a  greater 
extent  than  is  attributable  to  dilution  only. 

The  reduction  in  color  effect  beyond  mere  dilution  is  very  noticeable  in  the 
case  of  the  intensely  red  ferric  sulfocyanide.  Magini  attributes  the  hyper- 


540  QUANTITATIVE    CHEMICAL   ANALYSIS. 

reduction  of  color  to  increasing  ionization,  but  Andrews  to  hydrolysis  (e.  g., 
Fe  2(CNS)6  +  6H2O  =  Fe2(OH)6  -f  6HCNS) .  Gladstone,  from  his  experiments 
on  the  subject,  concludes  that  the  amount  of  ferric  sulfocyanide  produced  on 
mixing  a  ferric  salt  with  potassium  sulfocyanide  is  affected  by  the  nature  of 
every  substance  present,  by  the  mass  of  the  different  substances  present,  and 
temporarily  by  variations  of  temperature,  and  that  in  the  case  of  color  pro- 
duced by  a  chemical  reaction,  the  relative  amounts  of  salts  remaining  intact 
depends  on  their  proportion. 

Vernon  f  made  an  extended  investigation  in  relation  to  this  phenomenon, 
comparing  some  35  solutions  of  colored  salts  to  learn  if  dissociation  in  dilute 
solutions  progressed  regularly .  An  abridged  table  of  his  results  and  a  few 
excerpts  from  his  papers,  which  should  be  read  in  full,  are  here  given. 


Dilutions  

1.    2.5 

5. 

10 

50 

100    300 

5OO    700    1000 

Copper  sulfate    . 

100    .... 

98.3 

95.1 

93.0      

Copper  acetate 

100 

82.7 

68.2 

37.1 

Nickel  sulfate  .... 

100     .... 

100.1 

98.5 

86.2 

Cobalt  sulfate  

100     .... 

90.8 

89.2 

77.8      .     . 

«          «     * 

100  0 

90  0 

89  2      83  7 

82  0      79  0      77  8 

Cobalt  chloride  

100     .... 

87.91 

76.4 

65.5 

Cobalt  nitrate  

100      ... 

93.1 

89.4 

79  2      .... 

Uranium  sulfate 

100 

97.1 

91  9 

86.7 

Chromic  sulfate  .  . 

100.0 
100.0 

94.4 

98.2 

99.8      .... 
97.4     96.2 

91.1 
99.0    104.7    110.2 

All  the  above  were  diluted  suddenly  except  those  marked  with  an  *  which 
were  diluted  gradually. 

"Almost  all  of  the  solutions  of  the  35  coloured  salts  examined  showed  con- 
siderable decrease  in  colour  effect  on  dilution,  due  in  all  probability  to  dis- 
sociation taking  place.  The  only  exceptions  are  certain  chromium  derivatives, 
the  colour  of  whose  solutions  on  gradual  dilution,  either  remains  constant 
or  increases  slightly.  When  their  solutions  are  suddenly  diluted,  different 
colour  values  are  obtained.  Potassium  permanganate  solution  only  shows  a 
very  slight  decrease  in  colour  on  dilution.  Taken  as  a  whole  ferric  salts  show 
the  greatest  decrease  in  colour  effect  on  dilution  of  their  solutions,  then  cobalt 
salts,  then  uranium  salts,  and  then  nickel  and  cupric  salts.  Generally  also 
organic  salts  show  the  most  decrease  of  colour  on  dilution,  chlorides  less, 

nitrates  still  less,  and  sulfates  the  least On  comparing  solutions  of 

different  salts  together  as  to  colour  effect,  it  is  found  that  the  values  obtained 
bear  considerable  resemblance  to  the  amounts  of  decrease  of  colour  the  solu- 
tions undergo  on  dilution,  organic  salts  possessing  the  greatest  colour 
effect,  chlorides  a  lesser  effect,  sulfates  and  nitrates  a  still  smaller  colour 
effect." 

"  From  the  table  [Zoc.  ci«.]  it  will  be  seen  that  the  solutions  of  the  five 
copper  salts  examined  decrease  in  colour  effect  on  dilution,  the  sulfate  the  least, 
then  the  chloride,  nitrate,  and  lastly  the  acetate  which  decreases  very  greatly 
on  dilution Nickel  salts  show  much  less  colour  decrease  than  cop- 
per salts,  neither  the  sulfate  nor  the  chloride  showing  more  than  a  barely  ap- 
preciable decrease  at  a  dilution  of  ten  litres  [a  normal  solution  diluted  ten 
times].  All  the  cobalt  salts  dissociate  considerably  more  than  the  corre- 
sponding salts  of  both  nickel  and  copper;  but  in  this  case,  though  the  sulphate 
is  the  least  dissociable,  it  is  the  chloride  and  not  the  nitrate  that  is  the  most 
dissociable.  The  same  thing  holds  for  uranyl  salts,  the  sulphate  being  less 
dissociable  than  the  nitrate,  and  this  again  than  the  chloride.  These  salts  dis- 


T  Chem.  News,  1892-2-105, 152  and  198. 


NOTES    ON   THE   METHODS    OF   ANALYSIS. 

sociate  to  a  greater  extent  than  copper  salts,  but  not  so  much  as  cobalt  salts. 
The  acetate  of  uranium  solution  however,  shows  a  very  different  behavior 
from  that  of  acetate  of  copper,  for  it  is  the  least  dissociated  of  any  of  the  uranyl 
salts  examined." 

"  Chromium  salts  gave  very  extraordinary  results,  for  the  colour  effect  of  their 
diluted  solutions  was  found  to  depend  on  the  method  of  dilution,  according 

as  it  was  sudden  or  gradual It  will  be  seen  that  [on  gradual  dilution] 

chromic  sulfate  solution  decreases  slightly  in  colour  effect  till  a  dilution  of 
200  litres,  but  from  that  point  it  begins  to  increase  again,  till  at  1000  litres  it  is 
considerably  greater  than  the  decinormal  solution Such  extraordi- 
nary behavior  can  only  be  explained  on  the  supposition  that  hydrates  of  dif- 
ferent composition  and  different  colour  effect  are  formed,  according  to  the 
manner  of  dilution.  Iodine  dissolved  in  potassium  iodide  solution  acted  in  a 

similar  manner  to  chromic  salts The  results  as  a  whole  indicate  that 

dissociation  takes  place  in  solutions  on  the  one  hand,  and  hydrates  are  formed 
on  the  other,  so  it  would  seem  that  salts  exist  in  solution  in  both  these  condi- 
tions at  the  same  time". 

ts  The  general  conclusions  to  be  drawn  from  the  changes  in  colour  attending 
the  dilution  of  solutions  of  coloured  salts  are,  that  almost  all  salt  solutions 
show  a  decrease  in  colour  effect;  but  a  few,  notably  several  organic  salts  of 
iron,  show  a  considerable  increase  in  colour.  It  is  difficult  to  find  an  adequate 
explanation  for  these  changes  except  on  the  ground  of  electrolytic  dissocia- 
tion  The  changes  in  colour  effect  of  solutions  of  salts  on  dilution 

are  evidently  due  to  more  than  one  cause.  As  will  be  shown,  the  formation  of 
hydrates  will  account  for  them  in  part.  They  cannot  be  wholly  accounted  for 
in  this  way,  however.  Electrolytic  dissociation  seems  to  be  the  only  ground 
left  on  which  an  explanation  is  possible  ". 

The  practical  deductions  from  the  foregoing  are  that  (1),  the  concentrations 
of  the  standard  and  sample  as  regards  the  colorant  should  be  practically  iden- 
tical at  the  time  of  final  comparison,  and  (2),  that  the  compositions  as  regards 
the  other  constituents  should  not  greatly  differ.  The  first  condition  can  be 
nearly  enough  approached  by  compounding  as  indicated  by  the  result  of  a  pre- 
liminary test,  and  the  latter  by  suitable  additions  to  the  standard  or  sample  or 
removal  of  associates  specific  to  cither  —  this  precaution  however  is  in  many 
cases  practically  unnecessary.  It  must  be  remembered  that  in  practical  colori- 
metry  the  subject  is  not  usually  a  pure  chromogen  and  hues  rather  than  simple 
colors  are  to  be  compared. 

Lovibond*  proposes  that  curves  be  plotted  from  the  readings  of  his  tinto- 
meter for  colored  liquids  at  varying  concentrations,  when  it  will  be  found  that 
each  liquid  will  generate  a  specific  curve.  In  different  samples  of  a  commercial 
article  much  may  be  learned  from  the  divergencies  of  the  respective  curves  as 
to  their  relative  character  and  value  for  a  given  purpose ;  thus,  the  results  given 
by  malts  in  brewing  "  can  be  predicted  almost  with  certainty,  and  this  is  true 
of  other  substances  whose  curves  have  been  thoroughly  worked  out".  The 
curves  plotting  the  inharmonious  progression  of  depth  and  density  of  solutions 
of  dye-stuffs  are  said  to  be  characteristic  and  to  serve  for  distinguishing 
different  dyes. 

Unlike  volumetric  analysis,  it  would  appear  that  the  accuracy  of  colori metric 
determinations  is  generally  underestimated.  It  is  true  that  the  neglect  of  such 
precautions  as  have  been  pointed  out  will  inevitably  vitiate  the  results,  and 
that  certain  chromatic  bodies  are  unsuited  for  comparison  by  reason  of  fluores- 


*  Journ.  Socy.  Ohem.  Ind.  1898—207. 


542  QUANTITATIVE    CHEMICAL    ANALYSIS. 

cence  or  dichroism,  yet  as  technical  methods  go,  under  reasonable  conditions 
and  care,  the  results  are  quite  exact  enough  for  practical  ends.  The  limits  of 
accuracy  under  favorable  conditions  have  been  variously  stated  as  lf  an  aver- 
age of  one  per  cent",  "two  per  cent  or  less",  and  by  a  less  conservative 

writer  "  as  close as  one-tenth  of  one  percent".    In  assigning  limits 

it  must  of  course  be  taken  into  consideration  that  in  comparison  of  colors  no 
two  visions  are  of  exactly  the  same  acuteness ;  still  there  are  comparatively 
few  who  cannot  distinguish  fairly  closely  related  depths  or  shades  of  a  color. 

In  consequence  of  the  highly  tinctorial  qualities  of  many  bodies,  either 
inherent  or  that  can  be  generated  or  enhanced  by  some  chemical  transforma- 
tion, colorimetric  methods  can  often  be  applied  to  smaller  weights  than  could 
possibly  suffice  for  methods  of  other  kinds.  It  is  also  well  adapted  to  the 
determination  of  small  proportions  of  colorific  metals,  as  a  suitable  standard 
is  easily  prepared  from  the  pure  metal  or  one  of  its  definite  compounds. 

ATTRIBUTIVE   METHODS. 

Frequently  in  technical  and  sometimes  in  scientific  analysis,  one  meets  with 
mixtures  where  an  actual  separation  of  the  constituents  is  difficult  or  impos- 
sible by  any  known  means,  and  recourse  is  had  to  methods  based  on  the 
determination  of  some  physical  or  chemical  constant  of  the  mixture,  the 
resultant  of  a  differential  common  constant  of  the  constituents.  And  outside 
of  the  many  instances  where  it  is  his  sole  recourse,  in  many  cases  the  scheme 
is  so  profitable  in  point  of  time,  labor  or  attention,  that  the  chemist  will  adopt 
it  in  preference  to  a  direct  method. 

From  the  similarity  of  the  calculations  involved,  methods  based  on  the 
determination  of  a  common  constituent  of  two  admixed  bodies  are  included 
under  attributive  methods  in  Chapter  7. 

For  practical  purposes  mixtures  of  two  or  three  analogous  bodies  may  be 
divided  into  three  classes:  (1),  those  where  the  constant  of  any  mixture  varies 
directly  with  the  proportions  of  the  constituents;  in  other  words,  that  the 
curve  whose  ordinates  represent  units  of  the  constant,  and  abscissae  the 
relative  percentages  of  the  constituents  of  the  mixture  is  a  straight  line  or 
practically  so;  (2),  where  from  inter-reaction,  the  constants  of  the  mixtures 
do  not  vary  directly  with  the  ratios  of  the  constituents;  and  (3) ,  where  for  a 
limited  space  the  curve  of  (2)  is  practically  a  straight  line. 

The  laws  governing  the  inter-reaction  of  mixtures  are  but  imperfectly  un- 
derstood and  as  we  cannot  confidently  forecast  into  which  of  the  above  classes 
a  given  mixture  will  fall,  direct  experiment  must  be  resorted  to  for  positive 
information.  In  (2)  and  (3)  the  curve  is  distinctive  for  each  binary  mixture 
and  is  only  to  be  plotted  from  sufficiently  extended  data. 

In  complex  mixtures  there  is  often  some  uncertainty  as  to  whether  the  result- 
ant of  the  constants  of  two  bodies  is  not  modified  by  the  presence  of  some 
associate  in  the  mixture.  This  interference  has  sometimes  been  confounded 
with  an  inter-reaction  of  the  active  bodies  themselves,  the  correct  reason  ap- 
pearing only  on  examination  of  carefully  prepared  pure  mixtures. 

The  general  formula  (page  155)  for  a  binary  mixture  is  X  =  100          ,    where 

a  —  6 

Xis  the  percentage  of  one  constituent;  a  and  b  the  constants  of  the  constitu- 
entSj  and  d,  the  constant  of  the  mixture.  On  the  presumption  that  the  three 
values  can  be  determined  with  absolute  accuracy,  it  is  immaterial  how  little 
a  and  6  differ.  But  as  there  is  always  some,  and  frequently  a  considerable  in- 
accuracy, the  more  divergent  these  values  the  less  is  X  affected  by  an  error  on 


NOTES    ON  THE    METHODS    OF    ANALYSIS.  543 

any  of  the  three;  thus,  the  lesser  value  &  being  constant,  d  increases  in  pro- 
portion with  a,  and  the  numerical  va'lues  of  the  terms  of  the  fraction  _  become 

a 

correspondingly  greater  in  relation  to  —  ;  hence  an  equal  error  in  the  determi- 

0 

nation  of  d  is  of  less  importance. 

In  class  (2)  experiments  determining  the  amount  of  divergence  of  d  from 
the  calculated  values  will  decide  to  what  extent  interpolation  can  be  carried 
compatible  with  fair  results.  Where  a  and  6  differ  considerably,  at  a  certain 
range  in  the  proportions  of  a  mixture  the  consecutive  values  of  d  may  differ  so 
slightly  that  no  confidence  can  be  placed  in  the  results.  This  range  may 
be  but  small,  or  may  cover  nearly  all  percentages  of  one  constituent.  Usually 
the  curve  of  the  constants  traced  by  consecutive  proportions  of  the  constit- 
uents becomes  far  more  abrupt  near  one  or  both  ends,  though  the  reverse  is 
sometimes  true. 

Errors  in  fixing  the  constants,  outside  of  the  ordinary  analytical  deviations, 
may  arise  from  the  difficulty  of  obtaining  the  two  constituents  in  a  pure  state, 
or  from  the  variance  due  to  their  being  (1),  of  different  origin  or  changed  by 
age,  mode  of  preservation,  degree  of  refinement,  etc.;  (2),  both  members  of  a 
group  of  allied  bodies  that  possess  certain  common  characteristics  complicat- 
ing the  determinations;  or  (3),  each  constituent  itself  a  complex  mixture  of 
somewhat  indefinite  composition,  where  the  constants  may  on  the  one  hand  be 
confined  to  quite  narrow  limits,  the  mean  of  the  individual  members,  or  on  the 
other  hand  the  variations  be  so  great  as  to  make  an  attributive  method  value- 
less. In  dealing  with  any  of  the  above  the  chemist  will  be  prudent  if  he  per- 
sonally determines  the  constants  of  a  and  b  under  the  same  conditions  and 
npon  the  same  materials  (as  far  as  possible)  that  form  the  mixture  in  question, 
rather  than  depend  upon  the  data  of  others.  In  natural  products  where  the 
value  of  a  or  6  is  subject  to  large  variations,  it  is  also  important  that  the 
chemist  learn  whether  the  sample  is  a  mixture  of  but  two  individual  bodies,  or 
if  one  constituent,  or  both,  is  itself  a  composite,  in  the  latter  case  accepting 
only  the  mean  of  several  concordant  determinations  made  on  different  parts  of 
the  sample. 

The  identity  of  an  unknown  compound  or  mixture  that  may  be  any  one  of 
a  group  or  class  of  allied  bodies  may  be  inferred  from  a  comparison  of  the 
constants  with  tables  compiled  from  previous  observations  for  members  of 
that  class  (page  156.)  A  deduction  is  the  more  dependable  (1),  the  less  com- 
plex is  the  body  in  question  and  the  less  its  properties  are  dependent  on  the 
past  history  of  the  body;  (2),  the  greater  the  number  of  different  constants 
that  can  be  determined  and  compared,  and  their  reasonably  close  agreement 
with  established  data;  (3),  the  more  nearly  the  material  operated  on  and  the 
details  of  the  determinations  approach  those  of  the  compilers  of  the  tables  of 
constants. 

As  a  class,  attributive  methods  afford  reliable  and  fairly  accurate  results  on 
mixtures  of  two  active  bodies  alone  or  on  one  or  two  active  bodies  in  presence 
of  others  that  have  an  absolute  or  practical  zero-value  for  the  selected  prop- 
erty. Theoretically  the  proportions  of  a  mixture  of  n  bodies  can  be  cal- 
culated from  n  —  1  properties,  but  practically  no  higher  than  a  ternary  mixture 
is  attempted  on  account  of  the  great  number  of  determinations  required  for  the^ 
calculations  with  the  liability  of  some  one  or  more  being  incorrect;  moreover 
for  a  given  material  there  can  but  rarely  be  found  more  than  two  properties 
whose  constants  are  at  once  sufficiently  divergent  and  exactly  measureable. 

An  advantage  over  other  classes  of  methods  is  that  in  most  physical  methods 


544  QUANTITATIVE    CHEMICAL    ANALYSIS. 

but  a  comparatively  small  amount  of  material  is  needed  for  a  determination 
and  that  it  remains  intact  and  available  for  further  tests,  a  feature  appreciated 
where  the  supply  of  material  furnished  the  chemist  is  limited  by  reason  of  its 
rarity  or  high  cost;  also  where  the  integrity  of  the  material  in  its  original 
form  is  not  to  be  disturbed  or  where  it  cannot  be  removed  from  an  established 
locality  —  here  certain  physical  methods  (e.  g.t  electrical  conductivity)  can 
sometimes  be  applied.  But  unless  the  material  is  known  to  be  homogeneous 
one  must  guard  against  an  illusive  conclusion  by  making  several  determina- 
tions on  different  parts  of  the  material. 


Of  the  errors  that  may  be  incurred  during  an  analysis  some  are  observed  at 
once,  others  may  pass  unnoticed  should  the  attention  of  the  operator  be 
diverted  .for  the  moment,  and  can  only  be  located,  if  at  all,  by  some  palpably 
erroneous  result  manifest  after  the  analysis  is  finished.  It  is  within  the  ex- 
perience of  every  chemist  that  at  times  the  result  of  an  analysis  is  unaccount- 
ably so  far  from  what  was  expected  that  he  is  puzzled  to  account  for  the 
cause,  unaware  of  any  slip  on  his  part  that  could  explain  the  failure. 

Both  personal  errors  and  those  inherent  to  the  method  may  be  such  that  a 
loss  during  one  operation  will  be  measureably  compensated  by  a  gain  in 
another  and  the  final  result  be  not  greatly  vitiated;  but  it  is  more  usual  that 
they  tend  in  the  same  direction  and  are  cumulative.  And  it  must  be  remem- 
bered that  the  effect  of  a  loss  or  gain  in  any  one  operation  may  not  be  confined 
to  the  result  on  a  single  constituent  but  extend  to  subsequent  determinations 
as  well. 

The  operative  errors  have  been  discussed  in  Chap.  9 ;  let  us  consider  those 
arising  from  other  sources. 

1.  The  material  for  analysis  may  be  of  a  composition  not  covered  by  the 
method.  The  percentage  of  the  constituent  determined  may  be  greater  or  less 
than  was  contemplated  by  the  deviser;  some  exceptionally- occurring  interfer- 
ing body  may  be  contained,  or  one  absent  that  is  essential;  again,  the  physical 
properties  of  some  one  of  a  class  of  bodies  may  so  largely  differ  from  others  as 
to  exclude  methods  dependent  to  any  degree  on  the  constancy  of  the  physical 
condition  of  the  substance. 

Many  methods  of  quantitative  analysis  are  adaptations  of  qualitative  tests. 
Where  the  presence  of  an  element  is  manifested  by  the  formation  of  a  precip- 
itate with  a  particular  reagent  it  may  be  presumed  that  the  reaction  can  be 
applied  for  the  determination  and  perhaps  the  separation  of  the  element;  or 
an  exact  reaction  may  be  made  the  basis  of  a  volumetric  assay.  Yet  there  are 
many  reactions  unsuited  to  this  conversion,  disqualified  by  reasons  of  less 
or  no  consequence  in  qualitative  analysis;  thus,  indefinite  composition  of 
a  precipitate,  incomplete  insolubility,  a  state  of  agglomeration  opposing  easy 
filtration  and  washing,  alteration  on  heating;  in  colorimetry,  a  color  developed 
that  is  transient  or  unsuited  for  comparison;  etc. 

A  quantitative  should  be  preceded  by  a  complete  qualitative  analysis  that 
one  may  proceed  with  the  separations  and  determinations  in  a  methodical  way; 
in  many  cases  it  is  imperative  for  assurance  of  the  absence  of  whatever  might 
interfere  with  the  course  reactions.  Under  certain  circumstances,  however,  a 
qualitative  examination  can  be  abridged  or  entirely  omitted;  thus 

A.  The  composition  or  nature  of  the  commonly  occurring  impurities  may  be 
known  as  nearly  as  need  be  from  the  origin  or  nature  of  the  substance,  as  in 
minerals,  commercial  salts,  alloys,  drugs.  The  habitus  or  physical  character- 
istics—  color,  taste,  transparency,  fluidity,  odor,  peculiarities  of  aggregation, 


NOTES    ON   THE   METHODS    OF    ANALYSIS.  545 

etc.,  may  indicate  certain  associates  or  exclude  those  but  occasionally  met 
with.  In  proportion  as  one  becomes  conversant  with  chemical  technology  can 
he  pronounce  with  confidence  on  the  nature  and  composition  of  a  complex 
commercial  body,  and  in  doubtful  cases  decide  from  a  few  special  tests  what 
otherwise  could  only  be  learned  from  a  systematic  examination. 

B.  The  composition  of  an   artificial  mixture  may  be  inferred  from  the  con- 
stitution of  other  articles    employed    for  the  same  or    a    similar  purpose; 
many  classes  of  commercial    articles  designed  for  special  applications   are 
characterized  by  a  general  similarity  in  composition. 

The  limit  of  the  cost  of  production  or  manufacture  of  a  commercial 
article  as  fixed  by  competition  forbids  the  admission  of  the  more  expensive 
ingredients  or  those  that  entail  manipulations  of  an  extensive  or  unusual 
kind.  And  adulteration  or  an  inferior  or  debased  quality  is  to  be  suspected 
when  an  article  is  offered  at  a  price  considerably  below  that  ruling  on  the 
open  market.  Practically,  adulteration  is  only  practiced  where  the  cost  of 
production  can  be  so  far  reduced  that  the  profit  is  a  sufficient  inducement  to 
risk  detection  and  exposure,  except  cases  where  the  difficulty  of  distinguish- 
ing the  adulterant  from  the  genuine  makes  sophistication  a  comparatively 
safe  proceeding. 

C.  The  substance  may  have  been  subjected  to   some  physical  or  chemical 
treatment  —  e.  g.t  evaporation,   fusing,  washing,   sublimation,  aeration— that 
would  perforce  have  removed  certain  bodies  had  they  been  originally  present. 
Again,  that  a  certain  compound  is  contained  in  a  mixture  may  be  prima  facie 
evidence  against  the  co- existence  of  certain  other  bodies  that  would  act  chem- 
ically or  mechanically  on  the  former,  or  modify  some  of  its  manifest  qualities; 
per  contra,  there  may  be  implied  or  suggested  an  associate  essential  to  the 
integrity  of  the  first  or  conferring  or  enhancing  some  particular  property,  or 
as  a  coadjuvant.     How  often  are  these  truisms  overlooked! 

D.  Some  simple  preliminary  operation  may  reveal  the  constitution  as  nearly 
as  need  be.    Thus,  the  mixture  may  be  treated  with  a  diluent  or  a  selective 
solvent  to  take  up  some  constituent  that  by  its  mass,  opacity,  or  intense  color 
obscures  its  associates.    If  the  constitution  of  the  residue  cannot  be  easily 
recognized  on  inspection  by  the  shape,  color,  or  other  peculiarities  of  the  par- 
ticles, it  may  be  examined  under  the  microscope,  separated  into  layers  by 
agitation  with  water,  or  otherwise. 

E.  In  an  assay  a  specific  reagent  or  some  physical  process  may  be  applied 
that  will  affect  only  the  body  it  is  desired  to  determine,  or  perhaps  a  few  others 
also  that  may  afterward  be  looked  for  and   separated;  or  a  certain  operation 
may  withdraw  all  or  nearly  all  of  the  other  constituents  of  the  mixture. 

But  in  drawing  a  conclusion  from  any  of  the  above,  due  caution  will  often 
save  time  and  trouble,  for  it  is  well  to  recognize  the  possibility  of  the  presence 
of  what  is  least  suspected. 

2.  The  method,  designed  only  for  rapid  approximate  work,  may  fail  where 
greater  accuracy  is  essential.    The  temptation  to  adopt  such  methods  for  im- 
portant determinations  is  greatest  in  industrial  analysis  where  the  chemist  is 
often  burdened  with  more  than  he  can  properly  attend  to  in  the  limited  time 
allowed,  but  while  the  task  may  be  lightened  thereby,  the  substitution  is  always 
attended  with  hazard  and  may  at  any  time  prove  a  source  of  anxiety,  perhaps 
humiliation. 

3.  The  tables  of  the  atomic  weights  of  the  elements  are  periodically  revised 
to  conform  with  later  and  presumably  more  reliable  data.     With  each  revision 
the  values  assigned  to  some  of  the  elements  are  more  or  less  altered  —  the 
common  elements  but  slightly  if  at  all  —  and  an  analysis  calculated  accord- 

35 


546  QUANTITATIVE    CHEMICAL    ANALYSIS. 

ing  to  the  figures  of  different  tables  may  show  somewhat  different  re- 
sults. In  a  few  instances,  calculations  from  the  most  divergent  of  the  figures 
on  one  or  more  elements  may  show  considerable  variations.*  But  generally 
the  difference  is  of  less  moment  than  might  be  supposed,  for  as  the  determina- 
tion of  an  atomic  weight  follows  the  lines  of  a  quantitative  analysis,  whenever 
there  is  available  for  an  element  a  method  of  analysis  so  exact  that  the  result 
would  be  vitiated  by  a  slight  change  in  the  atomic  weights  concerned  in  the 
calculations,  it  will  be  found  that  a  correspondingly  exact  process  has  been 
used  for  the  determination  of  the  latter,  and  the  more  recent  values  are  almost 
identical.  Conversely,  when  recent  values  for  any  element  markedly  differ, 
the  inference  is  that  no  exact  methods  are  known  for  the  analysis  of  bodies 
containing  it.  However,  it  is  always  the  best  plan  to  compute  according  to  the 
latest  authorized  data. 

4.  Exceptionally  a  determination  may  be  complicated  by  certain  abnormal 
manifestations,  phenomena  apparently  at  variance  with  chemical  laws,  or  by 
some  irregular  or  capricious  deportment  manifest  under  ordinary  working 
conditions. 

It  is  well  known  that  under  certain  conditions  confined  within  narrow  limits 
issue  precipitates  of  anomalous  composition  or  aggregation  or  segregating 
more  slowly  than  is  usual;  that  in  a  solution  there  may  exist  between  two 
elements  a  state  of  libration  immediately  overset  by  the  introduction  of  a 
minute  excess  of  either,  or  a  reaction -lag  terminated  by  the  addition  of  a  trace 
of  one  of  the  products;  that  reactions  commonly  expressed  by  a  single  equa- 
tion are  not  exhibited  as  an  instantaneous  transposition  but  as  several  frac- 
tions or  installments,  a  major  preceded  or  followed  by  one  or  several  minor; 
and  that  the  normal  conduct  of  a  reaction  may  be  profoundly  modified  by  the 
presence  of  an  apparently  inconsiderable  amount  of  a  foreign  body  uncon- 
nected with  the  reaction. 

In  organic  analyses  there  must  be  recognized  the  influence  of  some  prelim- 
inary treatment,  mode  of  extraction  of  a  constituent,  age,  etc.,  to  hinder  com- 
plete precipitation  or  prevent  it  entirely;  and  it  must  be  remembered  that  these 
influences,  of  the  greatest  importance  in  the  determination,  may  not  be  mani- 
fested to  such  an  extent  as  to  prevent  recognition  of  the  body  in  the  previous 
qualitative  examination. 

The  physical  constants  of  certain  bodies  may  be  largely  altered  by  a  tem- 
porary change  in  the  molecular  arrangement,  not  returning  to  the  normal  for 
many  hours  or  even  days.  Certain  combinations  of  a  metal  with  animal  mat- 
ter, induced  by  passing  through  the  living  body  or  by  close  contact  with  the 
tissues  immediately  after  death,  exhibit  abnormal  reactions,  and  it  is  said  that 
the  chemical  activity  of  proteids  of  certain  living  animal  matter  is  far  more 
intense  than  when  derived  from  the  cadaver. 

The  mutual  action  of  certain  admixed  bodies  to  lessen  the  respective  solu- 
bility or  insolubility  in  a  solvent  is  well  known.  Thus,  in  the  separation  of 
the  rare  earths,  some  are  insoluble  in  a  solution  of  potassium  sulfate,  others 
freely  soluble,  but  when  members  of  each  class  are  in  admixture  the  sharp  line 
of  demarcation  is  suppressed,  some  minor  form  of  adhesion  appears,  and  only 
imperfect  results  can  be  had  in  one  operation.  Zirconia  and  titanic  acid  con- 
siderably modify  the  reactions  of  each  other,  as  do  ruthenium  and  iridmm;  in 
a  mixture  of  the  two  alkaloids,  quinine  is  less  and  cinchonidine  more  soluble 
in  ether  than  when  alone.  Paraffin  (parum-affinis),  so  indifferent  to  reagents 
even  when  highly  subdivided,  is  noticeably  acted  on  when  diluted  with  bees- wax. 


*  Chem.  News,  1896—2—143;  Crookes  Select  Methods,  16. 


NOTES   ON   THE   METHODS   OF    ANALYSIS.  547 

The  same  effect  may  be  noted  where  the  process  of  solution  involves  a  mani- 
fest reaction;  platinum  is  insoluble  in  nitric  acid,  yet  when  alloyed  with  a  large 
proportion  of  silver  both  dissolve  readily  and  completely;  the  conversion  of 
strontium  sulfate  to  carbonate  is  prompt  and  thorough  when  alone,  but  less  so 
if  in  admixture  with  barium  sulfate,  while  at  the  same  time  the  latter  body  is 
acted  on  to  some  extent ;  etc. 

And  generally,  there  must  be  recognized  the  influence  of  associated  bodies 
and  their  derivatives  to  modify  the  chemical  activity  of  reacting  elements  and 
disturb  the  normal  exhibition  of  the  reactions,  even  to  induce  a  reaction  the 
reverse  of  the  normal,  as  an  oxidation  instead  of  a  reduction. 

Phenomena  such  as  mentioned  ^above  are  so  striking  as  to  challenge  the  notice 
of  even  the  most  inattentive,  yet  who  can  say  where  may  hot  exist  the  same  or 
like  influences  effective  to  a  lesser  degree,  unnoticeable  though  not  to  be 
ignored,  and  perhaps  where  least  expected?  It  is  more  than  possible  that  many 
of  the  irregular  manifestations  that  at  one  time  or  another  perplex  every 
analyst  can  be  charged,  in  part  at  least,  to  influences  like  these  whose  exhibi- 
tion ordinarily  is  minimized  or  suppressed  by  unfavorable  conditions,  and 
be  readily  explainable  were  the  extent  of  the  influences  and  the  conditions 
favoring  their  assertion  but  known.  Even  in  the  extreme  case  of  a  synthetic 
proof,  though  so  compounded  as  to  differ  but  little  from  its  natural  prototype, 
may  yet  be  affected  by  the  unavoidable  variation  in  composition  to  a  degree 
greater  than  the  total  of  the  errors  it  is  intended  to  measure. 

In  any  event,  the  wisdom  of  proceeding  cautiously  when  dealing  with  un- 
familiar material  is  apparent. 

5.  In  this  connection  may  be  considered  the  personal  equation  of  the  chemist, 
his  interpretation  of  general  directions,  or  certain  peculiarities  of  manipulation 
that  have  become  habitual.  Though  in  the  main  of  small  import,  they  may  at 
times  seriously  affect  the  outcome  of  an  analysis;  for  example,  an  unqualified 
direction  to  concentrate  a  liquid  by  evaporation  would  be  understood  by  some 
to  allow  a  brisk  boiling  down,  while  others  would  restrict  the  heat  to  that  of 
the  water  bath,  with  not  infrequently  a  marked  difference  in  the  composition 
of  the  resulting  syrup;  the  same  is  true  of  many  other  operations. 

It  is  frequently  remarked  that  a  chemist  will  hold  to  methods  regarded  by 
most  as  antiquated  and  inferior  to  those  in  more  general  use.  That  they  are 
equally  successful  in  his  hands  can  be  attributed  to  certain  peculiarities  of 
manipulation  or  a  close  attention  to  details  apparently  so  insignificant  as  to  be 
overlooked  by  others,  but  of  whose  importance  he  is  aware  through  long  ex- 
perience. Doubtless  conservatism  may  be  carried  to  an  extreme  in  some  cases, 
nevertheless  it  is  soon  learned  that  in  analysis  as  elsewhere,  novelty  does  not 
always  imply  superiority. 

The  question  may  arise  as  to  how  far  it  is  advisable  for  the  beginner  to 
conform  his  modes  of  manipulation  to  the  directions  of  a  text- book  or  to 
imitate  the  practice  of  one  more  experienced.  That  a  chemist  of  undoubted 
skill  performs  a  certain  operation  of  the  routine  of  analysis  in  a  particular 
way,  insists  on  the  adoption  of  seemingly  unimportant  precautions,  or  does 
not  hesitate  to  neglect  measures  that  are  commonly  believed '  the  part  of  pru- 
dence and  without  apparent  detriment  to  the  correctness  of  his  work,  does  not 
imply  that  the  more  usual  practices  should  be  renounced,  but  rather  illustrates 
the  flexibility  of  analytical  practice  in  general,  the  possibility  of  a  wide  vari- 
ance in  details  compatible  with  satisfactory  results.  And  while  the  student 
must  perforce  follow  the  standard  practice  to  a  great  extent  he  should  never- 
theless endeavor  to  individualize  his  work  as  far  as  may  be,  always  with  the 
view  of  reducing  mechanical  or  other  errors  to  a  minimum.  Of  course  where 


548  QUANTITATIVE    CHEMICAL    ANALYSIS. 

specific  directions  are  laid  down  in  a  scheme  of  analysis  it  is  presumed  that 
the  author  had  good  reasons  for  their  interjection  and  it  is  well  to  follow 
them  without  deviation — at  least  when  it  is  not  considered  worth  the  while 
to  investigate  as  to  their  necessity. 

6.  Finally,  the  method  itself  may  be  at  fault,  and  this  more  often  than 
would  be  expected. 

As  regards  the  quality  of  accuracy  it  may  be  said  that  the  capabilities  of  the 
ordinary  methods  are  more  apt  to  be  overestimated  than  underestimated. 
For  it  must  be  remembered  that  a  new  method  is  judged  mainly  by  the  test 
analyses  appended  to  the  description  of  the  method.  These  analyses,  intended 
to  support  the  claim  of  the  method  to  consideration,  are  always  made  under 
the  m»st  advantageous  auspices  in  all  respects;  yet  too  often  only  the  most 
favorable  of  the  results  obtained  appear,  not  that  the  suppression  of  those 
less  flattering  was  done  to  conceal  the  shortcomings  of  the  method,  but 
rather  from  the  natural  proneness  of  the  inventor  to  discover  some  flaw  in 
manipulation  or  elsewhere  sufficient  to  warrant  their  omission.  Later,  the 
few  who  are  in  a  position  to  critically  examine  the  method  and  compare  it  with 
others  may  not  feel  called  on  to  express  their  conclusions  regarding  it,  or  if  so, 
their  criticisms  may  not  obtain  the  publicity  accorded  the  original  descrip- 
tion. 

Usually  a  method  for  the  analysis  of  an  inorganic  body  or  a  substance  of 
commercial  value  will  sooner  or  later  be  judged  on  its  merits  alone  and  take 
its  place  among  those  having  the  confidence  of  analysts  in  general,  or  join  the 
vast  array  of  the  doubtful  or  condemned.  Less  readily  classified  are  those 
which  measure  not  a  stable,  active,  definite  compound  or  element  but  some  in- 
definite, complex,  or  ill-understood  principle,  methods  without  pretensions  of 
determining  a  single  chemical  compound,  but  the  results  expressing  only  the 
relative  value  of  the  substance  analyzed  —  one  of  this  class  is  often  the  subject 
of  extended  and  perhaps  acrimonious  controversy. 

Again,  a  technical  method  may  be  unsuited  to  the  material  in  hand,  it  having 
been  designed  to  cover  only  a  special  substance  or  class  of  substances  of  which 
an  analysis  is  less  frequently  called  for  either  on  account  of  a  limited  use  in 
the  arts  or  that  the  constituent  proposed  to  be  determined  has  but  a  small  in- 
fluence on  the  physical  properties  or  commercial  value  of  the  substance  con  - 
taining  it,  and  although  excellent  from  every  point  of  view,  the  method  does 
not  attain  the  publicity  reached  by  those  of  a  broader  scope  and  therefore  is 
less  likely  to  attract  the  attention  of  those  who  would  modify  it  to  answer  for 
other  and  more  general  purposes. 

Of  the  defects  sufficient  to  condemn  a  method  for  practical  use,  perhaps  the 
most  common  can  be  charged  to  the  fact  that  the  originator,  fearful  as  to  his 
title  to  priority,  has  hastened  to  announce  his  invention  before  an  exhaustive 
examination  has  confirmed  its  worth,  claiming  general  applicability  from 
results  on  but  a  special  class  of  bodies;  assuming  that,  with  a  few  modifica- 
tions, a  method  known  to  be  qualified  for  the  analysis  of  one  substance  will  be 
equally  suitable  for  a  similar  one,  and  taking  no  cognizance  of  the  effect  of 
associated  constituents.  To  what  extent  an  associate  may  exert  an  influence 
on  the  reactions  of  others  has  already  been  stated. 

In  other  instances  the  deviser  has  apparently  assumed  that  methods  will 
always  admit  of  reversal,  the  reagent  becoming  the  compound  determined,  or 
that  specific  methods  of  determination  can  invariably  be  applied  for  separation, 
and  has  considered  a  practical  trial  superfluous.  Absurdities  are  often  the 
issue  of  this  expedient  of  writing-desk  invention;  as  an  example  may  be  cited 
the  directions  formerly  laid  down  by  a  standard  work  on  pharmaceutical 


NOTES    ON    TH«E    METHODS    OF    ANALYSIS.  549 

practice  for  determining  the  purity  of  commercial  oxalic  acid  by  titration  with 
standard  potassium  permanganate,  while  under  the  directions  for  standardizing 
the  latter,  advised  the  use  of  commercial  oxalic  acid!  Such  an  oversight  could 
hardly  have  remained  undetected  had  but  a  single  trial  been  made.  Certainly 
what  is  worth  publishing  is  worth  at  least  one  confirmation. 

Again,  variations  in  the  physical  structure  of  a  body  may  greatly  modify  its 
solubility  or  decomposibility,  certain  specimens  yielding  readily  to  a  solvent 
or  flux,  others  resisting  prolonged  treatment,  and  it  cannot  be  doubted  that 
many  schemes  advised  for  the  resolution  of  a  refractory  body  (chromite,  for 
example)  have  been  tried  only  on  one  of  the  more  easily  decomposed  varieties. 

The  sample  originally  tested,  if  a  natural  product,  may  have  been  freshly 
gathered  and  analyzed  near  the  place  of  its  growth,  or  if  a  manufactured 
article,  tested  immediately  following  production;  other  samples  reaching  a 
Chemist  may  have  been  altered  by  age,  exposure,  or  internal  reactions,  and  if 
analyzed  by  the  same  method,  that  may  be  chemical  or  physical,  do  not  furnish 
equally  satisfactory  results.  And  if  the  proportion  of  the  leading  constituent 
of  a  substance  is  arrived  at  by  a  computation  from  the  proportion  of  some 
constant  associate,  it  is  often  doubtful  whether  the  latter  may  not  have 
been  profoundly  altered  by  the  influences  mentioned  above  or  by  others  equally 
potent. 

Or  granted  that  the  method  is  correct  in  theory  and  practically  serviceable, 
the  published  description  may  be  anything  but  lucid.  Details  that  might 
safely  be  left  to  the  discretion  of  the  chemist  are  iterated  to  the  extent  of 
confusing  and  distracting  his  attention  from  the  more  vital  points  that  are 
consequently  overlooked.  The  importance  of  clearness  and  brevity  in  this 
regard  is  brought  home  most  forcibly  to  whoever  has  occasion  to  search 
through  many  volumes  of  a  journal  —  perhaps  in  an  unfamiliar  tongue — for 
information  on  a  subject  not  appearing  in  their  indices.  It  is  certainly  de- 
pressing to  read  through  pages  of  closely  printed  matter  to  find  at  last 
that  the  prolix  whole  but  describes  a  trifling  modification  of  dubious  worth 
that  might  well  have  been  compressed  to  the  compass  of  a  single  paragraph. 

Or  the  fault  may  lie  in  the  other  extreme;  the  fundamental  principles 
may  not  be  elucidated  with  sufficient  clearness  —  often  they  are  omitted  en- 
tirely, the  writer  evidently  taking  it  for  granted  that  the  reader  is  so 
familiar  with  the  subject  that  their  incorporation  would  be  superfluous, 
forgetting  that  a  thorough  knowledge  of  one  specialty  in  addition  to  a  gen- 
eral acquaintance  with  other  departments  is  all  that  can  be  expected  of  the 
average  analyst;  or  some  important  details  may  have  been  omitted  or 
passed  over  with  a  bare  mention.  Where  brevity  necessitates  ambiguity  or 
the  omission  of  essential  information  we  can  well  tolerate  a  more  diffuse 
style  —  even  "the  almost  epic  breadth  in  which  our  continental  brethren 
indulge  ". 

The  absolute  or  relative  accuracy  of  a  method  for  the  determination  of  a 
constituent  of  a  given  material  may  be  tested  in  several  ways. 

1.  For  a  method  designed  for  the  determination  of  the  constituents  of  a 
chemical  compound,  the  agreement  of  the  results  with  the  percentages  calcu- 
lated from  the  formula  of  the  compound  is  a  measure  of  the  correctness  of  the 
method  as  practiced  under  the  degree  of  skill  and  experience  possessed  by  the 
operator. 

2.  By  synthesis.    A  known  weight  of  the  pure  constituent  to  be  determined 
is  compounded  with  such  others  as  are  contained  in  the  substance  analyzed  in 
approximately  the  same  proportions,  and  an  analysis  made  on  the  mixture  by 


550  QUANTITATIVE    CHEMICAL    ANALYSIS. 

the  method  to  be  tested.  This  scheme  is  the  most  unequivocal  of  any  except 
that  of  (1;,  but  unfortunately  cannot  be  availed  in  all  cases,  for  not  only  must 
the  actual  proximate  composition  be  known  with  certainty,  but  it  is  often  im- 
possible to  reproduce  the  peculiar  combinations  and  physical  conditions  of  the 
original.  As  to  what  extent  this  is  essential  for  the  purpose  in  any  given  case 
must  be  left  to  the  discretion  of  the  chemist. 

Sometimes  the  constituent  to  be  determined  can  be  wholly  extracted  from 
the  substance  without  material  alteration  of  the  remainder,  after  which  a 
known  weight  of  the  pure  constituent  maybe  added  and  the  analysis  proceeded 
with. 

A  plan  frequently  adopted  is  to  make  two  determinations,  one  on  the  sub- 
stance itself,  the  other  after  the  addition  of  a  known  weight  of  the  pure  con  - 
stituent.  The  difference  between  the  two  results  will  approach  the  added 
weight  in  proportion  to  the  accuracy  of  the  method. 

3.  The  most  usual  method    is    that  of  comparison  of  the  results  afforded 
on  a  given  sample  by  the  method  in  question  with  those  by  another  differ- 
ing in  principle  or  at  least  in  conduct,   and    of   good  repute  as  to  accuracy 
and  reliability.    The  conclusions    are  the  more  convincing  where  the  means 
of  the  constant  errors  of  the  two  methods  tend  in  opposite  directions. 

4.  In  a  gravimetric  analysis  the   final   product   as  weighed    is  again  put 
through  the  same  operations  as  in  the  analysis,  then  reweighed,  the  loss  or 
gain  presumed  to  be  in   a    measure    equal   to  that  incurred  in  the  original 
analysis.    But  here  the  modifying  effect  of  the  associates  found  in  the  original 
sample  is  absent,  an  omission  that  may   sometimes  vitiate  the   result  to  a 
greater  extent  than  the  inherent  errors  of  the  method  itself. 

5.  Through  uniformity  of  manipulation  and  details  in  the  conduct  of  duplicate 
analyses,  even  a  highly  defective  method  may  give  results  that  agree  closely 
though  both  far  from  the   truth.    But    if    unequal    (and  preferably  greatly 
differing)  weights  of  the  sample    are   taken    for    the  determination  and  the 
details  varied  where  the  largest  errors  are  supposed  to  lie,  a  fair  idea  of  the 
standing  of  the  method  can  be  argued  in  most  cases. 

6.  In  default    of  any  direct  means     of  comparison,    the  method  may  be 
scrutinized  in  the  light  of  experience  with  other  methods  similar  in  character, 
noting  where  errors  are  most  likely  to  occur  and  their   probable  magnitude. 
It  may  be  possible  to  prove  by  a  few  experiments  that  the  errors  are  inconse- 
quential, or,  taking  into  consideration  the  nature  of  the  substance  in  hand 
and  the  degree  of  accuracy  attainable  by  other  methods,  that  they  can  be  neg- 
lected.   On  the  other  hand,  their  gravity  may  be  so  apparent  as  to    discredit 
the  method,  at  least  for  more  than  approximations. 


It  does  not  always  follow  tha*t  the  directions  formulated  in  the  description 
of  a  method  are  best  suited  to  a  particular  analysis.  Although  for  many 
bodies  we  have  as  yet  only  arbitrary  codes  that  are  to  be  followed  literally, 
yet  the  majority  of  methods  will  allow  some  and  often  a  great  variation  in 
technic  without  lessening  their  accuracy;  and  the  more  generally  applicable  a 
method,  the  greater  the  probability  that  it  can  be  modified  with  advantage  for 
a  given  determination. 

When  one  would  essay  the  emendation  of  the  details  of  a  method  he  should 
bear  in  mind  two  matters  of  importance,  first,  that  one  particular  flaw  may  be 
so  significant  as  to  make  useless  any  effort  to  improve  other  particulars  while 
it  remains,  and  even  when  the  method  is  free  from  any  one  pronounced  defect, 
nevertheless  there  is  always  some  weakest  link,  some  detail  wherein  lies  the 


NOTES    ON   THE    METHODS    OF    ANALYSIS.  551 

greatest  imperfection  or  liability  to  error,  and  to  this  should  his  attention  be 
first  directed;  second,  that  a  change  apparently  advantageous,  may  originate  a 
defect  of  a  different  nature  greater  than  the  one  sought  to  be  rectified.  Yet 
often  are  such  obvious  cautions  neglected. 

And  in  the  application  of  analytical  methods,  along  the  line  of  theoretical 
correctness  there  is  a  point  where  practicability  ends,  a  limit  specific  to  the 
method  and  the  individual,  and  so  important  is  the  ability  to  recognize  this 
boundary  between  the  possible  and  the  feasible  that  it  may  justly  be  deemed 
the  highest  accomplishment  of  the  practical  analyst. 

The  errors  inherent  to  a  method  may  to  some  degree  be  guarded  against  or 
corrected  for  as  follows. 

1.  All  unnecessary  operations  should  be  omitted.    Thus,  by  a  little  foresight 
or  care  the  bulk  of  a  solution  may  be  kept  so  small  that  an  evaporation  can 
often  be  dispensed  with  or  deferred  to  near  the  close  of  the  analysis,  and  the 
losses  or  gains  incurred  in  this  operation  concentrated  on  one  constituent, 
perhaps  one  less  important  than  others,  instead  of  being  distributed  among 
several ;  by  combining  two  ^precipitations,  one  filtration  may  be  dispensed 
with;  etc. 

It  is  not  uncommon  that  the  specific  directions  enumerated  as  being  essen- 
tial to  the  successful  conduct  of  a  method  are  in  reality  but  a  record  of  those 
employed  in  the  few  experiments  made  to  substantiate  the  claims  of  the 
author.  Handed  down  from  one  text-book  to  another,  they  are  regarded  as 
part  and  parcel  of  the  method  until  finally  shown  by  some  skeptical  investi- 
gator to  be  of  no  special  importance  and  often  to  be  changed  with  advantage. 

2.  Certain -manipulations  in  the  course  of  analysis  can  hardly  be  performed 
without  some  mechanical  loss  unless  great  care  is  exercised,  and  in  so  far  de- 
tract from  any  method  that  may  direct  them ;  such  are  the  trituration  of  a 
powder  in  a  mortar  alone  or  with  a  liquid  and  transference  to  and  fro;  mixing 
powders  by  grinding;  the  mechanical  removal  of  a  precipitate  from  a  filter; 
elutriation;  deflagration;  etc.    Though  sometimes  unavoidable,  it  is  well  to 
seek  whether  some  less  hazardous  process  will  not  accomplish  the  same  pur-' 
pose,  for  one  of  these  may  have  been  specified  simply  for  the  reason  that  the 
deviser  was  familiar  with  it  through  practice  in  some  branch  of  practical  chem- 
istry or  pharmacy  where  the  operation  is  of  frequent  occurrence. 

3.  With  equal  errors  in  manipulation,  the  results  by  a  given  method  are  ac- 
curate in  proportion  to  the  amount  of  material  operated  on,  and  frequently  the 
weight  directed  by  a  method  may  be  materially  increased  with  advantage.    In 
certain  cases  however  there  are  valid  reasons  for  taking  a  comparatively  small 
weight;  as  where  the  supply  of  material  is  limited  or  very  costly;  with  mate- 
rials dangerous,  as  high  explosives,  or  offensive ;  where  a  tedious  preparation, 
as  exceptionally  fine  grinding,  is  needed ;  or  where  solutions  must  be  very 
highly  diluted,  or  bulky  precipitates  are  formed. 

4.  A  modification  of  the  routine  for  the  separation  of  the  constituents  of  a 
complex  mixture  may  give  more  accurate  results  than  where  the  usual  sequence 
ia  followed.    Conditions  that  will  decide  whether  the  order  should  be  changed 
are  the  relative  weights  of  the  constituents,  their  relative  importance  for  prac- 
tical purposes,  the  difficulty  or  inconvenience  of  separating  any  two,  the  ac- 
curacy required,  etc.    Similar  considerations  will  decide  whether  a  given  con- 
stituent is  best  determined  as  separated  in  the  regular  course  of  analysis  or  by 
specific  treatment  of  a  separate  portion  of  the  original  material. 

6.  Some  of  the  factors  tending  to  magnify  a  result  may  be  allowed  for  by  run- 
ning a  blank  determination  along  with  the  analysis,  identical  with  it  in  every 
way  except  in  the  omission  of  the  substance  analyzed.  At  first  sight  this  would 


552  QUANTITATIVE    CHEMICAL    ANALYSIS. 

seem  to  offer  a  means  of  correction  comprehensive  and  exact,  yet  its  utility 
is  more  apparent  than  real,  since  it  does  not  as  a  rule  cover  the  faults 
common  to  most  analyses.  Moreover  the  absence  of  the  substance  to  be  an- 
alyzed is  per  se  capable  of  altering  the  conditions  of  the  analysis;  for  example, 
a  flocculent  precipitate  will  mechanically  carry  down  and  retain  colloidal 
matter  which  in  a  blank  determination  would  remain  suspended  in  the  solution. 

A  parallel  or  control  determination  or  essayette  is  far  better  adapted  to  the 
purpose,  especially  in  organic  assaying,  though  even  in  a  synthetic  proof  a 
difference  in  deportment  may  be  shown,  due  to  an  imperfect  admixture  of  the 
constituents  as  compared  with  the  natural  substance,  one  prepared  by  precipi- 
tation, etc. 

6.  The  result  of  a  determination  may  legitimately  be  altered  for  only  those 
errors  whose  nature  and  extent  are  definitely  known  or  for  which  the  proper 
allowance  has  been  previously  ascertained.  In  general,  a  method  that  requires 
the  result  to  be  empirically  corrected  may  be  considered  in  so  far  defective. 

The  correction  for  a  loss  or  gain  in  a  determination  may  be  ascertained  from 
(1),  theoretical  deductions;  (2),  data  secured  by  a  subsequent  operation  either 
of  the  regular  course  of  analysis  or  a  special  test;  (3),  the  loss  or  gain  sus- 
tained by  a  synthetic  proof  or  a  known  weight  of  the  pure  compound,  when 
submitted  to  the  regular  analysis;  (4),  the  loss  or  gain  sustained  when  a 
weighed  precipitate  is  put  through  the  same  process  as  the  original  sample; 
(5),  the  deviation  of  the  result  on  an  average  sample  of  the  substance  from 
that  afforded  on  the  same  sample  when  analyzed  by  another  method.  Of  the 
above,  only  the  first,  depending  on  stoichiometrical  laws,  can  be  considered 
unexceptionable  as  not  being  subject  to  variation  through  the  personal 
equation.  Should  the  correction  be  in  the  shape  of  an  addition  or  deduction 
of  a  fixed  quantity,  the  technic  must  be  prosecuted  under  at  least  fairly 
corresponding  conditions  to  those  employed  in  ascertaining  the  correction. 

The  correction  is  usually  applied  numerically  at  the  time  of  calculation  of 
the  results,  adding  or  deducting  the  quantity  calculated  or  otherwise  ascer- 
tained to  the  weight  of  a  precipitate  or  residue  or  the  volume  of  a  liquid  or 
gas.  In  a  few  methods  there  is  added  while  the  analysis  is  in  progress  a 
weighed  amount  of  the  constituent  determined  or  of  some  analogous  body 
to  lessen  or  counteract  a  specific  effect  of  an  interfering  constitaent,  or  more 
rarely,  to  increase  its  potency  up  to  a  known  or  measurable  extent. 

In  gravimetric  analysis  the  most  common  corrections  are  the  deductions 
for  the  weight  of  the  ash  of  filter  paper  and  impurities  in  reagents,  and 
additions  for  losses  due  to  solubility  and  by  volatilization  on  evaporation  of 
solutions  or  ignition  of  precipitates;  in  volumetric  analysis  for  the  temper- 
ature of  standard  solutions,  the  excess  of  titrand  to  show  the  end-point,  and 
for  the  space  occupied  by  a  precipitate  or  residue  in  a  liquid  to  be  divided ; 
in  gasometry,  for  aqueous  vapor  in  a  gas;  in  colorimetry,  for  the  effect  of 
a  minor  colorant;  and  in  physical  determinations,  for  the  usual  experi- 
mental variances. 

A  correction  for  the  incomplete  insolubility  of  a  precipitate  in  an  aqueous 
solution  based  on  its  coefficient  of  solubility  in  water  is  not  to  be  depended  on 
since  the  solubility  is  sometimes  lessened,  sometimes  increased,  by  the  excess 
of  the  precipitant  or  other  dissolved  compounds;  similarly,  the  rate  of  solu- 
bility of  a  gas  in  a  liquid  is  modified  by  the  dissolved  matter  of  the  latter  as 
also  by  the  duration  and  closeness  of  their  contact  and  the  presence  of  another 
dissolved  gas.  Where  the  solubility  of  a  precipitate  is  too  great  to  be  neg- 
lected, the  loss  is  found  from  a  parallel  .determination  on  the  pure  constituent 
under  conditions  as  nearly  identical  with  the  analysis  as  possible ;  this  where 


NOTES    ON   THE    METHODS    OF   ANALYSIS.  553 

the  better  plan  of  previously  saturating  the  liquid  with  the  same  compound  as 
the  precipitate  at  the  temperature  of  the  analysis  cannot  be  availed. 


Before  attempting  an  analysis  by  a  method  with  which  he  is  unacquainted, 
the  student  should  first  study  the  principles  until  clearly  understood,  then 
critically  examine  the  details,  endeavoring  to  determine  the  nature  of  the 
errors  likely  to  be  encountered  and  how  far  they  will  severally  and  conjointly 
affect  the  result;  whether  any  two  act  in  opposite  directions  and  tend  to  off- 
set each  other,  or  those  likely  to  prove  serious  may  not  be  eluded  by  modifi- 
cation or  suppression  of  certain  operations;  if  the  quantity  of  sample 
directed  is  sufficiently  large  for  the  degree  of  accuracy  required  yet  not  too 
great  for  convenience  and  reasonable  speed;  and,  should  any  special  weights 
of  reagents  be  advised,  that  they  are  adequate  but  not  unreasonably  exces- 
sive and  not  mis-stated  through  typographical  errors;  etc. 

As  an  example  let  us  scrutinize  the  details  formulated  for  the  exercise  on 
page  245,  the  determination  of  nitrogen  in  ammonium  sulfate.  A  weight  of 
.200  gram  of  pure  ammonium  sulfate  is  decomposed  in  a  nitrometer  by  a 
solution  of  10  grams  of  sodium  hydrate  and  2.5  Cc.  of  bromine.  Nitrogen  is 
formed  by  the  decomposition  of  the  salt  — 

6NaOH  +  6Br  =  SNaOBr  -f  3SaBr  -f  3H20;  and 

(NH4)2SO4  -+-  2NaOH  -f  SNaOBr  =  N2  -+-  3NaBr  -f-  Na2SO4  +  5H2O, 
and  is  measured  over  water  and  the  weight  of  nitrogen  calculated  from  its 
volume  by  the  equation 

;    where*  is    the 


percentage  of  nitrogen  in  the  salt;  V,  the  observed  volume  of  gas;  .5  Cc., 
the  correction  for  the  absorption  of  nitrogen  in  the  liquid;  B,  the  height  of 
the  barometer  in  millimeters;  F,  the  tension  of  aqueous  vapor  at  the  ob- 
served temperature;  t,  the  temperature  of  the  room  in  degrees  Cent.;  and  #, 
the  weight  of  the  sample. 

1.  Will  the  weight  of  .200  gram  of  ammonium  sulfate  liberate  a  volume  of 
nitrogen  suitable  for  measurement  in  a  burette  of  50  Cc.  capacity,  even  at  the 
extremes  of  high  temperature  and  low  pressure  —  say  30  °  Cent,  and  700  Mm.  of 
mercury  —  to  be  met  with  in  practice? 

Since  (NH4)2S04  (132.214)  liberates  N2  (28.08)  we  calculate  that  .200  gram  of 
the  salt  evolves  .042477  gram  of  nitrogen,  and,  as  one  cubic  centimeter  of  nitro- 
gen weighs  .0012562  gram  under  normal  conditions,  a  volume  of  33.81  Cc.  This 
corresponds  to  a  volume  of  42.68  Cc.  measured  moist  at  30°  Cent,  and  at 
700  Mm. 

2.  Are  the  proportions  of  the  reagents  as  stated  sufficient  for  the  weight  of 
ammonium  sulfate  directed? 

Uniting  the  equations  given  we  have 

(NH4)2SO4  +  6Br  -f  8NaOH  =  N2  -f  6NaBr  -f  Na2SO4  -f  8H2O. 
132.214          479.7      320.464      28.08 

From  the  above  equation  it  can  be  calculated  that  .200  gram  of  the  salt  is 
decomposed  by  .726  gram  of  bromine  with  .485  gram  of  sodium  hydrate  ;  as 
there  are  directed  fully  ten  times  these  weights  the  necessary  excess  is  amply 
provided  for. 

We  may  now  estimate  the  effect  of 

1.  Impurities  or  moisture  in  the  ammonium  sulfate. 

Simply  that  for  every  milligram  of  impurity  in  the  .200  gram,  the  percentage 


554  QUANTITATIVE    CHEMICAL    ANALYSIS. 

of  nitrogen  is  reduced  by  .005  of  21.24  per  cent  (the  theoretical  yield)  equal  to 
.11  per  cent  — this  on  the  supposition  that  the  impurity  is  such  that  no  nitro- 
gen is  evolved  from  it  by  sodium  hypobromite,  for  were  it  another  salt  of 
ammonium,  as  the  chloride  or  nitrate  the  percentage  of  nitrogen  would  be 
increased  instead  of  diminished. 

2.  An  incorrect  weight  due  to  a  defective  balance  or  weights,  or  an  error 
in  casting  up  the  weights. 

Similarly,  each  milligram  over  or  under  the  true  weight  raises  or  lowers  the 
nitrogen  by  .11  per  cent. 

3.  Defective  calibration  of  the  thermometer  or  a  misreading. 

From  the  equations  ante  it  may  be  calculated  that  the  theoretical  yield  of 
gas  from  .200  gram  of  the  salt  measures  37.14  Cc.,  at  a  temperature  of  20  o  and 
a  pressure  of  760  Mm.  and  saturated  with  water  vapor,  and  at  21  o  the  volume 
would  be  37. 33  Cc.  Here  the  difference  corresponding  to  one  degree  is  .19  Cc. 
Since  one  Cc.  of  nitrogen  under  normal  conditions  corresponds  to  .63  per  cent 
nitrogen  in  .200  gram,  then  .19  Cc.  equals  .12  per  cent,  the  error  incurred  by  a 
deviation  of  one  degree  Cent. 

4.  A  change  in  temperature  between  the  two  readings  of  the  thermometer. 
Here  the  apparatus  may  be  likened  to  an  air-thermometer;  if  the  volume  of 

gases  in  the  bottle,  burette  and  connecting  tube  is  say  100  Cc.  at  20°  and  760 
Mm.  moist,  the  expansion  for  one  degree  is  .50  Cc.,  and  since  there  is  a  differ- 
ence of  one  per  cent  in  the  result  for  a  variation  of  1.75  Cc.  under  these  con- 
ditions, each  degree  of  rise  or  fall  in  the  interval  between  the  readings  makes 
a  difference  of  about  .29  per  cent  in  the  result;  the  expansion  of  the  solution 
for  one  degree  is  about  .02  Cc.  per  100  Cc.,  compensated  in  a  measure  by  the 
expansion  of  the  glass. 

5. -Defective  calibration  of  the  barometer  or  a  mistake  in  reading  it. 

As  in  (3),  37.14  Cc.  of  moist  nitrogen  at  20°  and  760  Mm.  equals  21.24  per 
cent  of  nitrogen  in  the  ammonium  sulf  ate ;  if  the  pressure  should  be  incorrectly 
observed  as  761  Mm.,  the  calculated  percentage  would  be  21.27,  a  difference 
of  .03  per  cent  for  one  millimeter. 

6.  A  change  in  atmospheric  pressure  in  the  interval  between  the  two  read- 
ings of  the  barometer. 

As  in  (4),  taking  the  volume  of  the  gases  to  be  100  Cc.  at  20°  and  760  Mm. 
moist,  an  increase  in  pressure  to  761  Mm.  would  diminish  the  volume  to  99.87 
Cc.,  a  difference  of  .13  Cc.  This  corresponds  to  about  .08  per  cent  of  nitrogen 
for  each  milimeter  of  mercury. 

7.  Absorption  of  nitrogen  by  water  in  the  burette. 

The  trapping  water  measures  say  100  Cc.,  and  as  the  coefficient  of  absorption 
for  nitrogen  at  20°  is  .014031,  were  the  water  air-free  only  1.4  Cc.  could  pos- 
sibly be  absorbed.  As  the  water  is  largely  saturated  with  air,  the  surface  ex- 
posed to  the  gases  but  small,  and  the  time  of  contact  limited,  only  a  fraction  of 
this  volume  can  be  taken  up. 

8.  Retention  of  nitrogen  in  the  generating  fluid. 

The  correction  based  on  direct  experiment  is  .5  Cc.,  which  is  about  the  same 
volume  as  would  be  absorbed  were  the  liquid  water  and  the  gas  pure  nitrogen. 
It  is  probable  that  the  volume  named  is  correct  within  ±  .1  Cc. 

We  may  conclude  from  this  data  that  the  source  of  the  largest  error  is  likely 
to  be  a  variation  of  the  temperature  of  the  room  wherein  the  analysis  is  done, 
and  hence  great  care  should  be  exercised  against  draughts,  currents  of  hot  air 
from  burners,  heat  radiated  from  the  body,  etc.  If  there  is  a  change,  notwith- 
standing these  precautions,  the  approximate  volumes  of  the  air  and  air  plus 
nitrogen  may  be  found  and  both  readings  reduced  to  the  normal.  It  follows 


NOTES    ON    THE    METHODS    OF   ANALYSIS.  555 

that  the  smaller  the  volume  of  the  gases  the  less  will  the  reading  be  altered  by 
a  given  variation  in  temperature,  hence  the  bottle  and  the  upper  ungraduated 
part  of  the  burette  should  be  as  small  as  practicable. 

Usually  the  barometer  will  not  rise  or  fall  appreciably  during  a  test;  if 
so,  the  effect  of  the  difference  may  be  eliminated  by  the  reduction  as  stated 
above. 

As  the  salt  is  readily  purified  and  not  hygroscopic,  the  errors  of  (1)  and 
(2)  need  only  be  referred  to  in  the  way  of  a  caution,  and  with  reasonable  care 
will  never  be  incurred.  The  figures  of  (3)  and  (5)  point  to  the  necessity  of 
knowing  by  actual  test  or  comparison  with  normal  instruments  that  the  ther- 
mometer and  barometer  are  correctly  calibrated. 

Following  the  general  rule,  the  accuracy  of  the  determination  is  increased 
by  a  larger  weight  of  sample  with  a  proportionally  larger  burette. 

On  the  same  lines  as  the  above  should  all  gravimetric  or  other  methods  be 
examined ;  the  items  to  be  scrutinized  will  readily  suggest  themselves  to  the 
student.  It  must  be  observed  however  that  whenever  more  than  one  element 
or  compound  is  to  be  determined  in  the  same  weighed  portion  of  a  sample, 
certain  errors  may  entail  to  the  determinations  following  the  one  in  question. 


In  an  analysis  of  a  chemical  compound  of  known  purity  the  approximation 
of  the  percentage  of  each  constituent  to  that  calculated  from  the  formula  of  the 
compound  may  be  taken  as  a  measure  of  the  trustworthiness  of  the  method 
employed,  assuming  reasonable  skill  and  care  on  the  part  of  the  analyst.  No 
such  corroboration  is  to  be  had  for  such  indeterminate  mixtures  as  most  tech- 
nical products  and  articles  of  commerce,  and  beyond  a  presumption  from  a 
knowledge  of  their  usual  composition,  one  can  be  guided  only  by  the  agree- 
ment of  duplicate  analyses.  Should  the  analysis  be  a  complete  one,  the  cor- 
rectness will  also  be  indicated  by  the  equality  of  the  sum  of  the  weights  of  the 
constituents  with  the  weight  of  the  substance  taken  for  analysis;  in  other 
words,  when  the  sum  of  the  percentages  is  exactly  one  hundred.  This  rarely 
happens,  and  then  only  by  chance,  except  of  course  when  one  constituent  is  de- 
termined by  difference.  Presumably  the  nearer  the  sum  approaches  the  theo- 
retical total,  the  more  accurate  are  the  several  determinations;  but  neither  is 
this  evidence  conclusive,  since  a  large  positive  error  on  one  may  have  offset 
an  equal  negative  error  on  another,  and  so  the  total  be  not  seriously  affected. 
Similarly,  determinations  made  in  duplicate  may  closely  agree  though  each  have 
been  vitiated  by  a  defect  in  the  method,  impurities  in  a  reagent,  etc. 

As  a  rule  a  direct  method  is  the  most  unequivocal,  though  quite  as  correct 
results  may  be  obtained  by  the  indirect  methods  and  with  more  expedition  and 
less  labor;  the  field  of  the  latter  however  is  comparatively  quite  limited.  The 
accuracy  of  an  estimation  by  difference  depends  of  course  on  the  correctness 
of  the  determination  of  all  the  other  constituents,  and  is  unallowable  for  one 
forming  only  a  small  yet  important  component  of  a  material,  as  a  cumulation 
of  errors  on  the  others  would  seriously  affect  or  even  extinguish  it.  On  the 
other  hand,  where  an  element  or  compound  forms  almost  the  whole  of  a  material, 
the  percentage  as  found  by  difference  may  be  more  exact  than  can  be  expected 
from  the  most  careful  direct  determination  —  instance  the  metallic  lead  in  the 
refined  lead  of  commerce  where  the  sum  total  of  the  impurities  may  not  exceed 
.03  of  one  per  cent. 

On  examining  records  of  analyses  of  substances  of  every  variety,  made  by 
chemists  of  undoubted  skill  and  long  experience  in  their  special  departments, 


556  QUANTITATIVE    CHEMICAL    ANALYSIS. 

and  with  every  facility  at  their  command,  we  find  that  on  the  whole  under 
these  most  favorable  conditions  a  result  of  from  99.80  to  100.20  per  cent  of 
the  element  or  compound  determined  is  considered  excellent;  from  99.50  to 
100.50,  quite  satisfactory;  and  a  variation  of  one  per  cent  or  much  more  is 
not  uncommon.  Of  course  when  the  body  determined  forms  only  a  part  of 
the  substance  analyzed  (as  is  always  the  casein  practical  analyses)  the  error 
is  apparently  reduced  in  proportion,  and  for  comparison  with  the  theoretical 
content  all  the  percentages  should  be  recalculated  to  a  basis  of  100.  It  is  plain, 
however,  that  no  comprehensive  conclusions  can  be  drawn  from  results  on 
special  material,  and  the  figures  given  above  can  only  be  taken  as  a  guide  in  a 
general  way. 

Of  late  years  there  have  been  many  attempts  made  to  define  the  limits  of  al- 
lowable inaccuracy  for  special  determinations.  A  few  of  these  are  quite  mod- 
erate in  their  demands,  but  more  often  the  bounds  have  been  made  so  narrow 
as  ordinarily  to  be  attained  only  by  the  specialist  working  under  the  most 
favorable  circumstances  —  seldom  by  the  general  analyst  less  familiar  with  the 
methods  and  perhaps  handicapped  by  want  of  appropriate  appliances  and  fre- 
quent distraction  of  his  attention.  Reasonable  limits  would  appear  to  be  the 
extremes  of  a  large  number  of  determinations  made  by  different  chemists  of 
moderate  skill  and  experience  following  one  or  more  methods  as  the  case  may 
be,  expunging  any  results  distrusted  by  their  authors  or  clearly  abnormal. 
But  it  will  often  be  found  that  the  limits  laid  down  by  even  the  most  liberal  of 
these  are  more  closely  drawn  than  the  variations  of  a  symposium  of  this  kind 
would  justify,  and  we  must  conclude  that  they  are  based  chiefly  on  the  most 
concordant  of  a  number  of  duplications  from  the  hands  of  those  well  acquainted 
with  the  methods  for  the  particular  substance  in  hand;  while  not  a  few  are 
apparently  only  the  expression  of  a  personal  opinion,  unsupported  by  adequate 
experimental  data. 

On  the  whole,  while  they  may  be  interesting  to  the  beginner  as  evidencing 
the  possibilities  of  a  refined  analysis,  beyond  this  their  value  is  seriously 
lessened  by  the  objections  mentioned  above. 

Examples  of  some  comparative  analyses  made  by  a  number  of  chemists  using 
different  methods  or  modifications  for  one  sample  may  be  of  interest;  several 
of  these  have  not  been  heretofore  published  for  certain  reasons.  However  the 
important  proviso,  namely,  that  the  parts  of  the  original  material  sent  out  for 
analysis  were  assuredly  of  identical  composition  is  in  many  cases  open  to 
doubt;  indeed,  where  fine  subdivision  of  a  heterogeneous  solid  is  not  practi- 
cable, there  will  always  remain  a  suspicion  that  the  portion  received  by  one 
chemist  may  differ  from  that  of  another  so  far  as  to  cause  a  considerable 
difference  in  their  results. 

1.  Thackray  distributed  two  sets  of  drillings  of  medium  hard  Bessemer 
steels.     On  the  first  sample  23  chemists  reported  36  determinations  of  phos- 
phorus ranging  from  .045  to  .055  per  cent;  60  per  cent  of  the  determinations 
were  between   .049  and    .052,   and  the  general    average  was  .0498.    On  the 
second  steel  23  chemists  reported  36  determinations  from  .076  to  .09i;  75  per 
cent  of  the  results  ranged  from  .080  to  .086,  and  the  general  average  was  .0838. 
Drillings  of  a  steel  plate  were  sent  by  Jones  to  9  chemists  who  made  19  de- 
terminations of  phosphorus;  the  highest  was  .067,  the  lowest   .060,  and  the 
average  .064.    There  were  14  results  between  .062  and  .065. 

2.  On  a  Bessemer  pig  iron  from  Bachman,  phosphorus  determinations  wer«3 
made  by  16  chemists  who  returned  42  determinations  from  .096  to  .165.     He 
remarks:  "  To  sum  up,  I  flud  of  three  chemists  working  the  acetic  and  citric 
acid  method,  two  are  wrong.    Of  three  who  worked  the  direct  molybdate 


NOTES  ON  THE  METHODS  OF  ANALYSIS.          557 

method,  two  are  wrong.  Of  two  working  the  modification  of  the  molybdate- 
magnesia  method  in  which  there  is  a  large  quantity  of  chlorides  present  with 
the  nitric  solution  when  phosphorus  is  precipitated,  both  are  wrong ;  and  of 
ten  working  the  method  so  that  there  is  only  nitric  acid  and  ammonium  nitrate 
present  with  the  ferric  solution,  ntne  are  within  the  limits  of  error  "  [appar- 
ently rfc  ,005  percent].* 

3.  On  two  bottles  of  drillings  "from  the  same  plate  of  open-hearth  steel" 
sent  out  by  Kent,  11  chemists  returned  23  determinations  of  manganese  rang- 
ing from  .30  to  1.14  per  cent  (!). 

4.  Samples  of  a  low  grade  spiegel-eisen  prepared  by  Stone  were  analyzed  by 
18  chemists  according  to  12  methods  and  variations,  and  60  returns  tabulated 
omitting  some  that  were  doubtful.  Arranged  with  reference  to  the  methods  em- 
ployed, Williams'  volumetric  method  averaged  12. 85  per  cent  of  manganese  with 
extremes  of  12. 60  to  13.05 ;  other  volumetric  methods,  average  13.43,  extremes  13.02 
to  14.08 ;  gravimetric  methods  where  the  manganese  is  weighed  as  pyrophos- 
phate  averaged   13.43  with  extremes  of  12.92  to  13.84;  where  the  manganese 
was  precipitated  as  the  binoxide  and  weighed  as  trimanganlc  tetroxide  aver- 
aged 13.79  with  extremes  of  13.03  to  14.47;  all  methods  averaged  13.39  with 
extremes  of  12.60  to  14.47.    Hunt   (loccif)  in  discussing  the  results  observes 
that  it  was  "very  unfortunate  that   the  material  was  not  crushed  and  put 
through  a  one-hundred-mesh  instead  of  only  a  forty-mesh  sieve  "  before  dis- 
tribution, and  believes  that  imperfect  sampling  will  account  for  some  of  the 
discrepancies. 

5.  An  artificial  mixture  of  copper  with  various  other  metals,  arsenic,  sulfur, 
silica,  etc.,  representing  a  material  more  complex  and  difficult  of  analysis  than 
ordinarily  would  reach  the  chemist,  was  prepared  by  Eustis.    Seventeen  chem- 
ists reported  45  determinations  of  copper  by  seven  methods,  the  lowest  43.90  per 
cent,  the  highest  53.34  per  cent;  29  of  these  were  between  46.50  and  47.50.    On 
borings  of  pig  copper  sent  out  at  the  same  time  seven  chemists  returned  17 
tests  showing  91. 07  to  98. 17  per  cent  of  copper,  and  five  chemists  11  determina- 
tions from  94.38  to  94.92  per  cent. 

6.  A  large  number  of    assayers    examined    samples  of   copper  matte  and 
borings  of  ingot  copper  received  from  Raymond. f    A  summary  of  their  re- 
sults follows,  the  figures  for  gold  and  silver  in  ounces  per  ton  of  matte  and 
the  copper  in  percentages. 


Determns. 

Highest. 

Next. 

Lowest. 

Next. 

Average. 

Matte  —  silver 

26 

135.38 

131.22 

122.88 

123.03 

127.87 

u    —  gold 

26 

2.41 

2.40 

1.85 

2.05 

2.24 

^    «    —copper 

16 

55.17 

55.08 

50.55 

50.75 

54.11 

Borings  —  silver 

27 

164.14 

164.05 

147.40 

148.50 

157.29 

—  gold 

26 

.50 

.42 

.205 

.21 

.307 

'•       —  copper 

8 

98.46 

98.19 

97.04 

97.37 

97.69 

7.  Aground  oak-bark  was  divided  among  six  chemists  all  of  whom  were  more 
or  less  experienced  in  tan  analysis.  By  various  modifications  of  the  hide- 
powder  process,  16  determinations  showed  from  9.73  next  9.90,  to  11.42,  next 
11.40  percent  of  tannin.  By  Loewenthals  process  and  modifications, 7  results 
showed  from  6.40  next  6.49,  to  9.60  next  9.50  (Neubauer's  factor  ?).  By  various 
other  methods,  5  determinations  showed  5.11  next  5.70,  to  17.20  next  16.02.  The 
average  of  the  hide-powder,  Loewenthals,  and  other  methods  were  respectively 
10.01,  8.99,  and  11.39. 


*  Chem.News,  1889—2—115  and  131. 

t  Trans.  Amer.  Inst.  Mia.  Engrs.  1896—252. 


558  QUANTITATIVE    CHEMICAL    ANALYSIS. 

8.  Four  chemists  and  two  laboratory  assistants  assayed  a  liquid  proprietary 
medicine  for  morphine  sulfate.     Of  the  twelve  results  the  highest  (in  grains  per 
fluid  drachm)  was  .102  next  .098,  the  lowest  .067  next  .074,  and  the  average  of 
all  .085.    The  samples  sent  out  were  parts  of  a  mixture  of  several  bottles  of 
the  medicine. 

9.  A  sample  of  baking  powder  of  doubtful  quality  was  distributed  among 
7  chemists  for  analysis  including  a  determination  of  starch.    Of  10  results  the 
highest  percentage  of  starch  was  30.50,  next  29.91 ;  the  lowest  26.10,  next  27.72 ; 
and  the  average  28.84. 

10.  Not  to  unduly  extend  these  examples,  I  will  present  but  one  more,  that 
of  a  mixture  of  oils  prepared  with  great  care  to  insure  homogeneity.    Three 
lots  of  the  original  mixture  were  sent  out.    Of  the  first  lot,  15  determinations 
by  6  chemists  showed  from  98.72  to  103.42  per  cent  of  one  (unsaponifiable) 
oil  (calculated  to  the  basis  of  100  per  cent),  80  per  cent  of  the  15  being  within 
99  to  101. 5  per  cent.     The  second  lot  to  5  chemists  resulted  in   11  determina- 
tions ranging  from  61  to  177  per  cent,   and  only  45  per  cent  of  the   11   being 
within  96.5  to  103.5  per  cent.    Of  the  third  lot,  sent  to  8  chemists,  14  determina- 
tions were   returned  ranging  from  93  to  108  per  cent,  of  which  57  per  cent 
were  between  98  and  103  percent. 

It  will  be  seen  that  the  results  on  the  first  lot  agreed  remarkably  well  con- 
sidering the  nature  of  the  mixture,  the  third  was  only  fair,  while  the  second 
could  hardly  have  been  worse.  Let  those  reconcile  such  contradictions  who 
can  —  to  me  they  but  illustrate  the  futility  of  attempting  to  draw  conclusions 
from  data  so  influenced  by  the  personal  equation.  It  seems  folly  to  allow  the 
result  of  a  tyro,  perhaps  his  first  essay,  to  weigh  equally  against  one  of  a 
master  long  practiced  in  the  particular  field  of  analysis  relative  to  the  sample 
examined,  or  a  perfunctory  grind  against  a  careful  conscientious  effort  at  the 
best  possible  issue  —  yet  who  would  welcome  the  task  of  deciding  how  much 
weight  should  attach  to  the  respective  results  on  the  basis  of  compentency 
and  attention? 


Standard  methods.  There  are  many  determinations,  especially  in  proximate 
organic  analysis,  where  the  methods  are  so  complicated  or  defective  in  one 
way  or  another,  or  for  other  reasons,  that  it  is  often  difficult  for  different 
chemists  to  obtain  reasonably  concordant  results  when  working  on  one 
sample,  and  disagreements,  annoying  and  often  expensive,  may  arise  between  a 
buyer  and  seller,  each  party  insisting  on  the  recognition  of  the  analyses  most 
favorable  to  his  interest ;  and  although  such  differences  are  readily  understood 
by  those  acquainted  with  the  limitations  of  the  art,  they  are  certainly  not  con- 
ducive to  a  high  respect  for  the  utility  of  applied  chemical  analysis  in  general 
on  the  part  of  those  engaged  in  business  or  in  other  professions. 

To  provide  a  means  for  preventing  and  settling  disputes  of  this  kind  as 
far  as  possible,  a  number  of  chemists  engaged  in  any  one  line  of  technical 
analysis  may  agree  to  accept  as  an  arbiter  a  designated  method  whose  details 
are  specifically  set  forth,  the  results  by  this  method,  when  all  the  minutiae 
have  been  scrupulously  observed,  to  take  precedence  over  those  by  any  other. 
Should  the  method  gain  general  acceptance  among  trade  chemists  or  be 
indorsed  by  a  society  of  specialists,  it  is  known  as  an  "  official  method." 

However,  it  must  be  observed  that  the  advocates  of  this  measure  are  not  as 
yet  agreed  as  to  just  what  character  of  a  method  shall  be  made  standard  or 
official,  some  insisting  that  every  other  consideration  shall  be  subordinated  to 


NOTES    ON    THE    METHODS   OF   ANALYSIS.  559 

that  of  accuracy ;  others  propose  that  to  be  more  practically  useful  the  highest 
degree  of  accuracy  may  be  waived  if  necessary,  if  thereby  in  point  of  time  and 
attention  demanded,  the  method  may  become  a  practical  laboratory  process 
and  adoptive  to  the  exclusion  of  all  other  methods  for  both  routine  and  occa- 
sional analyses.  Others  again  would  go  so  far  as  to  simplify  it,  even  at  the 
expense  of  other  considerations,  to  the  extent  that  the  least  expert  should 
have  no  difficulty  in  following  the  directions.  More  conservative  commenta- 
tors point  out  that  as  the  object  is  primarily  the  unification  of  results,  it  is  a 
matter  of  indifference  whether  such  harmony  is  secured  by  means  of  similar 
or  identical  methods  or  otherwise. 

In  favor  of  the  establishment  of  standard  methods  it  is  urged  that  expense, 
friction,  delay  and  ill-feeling  are  avoided  by  providing  a  ready  and  certain 
means  of  adjusting  differences  in  a  commercial  transaction  or  wherever  values 
are  to  be  ascertained  or  confirmed;  that  the  thorough  investigation  to  which 
the  provisionally  established  methods  will  be  subjected  cannot  fail  to  determine 
what  details  are  essential  and  what  can  be  omitted  or  changed  to  advantage ; 
and  by  relieving  the  chemist  of  the  doubt  and  anxiety  engendered  when  the 
correctness  of  his  work  is  questioned  by  himself  or  others,  he  is  spared  the 
labor  of  many  confirmatory  analyses. 

But  there  are  not  a  few  chemists,  and  of  high  standing,  who  regard  the  matter 
with  disfavor,  believing  that  disputes  of  this  kind  can  be  readily  settled  by  a 
consultation  of  the  chemists  concerned  or  by  a  third  party  as  an  umpire.  Their 
objections  may  be  summarized  — 

1.  That  the  spirit  is  antagonistic  to  the  individuality  that  should  dominate 
the  conduct  of  an  intellectual  art,  evidenced  by  the  determined  opposition  that 
has  met  every  attempt  to  codify  the  practice  of  other  arts. 

2.  That  the  object  is  avowedly  the  unification  of  results,  while  a  more  ra- 
tional endeavor  would  be  the  discovery  and  perfecting  of  scientifically  accurate 
methods  —  the  former  does  not  always  imply  the  latter. 

3.  That  an  official  method  will  not  only  be  resorted  to  for  the  adjustment  of 
disputes,  but  for  various  reasons  will  be  adopted  by  a  large  proportion  of 
chemists  for  both  routine  and  occasional  analyses  to  the  exclusion  of  other 
methods.    It  will  be  the  exception  that  a  method  of  this  type  can  be  so  framed 
as  to  recognize  other  than  the  normal  and  usual  constituents  of  the  class  of 
substances  analyzed,  so  that  an  unusual  constituent  or  impurity  of  great  prac- 
tical importance  might  readily  pass  unsuspected  that  would  no  doubt  have  been 
discovered  had  several  different  methods  been  focused  upon  it  as  would  follow 
when  each  chemist  employs  such  a  method  or  modifications  as  he  believes  most 
competent  —  an  instance  is  cited  by  Allen.* 

4.  That  discrepancies  have  ever  been  an  incentive  to  investigation  as  to  their 
cause  and  remedy,  resulting  oftentimes  in  an  original  or  more  satisfactory 
method.     With  a  standard  method  in  vogue  this  stimulus  is  lacking,  and  it  is 
feared  that  more  will  rest  satisfied  with  the  probability  of  an  agreement  with 
other  chemists  than  will  undertake  the  task  of  confirming  their  results. 

5.  Possibly  what  appeals  with  most  force  to  some  who  dissent  is  the  recog- 
nition that  so  far  as  the  practice  of  any  art,  handicraft  or  business  is  reduced  to 
fixed  rules,  in  so  far  is  it  always  relegated  to  those  whose  only  qualifications 
are  the  ability  and  willingness  to  mechanically  follow  directions;  and  in  so 
far  as  the  practice  passes  into  the  hands  of  the  uneducated  and  descends  to 
the  plane  of  a  mere  subservience  to  recipes  will  scientific  research  in  the  field 


*  Journ.  Socy.  Chein.  Ind.  1884—2, 17. 


560  QUANTITATIVE    CHEMICAL    ANALYSIS. 

of  chemical  analysis  decline.  Moreover,  this  class,  satisfied  with  a  smaller 
return  for  their  services,  will  be  given  the  preference  in  industrial  laboratories 
over  those  who  justly  feel  that  the  time  and  outlay  for  a  liberal  technical  educa- 
tion should  be  recognized. 

Whatever  opinion  one  may  hold  as  to  the  wisdom  of  establishing  official 
methods,  it  is  certain  that  the  need  has  been  exaggerated,  more  especially  in 
the  field  of  metallurgical  analysis.  From  much  of  what  has  been  written  on 
the  subject  one  unfamiliar  with  technical  and  industrial  analysis  would  be  led 
to  suppose  that  in  transactions  based  on  analyses  of  commercial  materials, 
discordances  between  the  chemists  of  the  producer,  dealer  and  purchaser  are 
the  rule  rather  than  the  exception.  As  a  matter  of  fact  their  proportion  is  but 
small,  and  in  nearly  every  case  can  be  traced  to  one  of  three  causes :  imperfect 
sampling  done  by  ignorant  or  careless  samplers,  analyses  hurried  or  neglected 
through  press  of  other  work  or  made  by  incompetent  or  perfunctory  operators, 
and  the  effort  of  one  of  the  parties  to  the  transaction  to  obtain  an  unjust  ad- 
vantage of  the  other.  With  these  faults  a  standard  method  of  analysis  should 
have  nothing  to  do  —  the  remedy  should  come  from  other  directions. 

Taking  all  things  into  consideration,  a  conservative  view  would  seem  to 
sanction  the  temporary  establishment  of  standard  methods  for  all  determina- 
tions where  a  reasonably  accurate  method  is  lacking,  or  where  the  personal 
error  comes  greatly  in  evidence,  thereby  assuring  results  at  least  concordant. 
But  where  we  have  one  or  more  reasonably  accurate  methods  fora  determina- 
tion, it  would  appear  more  profitable  to  devote  the  time  and  labor  of  establish- 
ing a  standard  method  to  the  many  analytical  problems  for  whose  solution 
there  is  a  pressing  need.. 

Standard  materials.  Some  who  would  hesitate  to  indorse  the  establishment 
of  official  methods  have  proposed  the  preparation  and  distribution  of  standard 
lots  of  staple  articles  of  merchandise  or  technical  products  of  average  character 
and  quality.  Provided  in  large  quantities  with  ample  precautions  against 
heterogeneity,  a  portion  of  each  may  be  at  the  disp6sal  of  every  chemist  desir- 
ing it.  If  the  results  of  analyses  by  a  large  number  of  chemists  agree  with 
reasonable  closeness,  the  average  is  assumed  to  represent  the  true  composition, 
and  the  material  is  then  available  as  a  referendum  by  which  to  determine  the 
quality  of  a  new  method  and  its  rank  among  those  in  common  use,  or  in 
important  analyses,  as  a  substitute  for  a  synthetic  proof. 


Viewed  in  its  entirety,  one  cannot  fail  to  perceive  how  asymmetric  has  been 
the  growth  of  quantitative  analysis.  Along  certain  lines  the  progress  has 
indeed  been  rapid,  and  the  present  status,  if  not  all  that  could  be  desired,  may 
at  least  be  deemed  satisfactory.  In  other  departments  the  methods  are  fewer 
and  their  scope  more  restricted,  to  be  employed  with  caution,  and  the  results 
of  analysis  put  forth  under  the  proviso  that  great  accuracy  is  not  to  be 
expected. 

Again,  there  are  many  bodies  for  which  methods  of  analysis  will  be 
sought  for  in  vain,  or  at  best  but  a  few  dubious  makeshifts  unearthed 
by  patient  search  of  chemical  literature.  Were  these  bodies  of  minor 
importance  from  both  a  scientific  and  practical  point  of  view,  their  neglect 
would  be  easily  understood  and  of  less  consequence.  But  unfortunately,  a 
large  proportion  are  of  great  interest  and  value  to  metallurgy,  medicine,  ani- 
mal and  vegetable  physiology,  agriculture  and  other  sciences  and  arts,  and  to 


NOTES   ON   THE   METHODS   OF   ANALYSIS.  561 

special  departments  of  technology,  and  reliable  plans  for  their  separation  and 
determination  would  be  welcomed  as  the  key  to  many  of  the  problems  that  now 
appear  otherwise  unsolvable.* 

That  the  art  has  not  attained  the  breadth  and  congruity  that  could  reason- 
ably  be  expected  as  compared  with  other  arts  of  equal  age  may  be  due  in  part 
to  the  following : 

Excluding  physical  methods,  in  proportion  as  a  body  is  chemically  active 
and  pronounced  and  positive  in  its  relations  toward  reagents,  the  more 
numerous,  varied,  and  satisfactory  will  be  the  methods  that  can  be  and  will  be 
designed  for  its  determination.  Per  contra,  bodies  that  are  nearly  related  in 
chemical  properties  offer  peculiar  difficulties  toward  separation,  enhanced 
when  they  are  of  an  indifferent  or  negative  character. 

That  the  determination  of  a  body  will  further  some  practical  end  or  prove 
an  advantage  in  a  'financial  way  is  always  a  stimulus  to  the  invention  and 
perfection  of  methods  therefor.  So  potent  is  this  influence  that  the  great 
majority  of  methods  are  designed  for  a  direct  application  in  the  arts  or 
commerce.  This  would  be  of  no  particular  moment  were  it  not  that  such 
methods  are  for  the  most  part  special  in  their  nature,  unsuited,  without  more 
or  less  modification,  to  general  analysis ;  and  however  useful  to  the  specialist, 
cannot  prove  as  beneficial  to  the  art  as  would  those  more  comprehensive. 

The  subordinate  position  to  which  analysis  is  often  relegated  in  the 
curriculum  of  those  pursuing  a  scientific  course  of  study.  How  often  is  the 
"fetish  of  organic  research"  or  theoretical  or  physical  chemistry  allowed  a 
disproportionately  large  share  of  the  time  of  the  student  availiable  for 
chemical  study  and  practice.  It  is  difficult  to  assign  a  reason  for  this  pref- 
erenee;  the  unbiased  cannot  but  admit  that  quantitative  analysis  —  in  essence 
the  derivative  of  other  branches  of  chemistry  and  of  physics  —  affords  the 
student  a  broader  and  more  comprehensive  training  than  that  of  any  other 
department.  The  principles  of  the  atomic  hypothesis  and  the  laws  of  stoichi- 
ometry,  accepted  hitherto  by  the  student  as  abstract  propositions,  are  illustrated 
and  verified;  an  acquaintance  with  the  principles  of  general  chemistry  and 
many  of  its  practical  applications,  the  synthesis  and  purification  of  organic  and 
inorganic  compounds,  the  deportment  of  elements  and  compounds  singly  and 
conjointly,  are  all  to  be  acquired  by  whoever  essays  to  be  a  successful  analyst; 
in  the  practice  of  the  art  is  acquired  a  manual  training  in  the  use  of  instru- 
ments of  precision,  the  perceptive  faculties  are  sharpened,  and  the  discipline 
of  close  attention  to  details  and  habits  of  careful  observation  enforced  by  the 


*  For  example:  "  At  the  present  time  the  whole  subjsst  of  pepsin-assay  Is  in  a  very  un- 
satisfactory, not  to  say  discreditable  condition."  "  .  .  .  .  that  chemical  analysis  has 
failed  to  discover  any  process  whereby  the  physiological  action  of  these  [drugs  and  tox- 
ins] on  the  human  economy  can  be  preiloted  has  led  to  the  hardly  more  satisfactory 
physiological  assay  ".  "  The  skillful  adulteration  of  wine  is  extremely  difficult  to  detect 
by  chemical  analysis".  "  Perhaps  no  determination  is  more  unsatisfactory  than  the  one 
[nitric  acid]  in  question  ";  "  The  complete  analysis  of  a  colouring  matter  is,  generally 
speaking,  one  of  the  most  difficult  subjects  which  can  be  placed  before  a  chemist".  «'  No 
problem  of  greater  difficulty  will  confront  the  chemist  than  the  separation  of  such  complex 
bodies  [plant  constituents]  ".  "Results  [on  certain  animal  bases]  varying  among  them- 
selves over  100  per  cent.  ....  do  not  cause  us  to  have  a  very  high  respect  for  analyses  of 
this  kind."  "  As  a  consequence,  the  great  majority  of  the  published  determinations  of 
caffeine  [in  tea,  etc.]  are  completely  worthless.  ..."  Such  unanimity  as  to  the  want  of 
reliable  methods  for  many  well  known  articles  is  rather  depressing.  The  prevailing  pes- 
simism is  relieve 3  by  the  claim  of  Benedikt  (Die  Analyse  der  Fette  und  Wachsarten, 
Lewkowltsch's  translation)  that  the  analysis  of  fats  "  presents  an  almost  complete  sys- 
tem" though  unfortunately  he  appears  to  have  no  supporters  for  his  assertion,  at  least 
if  the  word  system  be  interpreted  to  imply  comprehensive  and  reasonably  exact  schemes 
for  the  analysis  of  complex  mixtures. 

30 


562  QUANTITATIVE    CHEMICAL    ANALYSIS. 

failure  inevitably  following  their  neglect.  Finally,  the  satisfaction  of  noting 
the  development  of  one's  skill  in  manipulation,  and  the  pleasure  derived  in 
confirming  deductions  from  a  theory  or  duplicating  the  work  of  abler  hands 
combine  to  make  the  study  attractive  as  well  as  instructive. 

The  paucity  of  reliable  methods  may  be  charged  in  part  to  the  misdirection  of 
the  efforts  of  the  practical  chemist.  How  often  are  time  and  labor  spent  in  the 
elaboration  of  the  minutiae  of  a  special  method  that  would  better  have  been 
directed  toward  solving  some  of  the  problems  that  so  frequently  perplex  the 
analyst.  Particularly  is  the  technical  and  industrial  chemist  remiss  through  his 
neglect  of  investigations  toward  the  discovery  of  methods  of  proximate  analysis 
for  inorganic  substances,  content  with  pursuing  the  conventional  routine  of  ele- 
mentary analysis. 


And  the  benefit  derived  from  the  invention  of  methods  of  proximate  inorganic 
analysis  would  not  be  confined  to  the  enrichment  of  analytical  resources  only, 
for  to  the  practice  of  metallurgy  and  kindred  industries  the  means  of  deter- 
mining the  proximate  composition  of  their  products  would  be  of  the  highest 
service. 

As  an  illustration,  witness  the  voluminous  literature  recording  perennial 
attempts  to  perfect  methods  for  the  determination  of  phosphorus  in  iron  and 
steel  that  shall  be  at  once  rapid,  accurate,  and  reliable.  Granted  that  such  a 
scheme  would  somewhat  reduce  the  expenses  of  a  technical  laboratory  or  per- 
haps be  desirable  in  other  ways,  it  is  certainly  a  matter  of  far  less  practical 
advantage  than  the  invention  of  methods  of  proximate  analysis  of  iron  and  steel, 
however  crude ;  for  on  metallurgical  grounds  it  appears  highly  probable  that 
the  proximate  composition  is  more  potent  in  establishing  the  physical  qualities 
of  the  metals  than  the  absolute  proportions  of  any  or  all  of  the  elementary  im- 
purities. Had  but  a  part  of  the  efforts  in  this  direction  been  turned  to  a  sys- 
tematic endeavor  to  demonstrate  the  composition  of  these  complex  alloys,  we 
might  be  confronted  with  fewer  mysterious  failures  in  service',  fewer  triumphant 
survivals  of  long  and  hard  usage  by  material  that  would  unhesitatingly  have 
been  rejected  as  unsafe  or  unsuitable  if  prejudged  by  an  ultimate  analysis.* 


For  the  future,  the  trend  of  progress  of  the  art  will  undoubtedly  be  in 
the  direction  of  proximate  analysis.  Ultimate  analysis,  both  organic  and  in- 
organic, has  reached  a  stage  where  it  may  be  left  to  the  steady  continuous 
growth  that  will  naturally  come.  But  the  call  for  proximate  methods  is  urgent 
for  there  are  problems  innumerable  in  all  departments  of  the  sciences  and  in- 
dustries that  can  only  be  attacked  through  a  knowledge  of  the  exact  proximate 


*  "  That  other  [than  mechanical  and  heat  treatment]  and  now  nnguessed  conditions 
profoundly  alter  both  the  mineral  species  and  the  structure  of  steel,  as  of  crystalline  rock, 
in  most  complex  ways,  is  indicated  by  the  utterly  anomalous  relations  between  the 
ultimate  composition  and  the  mechanical  properties  of  steel.  This  anomalousness  which 
has  puzzled  so  many  is  readily  explained  by  the  close  resemblance  between  the  conditions 
of  the  formation  of  rock  and  ingot  which  not  only  show  us  why  we  do  not  discover  these 
relations,  but  that  in  all  probability  we  never  can  from  ultimate  composition.  The  lithol- 
oglst  who  attempted  to-day  to  deduce  the  mechanical  properties  of  a  granite  from  its 
ultimate  composition  would  be  laughed  at.  Are  our  metallurgical  chemists  in  a  much 
more  reasonable  position? In  vain  do  we  flounder  in  the  sloughs  and  quag- 
mires at  the  foot  of  the  rugged  mountain  of  knowledge  seeking  a  royal  road  to  its  summit. 
If  we  are  to  climb,  it  must  be  by  the  precipitous  paths  of  proximate  analysis,  and  the 
sooner  we  are  armed  and  shod  for  the  ascent,  the  sooner  we  devise  weapons  for  the 
arduous  task, the  better."—  Howe. 


NOTES   ON   THE   METHODS    OF    ANALYSIS.  563 

composition  of  the  complex  materials  involved,  ultimate  analysis  failing  to 
afford  the  least  aid  in  their  solution.  Not  that  a  proximate  analysis  can  alone 
and  at  once  clear  up  the  many  vague  ideas  as  to  the  relations  of  these  complex 
bodies  or  interpret  their  frequent  anomalous  and  capricious  behavior  under 
certain  conditions,  but  through  the  information  afforded  the  microscopist, 
physiologist  and  technologist,  the  solution  will  be  made  possible. 

What  means  the  future  investigator  will  employ  to  accomplish  this  end  can 
only  be  conjectured,  A  number  of  analytical  processes  for  one  reason  or 
another  have  not  received  the  attention  they  deserve  and  have  been  ap- 
plied to  only  a  few  of  the  many  purposes  for  which  it  would  appear  that 
they  could  be  turned;  among  these  are  the  separation  of  inorganic  bodies 
by  immiscible  solvents,  now  so  successful  for  organic  compounds;  the 
electrolysis  of  organic  bodies,  yielding  liquid  or  gaseous  decomposition  prod- 
ucts; separation  by  electro-dissolution;  the  selective  action  of  certain 
reagents  in  solution  or  precipitation;  transformation  by  micro-organisms  or 
ferments;  the  measurement  of  progressive  inherent  or  incidental  changes  in 
organic  bodies ;  and  new  applications  of  cryoscopy,  capillarity,  osmose,  dis- 
sociation by  heat  in  vacuo,  etc.  Possibly  purely  mechanical  processes  may  be 
so  refined  as  to  be  capable  of  giving  reasonably  sharp  separations.  To  the 
study  and  practical  application  of  such  means  as  these  may  the  attention  of  the 
student  be  directed  with  a  better  prospect  of  success  than  in  attempts  to  apply 
the  more  familiar  principles. 

But  it  would  appear  that  our  present  resources  however  they  maybe  extended 
are  entirely  inadequate  to  the  task,  and  any  great  progress  in  this  direction 
must  come  through  the  discovery  and  application  of  new  principles  of  analysis. 


APPENDIX. 


TECHNICAL  AND  INDUSTKIAL  ANALYSIS. 


TECHNICAL   AND    INDUSTRIAL    ANALYSIS.  567 


APPENDIX. 

TECHNICAL  AND  INDUSTRIAL  ANALYSIS. 

Few  if  any  of  the  liberal  arts  have  contributed  so  largely  to  the  advancement 
of  the  sciences  and  arts  in  general  as  has  analytical  chemistry  and  its 
branches.  In  sanitary  science  and  hygiene  the  public  welfare  is  conserved  by 
the  detection  of  adulterated  foods  and  unwholesome  waters;  the  physician 
and  pharmacist  are  provided  with  materia  medica  of  trustworthy  strength  and 
purity;  the  agriculturalist  learns,  though  somewhat  imperfectly  as  yet,  what 
elements  of  the  soil  are  most  essential  to  specific  vegetable  growth  and  if  ab- 
sent or  exhausted,  and  the  nature  and  comparative  value  of  fertilizing  addi- 
tions ;  the  engineer  as  to  the  formulae  to  be  specified  for  the  composition  of 
metals  reliable  under  strain  and  shock,  the  calorific  value  of  fuels,  the 
quality  of  lubricants  and  anti -friction  bearings,  and  the  nature  of  boiler 
waters  and  their  correctives;  the  miner  and  smelter,  as  to  the  grade  of  ores, 
their  valuable  associates  and  detrimental  impurities,  and  the  processes  most 
suitable  for  their  reduction.  Besides  these  It  proves  oftentimes  an  almost 
unimpeachable  referee  in  legal  complications  deciding  contentions  that  are 
otherwise  unsolvable. 

Again,  in  technology  and  manufacturing  it  can  safely  be  said  that  every  de- 
partment of  industry,  outside  those  purely  mechanical,  has  profited  by  its  aid. 
It  is  not  difficult  to  perceive  how  a  knowledge  of  the  exact  composition  of 
such  articles  as  enter  into  the  processes  of  manufacturing  will  afford  a 
rational  basis  for  their  treatment  and  disposition,  and  to  this  end  has  the 
attention  of  manufacturers  been  directed  during  recent  years.  Less  familiar, 
however,  are  many  other  applications  which  from  their  specific  nature  or  for 
other  reasons  are  slow  to  become  known  to  the  public. 

The  material  with  which  the  practical  analyst  has  to  deal  may  be  roughly 
divided  into  two  classes.  The  first  includes  naturally  occurring  and  factored 
articles  of  which  he  is  to  determine  the  composition  and  advise  as  to  the 
value  and  adaptability  to  a  given  purpose,  or  to  certify  as  to  the  presence 
or  absence  of  deleterious  associates  or  adulterants.  Analyses  of  this  nature 
ordinarily  fall  in  the  province  of  the  commercial  or  sanitary  chemist.  To  the 
second  class  belong  the  raw  materials  and  intermediate,  final,  and  waste 
products  of  manufacturing  processes,  for  whose  analysis  provision  is  made 
in  the  way  of  special  laboratories  adjunct  to  the  manufactories. 


As  in  other  arts,  the  occupation  of  the  commercial  chemist  is  usually  con- 
fined for  the  most  part  to  some  one  analytical  specialty,  on  which  he 
endeavors  to  build  a  reputation  and  become  known  as  an  authority,  select- 
ing either  the  assaying  of  gold  and  silver  ores  and  their  products,  the  exami- 
nation of  natural  waters,  iron  and  steel,  sugar,  brewing  materials,  tanwares 
dyes,  etc.  In  proportion  as  his  reputation  for  ability  and  probity  enhances,  his 
counsel  is  sought  on  technical  questions,  the  advisability  of  investment  in  the 
numerous  projects  continually  being  proposed  to  the  capitalist,  and  as  an  expert 


568  QUANTITATIVE    CHEMICAL    ANALYSIS. 

in  the  valuation  of  mining  and  manufacturing  properties  and  other  lines  of 
business  enterprises. 

But  it  is  not  the  rule  that  he  can  command  so  large  a  clientele  in  his  chosen 
department  as  to  be  able  to  devote  his  time  to  it  exclusively,  and  he  must  be 
prepared  to  undertake  commissions  in  any  line  of  technical  investigation  and 
counsel  —  usually  more  remunerative  than  analysis  alone. 

The  occupation  of  the  sanitary  chemist  or  public  analyst  is  mainly  the 
examination  of  foods  and  condiments  for  adulterants  and  substitutions.  Many 
large  cities  and  some  States  have  established  laboratories  under  municipal  or 
State  control  and  employ  one  or  a  corps  of  chemists  who  periodically  report 
on  the  condition  of  the  water  supply  of  the  city  and  the  purity  of  the  ice  and 
foods  sold  in  the  open  market.  Of  the  latter,  milk,  so  easy  to  dilute,  naturally 
claims  the  most  attention,  although  the  substitution  of  butterine  for  butter,  and 
chicory  or  artificial  berries  for  coffee,  and  the  vending  of  exhausted  spices,  in- 
ferior drugs  and  proprietary  medicines  of  a  dangerous  character  are  not  uncom- 
mon. The  wholesomeness  of  potable  waters  is  often  to  be  passed  upon  with  rec- 
ommendations as  to  the  feasibility  of  purification  if  below  standard,  or  whether 
one  of  normal  purity  is  in  danger  of  contamination  by  reason  of  the  proximity  of 
drains  or  sewers.  In  addition  to  the  ordinary  duties  of  his  office  he  may  be 
asked  to  examine  the  ventilation  of  public  buildings,  to  superintend  the  disin- 
fection of  places  offensive  or  dangerous  to  health,  or  to  decide  whether  the 
manufacture,  storage  or  transportation  of  articles  that  are  inflammable,  ex- 
plosive or  offensive  should  be  permitted  within  certain  districts,  etc.  Super- 
vision by  competent  chemists  supported  by  police  authority  has  resulted  in 
abating  the  spread  of  certain  contagious  and  infectious  diseases  to  a  gratifying 
degree. 

The  bureaus  of  agriculture  in  many  countries  and  most  of  the  United  States 
have  instituted  laboratories  for  investigation  and  experiment  on  the  composi- 
tion and  aggregation  of  soils,  the  relative  food  value  and  yield  of  different 
varieties  of  plants  and  the  processes  of  manufacture  best  suited  for  their  prep- 
aration for  the  market  or  the  extraction  of  their  essential  principles,  the  com- 
parative value  of  fertilizers,  insecticides,  etc.,  and  kindred  problems.  Analysis 
is  confined  mainly  to  a  few  departments  except  where  needed  in  the  course  of 
investigations.  The  chemists  of  the  agricultural  stations  in  the  various  States, 
in  conjunction  with  those  of  the  United  States  government,  have  for  some 
years  maintained  an  association,  one  of  whose  objects  is  the  formulation  of 
official  methods  of  analysis  for  the  materials  and  products  of  agriculture. 
Several  other  countries  have  similar  organizations. 

Chemical  engineering.  There  are  many  lines  of  manufacturing  wherein  the 
processes  are  based  essentially  on  chemical  transformations  of  raw  material 
to  finished  product,  and  in  these  establishments  the  chemical  engineer  is  an 
important  official.  Again,  there  are  many  classes  of  factored  products  whose 
commercial  value  varies  in  proportion  to  their  purity  or  is  based  on  certain 
characteristics  of  color,  odor,  flavor,  clarity,  etc.,  such  as  crude  and  fine  chemi- 
cals, paints,  oils,  soaps,  beverages,  spices,  relishes,  medicinal  and  proprietary 
articles,  toilet  requisites  and  the  like,  and  to  these  his  services  are  not  less 
valuable. 

In  works  of  this  kind  all  the  processes  were  formerly  conducted  by  the  light 
ef  experience,  and  within  their  limitations,  and  so  long  as  the  established 
practice  remained  unchanged,  a  fair  and  not  infrequently  a  remarkably  good 
and  uniform  quality  of  the  various  products  was  ordinarily  attained.  But  to 
disturb  this  routine  there  would  occur  from  time  to  time  an  inability  to  secure 
raw  materials  from  the  source  formerly  constituting  the  entire  supply,  the  in- 


TECHNICAL    AND    INDUSTRIAL    ANALYSIS.  569 

stallation  of  more  modern  machinery  or  appliances,  a  change  in  the  personnel 
of  the  management,  a  demand  by  the  purchasers  for  some  change  in  the  com- 
position or  characteristics  of  the  products,  or  the  necessity  for  a  reduction  in 
the  cost  of  manufacturing  due  to  competition  or  an  advance  in  wages  or  the 
prices  of  raw  material.  At  such  times  many  difficulties  were  encountered  in 
modifying  the  general  conduct  or  details  of  the  processes  to  meet  the  new  con- 
ditions and  frequently  the  quality  of  the  product  suffered  in  the  interim. 

Under  the  modern  system  of  chemical  control,  changes  of  this  nature  can 
be  anticipated  and  provided  for,  and  come  into  effect  without  interfering 
with  either'the  quality  or  output  of  the  product.  And  although  in  some  in- 
dustries exact  information  as  to  the  reactions  on  which  the  conversions  are 
based  is  yet  wanting,  enough  is  known  to  outline  the  changes  and  to  learn  the 
effects  on  the  product  of  modifications  in  the  practice. 

Direction  of  both  the  chemical  processes  and  the  mechanical  appliances  is 
intrusted  to  the  chemical  engineer,  and  in  some  establishments  he  has  also 
a  general  supervision  of  the  labor.  He  must  be  well  acquainted  with  the 
details  of  manufacturing  and  machinery  to  be  competent  to  modify  and  improve 
them  as  circumstances  warrant,  to  plan  and  provide  for  the  manufacture  of 
new  products,  and  in  case  of  accident  to  contrive  temporary  expedients  that 
the  operation  of  the  plant  may  proceed  without  interruption. 

In  the  smaller  manufactories  such  a  position  can  be  filled  to  advantage  by 
one  person,  but  for  an  establishment  more  extensive  and  where  the  products 
are  numerous  and  varied,  many  believe  it  a  better  policy  for  the  chemist  to  dele- 
gate the  care  of  the  power  and  machinery  to  an  assistant,  a  competent  mechan- 
ical engineer.  For  in  these  days  of  rapid  development  and  sharp  competition 
better  service  can  undoubtedly  be  rendered  by  those  whose  training  and  experi- 
ence have  been  confined  to  one  specialty.  Few  there  are  who  can  master  two 
professions,  and  mediocrity  in  both  is  hardly  a  qualification  for  the  exacting 
demands  of  the  modern  factory. 

In  many  establishments  not  engaged  in  manufacturing  beyond  repairs  for 
the  maintenance  of  their  plants,  large  quantities  of  incidental  supplies  of 
various  kinds  are  purchased  as  needed.  Formerly  they  were  bought  under  the 
stipulation  that  the  quality  should  equal  that  of  some  well  known  make  recog- 
nized as  of  a  uniformly  high  degree  of  excellence.  On  receipt  of  a  purchase,  it 
was  inspected  by  some  employee  experienced  in  the  use  of  the  particular  article, 
and  his  decision  as  to  whether  it  conformed  to  the  grade  contracted  for  was 
conclusive  as  to  acceptance  or  rejection.  Sometimes  a  rough  and  often  per- 
functory practical  test  was  made  on  a  small  scale,  where  inspection  alone  left 
a  doubt. 

But  it  was  generally  admitted  that  such  a  superficial  examination  was  inade- 
quate, often  failing  of  vindication  when  the  article  was  put  into  practical 
use  and  incompetent  for  the  detection  of  the  many  ways  of  depreciating 
the  quality  without  affecting  the  general  appearance;  so  that  in  later 
years  the  purchaser  has  come  to  rely  more  and  more  on  chemical  anal- 
yses and  physical  tests,  which,  made  by  competent  inspectors,  will  often 
at  once  and  finally  settle  all  questions  as  to  quality,  purity,  and  com- 
mercial value.  And  generally,  the  consumer,  through  his  ability  to  dis- 
criminate between  the  wares  of  different  dealers,  receives  a  higher  grade 
and  more  uniform  quality  than  formerly,  for  the  fact  that  a  purchaser 
has  instituted  a  laboratory  soon  becomes  known  to  the  seller  with  the 
immediate  result  of  an  improvement  in  the  quality  of  supplies  furnished. 
And  constrained  by  the  demands  of  buyers  or  the  example  of  competitors  to 
be  able  to  certify  to  the  exact  quality  of  what  he  has  to  sell,  the  producer  or 


570  QUANTITATIVE    CHEMICAL    ANALYSIS. 

broker  must  resort  to  the  commercial  chemist  or  install  a  laboratory  of  his 
own. 

Within  recent  years  the  manufacturers  of  certain  products  have  become 
convinced  that  chemical  analysis  could  be  made  a  valuable  aid  in  the  conduct 
of  their  business,  and  through  having  at  first  an  occasional  analysis  made  by  a 
commercial  chemist,  have  found  it  good  policy  to  equip  and  maintain  works  - 
laboratories  wherein  one  or  more  chemists  are  engaged  in  analysis  and  the 
investigation  of  matters  appertaining  to  the  processes  in  use. 

In  every  manufacturing  or  refining  process,  exclusive  of  those  that  are 
strictly  mechanical,  the  progressive  changes  undergone  by  the  raw  material  in 
transformation  to  the  finished  product  may  be  followed  to  decide  what  par- 
ticular system  of  conversion  will  be  most  efficient  and  economical,  and  dis- 
cover where  modifications  can  be  introduced  with  advantage,  and  to  what  ex- 
tent waste  products  may  be  impoverished  or  turned  to  account.  And  a  lapse 
in  the  fidelity  of  the  workmen  or  an  impaired  condition  of  the  machinery  or 
other  appliances  or  an  increase  in  the  cost  of  production  at  some  stage  of  the 
process  may  frequently  be  exposed  or  traced  by  a  few  simple  chemical  tests. 
In  some  establishments  every  step  in  the  processes  of  manufacture  is  watched 
from  the  laboratory,  the  guides  of  tradition  and  personal  opinion,  often 
groundless  or  chimerical,  being  everywhere  supplanted  by  the  rational  and 
exact  basis  of  analytical  chemistry. 

It  is  probable  that  the  plan  of  analytical  control  originated  in  the  laboratories 
of  manufacturing  chemists,  though  carried  out  to  a  limited  extent  only,  ham- 
pered by  a  lack  of  adequate  methods  of  analysis.  Next  followed  the  smelters 
of  iron,  the  adoption  here  more  successful  since  the  processes  of  reduction 
deal  only  with  inorganic  bodies  and  the  chemical  reactions  of  smelting  are 
comparatively  well  known.  The  iron  and  steel  industries  still  lead  all  others 
in  the  number  of  chemists  employed  and  the  thoroughness  with  which  the 
technic  is  controlled.  Of  other  manufacturers  that  have  later  followed  their 
example  to  a  greater  or  less  extent  may  be  mentioned  the  producers  of  paints 
and  varnishes,  sugar  and  glucose,  explosives,  soap  and  candles,  glue,  rubber 
goods,  glass  and  pottery,  wood-pulp  and  paper,  dyes,  dyed  fabrics,  baking 
powder,  spices,  smelters  of  base  and  precious  metals,  refiners  of  oils,  meat 
packers,  etc. 

For  an  illustration,  the  routine  chemical  work  of  the  laboratory  of  an  iron 
and  steel  works  may  be  described  in  some  detail.  A  large  steel  works  pro- 
ducing rails  or  plates  may  regularly  employ  as  many  as  a  score  of  chemists 
occupied  about  as  follows : 

1.  In  each  cargo  or  shipment  of  iron  ore  received  is  determined  the  per- 
centage of  moisture,  metallic  iron,  phosphorus,  and  sulfur,  these  being  the 
most  important  constituents.    Silica,  the  earths,  and  other  constituents  are 
also  determined  on  each  cargo  unless  the  ore  comes  entirely  from  one  large 
mine,  when  less  frequent  tests  will  suffice.    The  receipts  of  fuel  — coke,  char- 
coal, or  anthracite  —  are  regularly  tested  for  sulfur  and  ash,  and  the  lime- 
stone for  lime,  magnesia,  silica,  and  sulfur,  and  for  other  impurities  when 
called  for.    In  the  spiegel-eisen   or  ferro-manganese   (the  carburetter)    the 
manganese,  phosphorus  and  silicon  are  the  constituents  usually  determined, 
the  carbon   and  iron  being  present  in  a  fairly  constant  proportion  in    any 
given  grade  (i.  e.t  percentage  of  manganese)  of  the  metal.     The  miscellaneous 
scrap  iron  or  scrap  steel  for  charging  the  open -hearth  furnace  is  sometimes 
analyzed,  but  usually  is  in  a  form  that  does  not  admit  of  fair  sampling. 

2.  The  pig-iron  made  by  the  blast  furnaces  is  regularly  analyzed,  for  bilicon, 
sulfur,  phosphorus,  and  manganese,  and  for  carbon  and  other  elements  as  may 


TECHNICAL   AND    INDUSTRIAL   ANALYSIS.  571 

be  desired.  In  works  where  the  iron  is  not  cast  into  pigs  but  is  carried  in  a 
molten  condition  from  the  blast  furnace  to  the  converter  or  open-hearth 
furnace,  the  silicon  and  sulfur  are  to  be  determined  as  quickly  as  possible 
after  the  metal  leaves  the  blast  furnace.  Of  the  furnace  slags  a  complete 
analysis  is  made  on  the  average  of  all  the  tappings  during  one  week,  or 
oftener  if  thought  advisable.  The  furnace  gases  are  regularly  or  occasionally 
examined  for  the  ratio  between  the  carbon  monoxide  and  carbon  dioxide. 

3.  From  every  heat  of  steel  from  the  converter  or  hearth  is  dipped  a  small 
ingot  which  is  tested  for  carbon,  silicon,  sulfur,  and  manganese,  except  where 
rail-steel  is  the  product,  when  the  average  of  the  heats  from  each  turn  of  twelve 
hours  is  usually  thought  sufficient  for  the  three  elements  last  named,  the  uni- 
formity of  the  steel  made  depending  largely  on  the  correctness  with  which  the 
ores,  fluxes,  and  metals  have  been  examined  and  weighed  into  the  furnaces. 
The  converter  and  open-hearth  slags  are  weighed  and    analyzed  occasion- 
ally as  a  check  on  the  inevitable  loss  by  permanent  oxidation  of  the  metal 
of  the  bath  and  from  metal  globules  retained  mechanically.    In  open-hearth 
furnace  practice,  rough  tests  of  the  metal  of  the  bath  are  made  periodically  at 
intervals  of  an  hour  or  less  to  determine  the  rate  at  which  the  carbon  is  being 
oxidized  to  carbon  monoxide  and  leaving  the  metal ;  and  where  the  hearth  is 
neutral  or  basic,  similar  tests  are  made  for  phosphorus  as  it  oxidizes  and 
passes  into  the  slag. 

Should  the  proportions  of  the  valuable  constituents  in  a  raw  material  be 
found  to  fall  below,  or  of  those  deleterious  to  exceed  the  limits  agreed  upon  at 
the  time  oi  purchase,  a  concession  in  price  is  demanded  or  the  entire  lot  is  re- 
jected. The  charges  of  ore,  limestone  and  fuel  fed  to  the  furnaces  are  cal- 
culated from  their  analyses  to  produce  the  composition  of  pig  iron  desired,  and 
to  furnish  a  slag  that  shall  be  fusible  at  a  moderate  furnace  temperature  and 
remove  as  much  as  possible  of  the  sulfur  of  the  charge,  thus  insuring  an  output 
limited  only  by  the  capacity  of  the  furnace,  and  produced  at  the  lowest  cost. 
From  the  analyses,  any  hitch  or  abnormal  working  of  the  furnaces,  cupolas  or 
vessels  is  shortly  detected;  and  the  product  can  be  placed  on  the  market  with 
confidence  that  the  impurities  are  within  the  stated  limits,  and,  as  far  as  com- 
position is  concerned,  will  uphold  the  reputation  of  the  makers.  And  since  a 
complete  chemical  record  is  kept  of  the  product,  in  the  event  of  a  failure  in 
service  resulting  from  undue  hardness,  rapid  wear,  heterogeneity,  or  similar 
fault,  the  precise  cause  of  failure  may  be  more  readily  located. 

4.  In  addition  to  the  above  are  examined  the  refractory  materials  used  in 
lining  the  furnaces  and  ladles;  the  coal  for  gas-producers  or  liquid  fuel  for 
boilers;  spittings  or  splashes  of  iron  and  steel  from  the  converter;  furnace- 
fume  ;  washes  for  coating  moulds ;  and  various  other  materials  and  by-products. 
Doubtful  or  apparently  incongruous  analytical  results  obtained  during  the  rapid 
routine  work  of  the  laboratory  are  to  be  repeated  by  more  accurate  methods. 
Special  analyses  are  to  be  made  of  iron  and  steel  that  is  designed  for  except- 
ionally severe  strain  or  wear  or  of  that  which  has  failed  in  service,  and  the 
products  of  competitors,  as  well  as  examinations  of  articles  of  the  most  varied 
nature  at  the  request  of  those  in  authority. 

All  this  calls  for  a  great  amount  of  analytical  work  that  is  turned  out  with  a 
rapidity  scarcely  credible  by  one  whose  experience  has  been  confined  to  analyses 
for  scientific  purposes  only  —  in  a  large  steel -works  the  number  of  determina- 
tions will  often  reach  as  high  as  150,000  per  annum.  Yet  the  not  inconsiderable 
cost  entailed  is  amply  repaid  by  the  certainty  and  uniformity  of  the  metal- 
lurgical practice  as  compared  with  the  rule-of-thumb  formerly  in  vogue. 

The  analyses  made  in  a  steel  works  may  be  divided  into  three  classes:  the 


572  QUANTITATIVE    CHEMICAL   ANALYSIS. 

first  comprises  those  where  the  highest  accuracy  is  demanded  and  where  ample 
time  is  allowed  the-analyst  to  secure  it;  in  the  second  are  those  that  should  be 
at  least  fairly  accurate,  yet  from  their  volume  and  the  immediate  need  of  the 
information  they  furnish  must  be  completed  in  a  limited  time;  and  lastly,  those 
that  shall  decide  the  further  treatment  or  disposition  of  intermediate  products 
liable  or  certain  to  rapidly  deteriorate  by  chemical  change  or  loss  of  heat  — 
here  immediate  returns  are  highly  desirable  if  not  imperative.  Even  a  rough 
approximation  if  quickly  known  may  be  of  great  assistance  to  the  management. 
Kecognizing  that  only  constant  practice  will  enable  one  to  attain  the  greatest 
rapidity  and  surety,  the  analytical  work  is  divided  in  such  a  way  that  each 
chemist  is  engaged  at  only  one  or  a  few  kinds  of  determinations  —  thus  three 
will  divide  the  day  of  twenty-four  hours,  each  making  color-carbon  tests  of 
the  steel  produced  during  his  turn  of  eight  hours;  three  others  the  silicon  and 
sulfur  in  furnace  iron;  one  chemist  analyzing  all  the  ore  received;  another  the 
slags,  limestone  and  coke;  and  so  on. 

The  great  volume  of  analyses  and  the  rapidity  with  which  they  are  turned 
out  has  given  rise  to  an  impression  that  most,  or  at  least  many,  of  the  results 
so  obtained  are  at  best  but  doubtful,  even  approximate  accuracy  being  sacri- 
ficed to  speed.  Doubtless  at  works  where  a  large  output  of  the  product  is 
the  highest  ambition,  with  quality  as  a  secondary  consideration,  the  same 
policy  extends  to  the  laboratory;  this,  unfortunately  common,  is  by  no 
means  universal,  'for  it  has  come  to  be  realized  that  a  result  to  any  extent 
doubtful  is  not  only  useless  but  misleading  as  well. 

As  to  the  comparative  accuracy  of  the  general  run  of  works'  analyses  to 
those  made  under  more  favorable  conditions  it  is  difficult  to  speak  positively. 
In  their  favor  there  is  to  be  considered,  first,  that  for  indefinite  periods  the 
composition  of  each  of  the  products  and  by-products  of  a  well  organized 
factory  is  so  uniform  or  approaches  so  closely  to  that  aimed  at  or  calculated 
(except  during  brief  periods  of  anomalous  conditions)  that  the  proportions  of 
the  various  constituents  are  confined  within  certain  fixed  maximum  and 
minimum  limits,  and  knowing  these,  the  works  chemist  can  simplify  and 
shorten  the  more  general  methods  or  safely  go  so  far  as  to  employ  those  worth- 
less for  other  than  the  specific  material  with  which  he  has  to  deal;  second, 
that  forms  of  apparatus  can  be  selected  or  specially  designed  that  are  con- 
venient and  adapted  to  the  particular  operations  of  an  analysis,  and  being 
permanently  arranged  are  in  readiness  at  all  times  for  immediate  use;  and 
lastly,  that  the  chemist,  engaged  continuously  in  one  line  of  work,  becomes 
familiar  with  all  the  details  and  can  carry  on  many  analyses  together  without 
loss  of  time  at  any  stage.  Under  these  circumstances  there  would  seem  to  be 
no  inherent  incompatibility  between  reasonable  accuracy  and  exceptional 
speed, 

The  chemist  is  occupied  mainly  in  the  capacity  of  analyst  in  metallurgical 
and  other  works  where  inorganic  substances  are  chiefly  dealt  with,  since  here 
the  materials  admit  of  rapid  and  exact  analysis  and  the  quality  of  the  output 
under  normal  conditions  of  manufacture  can  be  controlled  and  adapted  to  the 
market.  But  where  organic  bodies  are  the  subject,  as  in  tanning,  dyeing,  and 
the  like,  the  actual  chemical  changes  are  but  imperfectly  understood  and  dif- 
ferences in  the  composition  or  physical  character  of  the  raw  material  and 
adjuncts  or  a  variation  in  the  routine  of  the  processes,  though  perhaps  so 
slight  as  to  be  uonoticeable,  may  greatly  modify  the  habitus  and  general  quali- 
ties of  the  product.  Moreover,  being  often  bodies  highly  susceptible  to  chemi- 
cal change  and  prone  to  disorganize  spontaneously  or  through  external 
influences  and  so  deteriorate  or  develop  undesirable  properties  in  the  product, 


TECHNICAL  AND    INDUSTRIAL   ANALYSIS.  573 

a  large  amount  of  investigation  and  experiment  is  called  for.  To  deal  intelli- 
gently with  such  questions  the  chemist  must  be  so  conversant  with  every 
detail  of  the  processes  in  use  and  the  routine  of  the  manufacture  that  in 
the  laboratory  he  may  be  able  to  carry  a  sample  of  a  raw  material  or  interme- 
diate product  through  a  reproduction  in  miniature  of  the  whole  or  a  part 
of  the  process  and  to  judge  from  the  yield  and  character  of  the  product 
(corroborated  as  far  as  may  be  by  analysis)  as  to  the  completeness  and 
success  of  the  conversion,  and  to  note  any  unusual  behavior  or  character- 
istics that  may  have  to  be  considered  when  operating  on  a  larger  scale; 
or  to  convince  by  actual  demonstration  that  some  modification  or  rad- 
ical change  in  the  process  will  be  essential  or  advantageous  for  the  material 
in  hand. 

It  will  not  be  necessary  to  detail  the  occupation  of  the  chemist  in  the  different 
industries  of  this  kind;  in  all  he  is  expected  to  examine  and  value  the  raw 
materials  purchased  and  decide  as  to  their  suitability  for  the  purposes  intended, 
the  nature  of  the  impurities  as  regards  their  effect  on  the  processes  and  prod- 
ucts, liability  to  hasten  deterioration  on  keeping,  or  influence  in  other  wajs; 
whether  the  material  is  usable  in  the  form  or  state  received  or  better  after 
comminution,  purification,  drying,  etc.,  and  whether  adapted  for  treat- 
ment by  the  usual  processes  or  requiring  some  modifications;  and 
to  call  attention  to  any  peculiarities  or  unusual  qualities  he  may 
observe.  Further  he  is  periodically  to  test  the  products  in  suitable 
ways  that  he  may  certify  that  the  standard  of  quality  is  or  is  not 
maintained;  appraise  the  waste  products  and  advise  concerning  their 
disposition;  experiment  on  any  improvements  that  may  occur  to  him  or  be 
brought  to  his  notice;  and  in  general,  to  give  expert  opinions  on  any  subjects 
connected  with  the  chemical  side  of  the  manufactory.  Much  is  left  to  his  dis- 
cretion as  to  what  extent  it  is  necessary  to  carry  an  investigation  to  accomplish 
the  end  desired,  and  along  what  lines  it  is  to  be  conducted. 

Of  the  miscellaneous  industries  that  in  recent  times  have  come  to  employ  a 
large  number  of  chemists  may  be  mentioned  the  miners  of  iron  ore.  Here  the 
principal  occupation  of  the  chemist  is  in  analyzing  the  ore  shipments  for  their 
content  of  iron  and  other  important  constituents.  But  in  addition  in  many 
mines  the  output  is  divided  into  several  grades  classified  according  to  the  per- 
centage of  iron  or  some  other  constituent,  usually  phosphorus.  The  ore  deposit 
is  divided  into  sections  bounded  by  rock-seams  or  timbering,  and  the  surface  of 
ore  in  each  section  periodically  sampled  and  analyzed  and  the  miners  advised 
to  what  grade  the  ore  belongs.  Various  other  metalliferous  ores  are  mined  on 
a  similar  plan,  and  attempts  have  been  made  to  chemically  divide  the  yield  of 
coal  mines,  asphalt  beds,  clay  pits,  etc. 


The  technical  chemist  has  often  to  examine  mixtures  sold  for  sundry  pur- 
poses in  the  arts  that  are  wholly  or  partly  organic  and  whose  composition  is 
kept  secret  by  the  makers.  Such  are  proprietary  articles  designed  for  med- 
icinal or  toilet  purposes,  nostrums,  factitious  foods  and  condiments,  food 
preservatives,  water  purifiers  and  boiler  purges,  fuels,  metallurgical  fluxes, 
tempering  powders  and  physics,  cleaning  and  polishing  powders  and  pastes, 
«tc.,  etc.,  that  are  continually  brought  upon  the  market  vaunted  as  superior  to 
all  other  preparations  hitherto  employed  for  the  specific  purposes.  Some  of 
these  are  of  real  merit,  others  plainly  frauds  by  intention.  As  might  be  sup- 
posed, both  classes  are  as  a  rule  comparatively  simple  in  composition,  and  to 


574  QUANTITATIVE    CHEMICAL    ANALYSIS. 

those  having  some  experience  in  the  examination  of  mixtures  of  a  similar  kind, 
the  analysis  is  attended  with  no  great  difficulties. 

There  is  also  a  third  class,  unfortunately  too  common,  of  mixtures  com- 
pounded, often  in  good  faith,  by  those  wanting  in  chemical  knowledge  and  fre- 
quently in  practical  experience  as  well.  Like  the  poly-pharmacist,  they  en- 
deavor to  incorporate  every  ingredient  that  has  been  proposed  for  the  purpose 
with,  perhaps,  others  that  can  lay  no  claim  to  efficiency  and  apparently  chosen 
solely  for  their  cheapness  or  the  opportunity  to  purchase  a  supply  at  a  bargain. 
It  is  not  strange,  therefore,  that  it  is  often  beyond  the  ability  of  the  chemist  to 
solve  the  composition  of  such  a  hodge-podge,  and  any  statement  as  to  the  rela- 
tive proportions  of  the  constituents  but  little  better  than  a  guess. 

For  the  examination  of  material  of  this  kind  it  is  seldom  that  a  search 
through  chemical  literature  will  reveal  any  scheme  of  analysis  that  can  be 
adopted,  or  at  best  more  than  a  few  that  may  be  doubtful  or  discredited,  and 
the  analyst  must  exercise  his  chemical  knowledge  and  inventive  ability  to 
devise  one  that  shall  answer  the  purpose.  It  need  not  necessarily  be  so  com- 
prehensive as  to  include  every  constituent,  since  it  is  generally  left  to  his  dis- 
cretion as  to  what  extent  the  analysis  should  be  carried  to  supply  the  infor- 
mation desired  or  to  demonstrate  the  value  or  uselessness  of  the  article  for  a 
particular  end.  If  the  essential  ingredients  and  their  approximate  proportions 
can  be  discovered,  the  information  will  probably  be  sufficient  for  all  practical 
purposes,  and  to  this  end  should  the  attention  of  the  analyst  be  directed  — 
often  the  chemist  is  asked  merely  to  supply  a  recipe  for  the  preparation  of  an 
article  equally  as  efficient  for  a  given  purpose  as  the  sample  furnished  him;  with 
perhaps  the  stipulation  that  between  them  there  shall  be  a  close  resemblance  in 
appearance,  etc.,  for  reasons  not  difficult  to  surmise. 

However,  any  analytical  method  that  can  be  devised  will  often  prove  inade- 
quate for  either  a  qualitative  or  quantitative  analysis.  For  the  former  one 
must  rely  mainly  on  conclusions  drawn  from  the  general  appearance  and 
physical  qualities  of  the  sample,  and  nowhere  Is  the  faculty  of  acute  perception 
of  greater  service  than  in  the  identification  of  certain  constituents  from 
peculiarities  of  odor,  taste,  consistency,  hardness,  mobility,  appearance  when 
magnified,  etc.,  even  though  considerably  modified  by  other  ingredients  intro- 
duced for  legitimate  ends  or  to  conceal  evidences  of  sophistication  or  inferior 
quality.  Not  less  helpful  are  the  hints  afforded  by  a  familiarity  with  the  com- 
positionj  of  other  articles  in  common  use  for  the  same  or  similar  purposes  —  in 
fact  the  chemist  for  the  time  being  must  assume  the  rol£  of  botanist,  mineral- 
ogist, pharmacist,  metallurgist  or  what  not  according  to  the  problem  before 
him. 

For  the  quantitative  analysis  it  is  of  course  impossible  to  lay  down  any 
general  plans  of  proceedure  where  the  subjects  are  so  varied  in  character, 
but  it  may  be  said  that  for  the  most  part  an  actual  separation  of  all  the  con- 
stituents is  impracticable  and  that  attributive  methods  have  here  a  large  ap- 
plication ;  often  recourse  must  be  had  to  mechanical  separations,  sometimes  of 
the  crudest  nature.  In  the  case  of  liquids,  distillation  and  examination  of  the 
several  fractions,  especially  the  first,  may  give  information  as  to  volatile  con- 
stituents. For  powders  or  friable  solids  mechanical  separation  by  sifting 
through  various  sized  meshes,  elutriation,  vanning  or  jigging,  will  often  serve 
to  distinguish  most  or  all  of  the  constituents.  Colorimetric  methods  can 
sometimes  be  employed  for  one  or  more  members. 

A  scheme  that  has  much  to  commend  it  is  to  first  identify  as  many  of  the 
constituents  as  can  be  made  out,  then  compound  a  mixture  of  these  that  shall 
resemble  the  original  in  physical  properties,  altering  the  proportions  until  a 


TECHNICAL    AND    INDUSTRIAL    ANALYSIS.  575 

close  agreement  is  attained.  If  successful  the  approximate  composition  is 
already  known  and  may  be  corroborated  by  an  analysis  of  the  sample ;  but  if 
it  is  found  impossible  to  prepare  a  fair  duplicate  in  this  way,  the  differences 
noted  will  often  indicate  where  the  qualitative  deductions  were  incorrect  or 
suggest  the  nature  of  the  missing  associates.  And  occasionally  during  the 
succeeding  analysis  as  one  constituent  after  another  is  disposed  of,  artificial 
mixtures  should  be  compounded  and  compared  with  the  original  sample. 

Prior  to  the  actual  analysis  of  a  sample  of  this  kind  it  is  well  to  learn  the 
particulars  of  the  manufacture  as  far  as  they  are  made  public  or  can  be  gathered 
from  any  source,  the  selling  price,  the  directions  for  its  use  and  any  precau  - 
tions  in  the  application  enjoined,  for  what  particular  features  superiority  is 
asserted  and  what  defects  in  similar  articles  overcome.  The  general  habitus  of 
the  sample  furnished  should  be  noted  and  any  peculiarities  of  consistency,  color, 
odor  when  cold  or  on  heating,  heterogeneity,  alteration  on  exposure  to  light, 
air  or  moisture,  the  specific  gravity,  etc. 

Observations  such  as  the  above,  considered  individually  and  mutually,  will 
generally  furnish  clues  to  shape  the  course  of  the  succeeding  analysis.  A  prac- 
tical test  of  the  material,  a  study  of  the  composition  of  similar  articles,  their 
selling  price  and  the  relative  esteem  in  which  the  various  makes  are  held  by  the 
public,  and,  if  possible,  a  visit  to  the  manufactory  and  observation  of  the  pro- 
cesses of  manufacture  as  far  as  may  be  permitted,  may  result  in  much  pertinent 
information.  In  many  cases  it  will  be  found  more  expedient  to  conduct  the 
analysis  on  the  material  after  it  has  been  prepared  to  the  state  or  condition  as 
used  by  the  consumer  than  in  the  original  form  as  purchased  on  the  market. 
Finally,  a  consultation  with  a  chemist  whose  line  of  work  is  confined  largely  to 
materials  similar  to  the  one  in  hand  may  throw  more  light  on  an  obscure  point 
than  days  of  research. 

Investigations  of  this  kind  are  most  interesting,  and  although  at  times  the 
outcome  may  not  be  all  that  could  be  hoped  for,  cannbt  but  be  highly  instruc- 
tive to  the  patient  worker. 

A  class  of  articles  frequently  to  be  analyzed  by  the  technical  chemist  is  that 
of  homogeneous  mixtures  of  two  or  more  analogous  complex  bodies  incorpo- 
rated in  the  liquid  state.  Usually  the  identity  of  each  component  is  lost  as 
regards  any  means  of  direct  separation  now  known,  though  it  is  conceivable 
that  in  some  cases  perfect  amalgamation  is  long  deferred  and  that  processes 
of  differentiation  might  be  discovered  applicable  at  least  to  recent  mixtures. 
But  at  present  we  are  restricted  to  methods  based  on  (I),  the  determination 
of  a  normally  occurring  constituent  (either  originally  present  or  introduced 
in  the  technical  treatment  or  developed  by  age)  of  one  member,  absent  from 
the  other;  (2),  the  divergence  in  the  proportions  of  one  or  more  of  the 
common  constituents  of  the  members;  and  (3),  a  measurable  physical  constant 
of  unlike  values  in  the  members  and  persisting  in  the  mixture.  But  it  is  to  be 
noted  that  the  proportion  of  any  constituent  may  vary  between  wide  limits 
and  be  a  doubtful  quantity,  and  that  processes  of  preservation  or  purification 
may  alter  the  proportion  of  any  one  constituent  or  entirely  remove  it  from 
the  mixture,  or  modify  the  value  of  a  physical  constant. 

In  the  valuation  of  merchandise  by  analysis  two  cases  may  be  presented. 
One  is  where  the  material  examined  contains  but  one  constituent  of  value 
for  the  manufacturing  or  other  purpose  intended,  and  an  assay  of  the 
material  shows  at  once  its  money  value  as  far  as  constitution  is  concerned. 
Usually  the  material  is  priced  at  a  certain  sum  per  '  unit ',  a  unit  being 
one  per  cent  in  most  cases.  Exceptions  are  where  the  constituent's  in  two 
or  more  forms  or  combinations  that  are  of  unequal  value  for  the  purpose 


576  QUANTITATIVE    CHEMICAL    ANALYSIS. 

on  account  of  the  unlike  treatment  they  must  undergo  in  manufacturing,  or 
where  the  cost  of  conversion,  the  losses  sustained,  or  the  quality  of  the  prod- 
uct varies  with  the  relative  proportions.  For  example,  a  material  from  which 
the  constituent  is  to  be  extracted  by  an  acid,  one  part  readily  dissolves  in  a 
comparatively  dilute  acid,  the  other  only  in  strong  acid  and  incompletely  even 
after  long  digestion  at  a  high  temperature. 

The  other  case  is  that  of  material  containing  several  constituents  of  more 
or  less  value  for  the  purpose,  or  detrimental  as  the  case  may  be ;  it  is  usually 
a  difficult  problem  to  determine  to  what  extent  each  contributes  to  or  modi- 
fies the  value,  and  any  schedule  that  aims  to  fix  the  relative  values  of  the  con- 
stituents and  allow  a  calculation  of  the  absolute  or  relative  value  of  a  given 
lot  must  be  to  some  degree  arbitrary. 

The  conduction  of  a  sample  of  a  raw  material  or  intermediate  product 
through  the  routine  of  the  manufacturing  process  the  material  is  to  undergo 
must  be  done  with  much  circumspection,  and  the  results  liberally  interpreted, 
for  it  is  doubtful  whether  the  conditions  attending  factory  practice  can  ever 
be  exactly  duplicated  in  the  laboratory.  It  is  well  known  that  many  processes 
yield  products  that  differ  more  or  less  with  respect  to  the  amount  of  material 
operated  on,  certain  secondary  reactions  that  have  a  great  influence  on  the 
product  appearing  only  with  a  certain  minimum  of  material ;  the  unavoidable 
losses  are  nearly  always  less  in  proportion  to  the  weight  of  substance  treated ; 
the  application  of  heat,  refrigeration,  aeration,  extraction  by  solvents  and  like 
operations  proceed  more  uniformly  and  thoroughly  on  the  large  scale ;  these 
and  other  factors  tend  to  lessen  one's  confidence  in  deductions  from  tests  on 
the  small  scale.  Many  times  have  conclusions  drawn  from  laboratory  tests 
been  flatly  contradicted  when  the  same  operations  were  repeated  in  the  factory. 
Generally  speaking,  the  result  of  a  miniature  test  is  the  more  trustworthy  in 
proportion  to  the  amount  of  material  treated. 


As  a  prediction  of  the  result  of  an  analysis  based  on  the  general  appearance 
and  physical  properties  of  the  substance  analyzed  often  proves  fallacious,  con- 
versely due  caution  is  advisable  when  attempting  to  infer  mechanical  or  other 
characteristics  from  the  chemical  composition,  for,  contrary  to  what  is  antici- 
pated, an  analysis  alone  may  afford  no  information  whatever  as  to  the  general 
character  or  adaptability  for  a  given  purpose. 

Qualities  commonly  regarded  as  distinctive  and  characteristic  are  not  infre- 
quently qualified  or  suppressed  from  various  causes.  Thus  we  think  of  steel 
as  hard,  an  acid  sour,  silver  white,  and  quartz  vitreous;  yet  antecedent  thermal 
treatment,  degree  of  insolubility,  atomic  and  molecular  arrangement,  the  pres- 
ence of  associates  known  or  unsuspected,  etc.,  may  modify  or  efface  any  of 
these  attributes.  In  the  large  class  of  crypto -crystalline  and  flbro-crystalline 
bodies  the  influence  of  physical  structure  on  their  manifest  properties  is 
dominant  —  the  shape,  size,  and  juxtaposition  of  the  crystals,  their  regular  or 
segregated  distribution  through  a  matrix,  their  dowelling  or  interlacing  where 
vicinal,  the  effects  of  progressive  crystallization,  fibration,  granularity  from 
enveloping  shells,  etc.  Odor,  flavor,  aroma,  are  susceptible  of  modification  by 
physical  structure,  as  is  well  known,  and  color,  the  quality  so  often  relied  on  to 
establish  the  identity  or  composition  of  a  substance,  often  misleads.  Illustra- 
tions are  met  with  in  commercial  articles  made  up  by  different  manufacturers 
according  to  one  formula  but  markedly  unlike  in  appearance. 

In  face  of  such  disturbing  factors  it  is  a  question  how  far  a  knowledge  of  the 


TECHNICAL    AND    INDUSTRIAL    ANALYSIS.  577 

composition  will  afford  an  insight  to  the  properties  of  a  substance  or  justify 
an  opinion  as  to  its  practical  value  or  utility.  Plainly  a  proximate  analysis 
will  usually  be  a  far  more  substantial  basis  for  conclusions  than  an  ultimate 
one. 

Specifications.  That  the  seller  may  be  informed  as  to  the  grade  of  goods 
that  will  be  accepted  by  the  buyer  there  has  been  adopted  by  most  large  con- 
sumers the  plan  of  submitting  printed  specifications  that  detail  explicitly  what 
tests  and  inspections  each  purchase  must  pass.  According  to  the  nature  of 
the  article  the  tests  may  be  physical  or  chemical  or  both,  with  in  many  cases 
an  additional  stipulation  as  to  general  appearance  or  finish.  Following  are 
examples. 

SPECIFICATIONS  FOR  GALVANIZED  TELEGRAPH  WIRE. 

1.  The  weight  of  wire  per  mile  to  be  approximately  as  follows:  No.  4  gauge, 
730  Ibs.;  No.  6,  540  Ibs.;  No.  8,  380  Ibs.;  No.  9,  320  Ibs.;  and  No.  10,  260  Ibs. 

2.  The  tensile  strength  shall  not  be  below  2.5  times  the  weight  of  the  wire  in 
pounds  per  mile. 

3.  The  electrical  resistance  in  ohms  per  mile  at  68  °  Fahr.  shall  not  exceed 

4800 
the  fraction  -^-  where  W  is  the  weight  in  pounds  per  mile. 

4.  The  wire  to  be  circular  in  section,  soft  and  pliable,  and  have  a  smooth 
surface.    A  piece  6  inches  in  length  when  gripped  in  vises  and  twisted  must  not 
break  below  15  full  twists. 

5.  The  thickness  and  adhesion  of  the  galvanizing  will  be  tested  as  follows: 
the  wire  is  immersed  in  a  saturated  neutral  solution  of  copper  sulfate  for  one 
minute,  wiped  with  a  cloth  and  examined;  after  four  treatments  in  this  way  the 
wire  must  show  no  coating  of  copper.  * 

6.  Not  less  than  five  samples  will  be  taken  at  random  from  each  lot  of  wire 
received,  and  tested  by  an  inspector  representing  the  consignees  according  to 
the  above  specifications.    If  more  than  10  per  cent  of  the  samples  fail  to  meet 
the  requirements,  the  entire  lot  will  be  rejected  and  returned  to  the  manu- 
facturers. 

SPECIFICATIONS   FOR   OLEINE :    SPECIAL  GRADE. 

This  grade  of  oleine  will  be  bought  on  sample.  Any  barrels  of  a  shipment 
that  show  a  lower  quality  than  the  sample  furnished  will  be  returned  to  the 
shipper. 

1.  The  oleine  to  be  clear  at  50°    Fahr.,  the  color  light  to  medium  brown, 
and  the  gravity  at  60°  Fahr.  from  .890  to  .910. 

2.  The  ash  not  to  exceed  .5  per  cent. 

3.  The  total  free  acid  not  to  be  below  95  per  cent;  hydrocarbon  oils  not  over 
3  per  cent;  and  neutral  fat  not  over  3  per  cent. 

4.  The  free  mineral  acids  not  to  exceed  .6  per  cent,  and  the  oleine  not  to  con- 
tain more  than  traces  of  lead,  copper,  or  arsenic,  and  not  over  .3  per  cent  of 
iron  oxide. 

5.  Containers  to  be  plainly  marked  «'  Oleine  —  Special  grade".      Samples 
sent  with  quotations  not  to  be  less  than  one  pint. 

Usually  the  chemist  of  the  buyer  draws  up  the  specifications  for  the 
material  purchased,  availing  himself  of  the  advice  of  those  directly  concerned 
in  its  use  and  often  of  the  sellers  also.  The  specifications  are  revised  from 


*  The  zinc  coating  of  the  wire  reacts  with  copper  sulfate,  the  zinc  dissolving  and  being 
replaced  by  an  equivalent  weight  of  copper  In  a  loose  spongy  form  that  can  be  readily 
wiped  off.  Iron  and  steel  also  react  with  copper  snlfate,  but  in  this  case  the  copper  de- 
posit firmly  adheres  to  the  wire  and  cannot  be  wiped  off. 

37 


578  QUANTITATIVE    CHEMICAL   ANALYSIS. 

time  to  time  as  conditions  of  use  and  the  market  changes  and  practical  tests 
and  investigations  advise,  avoiding,  however,  frequent  and  radical  changes  as 
far  as  possible. 

To  formulate  a  specification  that  shall  be  equitable  to  manufacturer,  middle- 
man and  buyer  is  not  infrequently  a  matter  of  some  difficulty.  Many  adopt  the 
simple  plan  of  averaging  the  results  of  the  tests  of  all  purchases  during  sev- 
eral previous  years  that  have  proved  satisfactory  in  service;  others  select 
as  a  standard  the  products  of  some  one  manufacturer  of  acknowledged  high 
reputation.  And  a  few  appear  to  rely  entirely  on  their  own  opinions  however 
opposed  to  the  views  of  others  fully  as  experienced  and  perspicacious  as  them- 
selves. It  is  plain  that  to  proceed  intelligently  in  drawing  up  a  specification 
one  should  be  familiar  not  only  with  the  practical  use  of  the  material  but  with 
the  processes  and  limitations  of  manufacture  as  well.  And  all  specifications 
should  be  interpreted  not  as  hard  and  fast  rules  but  as  guides  to  be  relaxed  or 
modified  as  the  case  in  hand  appears  to  justify  —  yet  it  must  be  remembered 
that  if  the  inspector  is  to  be  strictly  impartial  and  favor  no  one  of  a  number  of 
competing  sellers,  he  must  fix  and  adhere  to  a  uniform  final  limit  with  all,  other 
things  being  equal.* 

Undoubtedly  there  have  been  issued  and  are  now  in  force  specifications  for 
certain  materials  whose  quality  would  be  better  determined  by  the  judgment 
of  those  familiar  with  the  purposes  for  which  they  are  intended.  For  in  many 
cases  those  particular  qualities  that  are  most  valuable  to  the  user  are  as  yet 
but  imperfectly  known  from  the  lack  of  systematic  and  conclusive  investigations. 
Again,  a  given  material  may  be  unusually  susceptible  to  the  treatment  it 
undergoes  during  its  application  and  while  in  service,  the  good  qualities  being 
conserved  and  the  faults  mitigated,  or  the  reverse,  according  to  the  skill  and 
care  of  whoever  is  directly  employed  in  its  application,  use  or  preservation. 
So  that  widely  differing  opinions  are  held  as  to  the  composition  or  physical 
properties  that  are  most  desireable;  and  manufacturers  of  certain  products 
have  repeatedly  claimed  that  the  stipulations  of  a  specification  of  this  kind 
could  be  met  by  material  of  a  lower  grade  than  they  would  be  willing  to  put  on 
the  market  admitted  as  their  own  product.  And  there  are  some  who,  with 
this  in  view,  would  do  away  with  specifications  largely  or  entirely,  asserting 
that  as  many  of  the  qualities  that  earn  for  a  material  a  deserved  reputation 
cannot  as  yet  be  reduced  to  set  phrases,  the  conscientious  manufacturer  has  no 
recognition  of  his  efforts  to  produce  a  superior  article. 

Yet  although  carried  to  an  extreme  by  the  injudicious,  there  can  be  no 
question  that  the  plan  of  purchasing  under  specifications  has  on  the  whole  been 
of  great  advantage  to  all  parties  concerned.  Restricted  to  requirements  that 
are  reasonable  and  generally  regarded  as  a  fair  criterion  of  quality  and  free 
from  individual  pet  theories,  there  can  be  raised  no  valid  objection  on  the 
part  of  the  seller,  and  where  close  limits  in  regard  to  such  matters  as  uni- 
formity, durability,  facility  of  application  and  the  like  can  only  be  laid  down 
somewhat  arbitrarily,  there  should  be  asked  no  more  than  what  is  considered 
as  moderate  and  reasonable  by  those  best  qualified  by  study  and  experience 
to  form  an  opinion  on  the  subject,  thereby  conserving  the  interests  of  the 
buyer  and  laying  no  undue  hardship  on  the  manufacturer. 


Adulteration.  A  large  share  of  the  occupation  of  the  technical  chemist  is  in 
the  examination  of  foods,  condiments  and  beverages,  drugs  and  proprietary 


*  The  Manufacture  and  Properties  of  Structural  Steel,  358. 


TECHNICAL    AND    INDUSTRIAL    ANALYSIS,  579 

medicines,  and  raw  materials  and  products  of  the  arts  for  evidences  of  an 
unadmitted  debased  quality. 

As  to  what  constitutes  a  prima  facie  case  of  adulteration  with  certain  articles 
is  to  some  extent  a  matter  of  opinion,  and  it  is  often  left  to  the  chemist  to 
decide  whether  a  certain  constituent  should  be  classed  as  an  adulterant 
within  the  legal  interpretation  of  the  term.  Practically,  the  laws  relating  to 
the  adulteration,  degradation  and  falsification  of  foods  and  drugs  are  based 
on  the  following  premises. 

1.  The  incorporation  of  any  foreign  substance  to  increase  weight,  bulk  or 
strength,  to  conceal  evidence  of  debased  or  inferior  quality,  or  to  confer  a 
fictitious  appearance,  flavor  or  odor. 

2.  The  omission  or  withdrawal  of  some  valuable  constituent,  wholly  or  in 
part. 

3.  The  presence,  either  originally,  developed,  or  by  addition,  of  a  poisonous 
ingredient  or  one  undoubtedly  injurious  to  health. 

4.  The  manufacture  of  any  product  from  diseased  or  tainted  flesh  or  decom- 
posed fruit  or  vegetables. 

5.  So  naming  or  describing  an  article  of  domestic  production  as  to  lead  the 
purchaser  to  infer  a  foreign  origin  or  that  the  article  is  of  some  well  known 
superior  grade  or  from  a  factory  of  established  high  reputation. 

In  the  case  of  drugs 

1.  If  when  retailed  for  medicinal  purposes  under  a  name  recognized  in  the 
Pharmacopoeia,  it  be  not  equal  in  strength  or  purity  to  the  standard  there  laid 
down. 

2.  If  when  sold  under  a  name  not  recognized  in  the  Pharmacopoeia  it  differs 
materially  from  the  standard  laid  down  in  approved  works  on  materia  medica 
or  the  professed  standard  under  which  it  is  sold. 

However,  the  addition  to  an  article  of  food  or  a  drug  of  some  foreign  body 
or  the  withdrawal  of  an  unimportant  constituent  for  purposes  of  preservation, 
to  allow  of  packing  or  transportation,  or  to  facilitate  the  subsequent  preparation 
for  comsumption,  cannot  be  considered  as  illegitimate,  nor  can  the  tinting  of 
an  article  with  a  color  to  add  to  its  attractiveness  or  for  other  reasons;  simi- 
larly there  may  be  allowed  a  reasonable  proportion  of  matters  ordinarily  included 
during  the  operation  of  collection,  transportation,  or  preparation  for  the  mar- 
ket, or  additidn?  to  inhibit  the  use  for  a  certain  purpose  while  not  interfering 
with  its  use  for  others,  or  to  retain  certain  important  properties  (e.  g.,  solu- 
bility). But  in  all  the  above  concessions  the  presence  of  such  additions  or  the 
removal  of  constituents  must  be  admitted  at  the  time  of  sale  unless  they  are  so 
customary  as  to  be  well  known  to  the  public  and  expected  by  the  purchaser, 
and  in  the  case  of  additions  to  foods  or  drugs,  that  they  are  wholesome  or 
innocuous. 

Articles  wholly  factitious  do  not  properly  come  under  the  definition  of  adul- 
terants though  taken  cognizance  of  in  the  laws  on  the  subject. 

The  extent  of  the  practice  of  adulteration  at  the  present  time  is  undoubtedly 
greatly  exaggerated,  for  the  proportion  of  adulterated  foods  and  beverages  on 
the  market  to  those  unquestionably  pure  is  far  less  than  is  popularly  believed. 
Many  products  reputed  to  be  most  subject  to  tampering,  are  in  reality  the 
purest,  and  many  of  the  adulterants,  if  not  wholly  imaginary,  are  seldom  or 
never  met  with  at  present. 

The  steady  decline  in  the  practice  of  adulteration  may  be  credited  in  part  to 
the  zeal  of  the  civil  authorities  co-operating  with  analysts,  and  partly  to  the 
continuous  reduction  in  the  price  of  raw  materials  and  improvements  in  the 
processes  of  preparation  and  manufacture  that  have  lowered  the  cost  of  the 


580  QUANTITATIVE    CHEMICAL    ANALYSIS. 

products  so  far  that  it  is  no  longer  profitable,  at  least  on  a  small  scale,  to  con- 
tinue the  use  of  adulterants  formerly  much  cheaper  than  the  article  itself.  And 
manufacturers  and  dealers  not  overscrupulous,  have  come  to  realize  that  in 
the  long  run,  the  profits  gained  in  a  fraudulent  way  do  not  compensate  for  the 
inevitable  loss  of  reputation.  Yet,  even  at  the  present  time,  it  is  remarkable 
how  great  ingenuity  is  displayed  by  some  who  devote  their  talents  to  dis- 
covering how  far  and  in  what  manner  adulteration  can  be  practiced  without 
likelihood  of  detection ;  and  it  is  alleged  that  in  some  establishments  chemists 
are  employed  chiefly  for  this  purpose. 

Where  adulteration  is  suspected,  the  sampling  of  the  article  should,  if  pos- 
sible, be  done  in  presence  of  the  health  oflicer  or  the  buyer  and  seller  or  their 
representatives,  and  the  sample  immediately  divided  into  three  equal  parts, 
one  for  each  party,  and  the  third  sealed  up  and  reserved  for  an  umpire.  Ma- 
terial that  is  wholly  or  partly  organic  should  as  a  rule  be  analyzed  as  soon 
after  sampling  as  practicable. 

It  is  of  course  a  great  advantage,  especially  in  legal  controversies,  if  the 
adulterant  can  be  separated  as  such  in  a  pure  state  and  produced  as  evi- 
dence. Even  approximate  separations,  yielding  the  adulterant  in  a  fairly  pure 
state,  such  as  obtained  by  fractional  solution,  distillation,  etc.,  may  answer 
the  purpose,  provided  the  distinguishing  properties  of  the  adulterant  are  not 
masked  by  the  bodies  remaining  associated  with  it. 

A  direct  separation,  however,  is^ften  impossible  for  the  reasons  that  the 
composition  of  the  adulterant  approaches  closely  to  that  of  the  original,  the 
chemical  and  physical  properties  of  the  two  are  similar,  or  marked  chemical 
characteristics  are  wanting  in  one  or  both.  These  and  other  causes  may  pre- 
vent a  direct  separation,  and  so  recourse  must  be  had  to  attributive  methods 
or  crude  mechanical  separations. 

A  determinable  chemical  constituent  may  be  normal  to  the  original  but  ab- 
sent from  the  adulterant,  or  vice  versa,  and  allow  a  fairly  accurate  determina- 
tion, calculating  from  the  constituent  to  the  compound  containing  it;  the  same 
is  true  of  an  associate  normally  present  in  either  in  a  fairly  constant  propor- 
tion. 

A  certain  definite  change  that  the  original  or  adulterant  undergoes  on  treat- 
ment by  some  regeant  with  increase  in  weight  or  volume  may  be  availed,  or  the 
new  combination  may  admit  of  direct  gravimetric  or  volumetric  determination. 

In  some  few  instances,  only  the  adulterant  reacts  directly  with  a  volumetric 
solution  or  dissolves  to  a  colored  solution  at  once  or  after  a  chemical  change, 
affording  an  easy  and  accurate  estimation.  The  possible  influence  on  the 
titration  or  the  colorimetric  comparison  by  the  constituents  of  the  original 
must  be  considered.  The  above  applies  as  well  when  only  the  original  reacts  or 
colors  the  solution. 

Attributive  methods  may  be  applied  where  a  constant  of  the  original  is 
determinable  and  the  same  constant  of  the  adulterant  is  nil,  or  the  reverse,  or 
where  both  possess  the  constant  but  in  widely  differing  ratios.  It  must  be 
remembered  however  that  the  calculated  figure  for  a  constant  of  two  admixed 
bodies  can  be  brought  to  equal  that  of  either  by  the  judicious  addition  of  a 
third,  hence  the  risk  in  attempting  to  pronounce  on  the  genuineness  of  a  sample 
from  the  determination  of  but  one  constant.  And  in  cases  where  the  constants 
do  not  differ  greatly,  the  maximum  of  the  lower  in  some  varieties  may  reach 
the  minimum  of  the  higher.  Obviously  the  more  extended  the  examination 
and  the  greater  the  number  of  constants  determined  the  more  easily  is  so- 
phistication detected,  and  the  more  confidently  may  the  chemist  pronounce  on 
the  purity  of  a  sample. 


TECHNICAL    AND    INDUSTRIAL    ANALYSIS.  581 

Microscopic  examination,  often  the  easiest  and  surest  means  of  detecting 
adulteration,  may  sometimes  be  applied  as  a  means  of  numerical  estimation, 
this  when  the  difference  in  appearance  between  the  original  and  adulterant  is 
great  enough  to  admit  of  a  sharp  distinction,  or  when  by  staining,  application 
of  polarized  light,  selective  solvents,  etc.,  can  be  brought  to  this  condition. 

Finally,  it  must  be  considered  that  in  the  more  scientific  modes  of  falsifi- 
cation the  adulterant  may  not  be  incorporated  in  the  form  usually  found  on  the 
market  but  only  after  some  preliminary  treatment  that  has  modified  its  con- 
stitution, appearance  or  reactions.  Hence  the  futility  of  attempting  to  draw 
conclusions  from  reactions  valid  only  with  the  adulterant  in  the  ordinary 
commercial  state. 

To  pronounce  positively  on  the  purity  of  many  organic  commercial  bodies 
is  often  difficult  and  it  is  sometimes  necessary  to  lay  down  a  standard  of 
purity  that  is  more  or  less  arbitrary.  This  applies  particularly  where  the 
methods  are  based  on  certain  chemical  or  physical  constants.  And  on  account 
of  the  difficulty  of  isolation  in  the  pure  state,  or  the  uncertainty  attending  a 
deduction  from  a  set  of  constants,  the  results  may  be  very  doubtful  if  not 
altogether  far  from  correct,  and  the  prudent  chemist  will  clothe  his  report  in 
language  that  will  allow  a  safe  margin  for  defects  in  analytical  methods,  and 
not  be  more  specific  in  designating  the  adulterant  than  his  tests  will  justify. 


At  times  one  may  hear  applied  analysis  deprecated  on  the  score  that 
industrial  analysis  in  general,  following  in  the  main  methods  that  are  fixed 
and  inelastic  —  perhaps  «  standard  '  —  in  no  way  aids  in  the  development  of 
chemistry  as  a  science  or  analysis  as  an  art,  and  opposes  the  breadtn  and  indi- 
viduality that  should  characterize  the  chemist;  as  evidence  is  cited  the  custom 
in  some  industries  of  intrusting  analytical  work  to  tyros  ignorant  of  the  sig- 
nificance and  purpose  of  the  operations  they  mechanically  perform,  lowering  the 
practice  to  a  mere  subservience  to  a  string  of  recipes.  A  few  would  even  go 
so  far  as  to  exclude  applied  analysis  from  the  list  of  intellectual  arts  and 
classify  it  as  a  handicraft  pure  and  simple. 

But  in  this  practical  age  it  will  hardly  be  contended  that  the  application  of 
the  principles  of  a  science  to  the  advancement  of  an  art  is  in  any  sense  "  de- 
rogatory" however  " unscientific"  it  may  be.  Such  ideas,  advanced  by  so 
small  a  minority,  would  hardly  merit  serious  consideration  were  they  not  urged 
by  a  few  whose  standing  in  the  chemical  world  gives  weight  to  their  opinions. 

In  defense  of  the  dignity  of  technical  analysis  *  it  may  be  said  that  objections 
like  these  are  advanced  the  more  confidently  in  proportion  as  the  critic  is  un- 
familiar with  the  nature  and  diversity  of  the  problems  to  be  solved  in  technical 
analysis  and  the  original  investigation  necessitated  by  the  limited  information 
to  be  gathered  from  the  literature  on  special  branches  of  technology.  As  to 
the  incompetence  of  many  employed  in  technical  work  it  may  be  answered  that 
the  practice  of  engaging  as  assistants  those  with  little  or  no  chemical  knowl- 
edge is  undoubtedly  on  the  wane.  It  is  well  that  such  is  the  case,  although 
at  present  usually  the  analytical  work  assigned  to  novices  is  of  the  simplest 
character,  and  so  monotonous  and  wearying  that  a  trained  analyst  would  en- 
dure it  only  so  long  as  compelled  by  stress  of  circumstances.  And  those  who 
are  content  to  remain  permanently  at  routine  analysis  are  not  of  the  class  from 
which  research  for  scientific  or  practical  ends  may  be  expected  whatever  the 


*  Journ.  Amer.  Chem.  Socy.  1898—81. 


582  QUANTITATIVE    CHEMICAL   ANALYSIS. 

position  in  which  they  may  be  placed.  Finally  it  must  be  considered  that 
through  a  steady  demand  for  those  having  at  least  an  elementary  education  in 
the  principles  and  practice  of  chemistry,  there  is  provided  for  others  —  per- 
haps those  who  most  decry  It  —  an  opportunity  and  facilities  for  the  prosecu- 
tion of  research  along  more  congenial  and  less  utilitarian  lines. 


To  the  student  who  proposes  to  adopt  chemistry  as  a  profession  a  few  hints 
as  to  the  opportunities  of  engaging  in  this  line,  the  prospects  of  advancement, 
and  the  course  of  study  that  will  aid  him  to  success,  may  be  helpful. 

Leaving  out  of  consideration  the  occupation  of  teaching,  two  courses  are 
open  for  the  graduate :  he  may  establish  a  public  laboratory,  or  become  an  em- 
ploye of  a  manufacturing  or  other  Industry. 

The  outcome  of  a  venture  of  the  first  kind  is  problematical.  To  build  up  a 
business,  the  essentials  are  a  location  within  convenient  reach  of  his  patrons, 
whether  his  clientele  is  the  general  public  or  those  engaged  in  a  special  line  of 
business,  an  extensive  personal  acquaintance  among  those  from  whom  he  may 
derive  patronage,  and  a  reputation  for  integrity  and  professional  ability.  At- 
taining these,  and  with  a  fair  share  of  enterprise,  discretion  and  business 
ability,  he  may  in  time  arrive  at  a  comfortable  income,  or  more  if  fortunate 
enough  to  secure  large  contracts.  But,  for  the  beginner,  probably  a  long 
period  will  elapse  before  he  can  gain  an  adequate  reputation  and  patronage 
and  the  experience  that  will  enable  him  to  advise  with  the  client  more  versed 
in  practical  than  scientific  affairs  and  to  whom  an  analysis  alone,  though  minute 
and  exact,  will  often  fail  to  give  the  information  he  seeks. 

Competition  is  as  evident  among  public  analysts  as  elsewhere,  and  he  will 
be  forced  to  recognize  if  not  to  meet,  the  rivalry  of  those  who  from  childhood 
have  been  accustomed  to  habits  of  severe  frugality  and  are  content  with  an  in- 
come below  what  the  average  American  would  consider  sufficient  for  a  bare 
existence,  and  to  secure  it  will  not  hesitate  to  reduce  their  fees  to  a  slight 
margin  above  laboratory  expenses. 

On  the  whole,  unless  the  possession  of  a  competency  from  other  sources  will 
assure  independence  during  the  first  years,  the  outlook  is  not  the  brightest ; 
more  have  failed  than  have  succeeded. 

On  the  other  hand,  should  he  decide  to  engage  as  a  works-chemist,  it  is 
likely  that  he  will  more  easily  find  employment  in  a  laboratory  where  a  number 
of  chemists  are  engaged,  since  changes  in  the  working  force  and  additions  are 
more  frequent  than  in  a  smaller  establishment.  Points  in  favor  of  beginning  in 
a  laboratory  of  this  kind  are  that  experience  and  technical  knowledge  are  not 
so  essential  as  elsewhere,  and  that  he  can  avail  himself  of  the  counsel  and 
assistance  of  his  older  associates.  If  his  tastes  incline  in  the  direction  of  the 
supervision  of  some  branch  of  a  manufactory,  the  opportunities  for  advance- 
ment to  such  a  position  are  exceptionally  good,  as  it  is  the  policy  of  the  ad- 
ministration of  many  works  to  promote  chemists  to  be  heads  of  departments  if 
found  to  possess  the  requisite  executive  ability. 

Against  such  a  position  is  the  monotony  of  the  continuous  grinding  out  of 
one  kind  of  determinations,  long  and  perhaps  unseasonable  hours,  and  a  salary 
that  may  hardly  exceed  that  of  an  office  clerk.  As  a  permanent  engagement 
there  is  little  that  is  attractive,  yet  as  an  antecedent  to  one  more  congenial  and 
lucrative  it  has  much  to  commend  it. 

In  the  majority  of  works- laboratories  the  number  of  analyses  required  is  not 
sufficient  to  justify  the  employment  of  more  than  one  chemist,  and  usually  the 


TECHNICAL    AND    INDUSTRIAL    ANALYSIS.  583 

analytical  work  is  more  varied  and  interspersed  with  practical  experiments, 
so  that  he  escapes  the  tedium  of  confinement  to  one  special  kind  of  analysis. 
Being  alone,  the  chemist  must  rely  more  on  his  own  ability  to  choose  and  adapt 
methods  best  suited  to  his  purposes,  and  has  usually  some  time  at  his  disposal 
for  investigation  and  experiment  in  this  direction.  The  opportunity  for  devis- 
ing improvements  and  economies  in  the  processes  of  manufacturing  or  in  other 
directions  is  greater  than  in  larger  works  provided  with  a  scientific  staff,  and 
where  study  and  experiment  have  already  so  far  perfected  the  technic  that  no 
betterment  seems  possible  short  of  a  radical  change. 

Detracting  from  such  a  position  is  the  comparative  isolation  of  the  chemist. 
Confined  for  the  most  part  to  the  laboratory,  he  is  deprived  of  association  with 
others  following  scientific  pursuits  and  the  benefit  of  their  counsel  and  en- 
couragement and  can  form  fewer  acquaintances  among  those  in  other  practical 
occupations  and  in  business  through  whom  advancement  most  frequently  comes. 
Another  drawback  is  the  injunction  of  secrecy  in  regard  to  the  details  of  the 
manufacture  and  innovations,  restraining  him  from  publishing  and  receiving 
credit  from  his  fellows  and  the  public  for  meritorious  work  he  may  do,  granted 
that  his  discoveries  are  not  so  specific  in  nature  as  to  be  of  no  general  interest. 

A  source  of  annoyance  or  worse  that  he  will  likely  encounter  will  be  the 
open  or  secret  jealousy  of  some  of  the  t( practical"  fellow-employees  of  the 
establishment,  who  fearing  loss  of  prestige  from  exposure  of  their  igno- 
rance or  errors,  are  ever  on  the  alert  to  belittle  and  oppose  any  really  advanta- 
geous step  the  chemist  may  propose,  and  as  the  advent  of  a  laboratory  is  to 
many  works  a  novelty  whose  usefulness  has  yet  to  be  proved,  the  administra- 
tion is  apt  to  view  it  as  a  rather  dubious  and  costly  experiment  and  display  a 
painful  regard  for  economy  in  fitting  up  laboratory  quarters,  the  purchase  of 
supplies,  and  particularly  toward  the  salary  of  the  chemist. 


Let  us  briefly  consider  the  question  so  important  to  one  contemplating  the 
adoption  of  the  practice  of  chemistry  as  a  vocation,  as  to  the  course  of  study 
in  general  and  particular  that  shall  best  fit  him  to  assume  the  duties  of  the 
technical  or  industrial  chemist. 

Unquestionably  the  subject  has  not  suffered  for  want  of  discussion.  It  has 
been  written  on  and  spoken  on  from  every  conceivable  point  of  view,  by 
those  who  from  a  long  and  comprehensive  acquaintance  with  the  correlation  of 
analysis  and  practical  affairs  and  the  possibilities  and  limitations  of  the 
college  and  technical  school  are  qualified  to  express  opinions  that  merit 
earnest  consideration,  and  by  others  whose  knowledge  of  either  or  both  is 
limited  to  vague  theories  gained  at  second-hand. 

As  might  be  supposed,  the  convictions  of  the  former  class  differ  greatly 
Some  advocate  a  course  confined  largely  or  exclusively  to  theoretical  chemistry 
and  synthetic  organic  experiment,  others  lay  stress  on  the  desirability  of  a 
maximum  of  special  practical  work  in  technical  analysis  and  the  study  of 
processes.  Out  of  the  voluminous  arguments  that  have  been  advanced  by 
the  adherents  of  either  side,  let  me  extract  from  the  writings  of  a  few  authors 
that  may  fairly  represent  the  extremes. 

The  first  is  from  a  letter  by  Prof.  Dr.  Ostwald  of  Leipzig.* 

"  When  the  student  [of  a  German  university]  has  finished  his  course  he  is 
still  entirely  free  to  choose  between  a  scientific  and  technical  career.  This  is  a 
very  important  point  in  our  educational  system;  it  is  made  possible  by  the  cir- 


*  Chemist  &  Druggist,  1896—353. 


584  QUANTITATIVE    CHEMICAL    ANALYSIS. 

cumstance  that  the  occupation  of  a  technical  chemist  in  a  works  is  very  often 
almost  as  scientific  in  character  as  in  a  University  laboratory.  This  is  connected 
with  a  remarkable  feature  in  the  development  of  technical  chemistry  in  Ger- 
many —  the  very  point  upon  which  the  important  position  of  chemical  manu- 
facture in  this  country  depends.  The  organization  of  the  power  of  invention  in 
manufactures  and  on  a  large  scale  is,  as  far  as  I  know,  unique  in  the  world's 
history,  and  it  is  the  very  marrow  of  our  splendid  development.  Each  large 
work  has  the  greater  part  of  its  scientific  staff  —  and  there  are  often  more  than 
100  Doctores  Phil,  in  a  single  manufactory  —  occupied,  not  in  the  management 
of  the  manufacture,  but  in  making  inventions.  The  research  laboratory  in  such 
a  work  is  only  different  from  one  in  a  University  by  its  being  more  splendidly 
and  sumptuously  fitted  than  the  latter.  I  have  heard  from  the  business  mana- 
gers of  such  works  that  they  have  not  unfrequently  men  who  have  worked  for 
four  years  without  practical  success ;  but  if  they  know  them  to  possess  ability 
they  keep  them  notwithstanding,  and  in  most  cases  with  ultimate  success  suffi- 
cient to  pay  the  expenses  of  the  former  resultless  years. 

It  seems  to  me  a  point  of  the  greatest  importance  that  the  conviction  of  the 
practical  usefulness  of  a  theoretical  or  purely  scientific  training  is  fully 
understood  in  Germany  by  the  leaders  of  great  manufactories.  When,  some 
years  ago,  I  had  occasion  to  preside  at  a  meeting,  consisting  of  about  two- 
thirds  practical  men  and  one-third  teachers,  I  was  much  surprised  to  observe 
the  unhesitating  belief  of  the  former  in  the  usefulness  of  entirely  theoretical 
investigations.  And  I  know  a  case  where,  quite  recently,  an  *  extraordinary  * 
professor  of  a  University  has  been  offered  a  very  large  salary  to  induce  him  to 
enter  a  works,  only  for  the  purpose  of  undertaking  researches  regarding  the 
practical  use  of  some  scientific  methods  which  he  had  been  working  at  with 
considerable  success.  .  .  .  You  will  excuse  my  boasting  about  our  German 
management  of  this  most  important  question  of  scientific  education.  It  is  no 
blind  admiration  without  criticism,  for  I  know  by  practical  experience  the 
management  in  other  countries  and  I  can  compare  them." 

The  second  an  extract  from  a  paper  by  Bancroft*  on  the  Relation  of  Physical 
Chemistry  to  Technical  Chemistry. 

"  ...  To  my  mind,  specialisation  and  research  work  do  not  give  the  proper 
training.  .  .  .  A  man  specializing  in  organic  chemistry  gets  a  training  in  manip- 
ulation and  in  methods  of  making  new  compounds;  in  addition,  he  increases 
his  knowledge  of  chemistry  and  of  chemical  phenomena.  This  work  qualifies 
him  to  meet  one  of  the  requirements  of  the  manufacturer;  he  can  make  himself 
valuable  in  discovering  new  and  useful  compounds,  and  in  working  out  new 
methods  of  making  compounds  already  known.  His  training  has  not  been  of  a 
nature  to  make  him  especially  valuable  in  improving  methods  ....  I  wish  to 
emphasize  the  fact  that  the  ideals  of  the  organic  chemist  are  not  the  ideals  of 
the  manufacturer,  and  that  a  training  in  organic  chemistry  is  not  the  best 
training  for  a  technical  chemist.  I  have  laid  stress  on  the  training  in  organic 
chemistry,  rather  than  on  the  training  in  inorganic  chemistry,  because  organic 
chemistry  rather  overshadows  inorganic  chemistry  in  most  of  our  universities 
and  colleges.  It  is,  however,  equally  clear  that  inorganic  chemistry,  as  now 
taught,  does  not  offer  the  ideal  training  for  a  technical  chemist Person- 
ally, I  do  not  believe  in  the  teaching  of  technical  chemistry  as  technical  chem- 
istry. To  my  mind,  a  comparison  of  German  results  with  English  results 
shows  very  conclusively  that  the  best  way  to  teach  technical  chemistry  is  to 
teach  scientific  chemistry The  whole  matter  can  be  summed  up  in  a  few 


*  Journ.  Amer.  Chem.  Socy.  1899—1101. 


TECHNICAL   AND    INDUSTRIAL   ANALYSIS.  5#5 

words.  A  good  training  in  physical  chemistry  is  the  best  possible  preparation 
for  a  technical  chemist;  .  .  .  .  "  , 

The  third,  from  a  communication  by  Percy  Williams,  of  Colorado,*  writing 
on  the  subject  of  the  technical  training  of  metallurgical  chemists. 

"  .  .  .  .  It  is  immaterial  how  careful,  accurate  and  conscientious  the 
new  graduate  may  be;  upon  his  advent  into  the  mine  or  smelter  laboratories 
he  is  simply  overwhelmed  by  the  amount  and  variety  of  his  first  day's  work, 
and  may  be  either  cast  adrift  to  hunt  up  another  position,  not  feeling  particu- 
larly encouraged  by  his  first  experience  at  practical  work,  or  if  he  is  particu- 
larly fortunate  he  may  be  retained  at  the  establishment  in  some  subordinate 
capacity  at  a  small  salary  and  given  an  opportunity  to  familiarize  himself  with 
those  countless  details  which  he  was  unable  to  acquire  in  his  school.  The 
opportunities,  however,  of  taking  a  post-graduate  course  in  some  smelting 
company's  laboratory  while  the  company  itself  pays  the  tuition  bills  are  rare, 
and  the  result  is  that  a  large  percentage  of  our  technical  graduates  must  under- 
go a  protracted  course  of  hard  knocks,  drifting  about  the  mining  districts,  re- 
ceiving a  smattering  of  valuable  experience  here  and  there  until  eventually 
they  find  themselves  sufllciently  practiced  to  hold  a  difficult  position  with  a 
smelter  and  able  to  make  accurately  a  hundred  analyses  every  day  if  need  be. 

It  is  but  justice  to  the  manager  to  admit  that  he  cannot  be  expected  to  spend 
time  and  money  in  allowing  his  laboratories  to  become  a  training  school  for 
young  men  fresh  from  their  universities  or  mining  schools;  indeed  he  has  the 
right  to  expect  that  reputable  mining  schools  shall  send  him  assistants  fully 
prepared  to  enter  upon  the  duties  required  of  them  and  earn  the  salaries  paid 
to  them. 

Yet  all  metallurgists  who  have  been  in  charge  of  any  of  our  large  smelter 
laboratories  for  any  length  of  time  know  that  nine  out  of  every  ten  men  who 
enter  their  offices  direct  from  school  prove  unsatisfactory,  however  great  their 
ability  and  earnestness  of  purpose  may  be,  because  they  become  at  once  con- 
fused by  the  amount  and  variety  of  work  which  must  be  accomplished.  They 
are  ignorant  of  most  of  those  important  little  details  of  manipulation,  familiarity 
with  which  alone  enables  the  chemist  to  become  a  rapid  analyst  without  making 
any  concessions  to  accuracy. 

....  Some  of  my  associates  will  think  the  picture  I  have  attempted  to  draw, 
illustrating  the  difficulties  besetting  the  path  of  the  improperly  trained  chemist, 
to  be  exaggerated ;  but  I  am  satisfied  that  the  chemists  themselves  who  have 
attempted  to  fill  responsible  positions  directly  after  their  graduation  will  agree 
that  I  have  not  overestimated  the  terrors  of  the  situation." 

The  fourth,  a  letter  from  the  manager  of  a  large  manufactory,  to  whose  acumen 
and  wide  experience  many  can  bear  witness  — 

"  ....  As  you  say,  I  have  exceptionally  good  opportunities  for  placing 
young  men  in  our  laboratory  with  a  view  of  promoting  them  later  on,  and 

observe  how  they  get  on  with  chemical  work For  some  years  I  have 

refused  to  take  on  any  graduate  who  has  not  had  a  year's  practical  experience, 
at  least,  in  my  line  of  work  after  he  has  graduated.  The  reason  is  that  I  am 
tired  of  the  trouble  and  confusion  that  a  beginner  always  causes.  Not  one  of 
them,  and  I  have  had  men  from  many  of  the  large  universities  abroad  and 
some  in  this  country,  but  what  was  deficient  in  what  we  needed  most,  I  mean 
that  there  was  a  want  of  a  broad  enough  knowledge  of  analytical  chemistry  to 
take  hold  of  the  analysis  I  wanted  and  carry  them  along  without  constant  su- 
pervision. Some  were  well  posted  in  assaying,  water  analysis,  etc.,  but  none 


*  Engineering  and  Mining  Journ.,  1897  —  477. 


586  QUANTITATIVE    CHEMICAL    ANALYSIS. 

of  them  had  any  practice  in  the  kind  of  work  done  here  [organic,  prin- 
cipally] .....  So  in  future  I  shall  let  the  *  other  fellow '  do  the  finishing  up 
of  their  education  and  when  they  have  spent  a  year  or  so  in  his  laboratory 
they  can  apply  to  me. 

Another  writer*  refers  in  the  same  strain  to  industries  wherein  the  chemist 
has  the  direction  or  oversight  of  a  department  of  the  works  — 

"  Some  years  ago  a  chemical  firm  in  one  of  our. Eastern  cities  was  desirous 
of  obtaining  the  services  of  a  chemist  who  should  take  charge  of  the  factory. 
Accordingly  advertisements  were  inserted  in  the  industrial  journals  for  a  man 
who  should  Hot  only  be  familiar  with  the  analytical  work  necessary,  but  who 
could  also  assume  the  responsibility  of  overseeing  the  plant,  checking  the 
running  of  the  various  proc'esses,  and  meeting  the  emergencies  that  are  con- 
stantly arising  in  operations  of  this  kind. 

A  large  number  of  answers  were  received.  Interviews  were  requested  with 
those  who,  from  their  letters,  appeared  to  be  the  most  likely  to  suit.  But,  as 
a  result,  it  soon  appeared  that  the  securing  of  a  competent  man  was  by  no 
means  an  easy  matter.  Some  of  the  applicants  whose  letters  were  most  assur- 
ing, turned  out  to  have  been  simply  laboratory  boys.  Others,  more  promising, 
were  of  foreign  birth,  but  unfamiliar  with  the  language  and  customs  of  their 
[adopted  ?]  country.  Some  were  undesirable  on  account  of  their  personal  man- 
ner or  character.  But  by  far  the  most  general  objection  was  that  the  knowl- 
edge and  experience  of  these  chemists  were  limited  to  the  field  of  analytical 
chemistry  and  to  the  work  of  the  laboratory.  t  They  were  entirely  familiar 
with  the  handling  of  beaker  glasses  and  funnels,  platinum  crucibles,  analytical 
balances,  burettes  and  flasks  But  in  the  matter  of  treating  material  in  large 
quantities,  and  obtaining  result's  in  the  factory,  they  came  up,  as  it  were, 
against  a  stone  wall.  Many  of  them^  in  fact,  were  literally  as  unfamiliar  with 
the  operations  of  a  chemical  plant  as  they  were  with  the  working  of  an  astro- 
nomical observatory." 

44  It  should  be  observed  that  the  case  here  described  is  by  no  means  an  isolated 
one.  There  is  reason  to  believe  that  there  is  hardly  a  large  chemical  manufac- 
turer in  the  country  who,  at  one  time  or  another  in  his  life,  has  not  had 
experiences  of  a  nature  similar  to  this  one." 

"  It  will  be  admitted  that  the  question  of  technical  education  is  a  most  im- 
portant one.  It  deserves  at  least  as  much  attention  in  the  United  States  as  it 
does  elsewhere,  on  account  of  the  remarkable  progress  and  development  of 

industrial  activity  here It  is  for  this  important  field,  then,  that  the 

universities  and  technological  schools  of  the  country  prepare  their  young  men. 
And  it  is  because  the  quality  of  this  technical  talent  is  so  frequently  below  what 
is  called  for  that  I  venture  to  draw  attention  to  certain  considerations  on  the 
subject  that  may  be  of  interest .  .  .  ." 

Between  such  diametrically  opposite  views  as  are  held  by  those  who  have  just 
been  quoted  all  shades  of  opinion  prevail,  and  it  is  not  likely  that  any  agreement 
can  be  reached,  however  far  the  discussion  may  be  extended.  It  will  be  noted 
however  that  each  writer  on  the  subject  has  in  view  a  particular  industry  or  class 
of  industries  for  which  he  opines  the  technical  chemist  should  be  specially  trained. 

Let  me  present  a  few  observations  that  seem  germane  to  the  subject. 

The  views  of  those  who  favor  a  course  of  instruction  mainly  or  exclusively 
confined  to  theory  and  original  investigation  in  organic  and  physical  chemistry 
have  in  most  cases  been  formed  through  a  study  of  the  conditions  of  manufac- 
ture and  trade  prevailing  in  foreign  countries.  In  America  the  situation  pre- 
sents many  and  obvious  differences. 


*  The  Leather  Manufacturer,  5—2. 


TECHNICAL    AND    INDUSTRIAL    ANALYSIS.  587 

The  manufacturers  of  this  country,  with  of  course  some  notable  exceptions, 
as  a  class  are  not  sc  inclined  to  devote  their  highest  efforts  to  attaining  and 
maintaining  an  unexcelled  and,  if  possible,  an  unrivaled  quality  in  their  wares. 
A  large  output  immediately  marketed,  offers  such  financial  inducements  that 
many  are  quite  indifferent  whether  the  product  of  competitors  equals  or  sur- 
passes their  own,  so  long  as  a  satisfactory  profit  is  forthcoming.  Until  recent 
years,  also,  competition  has  not  been  so  keen  as  to  reduce  the  margin  of  profit 
to  a  point  where  strict  attention  to  the  minor  economies  of  manufacture  and 
the  utilization  of  waste  products  becomes  a  necessity. 

Again,  many  of  the  great  industries  of  Europe  have  their  counterparts  here 
only  in  a  small  way,  if  at  all  —  we  have  no  Merck,  no  Baeyer,  no  Analin  Fabrik 
that  can  absorb  hundreds  of  graduates  in  employment  in  research  work  along 
lines  almost  identical  with  their  exercises  in  the  universities.  American  estab- 
lishments of  this  kind  are  smaller  and  less  independent  financially,  and  their 
products  fewer  and  more  limited  to  the  staple  articles  in  common  use.  And 
the  general  policy  of  the  manufacturer  is  rather  in  the  direction  of  lowering 
the  cost  of  manufacture  of  his  products  by  high  organization  of  labor,  the  in- 
stallment of  labor-saving  machinery,  and  greater  rapidity  of  conversion,  than 
toward  the  discovery  and  invention  of  new  processes  and  products. 

Finally,  it  cannot  be  denied  that  in  America  science  and  its  followers  are  not  ac- 
corded that  universal  respect  that  is  so  plainly  noticeable  in  European  countries. 
From  various  causes,  there  is  here  a  disposition  among  the  so-called  practical 
men  to  disparage  or  deride  the  efforts  of  those  who  would  bring  scientific 
principles  and  manufacturing  practice  into  harmonious  association,  and  the 
depreciation  of  such,  expressed  with  characteristic  positiveness,  will  often- 
times be  more  convincing  to  the  uninformed  than  the  moderate  assertions  of 
the  more  liberal  and  well-informed. 

The  chemical  course  of  the  scientific  school  aims  to  combine  a  training  in 
chemistry  and  allied  subjects  with,  as  far  as  may  be,  a  preparation  for  prac- 
tical work  in  technical  laboratories,  in  addition  to  such  other  studies  as  it  is 
believed  will  conduce  to  a  broad  and  liberal  mental  training.  The  four  years 
course  pursued  at  the  leading  scientific  schools  is  made  up  usually  on  the  fol- 
lowing lines.  During  the  Freshman  year  general  chemistry  is  the  major 
study,  with  mathematics,  a  foreign  language,  and  elementary  physics  as 
minors;  in  the  second  year  these  subjects  are  continued  and  inorganic  quali- 
tative analysis  begun.  Inorganic  quantitative  analysis  is  the  main  require- 
ment during  the  Junior  year,  with  exercises  in  the  gravimetric  and  volumetric 
determinations  of  the  common  metals,  electrolytic  assays,  and  gas  analysis. 
In  the  Senior  year  are  taken  up  technical  and  manufacturing  chemistry, 
metallurgy,  and  technical  analysis,  with  more  or  less  synthetical  organic 
work  including  ultimate  organic  analysis  —  the  latter  forming  the  greater  part 
of  the  year's  employment  in  some  institutions.  The  practice  in  technical 
analysis  is  usually  in  the  line  of  examinations  of  iron  and  steel,  ores,  waters, 
foods,  and  dairy  products.  « 

A  comprehensive  acquaintance  with  general  chemistry  is  indispensable  to 
the  progressive  technical  chemist  in  whatever  line  of  technical  work  he  may 
be  engaged.  For  so  intimate  is  the  connection  between  the  principles  there 
enunciated  and  illustrated  and  the  practice  of  analysis  and  technology  that  one 
not  well  grounded  therein  must  necessarily  be  mechanical  in  whatever  he 
essays,  lacking  the  confidence  to  leave  the  beaten  path  of  detailed  methods 
and  practice. 

Qualitative  analysis  acquaints  the  student  with  the  properties  of  the 
common  elements  and  their  chemical  reactions  and  affords  practice  in  many 


588  QUANTITATIVE    CHEMICAL    ANALYSIS. 

of  the  manipulations  used  in  quantitative  analysis,  and  as  such  is  the  logical 
introduction  to  the  latter.  Of  late  years  the  tendency  is  to  shorten  the  time 
devoted  to  qualitative  work  to  the  benefit  of  quantitative,  which  is  without 
doubt  a  move  in  the  right  direction,  considering  how  short  a  time  at  best  can 
be  allowed  the  latter.  For  some  rather  obscure  reason  instruction  in  qualita- 
tive analysis  has  usually  been  confined  to  the  common  metals  and  acids  to  the 
exclusion  of  organic  bodies  to  which  a  considerable  part  of  the  course  might 
be  devoted  with  advantage. 

The -allied  branches  —  physics,  mineralogy,  metallurgy,  electro-chemistry, 
pharmaceutics,  etc.,  all  contribute  in  some  degree  to  the  attainments  of  the 
student  and  prove  of  special  practical  use  in  future  life. 

Applied  chemistry,  describing  the  application  of  chemical  principles  to 
manufacturing  and  the  arts,  is  of  value  in  disclosing  the  possibilities  in 
manufacturing  processes  and  drawing  attention  to  the  prime  consideration  of 
every  process,  that  of  cost  of  installation  and  prosecution.  Especial  attention 
should  be  paid  to  the  modern  practices  that  have  almost  revolutionized  many 
industries,  and  to  comparison  with  those  antiquated  and  obsolete. 

As  an  auxiliary  study  I  would  strongly  recommend  the  details  of  technology, 
meaning  by  this  the  particulars  of  the  preparation,  application  and  use  of  the 
different  varieties  of  such  materials  as  are  common  to  all  manufacturing  estab- 
lishments. However  well  instructed  in  chemistry  and  general  technology  the 
graduate  may  be,  it  is  hardly  probable  that  on  entering  the  laboratory  of  a  man- 
ufactory he  will  be  able  to  suggest  any  important  changes  in  the  routine  of  man- 
ufacture or  will  have  gained  the  confidence  of  the  management  to  the  extent 
that  it  will  entertain  on  his  advice  alone  any  proposition  involving  the  expend- 
iture of  a  considerable  sum  or  any  radical  modification  of  a  process.  Many 
chemists  who  have  attempted  to  introduce  material  changes  before  gaining  a 
thorough  acquaintance  with  the  processes  involved  have  met  only  failure  and 
derision. 

But  in  every  manufactory  there  are  a  large  number  of  minor  matters  that  may 
well  engage  the  attention  of  the  young  chemist,  points  where  there  has  been  no 
investigation  heretofore  or  at  least  none  sufficiently  thorough.  He  will  doubt- 
less find  that  there  are  in  use  many  materials  that  while  answering  their 
respective  purposes  fairly  well  can  often  be  improved  on  or  replaced  by  some- 
thing better  or  cheaper;  or  the  grade  of  purchases  may  be  variable,  some- 
times up  to  standard,  sometimes  below. 

Such,  for  example,  are  paints  for  special  localities  exposed  to  the  weather, 
gases,  or  gritty  dust;  the  coal  for  steam  generating,  the  water  used  for 
boilers  or  cleansing  and  its  purification  if  necessary;  the  lubricants  for  vari- 
ously weighted  journals;  iron  and  brass  castings;  antifriction  metals;  illum- 
inating oils;  waste  and  packing,  etc.  The  examination  of  such  articles  and 
the  endeavor  to  correct  their  faults  or  replace  with  something  better  cannot 
fail  to  result  in  an  aggregate  improvement  or  saving  that  will  alone  repay  the 
cost  of  the  laboratory  and  the  salary  of  the  chemist.  One  who  is  alert  and 
studious  can  always  find  sufficient  subjects  for  investigation  to  profitably 
occupy  a  large  share  of  his  time  while  incidentally  gaining  that  intimate 
knowledge  of  the  principles  and  practice  of  the  routine  of  manufacture  that 
will  restrain  him  from  proposing  or  assenting  to  changes  that  are  impracticable 
or  useless. 

During  the  first  years  of  practical  life  the  chemist  should  endeavor  to  master 
the  business  principles  that  govern  every  successful  industrial  plant  even 
though  he  should  have  no  expectation  of  entering  a  position  where  he  would  be 
called  on  to  apply  them.  For  as  in  the  discussion  of  matters  appertaining  to- 


TECHNICAL    AND    INDUSTRIAL    ANALYSIS.  589 

his  engagement,  he  is  brought  into  personal  contact  with  those  in  authority, 
their  estimate  of  his  ability  and  value  in  his  position  will  often  be  formed 
more  in  proportion  to  his  familiarity  with  practical  affairs  than  from  his  strictly 
scientific  attainments  however  great.  And  this  is  also  true  of  others  with 
whom  he  may  have  business  relations.  Even  so  small  an  accomplishment  as 
the  ability  to  use  the  customary  trade-names  when  speaking  of  tools  and  the 
details  of  machinery  will  gain  for  him  a  certain  respect  among  workmen  that 
is  desirable  when  he  would  seek  their  advice  or  co-operation,  often  most 
valuable. 

Up  to  recent  years  a  student  had  necessarily  to  prepare  himself,  as  far  as 
possible,  for  every  line  of  chemical  work.  For  of  the  industries  of  this  coun- 
try commonly  employing  chemists,  in  but  two  or  three  were  the  plants  so 
numerous  and  extensive  as  to  warrant  one  in  confining  his  studies  and  exer- 
cises in  analysis  and  technology  to  the  class  of  work  there  pursued,  with 
a  reasonable  expectation  of  obtaining  employment  therein.  Moreover,  in  these 
industries  a  permanent  position  is  less  certain  than  in  others,  the  one  pecu- 
liarly susceptible  to  the  fluctuations  of  the  market,  responding  to  the  earliest 
hint  of  a  general  business  depression  by  curtailing  or  ceasing  operations  for 
an  indefinite  period;  the  other  liable  at  any  time  to  face  the  exhaustion  of  its 
natural  supplies.  One  who  was  so  fortunate  as  to  know  In  advance  in  what 
particular  line  of  industry  he  would  be  employed  and  the  nature  of  the  ana- 
lytical and  other  work  he  was  to  undertake,  had  of  course  no  difficulty  in 
arranging  the  latter  part  of  his  educational  course  along  the  same  lines.  But 
the  majority  of  students  had  no  such  assurance,  and  after  graduation  cast 
about  for  employment  and  accepted  what  offered.  With  this  prospect  in  view 
he  has  had  to  consider  what  studies  afforded  the  best  training  in  the  way  of 
enabling  him  to  enter  wherever  opportunity  offered  with  a  fair  promise  of  a 
successful  career. 

But  with  the  advent  of  a  more  rational  scientific  operation  of  manufacturing 
plants,  the  generally  admitted  value  of  chemical  analysis  to  the  buyer  of  mate- 
rials, and  the  substantial  interest  manifested  in  the  application  of  chemistry 
to  the  arts,  the  field  open  to  the  chemist  has  widened  until  he  can  now  prepare 
for  a  single  department  with  reasonable  assurance  of  finding  employment 
therein. 

It  may  be  conceded  that  no  course  of  study  of  the  usual  collegiate  length  can 
possibly  constitute  an  adequate  preparation  for  the  immense  variety  of  analyt- 
ical work  that  is  demanded  in  the  different  industries  the  graduate  may  enter. 
It  is  impossible  for  even  the  most  gifted  and  industrious  to  become  an  expert 
in  assaying  ores,  analyzing  iron  and  steel,  dyestuffs,  fertilizers,  drugs  and 
Pharmaceuticals,  dairy  products,  explosives,  oils,  and  the  long  list  of  other 
materials,  each  class  demanding  considerable  special  practice  ere  confidence  in 
the  analytical  results  is  obtained.  It  matters  little  how  expert  the  chemist  may 
be  in  one  line,  the  difficulties  he  encounters  in  essaying  another  can  only  be 
surmounted  by  special  study  and  practice  —  the  less,  of  course,  in  proportion 
as  the  general  principles  of  analysis  have  been  mastered  and  dexterity  in  manip- 
ulation acquired. 

Formerly,  the  student  had  little  voice  in  the  selection  of  the  studies  that 
should  comprise  a  chemical  course;  a  single  rigid  routine  was  insisted  on,  re- 
gardless of  the  purposes  and  prospects  of  the  student,  and  optional  branches 
were  few.  Latterly,  a  more  liberal  policy  obtains  in  the  more  progressive 
schools;  who,  while  insisting  on  the  pursuance  of  a  combination  of  studies  that 
is  designed  to  give  breadth  and  mental  culture,  yet  allow  more  or  less  change 
by  the  substitution  of  special  branches,  and  where  a  degree  in  course  is  not  to 


590  QUANTITATIVE    CHEMICAL    ANALYSIS. 

oe  conferred,  offer  special  courses  adapted  to  qualify  the  student  for  engaging 
in  any  special  industry  he  may  elect.  In  making  such  a  selection,  the  experi- 
ence and  foresight  of  those  who  have  arranged  the  courses  should  be  given  due 
consideration,  and  their  advice  regarding  the  curtailing  or  omission  of  certain 
studies  and  the  substitution  of  others  should  be  sought  before  a  decision  is 
made. 

Finally,  there  can  be  no  doubt  that  in  future  a  more  thorough  grounding  in 
technical  analysis  will  be  required  of  the  industrial  chemist  than  has  been  the 
rule  in  the  past.  The  complaint  of  Williams  as  to  the  inability  of  the  recent 
graduates  of  technical  schools  to  cope  with  the  routine  work  of  the  smelter 
laboratory  is  no  doubt  well  founded.  Much  the  same  condition  has  prevailed  — 
and  still  is  in  evidence  to  some  extent  —  in  iron  and  steel  laboratories,  and 
those  in  charge  of  various  other  industries  have  assured  me  that  the  same  is 
true  within  their  observation,  and  that  his  strictures  voice  the  prevailing 
sentiment  among  manufacturers  in  general. 

Nevertheless  while  it  is  reasonable  that  an  employer  may  insist  that  an  ap- 
plicant for  the  position  of  chemist  shall  be  acquainted  with  the  general  principles 
of  chemical  processes  and  the  analysis  of  the  materials  treated,  and  cannot  be 
censured  for  engaging  a  chemist,  as  he  would  any  other  employ^,  on  the  pre- 
sumption that  he  is  master  of  his  trade  and  prepared  to  immediately  render  an 
equivalent  for  his  wage,  he  cannot  in  fairness  expect  a  familiarity  with  the 
minutiae  and  special  practice  of  the  laboratory  of  a  particular  plant  nor  with 
analyses  that  involve  unusual  manipulations  or  in  which  a  knowledge  of  the 
properties  of  uncommon  organic  compounds  is  essential  to  their  determination. 
In  no  two  works  are  the  materials  to  be  analyzed  exactly  the  same ;  the  methods  of 
analysis  that  are  deemed  most  suitable  differ  more  or  less ;  the  apparatus  provided 
varies  in  construction  —  these  and  other  important  matters  may  be  so  unlike 
as  to  confuse  and  discourage  the  beginner  and  embarrass  for  a  time  even  one  of 
long  experience.  A  reasonable  time  should  be  allowed  the  chemist  to  become 
acquainted  with  his  surroundings  and  the  special  routine  required,  and  it  should 
not  be  expected  that  a  recent  graduate  shall  be  so  well  trained  in  any  given 
line  as  to  be  capable  of  immediately  performing  as  great  an  amount  of  analytical 
work  as  one  with  months  or  years  of  special  practice. 

There  are  many  manufacturers  and  dealers  in  crude  and  factored  articles 
to  whom  the  utility  of  the  laboratory  has  yet  to  be  demonstrated.  To  the 
more  progressive  the  advantage  is  already  manifest,  ultimately  all  will  admit 
it,  but  it  is  undeniable  that  progress  in  this  direction  has  been  far  slower 
than  could  reasonably  be  expected.  The  delay  may  be  traced  partly  to  the 
caution  and  conservatism  of  the  proprietors  who  view  the  project  as  a  some- 
what costly  and  doubtful  experiment;  but  it  is  undeniable  that  much  of  this 
hesitation  is  the  outcome  of  the  failures  of  pioneer  chemists,  unequal  to 
the  duties  of  their  positions  and  ignorant  of  the  principles  governing  the 
operation  of  a  successful  manufacturing  plant. 

All  will  concede  that  a  course  of  instruction  devoted  largely  or  entirely  to 
theoretical  chemistry  and  work  in  organic  synthesis  is  not  to  be  undervalued 
for  the  knowledge  and  mental  discipline  it  affords,  nor  is  this  less  true  where 
physical  chemistry  is  the  main  topic.  But  where  an  education  must  be  to  a 
great  extent  a  means  to  an  end,  where  one  must  step  from  the  college  door 
into  the  technical  laboratory,  the  inadequacy  of  such  a  course  of  training 
is  apparent  to  all  familiar  with  the  duties  of  the  technical  chemist. 

I  would  emphasize  this  point,  recalling  a  score  of  failures  that  have  come 
under  my  observation;  graduates  of  universities  of  high  repute  at  home 
and  abroad,  men  of  undoubted  ability  and  well  grounded  in  theoretical  and 


.     TECHNICAL   AND    INDUSTRIAL   ANALYSIS.  591 

organic  chemistry  make  up  a  good  share* of  the  list*.  Entering  a  works- 
laboratory  where  all  is  severely  practical,  he  has  been  confronted  by  a  first  in- 
stallment or  an  accumulation  of  material  to  be  analyzed  formidable  enough 
even  to  one  of  long  experience.  Often  mixtures  of  divers  kinds  were  com- 
pounded by  the  manager  or  samples  selected  that  had  been  previously  ex- 
amined by  experts,  these  to  be  analyzed  at  once  and  the  results  to  closely  tally 
with  the  synthesis  or  previous  analyses.  Is  it  to  be  wondered  at  that,  distrust- 
ful of  what  ability  he  really  possessed,  the  chemist  has  resigned  forthwith, 
seeing  the  futility  of  attempting  the  task  before  him,  or  a  little  later  is  dis- 
missed as  Incompetent;  and  discouraged  and  disheartened,  has  abandoned 
further  attempts  and  turned  to  a  pursuit  less  fraught  with  difficulties  to  the 
beginner? 

And  what  is  more  to  be  deplored,  the  failure  of  his  initial  effort  cannot  but 
raise  a  doubt  on  the  part  of  the  administration  as  to  the  advantage  of  chemi- 
cal investigation  and  control  to  the  manufacturer,  especially  if  the  laboratory 
is  an  innovation,  and  it  is  not  probable  that  the  experiment,  successful  only 
in  engendering  the  disappointment  of  the  managers  and  the  derision  of  the 
workmen,  will  be  repeated,  at  least  until  a  change  of  administration;. nor  will 
it  encourage  the  institution  of  laboratories  in  other  works  of  the  same  or  a 
similar  kind. 

It  is  true  that  those  exceptionally  gifted  with  perseverance  and  self-confi- 
dence will  overcome  these  difficulties  and  ultimately  succeed,  and  others  from 
the  favoring  circumstances  of  entering  large  laboratories  as  assistants  and  at 
first  assigned  simple  routine  work,  or  through  the  lenience  of  employers  are 
able  to  conceal  their  incompetency  for  the  time  being.  But  many  times  will 
these  have  cause  to  feel  and  regret  the  handicap  of  unfamiliarity  with  matters 
directly  touching  their  employment. 

The  opportunities  for  engaging  in  technical  and  industrial  chemistry  are  at  the 
present  time  fully  as  great  as  in  any  other  profession.  Industries  already  more 
or  less  under  chemical  control  are  increasing  their  chemical  staffs,  and  others 
only  await  the  coming  of  those  able  to  demonstrate  the  value  of  the  art  to  their 
special  practice.  The  field  is  wide  and  by  no  means  overcrowded. 

But  the  preparation  of  the  applicant  must  be  adequate  and  appropriate  to 
the  specific  task  he  is  to  assume.  No  mere  smattering  of  the  principles  and 
practice  will  suffice.  Equipped  with  a  broad  and  comprehensive  acquaintance 
with  technical  analysis  and  its  applications  in  general  and  a  specific  branch 
in  particular,  and  with  a  fair  share  of  self-confidence  and  tact,  he  may  enter 
his  chosen  line  of  technical  analysis  with  a  reasonable  expectation  of  imme- 
diate success  —  without  these  qualifications  his  career  will  likely  be  short  and 
disappointing  or  at  best  bestrewn  with  formidable  difficulties. 

And  with  the  extension  of  chemical  control  directed  by  those  whose  training 
has  been  both  broad  and  specific  there  will  result  a  better  appreciation  on  the 
part  of  employers  of  the  dignity  and  value  of  the  art  and  its  followers ;  more 
intimate  business  relations  between  them  and  the  chemist  will  be  established 
without  the  intervention  of  minor  officials  who  so  often  restrict  and  hamper 
the  successful  operation  of  the  laboratory  through  their  inability  to  appreciate 
its  scope  and  possibilities.  On  the  other  hand  the  occupation  of  the  chemist 
will  be  less  in  the  direction  of  routine  work  of  the  laboratory,  the  minutiae  of 
analyses,  and  the  search  for  petty  economies  of  time  and  material,  and  more 
in  researches  and  experiment  in  the  application  of  chemistry  to  manufacturing 
and  trade. 


592 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


TABLES. 

TABLE  OF  ATOMIC  WEIGHTS. 
Oxygen  =16. 


Aluminum 

Antimony 

Argon 

Arsenic 

Barium 

Bismuth 

Boron 

Bromine 

Cadmium 

Caesium 

Calcium 

Carbon 

Cerium 

Chlorine 

Chromium 

Cobalt 

Columbium 

Copper 

Erbium 

Fluorine 

Gadolinium 

Gallium 

Germanium 

Glucinum 

Gold 

Helium 

Hydrogen 

Indium 

Iodine 

Iridium 

Iron 

Krypton 

Lanthanum 

Lead 

Lithium 

Magnesium 

Manganese 

Mercury 

Molybdenum 


Al 

Sb 

Ar 

As 

Ba 

Bi 

Bo 

Br 

Cd 

Cs 

Ca 

C 

Ce 

Cl 

Cr 

Co 

Cb 

Cu 

Er 

Fl 

Gd 

Ga 

Ge 

Gl 

Au 

He 

H 

In 

I 

Ir 

Fe 

Kr. 

La 

Pb 

Li 

Mg 

Mn 

Hg 

Mo 


27.1 

Neodymium 

Ne 

120.4 

Neon 

No 

39.92 

Nickel 

Ni 

75.0 

Nitrogen 

N 

137.4 

Osmium 

Os 

208.1 

Oxygen 

0 

11.0 

Palladium 

Pd 

79.95 

Phosphorus 

P 

112.4 

Platinum 

Pt 

132.9 

Potassium 

K 

40.1 

Praseodymium 

Pr 

12-0 

Rhodium 

Rh 

139.0 

Rubidium 

Rb 

35.45 

Ruthenium 

Ru 

52.1 

Samarium 

Sm 

59.0 

Scandium 

Sc 

93.7 

Selenium 

Se 

63.6 

Silicon 

Si 

166.0 

Silver 

Ag 

19.05 

Sodium 

Na 

156.0 

Strontium 

Sr 

70.0 

Sulfur 

S 

72.5 

Tantalum 

Ta 

9.1 

Tellurium 

Te 

197.2 

Terbium 

Tb 

3.96 

Thallium 

Tl 

1.008 

Thorium 

Th 

114.0 

Thulium 

Tu 

126.85 

Tin 

Sn 

193.1 

Titanium 

Ti 

56.0 

Tungsten 

W 

81.7 

Uranium 

U 

138.6 

Vanadium 

V 

206.92 

Xenon 

X 

7.03 

Ytterbium 

Yb 

24.3 

Yttrium 

Yt 

55.0 

Zinc 

Zn 

200.0 

Zirconium 

Zr 

96.0 

TABLE  2. 


143.6 
19.94 
58.7 
14.04 
191.0 
1H.O 
107.0 
31.0 
194.9 
39.11 
140.5 
103.0 
85.4 
101.7 
150.3 
44.1 
79.2 
28.4 
107.92 
23.05 
87  6 
32.07 
182.8 
127.5 
1GO.O 
204.15 
232.6 
170.7 
119.0 
48.15 
184  0 
239.6 
51.4 
128  0 
173  2 
89.0 
65.4 
90.4 


Metric  and  English  Weights  and  Measures. 

1.  Standard  of  length,  the  Meter  =  1.09361  yards.     One  foot  =  .3048  meter. 
One  centimeter  =  .39371  inch.      One  inch  =  2.54  centimeters.      One   milli- 
meter =  .039371  inch.     One  inch  =  25.40  millimeters. 

2.  Standard  of  surface,  the  Square  Meter  =  10.764  square  feet.    One  square 
foot  =  .0929  square  meter.    One  square  centimeter  =  .155  square  inch.    One 
square  inch  =*  6.4513  square  centimeters. 

3.  Standard  of  capacity,  the   Cubic  Meter  =  35.316  cubic  feet.    One  cubic 
foot  =  .02832  cubic  meter.    One  cubic  centimeter  =  .06103  cubic  inch.     One 
cubic  inch  =  16.383  cubic  centimeters.     One  liter  =  1000  cubic  centimeters  = 
1.0567  wine  quarts  =  33.84  fluid  ounces  Apoth.     One  cubic  centimeter  ==  .03382 
fluid  ounce  Apoth.     One  gallon  (U.  S.)  =  3.785  liters.     One  quart  (U.  S.)  = 
946.5  cubic  centimeters.     One  fluid  ounce  Apoth.  =  29.57  cubic  centimeters. 
One  fluid  drachm  Apoth.  =3.696  cubic  centimeters. 


TABLES. 


593 


4.  Standard  of  weight,  the  Gram  =  15.432  Troy  grains  =  .2572  Apoth. 
drachm  =  .03527  Avoirdupois  ounce  =.03215  Troy  ounce.  One  kilogram  = 
2.205  Avoirdupois  pounds  =  2.6792  Troy  pounds.  One  Troy  grain  =  .064799 
gram.  One  Troy  drachm  =  3.887  grams.  One  Troy  ounce  =  31. 10  grams. 
One  Avoirdupois  pound  =  453.55  grams.  One  Troy  .pound  =  373.22  grams. 
One  Avoirdupois  ounce  =  28.35  grams. 

One  Avoir,  oz.  =  .91146  Troy  oz.     One  Troy  oz.  =  1.09714  Avoir,  ozs. 

One  Avoir.  Ib.  =  1.21528  Troy  Ibs.    One  Troy  Ib.  =  .82286  Avoir.  Ib. 

Prefixes  in  the  metric  system.  Deka-,  ten;  Hecto-,  one  hundred;  Kilo-, 
one  thousand;  Deci-,  one-tenth;  Centi-,  one-hundredth;  Milli-,  one-thou- 
sandth. These  apply  to  all  the  standards. 

TABLE  3. 
Weight-in  Grams  of  1000  Cc.  of  Gas  at  Zero  Cent,  and  760  Mm.  of  Mercury. 


Hydrogen 0896 

Methane 7190 

Ammonia 7707 

Water 8063 

Acetylene 1.1650 

Ethylene 1.2510 

Carbon  monoxide 1.2513 

Nitrogen 1.2562 

Air 1.2939 

Ethane 1.3404 

Oxygen 1.4298 

Hydrochloric  acid 1.6131 


Nitrogen  protoxide 1.9746 

Carbon  dioxide 1 .9772 

Alcohol 2.0862 

Cyanogen 2.3360 

Sulfur 2.8430 

Sulfur  dioxide 2.8689 

Chlorine 3. 1801 

E  ther 3 . 31 70 

Chloroform 4.4507 

Bromine 6.8697 

Mercury 9.0210 

Iodine 1 1.2710 


TABLE  4. 
Volume  and  Density  of  Water  at  Different  Temperatures. 

(ROSSETTl). 


Temp. 

Volume  of 
Water 

Sp.  Gr.  of 
Water 

Temp, 
on 

Volume  of 
Water 

Sp.  Gr.  of 
Water 

oC. 

(atO°  =  l). 

(atO«=l). 

WO. 

(at  0°  =  1). 

(atO«  =  l). 

0 

1.00000 

.000000 

19 

.00141 

0.998588 

1 

0.99994 

.000057 

20 

.00161 

0.998388 

2 

0.99990 

.000098 

21 

.00183 

0.998176 

3 

0.99988 

.000120 

22 

.00205 

0.997956 

4 

0.99987 

.000129 

23 

.00228 

0.997730 

5 

0.99988 

.000119 

24 

-.00251 

0.997495 

6 

0.99990 

.000099 

25 

•00276 

0.997249 

7 

0.99994 

.000062 

26 

.00301 

0.996994 

8 

0.99999 

.000015 

27 

.00328 

0.996732 

9 

.00005 

0.999953 

28 

.00355 

0.996460 

10 

.00012 

0.999876 

29 

.00383 

0.996179 

11 

.00022 

0.999784 

30 

.00412 

0.99589 

12 

.00032 

0.999678 

40 

.00757 

13 

.00044 

0.999559 

50 

.01182 

14 

.00057 

0.999429 

60 

.01678 

15 

.00071 

0.999289 

70 

.02243 

16 

.00087 

0.999131 

80 

.02874 

17 

.00103 

0-998970 

90 

.03554 

18 

1.00122 

0.998782 

100 

.04299 

594 


QUANTITATIVE   CHEMICAL   ANALYSIS. 


TABLE  5. 

Formulae  for  Converting  the  Reading  of  One  Thermometer  to  Another. 
Fahrenheit  to  Centigrade,  subtract  32  and  multiply  by  .5556. 
Centigrade  to  Fahrenheit,  multiply  by  1.8  and  add  32. 
Fahrenheit  to  Reaumur,  subtract  32  and  multiply  by  .4444. 
Reaumur  to  Fahrenheit,  multiply  by  2.25  and  add  32. 
Centigrade  to  Reaumur,  multiply  by  .8. 
Reaumur  to  Centigrade,  multiply  by  1.25. 

Degrees  Centigrade  Corresponding  to  Degrees  Fahrenheit. 


Fahr. 

Cent. 

Fahr. 

Cent.  Fahr. 

Cent. 

Fahr. 

Cent. 

Fahr. 

Cent. 

60 

10.0 

60 

15.6 

70 

21.1 

80 

26.7 

90 

32.2 

51 

10.6 

61 

16.1 

71 

21.7 

81 

27.2 

91 

32.8 

52 

11.1 

62 

16.7 

72 

22.2 

82 

27.8 

92 

33.3 

53 

11.7 

63 

17.2 

73 

22.8 

83 

28.3 

93 

33.9 

54 

12.2 

64 

17.8 

74 

23.3 

84 

28.9 

94 

34.4 

55 

12.8 

65 

18.3 

75 

23.9 

85 

29.4 

95 

35.0 

56 

13.3 

66 

18.9 

76 

24.4 

86 

30.0 

96 

35.6 

57 

13.9 

67 

19.4 

77 

25.0 

87 

30.6 

97 

36.1 

58 

14.4 

68 

20.0 

78 

25.6 

88 

31.1 

98 

36.7 

59 

15.0 

69 

20.6 

79 

26.1 

89 

31.7 

99 

37.2 

TABLE  6. 
Corresponding  Heights  of  the  Barometer  in  Millimeters  and  Inches. 


Mm. 

Inches. 

Mm. 

Inches. 

Mm. 

Inches. 

Mm. 

Inches. 

Mm. 

Inches. 

730 

28.74 

740 

29.13 

750 

29.53 

760 

29.92 

.770 

30.32 

731 

.78 

741 

.17 

751 

.57 

761 

.96 

771 

'  .36 

732 

.82 

742 

.21 

752 

.61 

762 

30.00 

772 

.39 

733 

.86 

743 

.25 

753 

.65 

763 

.04 

773 

.43 

734 

.90 

744 

.29 

754 

.69 

764 

.08 

774 

.47 

735 

.94 

745 

.33 

755 

.73 

765 

.12 

775 

.61 

736 

.98 

746 

.37 

756 

.76 

766 

.16 

776 

.55 

737 

29.02 

747 

.41 

757 

.80 

767 

.20 

777 

.59 

738 

.06 

748 

.45 

758 

.84 

768 

.24 

778 

.63 

739 

.10 

749 

.49 

759 

.88 

769 

.28 

779 

.67 

TABLE  7. 

.0012562 
Equivalent  of  the  Fraction  760  ^  +  Q0367t)  (page 


Tern. 

Equivalent 

Tern. 

Equivalent 

Tern.   0 

Equivalent 

15 

.0000015667 

21 

.0000015346 

27 

.0000015039 

16 

.0000015612 

22 

.0000015294 

28 

.0000014989 

17 

.0000015558 

23 

.0000015242 

29 

.0000014939 

18 

.0000015505 

24 

.0000015191 

30 

.0000014890 

19 

.0000015452 

25 

.0000015140 

31 

.0000014841 

20 

.0000015399 

26 

.0000015089 

32 

.0000014792 

TABLE  8. 
Tension  of  Aqueous  Vapor  in  Millimeters  of  Mercury. 


°  Cent. 

Mm. 

o  Cent. 

Mm. 

0  Cent. 

Mm. 

o  Cent. 

Mm. 

0  Cent. 

Mm. 

10. 

9.16 

.15. 

12.70 

20. 

17 

.39 

25. 

23 

.55 

30. 

31.55 

10.5 

9.47 

15.5 

13.11 

20.5 

17 

.94 

25.5 

24 

.26 

30.5 

32.46 

11. 

9.79 

16. 

13.54 

21. 

18 

.50 

26. 

24 

.99 

31. 

33.41 

11.5 

10.12 

16.5 

13.97 

21.5 

1!) 

.07 

26.5 

25 

.74 

31.5 

34.37 

12. 

10.46 

17. 

14.42 

22. 

19 

.66 

27. 

26 

.51 

32. 

35.36 

12.5 

10.80 

17.5 

14.88 

22.6 

20 

.27 

27.5 

27 

.29 

32.5 

36.37 

13. 

11.16 

18. 

15.36 

23. 

20 

.89 

28. 

28 

.10 

33. 

37.  U 

13.5 

11.53 

18.5 

15.85 

23.5 

21 

.53 

28.5 

28 

.93 

33.5 

38.47 

14. 

11.91 

19. 

16.35 

24. 

22 

.18 

29. 

29 

.78 

34. 

39.57 

14.5 

12.30   19.5 

16.86   24.5 

22 

.86   29.5 

30 

.65 

34.5 

40.68 

TABLES.  595 

TABLE  9. 
EMPIRICAL  VOLUMETRIC   SOLUTIONS. 

Let  it  be  required  to  make  up  of  a  reagent  a  a  solution  of  which  one  cubic 
centimeter  shall  react  with  6  grams  of  an  element  or  compound  c;  then  6  X  f is 
the  theoretical  weight  in  grams  of  a  to  be  dissolved  to  one  liter. 

a  c  F 

Arsenious  oxide Iodine 390.2 

"  "     Chlorine 1396.3 

Barium  chloride  cryst Sulf uric  acid  (H2SO4) 2491 .0 

Barium  hydrate  cryst Nitric  acid  (HNOs) 2502.4 

"  "         "     Hydrochloric  acid 4327.5 

"  "          "     Carbon  dioxide 7172.5 

Ferrous  sulfate  cryst Potassium  permanganate 8797.1 

Hydrochloric  acid Sodium  carbonate,  anhydrous 687.2 

ts  "    Potassium  carbonate ...     527.5 

"  "     Potassium  hydrate 649.7 

"  "     Sodium  hydrate 910.1 

"  k<     Nitrogen  (as  ammonia) 2596.7 

"  "     Ammonia  (NH3) 2136.5 

Iodine Sodium  thiosulfate  anhydrous 801.6 

"     "  "         crystallized 510.8 

"     Hydrogen  sulflde 7442.9 

"     Sulfur  dioxide 3959.7 

"     Arsenious  oxide 2562.6 

lf     Tin  (as  stannous  chloride)  2131.9 

Iron  (as  ferrous  salt) Nitric  acid  (HNO3) 2664.7 

"    "        "          "      Manganese  dioxide 1287.4 

"    "        "          4I      Potassium  permanganate 1770.9 

"    "        "          "      Potassium  bichromate 1141.2 

"    "        "          "      ...'. Chromium  trioxide 1678.3 

Oxalic  acid  cryst Potassium  hydrate 1123.1 

"         "        "     Manganese  dioxide 1448.8 

"         "        "    Sodium  hydrate 1573.3 

"         "        «    Ammonia  (NHS) -3693.4 

"         "        "     Nitrogen  (as  ammonia) 4488.9 

Potassium  permanganate Iron  (as  ferrous  salt) 564.7 

"  «  Hydrogen  peroxide 1859.2 

Oxalicacid  (H2C2O4) 702.6 

"  Oxalic  radical  (C204) -     .     718.7 

"  •«  Manganese  (by  precipitation) 1916.5 

".  Nitrous  acid  (HN02) 1344.2 

"  Calcium  oxide  (as  oxalate) 1127.3 

44  "  Molybdenum  sesquioxide 790.6 

"  ('  Potassium  ferrocyanide,  anhyd 85.8 

Potassium  hydrate Sulfuric  acid  (H2SO4) 1144.3 

"  " Hydrochloric  acid 1539.3 

"  "        Nitric  acid  (HNOs) 890.1 

"  "         Acetic  acid  (HC2H302) 934.8 

Potassium  bichromate Iron 876.2 

"  "  Lead 711.4 

•«  "  Potassium  iodide 295.7 

"  "  Glycerol 7462.0 

Potassium  chromate Barium 1414.3 


596  QUANTITATIVE    CHEMICAL    ANALYSIS. 

Potassium  chromate Lead 939. 1 

Potassium  iodide Iron 2963.6 

"              "      Copper 5218.9 

"              "       Chlorine 4681.5 

"             «       Bromine 2075.8 

Potassium  cyanide Copper 4097.5 

Potassium  sulfocyanide Silver 900.9 

"                  "          Copper  (cuprous) 1528.6 

Silver  nitrate Chlorine 4794.3 

te           "     Bromine 2125.8 

"            «     Iodine 1339.9 

Sodium  carbonate,  anhyd Hydrochloric  acid 1455 . 1 

"              "               "      Sulfuric  acid  (H2S04) 1081.7 

"              "               "      Ni trie  acid  (HN03) 841.4 

Sodium  chloride Silver 542.1 

Sodium  thiosulfate  cryst Iodine 1957.6 

"  «<  "    Chlorine ....7004.8 

«               "             "    Bromine 3105.9 

Sodium  sulflde  (Na2S) Copper 1229.1 

«            tt           tt      Lead 377.8 

"            "            «<      Zinc 1195.3 

Sulfuric  acid  (H2SO4) Potassium  hydrate 873.9 

"        «<          "         Ammonia  (NH3) 2874.0 

"        u          "        Sodium  hydrate 1224.3 

"        "          "         Lead 474.0 

"        «          «•         Barium 713.8 

Stannous  chloride Iron  (as  ferric  salt) 1695.5 

Conversely,  if  the  concentration  n  of  a  solution  of  a  is  known,  the  value  of  b 

is  p  .    And  if  the  titre  of  a  solution  of  a  is  that  one  cubic  centimeter  is  equal 

to  b  gram  of  an  element  or  compound  c,  the  strength  against  another  reacting 
element  or  compound  c' may  be  calculated— 4et/ and  /'be  the  numbers  in 

column  F  corresponding  to  c  and  c ',  then  ?/      grams  of  c '  reacting  with  the 
solution  of  a. 


To  make  up  a  solution  of  a  definite  oxidizing  or  reducing  power  in  terms  of 
oxygen,  that  is,  of  a  reagent  a  whose  solution  shall  contain  b  grams  of  oxygen 
or  its  equivalent  in  one  cubic  centimeter,  or  if  a  reducer,  one  cubic  centimeter 
shall  combine  with  b  grams  of  oxygen  or  its  equivalent.  The  weight  of  a  to  be 
dissolved  to  one  liter  is  b  X  &• 

Barium  peroxide  (with  H2SO4)  Available  oxygen 10587.5 

Chromium  trioxide u  "     4170.8 

Hydrogen  peroxide "  '•     2126.0 

Potassium  chlorate "  "     2553.3 

Potassium  permanganate "  "     3952.8 

Potassium  bichromate "  "     6133.8 

Potassium  chromate "  "     8096.7 

Sodium  peroxide u  "     4881.2 

Bromine Equivalent  to  oxygen 9993.7 

Chlorine "  "  .- 4431.3 

Iodine  "  '*  15856.2 


TABLES. 


597 


Iron  (in  ferric  salts) Equivalent  to  oxygen 7000.0 

Mercuric  chloride "  "  33862.5 

Ammonium  oxalate  cryst Combines  with  oxygen 8885.0 


Ferrous  sulf ate  cryst 

Iron  (in  ferrous  salts) 

Oxalic  acid  cryst 

Potassium  ferrocyanide  cryst. . . 

Potassium  nitrate 

Sodium  sulf  ate  cryst 

Sulfurous  acid  (SO2) 

Tin  (as  stannous  chloride)  .... 


34772.7 
7000.0 
7878.0 

52841.0 
5321.9 

15767.6 
4004.4 
7437.5 


INDEX. 


599 


INDEX. 


Acetic  acid 225 

Acetone  in  urine 505 

Acetylation 315 

Acetyl  value,  oils 456 

Acidimetry 221 

Acid  value,  oils  45 

Acid,  standard 222 

Aconite,  analysis 418 

Adjusting  balance 37 

Adhesion 170 

Adulteration 578 

Air,  analysis 245 

Air-bath 25 

Air-pump,  water 102 

Agate  mortar 23 

Albumin  in  urine 501 

Alcohol 213 

Alcohol,  determination 393 

Alcohol,  reagent 206 

Alcohol,  methyl 396 

Alcohol,  amyl 397 

Alkali,  standard 222 

Alkalies,  determination 252 

Alkalimeter 12 

Alkaloids 409 

Alkaloids,  table  of 409 

Alkaloids,  extraction 411 

Alkaloids,  indicators 413 

Alizarin 511 

Alumina  in  ores 355 

Alumina  in  silicates 252 

Ammonium  salts,  reagents 207 

Ammonium  sulfate 245,  553 

Analytical  balance 29 

Analysis,  accuracy 526 

Analysis,  colorimetric 259 

Analysis,  volumetric 110 

Apparatus,  extraction 78 

Apparatus,  gasometric 139 

Apparatus,  percolation 52 

Aqueous  vapor,  tension 183,  594 

Arsenic  in  iron 348 

Arsenic  in  ores 356 

Asbestos  filter 91 

Ash,  filter 104 

Ash,  determination ....  105 


Assay  balance 33 

Assay,  fire 268 

Assay  weights 41 

Atomic  weights,  accuracy..  ..526,  546 

Atomic  weights,  'table 592 

Attributive  methods 155,  542 

Attributive  methods,  physical ....  155 
Attributive  methods,  chemical  —   171 

Back  titration 130 

Balance,  assay 33 

Balance,  analytical 29 

Balance,  equation  of 34 

Balance,  testing 41 

Barium  chloride 238,  207 

Barium  sulfate,  occlusion 632 

Barium  hydroxide,  reagent 207 

Base  metals,  assay 275 

Batteries 279 

Battery  fluid,  electropion 207 

Beakers 49 

Beeswax 466 

Berberine 227 

Blank  analysis 9 

Blast  lamp 102 

Bomb,  calorimetric 300 

Bleaching  powder 325 

Boat 298 

Blowpipe  assay 274 

Blyths  digester 51 

Bromine,  reagent * 208 

Bruehls  apparatus 68 

B  ticking  board 23 

Bunsen  burner 58 

Bunsen  pump..... 93 

Buntes  burette 147 

Burettes ill 

Burette  stand 114 

Burning  filters 104 

Butter 486 

Butter  adulterations 488 

Camera  . .  > 260 

Caffeine 228 

Calcium  carbonate,  reag 208 

Calculation 174 


600 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


Capillarity 168 

Carbon  determination 295 

Carbon  in  iron 344,  347 

Carbonates,  analysis 12 

Carbohydrates 427 

Carbonic  acid  in  water 368 

Carius  method 307 

Carmichael  filter 94 

Casseroles 60 

Cellulose 448 

Centrifugal  machine 86 

Chlorate  determination 229 

Chloroform,  reagent 208 

Chlorimetry 322 

Chloral 223 

Chlorine  determination 239 

Chloroplatinic  acid,  reag 208 

Chrome  yellow 23 1 

Chromic  acid,  oxidation 302 

Chromometer 263,  265 

Cinchona  analysis 418 

Coal 359 

Coffee 217 

Color  phenomena 537 

Colorimetry 259 

Colorimeters 263 

Colorimetric  methods 536 

Coal  gas ....  * 152 

Co-precipitation 96,  631 

Combustion  furnace 296 

Computations 174 

Condenser 63 

Continuous  percolation 52 

Congealing  point 163 

Copper  determination 249,  348 

Creatinin  504 

Crucibles,  clay 269 

Crucibles,  Gooch 92 

Crucibles,  platinum 100 

Crucibles,  porcelain 101 

Crusher,  ore 22 

Crucible  fusion 268 

Cryoscopy 491 

Crystallization,  f ract 83 

Crystallizing  dishes 60 

Cupellation 272 

Decantation 86 

Densimetric  methods 186 

Desiccator 26 

Determination,  blank 9 

Determination,  parallel . . .  ° 9 

Dialysis 84 


Difference,  analysis  by 555 

Diamond  mortar 21 

Dishes,  evaporating 60 

Distillation 62 

Distillation,  destructive 65 

Distillation,  fractional 65 

Distillation  in  vacuo 64 

Distilled  water 211 

Double  filters 95 

Drugs,  extraction 411 

Drying 25 

Drying  filters 100 

Duclaux  method 320 

Dyestuffs 506 

Dye-test 507 

Eau  de  Javelle 327 

Electrolysis 278 

Electrodes 282 

Elementary  organic  anal 295 

Emmerlings  tube 56 

Empirical  solutions 182 

Empirical  solutions,  table 595 

Elaidin  test 455 

End  point,  titration 117 

Erlenmeyer  flasks . . .  0 49 

Erdmanns  float 113 

Errors  in  analysis 190,  544 

Ether 220 

Ether  value 457 

Eudiometer 139 

Evaporation 57 

Evaporating  dishes 60 

Evaporation  in  vacuo 60 

Exercises 213 

Extraction  apparatus 52 

Extracted  filtered  paper 89 

Extraction  of  liquids 78 

Extractive,  malt 445 

Fats 452 

Fat  in  milk 479 

Fehlings  solution 430 


Fertilizers 382 

Fibers,  mixed,  analysis 450 

Filtration 89 

Filtration,  rapid 93 

Filtration,  liquids §5 

Filter  paper 89 

Filter  stand 90 

Filter  pump 93 

Filter  ash 89,  104 


INDEX. 


601 


Filters,  burning 103 

Filters,  weighed 100 

Fire  assay 268 

Fixed  carbon 360 

Flash  point 1 70 

Flash  point,  oils 462 

Flasks 49 

Flasks,  volumetric 117 

Float 113 

Fluxing 55 

Forge  scale 230 

Formulae  calculation 177 

Fractional  distillation 65 

Fractional  solution 75 

Fractional  precipitation 83 

Funnels 90 

Furfurol 434 

Fusion  of  silicates 255 

Fusel  oil 393 

Galena 237 

Gas  analysis 139 

Gas  balance 145 

Gas,  reduction  to  normal 183 

Gas  generators 72 

Gases  in  solids 150 

Gas  pipette '. 140 

Gas  volumes,  calculation 183 

Gases,  weight  of 593 

Gasoline  gas  burners 58 

Gelatin,  absorption  of  tannin 424 

Ginger 218 

Glass,  chemical 61 

Glass,  dissolved 61 

Glass  mortars 23 

Glucose 439 

Glycerol 405 

Glycerol  in  wine 407 

Gold  assay... 274 

Goetz  tube 15,  340 

Gooch  crucible 92 

Graduated  glassware 110 

Graphite  in  iron 347 

Gravimetric  analysis 9 

Gravimetric  methods 529 

Greiners  burette 113 

Grinding  machines 23 

Guarana 227 

Halogen  absorption 455 

Halogen  determination 807 

Hardness  of  water 372 

Heating  in  tubes 50 


Hempels  desiccator 26 

Hide  powder 423 

Hogarths  flask . . . .. 160 

Hot  filtration 90 

Hot  plate 57 

Hydrastis 227 

Hydrochloric  acid,  reag 208 

Hydrogen  peroxide,  reag 209 

Hydrogen  sulflde  apparatus 72 

Hydrometer 159 

Hygroscopic  bodies,  weighing ....  46 

Ignition 100 

Ignition  in  gas 107 

Igniting  precipitates 102 

Illuminating  gas,  analysis 152 

Illuminating  oils,  analysis 461 

Impurities  in  precipitates 108 

Indicators 120 

Indirect  analysis  12 

Indigo 513 

Indigo,  artificial 514 

Inversion  of  sugar 434 

Iron,  analyses 328 

Iron,  colorimetrically 250 

Iron  scale 230 

Iron  manufacture 329 

Iron,  silicon  in , 219 

Iron,  volumetric    determination, 

230,  351 

Iron  wire 209 

Iron    ores 350 

Iron  mortar 22 

Jars,  measuring 117 

Jars,  precipitating 86 

Keisers    apparatus 143 

Kellogg  lamp 59 

Kjeldahls  method 306 

Koettstorfers  number 240,  459 

Knapps  solution 433 

Knife-edge 29,34 

Lactobutyrometer 480 

Lakes 531 

Lard , 240 

Lead  carbonate 209,  215 

Lead,  refined 174 

Lead,  determination 238 

Leather 426 

Lemon  juice 223 

Levigation 23 


602 


QUANTITATIVE    CHEMICAL   ANALYSIS. 


Limits  of  error 555 

Lime,    determination 252 

Liquids,  sampling 20 

Litmus 122 

Limits  of  inaccuracy 556 

Lixiviation 75 

Loewenthals  method  422 

Lovibonds  tintometer  261 

Lunges  burette 149 

Lux  gas-balance 145 

Lunges  carbon  apparatus 149 

Magnesia,  determination 252 

Malt 444 

Malt  analyses 445 

Manganese  determination 243 

Manganese  in  iron 338 

Manganese  in  ore 355 

Manganese  ores 322,  350 

Manometer 299 

Mechanical  stirrer 49 

Measuring  flasks 117 

Measuring  jars 117 

Mercury  trough 14o 

Methods,  attributive 155 

Methods,  notes  on 521 

Mebus  method. , 186 

Metals,  gases  in 151 

Metals  and  acids 289 

Melting  point 163 

Methyl  orange 122 

Methyl  alcohol 396 

Metol 233 

Meyers  funnel 62 

Milk 476 

Minerals,  pulverizing 21 

Moist  combustion... 300 

Moisture  determination 360 

Mortars 22,  23 

Moores  apparatus 145 

Muffle 102,  27 1 

Muffle  furnace 271 

Muellers  apparatus 152 

Nickel -copper  alloy 247 

Nitrates  in  water. 877 

Nitrates  in  fertilizers 388 

Nitrites  in  water 379 

Nitrogen  determination 304 

Nitrogen  in  air 244 

Nitrogen  in  iron 152 

Nitrogen  in  fertilizers *388 

Nitrogen  in  water 377 


Nitric  acid,  reagent 209 

Nitroglycerin,  glycerol 408 

Nitrometer 144 

Normal  solutions ...  125,  180 

Normal  solutions,  table 126 

Notes  on  methods 521 

Occlusion  of  impurities 531 

Official  methods 558 

Oils,  mixed,  analysis 459 

Opium  analysis 420 

Ores,  sampling 18 

Ores,  powdering 22 

Ores,  iron  and  manganese 350 

Organic  analysis,  ultimate 295 

Organic  analysis,  proximate 311 

Orsats  apparatus 148 

Oven,  water 27 

Oxygen  in  water 374 

Pan-supports,  balance 32 

Paper,  filter 89 

Paper  pulp 89 

Parting 273 

Penetrability 169 

Permanganate,  analysis 242 

Permanganate,   reagent 210 

Permanganate,  standard 229 

Permanganate,  oxidation 188,  301 

Percolation 51 

Phenol 13 

Phenylhydrazin 434 

Phenol-phthalein 122,  209 

Phosphorus,  determination 309 

Phosphorus  in  iron 337 

Phosphoric  acid  in  ores 356 

Phosphoric  acid,  fertilizer 382 

Pipettes 115 

Pipettes,  assay 115 

Pipettes,  empirical 116 

Platinic  chloride 208 

Platinum  dishes 60 

Platinum  crucibles    100 

Polarization 165 

Polarimeter 165 

Porcelain  mortar 23 

Porcelain  crucibles 101 

Potash  in  fertilizers 386 

Potassium  chlorate 229 

Potassium  salts,  reagents 209,  210 

Potassium  permanganate,  anal...  242 

Potassium  hydrate,  standard.   ...  222 

Potassium  and  sodium,  separation  391 


INDEX. 


603 


Precipitation 68 

Precipitates,  drying 103 

Precipitates,  igniting t 102 

Precipitates,  change  on  ignition  . .  106 

Precipitates,  volume  of 88 

Precipitates,  impurities  in 108 

Precipitates,  filtering 95 

Preparation  of  sample 24 

Pressure  flask 50 

Proteids  of  milk 484 

Proximate  organic  analysis 311 

Pulverizing  solids 21 

Pump,  vacuum 93 

Purifying  compounds 24 

Pyrogallol,  reagent 210 


Quantitative  analysis. 
Quartation 


561 
273 


Radial  burner 59 

Raw  sugar,  analysis 438 

Reagents 205 

Reagents,  solutions 206 

Reagents,  calculation ....   183 

Receiver 64 

Reductor 341 

Residual  titration 130 

Resin  acids 473 

Reichenburgs  apparatus 150 

Refractive  index 167 

Reversal,  weighing  by 38 

Rider 40 

Rider  arm 32 

Roasting 100 

Routine  of  separation 529,  551 

Saccharimeter 165 

Sampling 18 

Sampling  machines 21 

Sampling  shovel 18 

Sachses  solution 433 

Sanitary  analysis 568 

Sand  filter 92 

Saponification 321 

Saponification  equivalent....  240,458 

Sand-bath 67 

Scheiblers  apparatus 149 

Schultze-Tiemanns  method 389 

Scale,  iron 230 

Scorification 270 

Sealed  tubes,  heating  in 50 

Segregation 19 

Separation 74 


Separation  of  organic  bodies 318 

Separation  of  sugars .  435 

Separation,  partial 85 

Separation,  mechanical 74 

Separation  by  distillation 81 

Separation  by  heat 80 

Separation  by  electrolysis 286 

Separation  by  extraction 77 

Separation  by  precipitation 82 

Separation  by  solution 74 

Separatory  funnel 78 

Siegurt  and  Duerrs  apparatus  ....  146 

Sifting 23 

Silicon  in  iron 332 

Silicon,  determination 219 

Silica  in  ores 354 

Silica  in  silicates 251 

Silicates 251 

Silver  nitrate,  reagent 210 

Silver  assay -  273 

Soaps 469 

Sodium  salts,  reagents 210 

Sodium  thiosulfate 234 

Sodium  chloride 216 

Solution 46 

Solutions,  normal 125 

Solutions,  standard 123 

Solvents 47 

Soxhlets  apparatus 53 

Specifications 577 

Special  balances 32 

Specific  gravity  of  gases 162 

Specific  gravity 157 

Specific  gravity,  formulae 185 

Sprengels  tube 158 

Spot  indications 119 

Spectrum  analysis 166 

Stand,  weighing 38 

Standardizing  solutions 123 

Standard  acid 221 

Standard  alkali 222 

Standard  solutions 123 

Standard  permanganate 229 

Standard  methods 558 

Starch 440 

Starch  sugar,  analysis 439 

Steel  works  analysis 670 

Steel,  manganese  in 235 

Steel  manufacture 328 

Steel  mortar 21 

Stirring  machine 48 

Still 212 

Substitution,  weighing  by 38 


604 


QUANTITATIVE    CHEMICAL    ANALYSIS. 


Sublimation 67 

Subnormal  solutions 126 

Sugars 427 

Sugar  in  urine 503 

Sugar  of  milk 485 

Sulfldes,  roasting 107 

Sulfur  in  coal 361 

Sulfur  in  iron 341 

Sulfur  in  ores 356 

Sulfur,  determination 277,  308 

Sulfuric  acid,  determination 233 

Sulfuric  acid,  standard 221 

Sulfuric  acid,  reagent 211 

Sulfurous  acid,  reagent 211 

Superphosphates,  analysis 390 

Tables 592 

Table,  normal  solutions 126 

Table,  indicators 121 

Tanning  extracts 425 

Tannins 421 

Terminology 8 

Testing  the  balance ....    41 

Testing  the  weights 43 

Testing  volumetric  ware 136 

Tension  aqueous  vapor 183,  594 

Thoerners  tube . . . , 56 

Tintometer , 261 

Titration 134 

Titration,  fractional 132 

Thermostat 27 

Titanic  acid 357 

Tobacco,  analysis 419 

Ton,  assay 41 

Torsion  balance 33 

Triangles , 102 

Turpentine 465 

Tube,  pressure 50 

Ultimate  analysis,  coal 364 

Ultimate  organic  analysis 295 

Units  of  electricity 281 

Urea 498 

Urine,  composition  o£ 493 

Uric  acid 497 

Vacuum  pump 93 

Vapor  temperature 169 

Vegetable  matter,  burning 105 

Vegetable  matter,  analysis 450 


Vibration,  weighing  by 38 

Vinegar 223 

Viscosity... 168 

Viscosimeter 168 

Voltaic  energy 170 

Volumetric  analysis 110 

Volumetric  apparatus 110 

Volumetric  methods 533 

Volumetric  solutions 127 

Volumetric  solutions,  normal  —  125 

Volumetric  solutions,  standard . . .  123 

Volumetric  analysis,  reactions ...  110 

Volumenometer 160 

Wash-bottle 97 

Washing  precipitates 96 

Washings,  testing 99 

Water,  natural 366 

Water  bath 59 

Water,  distilled 211 

Water,  weight  of 593 

Water  oven 27 

Water-level 27 

Weighed  filters 100 

Weight  of  water 593 

Weight  of  gases . .  593 

Weight  of  material  for  analysis..  45 

Weighing  bottle 46 

Weighing,  operation  of 37 

Weighing  by  reversal 38 

Weighing  by  substitution 38 

Weighing  by  vibrations 38 

Weighing  in  vacuo 39 

Weighing,  temperature  in 38 

Weights,  atomic,  table  of 692 

Weights,  accuracy 41 

Weights,  assay  41 

Weldons  mud 324 

Westphal  balance 159 

Wine,  analysis 401 

Will-Varrentrapp  method  305 

Wollastonite 25i 

Wood  fiber  analysis 450 

Xanthin  bases 505 

Zeisels  method 316 

Zinc,  reagent 212 

Zinc,  decomposing  sulfides 75 


YD  07384 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


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